Revision 8.2

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IDC Technologies Pty Ltd
PO Box 1093, West Perth, Western Australia 6872
Offices in Australia, New Zealand, Singapore, United Kingdom, Ireland, Malaysia, Poland, United States of America, Canada, South Africa and India

Copyright © IDC Technologies 2016. All rights reserved.

First published 2002

ISBN: 978-1-921716-33-1

All rights to this publication, associated software and workshop are reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. All enquiries should be made to the publisher at the address above.

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Acknowledgements

IDC Technologies expresses its sincere thanks to all those engineers and technicians on our training workshops who freely made available their expertise in preparing this manual.

Contents

Preface i


1 Introduction 1


1.1 Introduction 1

1.2 Definitions 2

1.3 Investigation after accidents and disasters 4

1.4 History 5

1.5 Equipment certification 9

1.6 Conclusion 10


2 Flammability Characteristics, Ignition Sources and the Use of Electricity 11


2.1 Flammability 11

2.2 Flammability information (gasses and vapours) 16

2.3 Dusts 17

2.4 Theory into practice 20

2.5 General sources of ignition 20

2.6 Use of electricity and its sources of ignition 21

2.7 Electrical protection (as opposed to explosion protection) 24

2.8 Toxicity hazard 25

2.9 Conclusions 25


3 Area Classification 27


3.1 General 27

3.2 Overview of the principles of safety 28

3.3 Definitions 28

3.4 The process of assessment 29

3.5 Normal and abnormal operation 32

3.6 Classification into zones 33

3.7 Area classification process 35

3.8 Openings 39

3.9 Ventilation 41

3.10 HAC calculation 43

3.11 Area classification of dust hazards 50

3.12 Responsibility and personnel involved 51

3.13 Documentation 51

3.14 Policy and guidelines for implementation 53

3.15 General information 54

3.16 Area classification standards 55

3.17 HAC examples 56

3.18 Dusts 58

3.19 Classification in North America 59

3.20 Conclusions 60


4 Explosion Protection Philosophy and Equipment Classification Systems 63


4.1 General 63

4.2 Classification concepts 64

4.3 Introduction to equipment certification 64

4.4 Temperature classification 65

4.5 Equipment grouping 67

4.6 Types of explosion protection 72

4.7 Overview of explosion protection theory 74

4.8 Brief comparison of types of protection 75

4.9 Mixed techniques 81

4.10 Dust explosion protection methods 81

4.11 Selection of the type of explosion protection 82

4.12 Conclusion 83


5 Protection Concepts: Type of Protection; ‘d’ 85


5.1 Name 85

5.2 Standards 85

5.3 Definition 86

5.4 Principle of operation 86

5.5 Types of flamepath joint and uses 89

5.6 Explosion pressure 93

5.7 Certification 94

5.8 Cabling requirements for Ex d 95

5.9 Design and type-testing 101

5.10 Installation and conditions of use 102

5.11 Regional variations in Ex d implementation 104

5.12 Illustrations of mechanical construction 104

5.13 Summary 107


6 Protection Concept ‘e’ 109


6.1 Name 109

6.2 Standards 109

6.3 Definitions 110

6.4 Principles of design for increased safety 110

6.5 Component certification 112

6.6 Internal requirements of Ex e 113

6.7 Ex e enclosures 114

6.8 Rotating electrical machines 115

6.9 Cable and glanding requirements 117

6.10 Periodic inspection requirements 119

6.11 Marking 120

6.12 Applications 120

6.13 Summary 121


7 Protection Concept ‘n’ 123


7.1 Name 123

7.2 Standards 123

7.3 Definitions 125

7.4 Principles of design 126

7.5 Construction 127

7.6 Additional means of protection 128

7.7 Applications 133

7.8 Installation 133

7.9 Inspection 134

7.10 Live working 134

7.11 Conclusions 136


8 Protection Concept ‘i’ Principles 137


8.1 Name 137

8.2 Standards 137

8.3 Definition 137

8.4 Origins of intrinsic safety 138

8.5 Principles 138

8.6 Electrical theory 140

8.7 Implementation of IS 146

8.8 The shunt diode safety barrier 150

8.9 Associated apparatus 161

8.10 Electrical apparatus in the hazardous area 162

8.11 Enclosures 171

8.12 Temperature 171

8.13 The Ex i systems concept 172

8.14 An Ex i ‘system’ 172

8.15 Assessment of safety 172

8.16 Simple apparatus 174

8.17 Safety parameters 174

8.18 Temperature classification of systems 175

8.19 Systems concepts in other standards 175

8.20 Conclusion 175


9 Protection Concept Ex p 177


9.1 Name 177

9.2 Standards 177

9.3 Principle of operation 179

9.4 Purging 181

9.5 Pressurisation 183

9.6 Variations 183

9.7 Application notes 186

9.8 Certification and documentation 188

9.9 Pressure / Flow failure 188

9.10 Ex p protection type px, py and pz 190

9.11 Conclusion 192


10 Other types of Ex Protection and their use in Combination 195


10.1 General 195

10.2 Ex o 195

10.3 Ex q 197

10.4 Ex m 198

10.5 Ex s 199

10.6 Multiple-certification 202

10.7 Selection of certification method 204

10.8 Equipment certified for dust 205

10.9 Conclusion 206


11 Earthing and Bonding 207


11.1 Earthing 207

11.2 Personnel safety 208

11.3 Hazardous area considerations 209

11.4 Earthing and bonding 210

11.5 Static electricity 213

11.6 Clean and dirty earthing 215

11.7 Electrical interference 216

11.8 Earthing terminology 218

11.9 Connection of earthing systems 221

11.10 Power supply systems 223

11.11 Portable equipment using batteries 225

11.12 Earthing arrangement standard solutions 226

11.13 Earth loops 228

11.14 Computer earthing 229

11.15 Surge protection systems 232

11.16 Summary 234


12 Installations 235


12.1 Introduction 235

12.2 General 237

12.3 IEC 60079-14: 2013 contents 238

12.4 Other relevant installation standards and codes 239

12.5 General requirements of the standard 239

12.6 Selection of electrical equipment 242

12.7 Dusts 245

12.8 Electrical supply systems 248

12.9 Commissioning 252

12.10 Installed arrangements 252

12.11 Summary 253


13 Inspection and Maintenance 255


13.1 Introduction 255

13.2 Standards 255

13.3 Scope of IEC 60079-17 257

13.4 Definitions 257

13.5 Layout of the standard 258

13.6 Types of protection 262

13.7 Insulation testing 262

13.8 Maintenance 263

13.9 Testing 264

13.10 Unauthorised modification 264

13.11 Earthing integrity verification 265

13.12 Need for inspection and maintenance 265

13.13 IEC inspection tables 272


14 Safe Working Practices 279


14.1 Introduction 279

14.2 Risk assessment 280

14.3 General rules 280

14.4 Danger signals of electrical malfunctioning 281

14.5 Recording of incidents and observations 282

14.6 Maintenance and safe practices 282

14.7 Training of personnel 285

14.8 Electrical fire and shock 285

14.9 Summary 286


15 Fault Finding and Testing 287


15.1 Fault finding 287

15.2 Fault finding routine 287

15.3 Electrical testing in hazardous area 289

15.4 Earth testing in a hazardous area 292

15.5 Repairs 295

15.6 Competency assessment 295

15.7 Summary 295


16 Standards, Certification, Marking, ATEX and DSEAR 297


16.1 Introduction 297

16.2 Progression of standards 297

16.3 A brief history of certification and approval 299

16.4 Types of certificates 300

16.5 Combined protection 302

16.6 Approval versus Certification 303

16.7 The IEC-Ex scheme 303

16.8 Conclusion 304

16.9 Course Conclusion 305


Appendix A – IEC Series Standard Titles for Explosive Atmospheres 307


Appendix B – Ingress Protection Code for Enclosures of Electrical Equipment 311


Appendix C – Case Studies 315


Appendix D – Hazardous Area Classification Tabulation Format taken from IEC60079-10 317


Appendix E – Practical Exercises for Hazardous Areas Course 323


Appendix F – ATEX: European Directives 343


Appendix G – DSEAR: Dangerous Substances and Explosive Atmospheres Regulations applied in the UK 371



Preface

This book provides delegates with an understanding of the hazards involved in using electrical equipment in potentially explosive atmospheres. It is based on the harmonised IEC 60079 Series of International Standards that have now replaced the older national Standards and are directly applicable to most countries in the world (including North America). Explosion-Protected installations can be expensive to design, install and operate. The wider approaches described in these standards can significantly reduce costs whilst maintaining plant safety. The book explains the associated terminology and its correct use. It covers from Area Classification through to the selection of explosion protected electrical apparatus, describing how protection is achieved and maintained in line with these international requirements. Standards require that engineering staff and their management are trained effectively and safely in Hazardous Areas and this book is designed to help fulfill that need.

This book is aimed at anyone involved in design, specification, installation, commissioning, maintenance, inspection or documentation of industrial instrumentation, control and electrical systems. This includes:

  • Tradespersons working in potentially explosive areas
  • Electrical and Instrument Tradespersons
  • Instrumentation and Control Engineers
  • Electrical Engineers
  • Instrumentation Technicians
  • Design Engineers
  • Managers with responsibility for hazardous areas

We would hope that you will gain the following from this book:

  • A good understanding of terminology used with Hazardous Areas
  • An understanding of the hazards of using Electrical equipment in the presence of flammable gases, vapours and dusts.
  • A basic knowledge of Explosion Protection to IEC Standards
  • The ability to do a simple hazardous area classification
  • Details of the types of apparatus that can be used in a given hazardous area
  • How to design and install safe working systems in hazardous areas
  • An understanding of the safety and operational aspects of hazardous areas
  • Knowledge of the system limitations in using hazardous areas protection
  • A brief review of the key areas of the national codes of practice

You will need a basic understanding of instrumentation and electrical theory for the book to be of greatest benefit. No previous knowledge of hazardous area installation is required.


1


Introduction

This manual accompanies the Hazardous Areas training course presented by IDC Technologies. In this first Chapter we begin by examining the legacy of learning from previous accidents. We present an overview of the background and history of Explosion Protection.

Learning objectives

  • To learn from previous disasters
  • To realise the risk posed by electrical equipment in hazardous areas
  • To understand the need for the development of Standards

1.1 Introduction

Any threat to ‘life, property and investment’ is said to constitute a ‘hazard’. In modern manufacturing industries there are many types of hazard. These are encountered in various ways. Each hazard poses a different level of threat.

Where materials that can be ignited are used as part of any industrial process, they are referred to as ‘flammable materials’ and precautions must be taken to prevent the inadvertent occurrence of explosion and fire.

In the design of a plant, the flammable material which may be in the form of a gas, vapour, mist or dust, can be confined, transported, processed or possibly released under different circumstances. In each situation, if it can form a ‘potentially explosive atmosphere’ (PEA) by mixing with air then the simultaneous presence of sources of ignition must be eliminated or adequately controlled. The design of an industrial plant or facility and the equipment and procedures used must render the plant as safe as is reasonably practicable.

The combination of scientific research, technological development and practical experience are the three key considerations in human endeavour to minimise risk. Risk Assessment is the process by which this learning is applied to the concept of safety to achieve what is judged to be at an acceptable level.

This course will study current thinking and practise on the protection of industrial plants to train delegates on the technical and organisational measures required for safety purposes. It focuses on the use of explosion protected equipment operating in hazardous areas. It is suitable for personnel involved in the following activities on industrial plant and equipment:-

  • Process Design
  • Selection
  • Installation
  • Operation
  • Inspection
  • Maintenance
  • Repair and overhaul and
  • Troubleshooting

1.2 Definitions

It is important to define and understand some of the key terms used in this subject:-

1.2.1 Fire and explosion

FIRE (Combustion) is the process of a flammable material undergoing a rapid oxidation reaction that results in the production of heat (and, generally, visible light).

EXPLOSION is the violent and sudden expansion of gases produced by rapid combustion. It is a strong force, producing noise and supersonic shock waves that can cause extensive mechanical damage by the uncontrolled release of energy.

Examples are: -

  • boiler explosion
  • combustion of a gas/air mixture
  • detonation of explosives

1.2.2 Hazard

Hazards are of two types, either ‘Natural’ or ‘Manmade’.

Natural ‘Hazards’, such as blizzards, flash floods, earthquakes, heat waves, hurricanes, tornadoes, volcanic eruption etc, cannot be prevented. Countermeasures can only be taken to minimise the consequences.

Manmade ‘Hazards’, such as the potential occurrence of explosion and fire in industrial situations, are that which this course seeks to address.

1.2.3 Hazardous area

A HAZARDOUS AREA in the context of this subject is:-

  • An area in which a flammable gas or vapour may be present in sufficient quantity to form a ‘potentially explosive atmosphere’

One is reminded that this may be only one of many ‘areas’ on an industrial site which might be prone to an accident or the onset of a potentially dangerous situation in a defined region, owing to the presence of other predominant risks. The ‘hazard’ about which this subject is concerned is for when a ‘fuel’ in the form of a gas, vapour mist or dust is present in ‘atmospheric air’ and a ‘source of ignition’ caused by the presence of electrical equipment occurs simultaneously.

1.2.4 Hazard

An explosion occurs when there is a convergence of three basic ingredients as depicted in the classic Fire Triangle analogy (see Figure 1.1):-

  • Fuel, any combustible material
  • Air with 21% Oxygen
  • Ignition source

Where electricity is in use, it is known that heat and sparking at sufficient levels can provide an adequate source of ignition.

Wherever combustible or flammable materials are stored, handled or processed there is an increased likelihood of leakage or ‘availability’ of the fuel and so it is necessary to be able to predict the circumstances of presence of the elements of the fire triangle. This is a form of Risk Assessment. Proper application is necessary to manage the hazard safely.

In practical terms, with the abundance of air around a process plant, adequate control must be exerted over the other two elements to reduce risk of explosion to acceptable levels.

Figure 1.1
The explosion triangle

1.2.5 Risk assessment

The process of Risk Assessment applied to Hazardous Areas is to define its nature and presence in a given location. Electricity is essential to industry but its use can generate heat or sparks that can ignite a potentially flammable atmosphere. Once defined, equipment and procedures suitable for use in such a hazard can be selected and operated safely. Examination of the issues and acceptable solutions are discussed in this course. The use of electrical equipment protection measures are well established but risk from non-electrical sources are now being included into Standards to be discussed.

Risk assessment must cover all sources of ignition.

After the occurrence of accidents and disasters, the human quest for safety forces thorough investigation by diligent scientific means to reveal likely or actual causes. “Root Cause Analysis” ensures that the set of conditions that has occurred to cause the disaster, are adequately understood. Appropriate precautions can then be taken to ensure that they cannot occur under the same circumstances again. In this way risk is managed and reduced.

The investigation must also take into account the actions of humans in relation to the circumstances of the accident. Causes of accidents have been shown to be by:-

  • Improper installation and selection
  • Lack of proper maintenance
  • Improper use
  • Carelessness or oversight
  • Ignoring warning signs of failure
  • Inadequate training and supervision

Such foreseeable and avoidable human errors must therefore be the subject of scrutiny and prevention.

1.3 Investigation after accidents and disasters

Industrial accidents involving explosion and fire will always be the subject of investigation. Lessons can be learned and so prevention knowledge and techniques can be further developed and improved. Cumulative knowledge now helps to fertilise the thinking processes and the approach taken on an international footing.

The Piper Alpha oil platform disaster occurred on 1st June 1988 in the North Sea off the coast of Scotland and is shown in Figure 1.2. The subsequent investigation uncovered how and why it happened exposing many bad practices owing to poor management. The report was responsible for initiating a dramatic change in the oil and petrochemical industries attitude to the management of safety.

Figure 1.2
The explosion triangle

This is one of many risks that the Owner of any industrial process on commercial premises must consider. The Owner, often referred to as the ‘Duty-holder’ in Law, must ensure that risks are adequately understood and therefore adequate precautions are in place to ensure safety to life, property and investment.

The term ‘Loss Prevention’ is a modern title applied to anybody within an organisation responsible for overseeing the wider implementation of safety. The ‘loss’ could be to any one or more of the three critical values of life, property and investment.

Accidents in the mining industry are still common with other recent deaths in China and other Far Eastern countries. Closer to home in the UK, the Senghenydd disaster, in South Wales on 14th October 1913, killed 439 miners. Investigation into this disaster and the subsequent research taught the mining industry a great lesson that was passed on internationally.

This has led to extensive research and detailed studies to assimilate knowledge in order to prevent explosions and fires in all types of industrial, commercial and domestic circumstances. ‘Hazardous environment’ found in various industries.

Globally, expertise is now shared to educate and prevent accidents. This is culminated in the International Standards to which industries work to maintain safety. The technology is currently based on the identification of the risk of an explosive atmosphere being present in a particular place. This is coupled with the identification of the likelihood of electrical equipment within the explosive atmosphere malfunctioning in a way that would cause it to become a source of ignition coincident with the presence of that explosive atmosphere. The objectives are not just to identify these coincidences but to utilise the information so obtained to influence the design of particular process plants and similar operational situations. This will help to minimize the risk of an explosion due to electrical installations. In this approach, the areas normally prone to have an explosive atmosphere due to the requirement of varies processes involved, are identified. Similarly, the areas where its likelihood is low but identifiable are marked up. It is needless to say that this is not an end in itself but should be deployed as a part of ‘Overall Safety Strategy’ for the plant.

1.3.1 Emergence of standards

The ability of electricity to cause ignition and trigger explosions has been understood since the turn of the twentieth century. Measures to control and minimise the risks have become part of the engineering discipline of design and to meet legislation for safety ever since.

Initially, countries developed their own regional practices independently of each other. The types of industry which were developed in that region depended on the natural resources available and hence the development of local expertise to deal with the hazards. In those early days the rules or practices were created by different organisations often depending on the system of Law in the country.

These have evolved into ‘Standards’ which will be introduced and discussed in this manual. Local Standards such as British Standards have merged into Regional Standards such as from Europe and then have become International Standards. This is discussed in detail in the Chapter on Standards. Examples are:-
UK: BS229, BS889, BS4683, BS5501
North America: NFPA70 – NEC Article 500
Europe: CENELEC EN50 Series
Global: International Electrotechnical Commission: IEC 60079 Series

1.3.2 Methodology

The Standards impose a methodology that looks not only at technical issues but management and control issues. Under the European ATEX Directives, discussed in detail later and adopted by many institutes, the route to safe analysis is based on:-

  • First step : identification of the risk of an explosive atmosphere being present in a particular area
  • Second step : eliminating the use of electrical equipment in such areas
  • Third step : if installation becomes essential: identify the likelihood of electrical equipment malfunctioning

Thus precautions can be taken to assess and prevent ignition occurring in such areas in the most logical and effective way.

An overall plant safety strategy must be developed of which Area Classification, the outcome of the risk assessment, must be part to ensure safety. Thus Explosion Protection techniques can be applied where equipment must function in the possible presence of a hazard.

1.4 History

The use of electricity in a potentially explosive atmosphere was first encountered in the coal mining industry and it is there where the first precautions were developed and implemented. Even before this the hazard of ‘fire-damp’ [methane] was recognised.

Miners observed that the presence of fire-damp would make the flames of the miners’ naked-flame lighting sources (oil lamps and candles) burn a different colour. This was also inadvertently the original hydrocarbon ‘gas detector’. Unfortunately, this was also the source of ignition causing many deaths. The mining authorities realised that burning off the methane collected in the seams of the mines would deal with the immediate problem at the start of a shift. Young miners were covered in wet sacking and would be induced to crawl down the tunnels with a long lighted taper held up towards the ceiling in the highest points thereby setting light to the gas. In later years, penitents, prisoners for serious crimes, were released to perform this service. This made the mines safe for miners to work. Although this was an effective method, it was somewhat barbaric in nature and fell into disrepute. Subsequently, methane and other noxious gasses were removed by improving ventilation.

After a disaster at the Feeling Coal Mine in Northumberland, UK on 25th May 1815 during which 92 miners died, Sir Humphry Davy (assisted by the young Michael Faraday) invented the Davy Safety Lamp (see Figure 1.3) which was first tested underground in the Hepburn Colliery, Tyne and Wear, on the 9th January 1816. This invention must have saved innumerable lives over the years.

Figure 1.3
Davy Safety lamp designs

Around the late nineteenth century and in the early part of the twentieth century the use of electricity in mining began, using d.c. supplies for lighting and motive power. The early equipment produced sparks and some explosions were caused, igniting methane and coal dust. Before World War II, extensive research work was done in Germany and the UK to prevent this and so came the development of a crude form of ‘flameproof enclosure’ suitable for sparking equipment. As a result of the Welsh Mining Disaster, mentioned earlier, the ignition capability of control and signalling systems was realised and so the concept of energy limited circuits became understood and was developed. The concept of ‘Flameproofing’ was to contain an ignition, preventing it from propagating into a hazardous area. Scientists and engineers, however, also knew that if the power and energy levels in circuits were regulated and limited then it could not cause ignition. This subsequently became known as ‘Intrinsic Safety’.

Originally these crude types of protection were developed specifically for the mining industry to be safe in methane and coal dust, but it was realised that the same approach could be used for the developing surface industry.

1.4.1 ‘Surface’ industries and area classification

It became apparent over time that, whereas in mines only coal dust and methane gas presented the hazardous conditions, on the surface a myriad of situations depending upon the type of industry and processes used. To ensure that appropriate precautions were taken, a risk assessment technique evolved to classify each ‘Hazardous Area’. This led to the process of ‘area classification’, that of defining where the hazard might be present, and equipment classification. This was to identify the risks associated with each type of explosive atmosphere and to choose equipment that was known to be safe in those areas.

1.4.2 UK and Europe

In the United Kingdom the first legislation covering the use of electrical equipment in explosive atmospheres came into being through ‘The Electricity (Factories Act) Special Regulations 1908 and 1944, Regulation 27, which states that –

‘All conductors and apparatus exposed to the weather, wet, corrosion, inflammable surroundings or explosive atmosphere, or used in any process or for any special purpose other than for lighting or power, shall be so constructed or protected, and such special precautions shall be taken as may be necessary adequately to prevent danger in view of such exposure or use.’

Even Regulation 6 of ‘Electricity at Work Regulation 1989’ is in the same spirit of placing the responsibility of achieving the objective on the owner of industry without specifying the methods to be adopted. In the UK, it was legal to use uncertified equipment in hazardous areas provided that it could be shown to be safe. Thus, as long as owner maintains adequate records of plant safety this clause gets satisfied.

This is in variance to the one being followed in USA and Germany and other parts of Europe, where specifics are also formulated. Both approaches have withstood the test of time and there is not much evidence of putting one method over the other.

In the UK much work has been done in the area of electrical installation safety in hazardous atmosphere by the Safety in Mines Research Establishment, the Electrical Research Association [now ERA Technology Ltd.], the Fire Protection Association, Institution of Fire Engineers, Loss Prevention Council and The Institute of Petroleum.

With the advent of automobiles and airplanes in the early 1920s, fuel refining began and increased in capacity very quickly. Volatile vapours from oil by-products and electrical sparks and heat did not mix safely! Fires and explosions were common in the industry. So the first hazardous area classification was invented about this time but it is thought that the Imperial Chemical Industries (ICI) Company had started to evolve the notion of Divisions. Division 1 described areas being normally hazardous.

In the wake of the mining disaster in South Wales, investigation into the cause and then further research pioneered by Newcastle University began the understanding of how Explosion Protection could be harnessed. Thus, a new industry with the goal of protecting electrical equipment in hazardous areas was born. Flameproof enclosures and simple intrinsically safe circuits were now being used, the first Standard for FLP (BS229) equipment being issued in 1928. Oil immersion followed and, together, these were the first types of protection developed.

World War II brought many changes in Europe and North America. Metal shortages in Europe prompted more plastic use in electrical equipment, and the first construction standards for explosion-protected electrical equipment appeared in Germany.

At about the same time, North American industries determined that hazardous area classifications needed to be expanded. A Division 2 was needed to describe locations that were not normally hazardous to allow use of less expensive equipment and less restrictive wiring methods.

1.4.3 The USA

In the United States of America the NFPA (National Fire Protection Association) was formed in 1896 with the aim to reduce the burden of fire on quality of life by advocating scientifically based consensus codes and standards. It also carries out research and education for fire and related safety issues. The Association was incorporated in 1930 under laws of the Commonwealth of Massachusetts.

The Electrical section was added in 1948. The National Electricity Code (NEC) under NFPA 70 defines rules and regulations regarding use of electrical equipment. The sections 500 through to 517 deal with installation, testing, operation and maintenance of electrical equipment in hazardous area.

In addition, various government laboratories, university laboratories, private and industrial laboratories do research and education in USA. One of the most prominent is the Underwriters Laboratories Inc. This was founded in 1894. It was originally conceived to serve the insurance industry as an arbitrator for safe practice but is now a not-for-profit corporation having as its sole objective the promotion of public safety through the conduct of –

“…scientific investigation, study, experiments, and tests, to determine the revelation of various materials, devices, products, equipment, constructions, methods, and systems to hazards appurtenant thereto or to the use thereof affecting life and property and to ascertain, define, and publish standards, classifications, and specifications for materials, devices, products, equipment, constructions, methods, and systems affecting such hazards, and other information tending to reduce or prevent bodily injury, loss of life, and property damage from such hazards.”

The role of Federal Government was minimal in Fire protection prior to 1974. However, in 1974 Congress passed the Federal Fire Prevention and Control Act. Under this act 12 Executive Branch departments and 10 independent agencies are supposed to administer the various provisions of the Act. The NFPA enjoys a co-operative relationship with these agencies. A number of agencies rely upon NFPA Standards and participate in the NFPA standards-making process.

In 1970, the Congress established the Occupational Safety and Health Administration (OSHA) within Department of Labour to oversee development and implementation of mandatory occupational safety and health standards – rules and regulations applicable at the workplace. The Mine Safety and Health Administration was established in 1977 with a functional scope similar to that of OSHA, but with a focus on mining industry.

In order to understand how the electrical code is evolving and what guides this evolution we need to look back in history and their development till date. In the early 1900s, when contractors were busy electrifying industrial buildings, electrical wires were run through existing gas pipes, resulting in today’s conduit system of wiring. This formed the basis of wiring in North America and the codes and standards were made to suit the safety requirement pertinent to these practices.

While this was being done on the American continent, the International Electro technical Commission (IEC) was founded in Switzerland in 1906. The IEC is supposed to be the “United Nations” of the electrical industry. Its ultimate goal is to unify worldwide electrical codes and standards. Few IEC practices were incorporated into the NEC or CEC mainly because North America operated on different voltages and frequencies than most of the rest of the world.

1.4.4 Advent of hazardous area in surface industries

In the 1960s, the European community was founded to establish free trade through Europe. To reach this goal, technical standards needed to be harmonized. As a result, the European Community for Electro technical Standardisation (CENELEC) was established as the Standards writing body for Europe.

By this time the German chemical industry had departed from the traditional conduit or pipe wiring system and migrated towards cable as a less expensive alternative. This wiring-method change led to the zone classification system later adopted in 1972 by most European countries in a publication known as IEC 79-10. This action led to the different methods of classifying hazardous areas as well as protective, wiring, and installation techniques, which form the basis of present IEC classification.

1.5 Equipment certification

It would be a very costly and time-consuming affair to test each electrical installation for safety. Standards could not be written for design validation and conformance of plant as it varies dramatically from one application to another. The nature of the flammable chemicals used would be different. Thus, in Europe, a system of certification evolved for the use of electrical equipment to be installed in a hazardous atmosphere of a Plant.

This technology primarily consists of –

  • The classification of the area of the Plant where the electrical equipment is needed.
  • The classification of the electrical equipment used in that area of the Plant.

The classification systems used for both must be the same so that it would be easy to determine if a particular piece of equipment would or would not be safe in a given area. The International Standards and Codes of Practice have been developed from the range of those used in individual countries. Obtaining agreement has not been easy. The benefit to the user is to provide a level of confidence for safe operation of electrical equipment under the specified conditions.

These classifications will be explained in some depth in this course, allowing an understanding of the application for given situations.

The certification process merely states conformance with the Construction Standard to which equipment has been assessed. It does not imply that the equipment is safe. A few examples of the marking of equipment are illustrated in Figure 1.4.

Currently, internationally acceptable markings are used to identify Explosion Protected (Ex) equipment. This leads to uniformity in the industry and gives a confidence level to the user, vis-à-vis, the suitability and integrity of quality and design and lessens the work of manufacturer in getting each piece approved all over the globe.

Figure 1.4
Typical certification logos

In Europe, under the ATEX Directives that came into force in member states on 1st July 2003, all ignition-capable equipment (Electrical and NON-electrical) that is sold into or used on plants requires assessment and certification to harmonised Standards if used in a Hazardous Area. The Directives, in line with the Standards, expect Plant management to keep records and documentation of all aspects of safety related plant and equipment so as to be able to demonstrate safety compliance.

The harmonised IEC 79 Series of Standards are recognised as IEC60079 and have been adopted in many countries including the USA who has now incorporated the relevant classification requirements into NEC. In time all countries claiming IEC compliance are expected to follow.

1.6 Conclusion

This chapter has introduced a wide range of subject matter which will be discussed in detail in subsequent chapters.

The overview and brief history of the development of the Standards given here will help to provide an appreciation of the depth of engineering that has been dedicated to the prevention of accidents.

There are many misconceptions on the principles in this subject that can undermine safety unless properly understood. The early separate development of Standards in different countries goes some way to explaining why the industry suffers from terminological inconsistency which has, in part, thought to have exacerbated these misunderstandings.

This Manual focuses on the harmonised International Standards as the primary source of information in an attempt to unify the terminology. There remain regional variations in the approach to Hazardous Area and the implementation of Ex equipment. These are mentioned and clarified where it is felt appropriate.


2


Flammability Characteristics, Ignition Sources and the Use of Electricity

This chapter reminds the reader of basic issues and definitions that will be required to understand the risks associated with Electrical Equipment being used in Hazardous Areas where Explosion Protection has been applied.

Learning objectives

  • To examine the relevant properties of flammable materials
  • To define the terminology used for these properties
  • To define the mechanisms that cause ignition
  • To remind the reader of important basic electrical terms

2.1 Flammability

‘Controlled’ combustion is merely ‘fire’ that has been harnessed for the benefit and ease of human convenience. Methane extracted from underground is piped to homes and businesses where the rate at which it is allowed to combust produces the right amount of heat needed to roast meat or run a gas-turbine alternator. It is said that the control of fire singles out human sophistication from other species on this planet.

In both examples, the gas is allowed to mix with air at a controlled rate at the point of combustion. The oxygen can then support combustion to produce the required amount of heat.

Where a gas is mixed with air and then a source of ignition is introduced, the burning is so rapid as to provide an ‘explosion’. This is ‘uncontrolled’ combustion which proves a threat.

Figure 2.1
Controlled and uncontrolled combustion

In Figure 2.1 the burning of gas at the outlet where it mixes with air shows controlled combustion but the example of a building fire is uncontrolled in that the rate of propagation is only limited by the ability of the flames to reach and consume unburned material. The fire would escalate until the fuel is exhausted or it is ‘brought under control’ by the Fire and Rescue Services.

In all cases this definition of ‘Flammability’, provided in various Standards is true:-

“A gas, vapour, liquid or solid that can react continuously with atmospheric oxygen and may therefore sustain fire or explosion when such reaction is initiated by a suitable spark, flame or hot surface.”

The important difference between controlled and uncontrolled combustion is purely the rate at which this happens. Burning some ‘fuels’ such as wood and coal naturally includes rate limiting. Solids and liquids do not combust and cannot explode in that state. It is only the gas which is liberated by the decomposition of the burning process itself that supports the combustion. A fuel is any flammable material that is in a gaseous state or that liberates gas as it is heated. Burning, then, may be regarded as a form of chemical decomposition.

Where gas is released into the atmosphere it will mix with the air and if in sufficient quantity, it will explode on contact with a source of ignition. This, by definition, is ‘uncontrolled combustion’, which must be avoided as extensive damage would be caused by the capacity to release energy. Where a gas/air mixture is present in its unburned state then it is referred to in the following text as a ‘hazard’, also being described as a ‘potentially explosive atmosphere’ (or p.e.a.).

2.1.1 Forms of flammable material

An important aspect of flammability is the distinction between gasses, vapours mists and dusts. Mists and dusts will be discussed later in this Chapter.

At normal pressures and temperatures (NTP), if a material is in its gaseous state, then mixture with air can occur at that temperature and a flammable atmosphere (or hazardous atmosphere) can be formed. Where a flammable material is only present in its liquid state, it presents no hazard. It is important to know under what conditions the transition will occur.

If the temperature of the liquid rises then increased levels of vapour will be emitted from the surface of the liquid and the concentration of vapour above the liquid will increase.

The critical conditions are explained in detail below.

2.1.2 Flashpoint for liquids

At normal ambient temperature, flammable materials in a gaseous state when released into the atmosphere will mix with air. If the material is in a liquid state at ambient temperatures, then knowledge of the Flashpoint is necessary to determine if there is a possibility that a flammable mixture will result.

Figure 2.1
Flashpoint and ignition temperature

As the temperature of a substance is elevated above its freezing point, it will turn into a liquid. Even at this temperature some vapour molecules will be given off naturally (Triple-point). As the temperature increases more vapour will be liberated. The process of converting a liquid to a gas by providing heat to increase the partial pressure at the surface of a liquid is known to chemists as ‘flashing’.

The ‘Flashpoint’ is of interest for hazardous area definition purposes because it is a temperature above which the liquid can form a hazard. The definition of Flashpoint is:-

  • “The minimum temperature of a liquid, at which, sufficient vapour is given off from the surface of that liquid in still air such that it may form an explosive gas-air mixture”.

The Flashpoint for each flammable material in liquid form is different. Where its temperature remains reliably below the Flashpoint then any vapour given off cannot form a hazard because the vapour present will be below the concentration level of the LFL, even though the presence of the gas may be detectable.

If the liquid was spilt onto a hot surface above the flashpoint, vapour would be liberated such that a hazard would be present in the vicinity of the spill.

2.1.3 Ignition temperature

Irrespective of the flashpoint for liquids, raising the temperature of a gas or vapour enough would encounter a point at which the material, when mixed with sufficient air, would ignite. This point is known as the Ignition Temperature which is defined as:-

  • “The lowest surface temperature to which the most easily ignitable mixture has to be exposed to cause ignition when tested as described in publication IEC 79-4”.

This is also known as Auto Ignition Temperature (AIT) or Spontaneous Ignition Temperature (SIT).

It is easy to become confused between Flashpoint and Ignition Temperature but it will be realised that there is a vast difference between them. Ignition temperature will ALWAYS be higher than Flashpoint. The Flashpoint can be below 0°C as well as above.

A further issue is the size of the surface area in contact with the flammable gas air mixture. As the size decreases in area, the temperature at which ignition takes place rises. The discovery of this fact is useful in the design of equipment and will be discussed in later Chapters.

2.1.4 Gas concentration

The concentration of any given gas or vapour in air must be within certain limits before it will form an ignitable mixture. Different gasses and vapours have different limits.

The flammability limits

These are the extremes of concentration within which the ignitable mixture of gas or vapour, in air, must be present in order to allow combustion to take place. Note that these are sometimes referred to as ‘Explosive’ limits.

Upper flammability limit [UFL or UEL]

The highest (richest) concentration of a given vapour in air above which ignition will not take place

Lower flammability limit [LFL or LEL]

The lowest (leanest) concentration of a given vapour in air below which ignition will not take place

Above or below these limits there is insufficient oxygen or insufficient fuel, respectively, to sustain combustion. The “Flammable Range” is said to be the concentrations between the upper and lower limits. In Figure 2.1 the flammable ranges of three particular gasses are shown graphically plotted against the amount of energy required to ignite them at various concentrations.

2.1.5 Ignition by energy

Between the Limits of flammability there exists a Most Easily Ignitable Mixture’ (MEIM) for each type of gas. This is at the bottom of each of the curves. Below this value ignition by energy cannot take place for any concentration of the gas mixture.

Figure 2.1
Flammability limits illustrated for Energy

For example, Hydrogen has a flammable range between 4% and 76% by volume of hydrogen in atmospheric air at normal temperature and pressure (NTP). Oxygen is normally at about 20.8% in air for the purposes of NTP. The MIE figure at the MEIM of 26% is taken as 20µJ. Other gasses and vapours all have different figures and there is no defined relationship known to scientist and chemists as some believe. Hydrogen is said to be the most easily ignitable gas known as far as spark energy is concerned.

2.1.6 Ignition by heat

Raising the temperature of a gas/air mixture that is within its flammable range, even in the absence of energy from sparks or flames, will eventually cause ignition. This is plotted on the graph in Figure 2.2. Notice that for hydrogen, the same flammable limits and shape of the curve applies. The minimum ignition temperature (MIT) is 560°C. So, although Hydrogen is the most easily ignitable gas by spark energy, it requires a very high temperature to ignite. There is no correlation between these characteristics.

The ignition curve characteristics for heat and energy are not symmetrical about the point of the MEIM.

Figure 2.2
Flammability limits illustrated for temperature

2.1.7 Relative vapour density

Gas or vapour released in to the atmosphere will have a tendency to rise or fall because of the effects of buoyancy in air. This is determined by its density and the most useful way to predict how it will behave is to compare it with that of Air. Thus it is expressed as Relative Vapour Density (RVD). It is defined as:-

  • “The ratio of the weight of one unit volume of gas or vapour compared with that of AIR at the same temperature and pressure”.

The value will determine how the gas or vapour will travel when released:-

  • If the value for a gas is greater than one (>1) it is heavier than air: the vapour would tend to sink towards the ground
  • If the value is less than one (<1) it is lighter than air: the vapour would tend to rise.

To some extent the gas (or vapour) will be carried by the movement of air so that a release would eventually become diluted by natural means. From a point of release the direction that the gas will move is of interest to determine where the hazard will be formed.

2.2 Flammability information (gasses and vapours)

Typical Data for gasses and vapours is taken from IEC79-20:-

Table 1
An extract from IEC60079-20 showing typical flammability data
Material Boiling Point Flashpoint RVD Ignition Temp. Flammable limits % in Air
  ºC ºC   ºC LFL UFL
Acetylene -83 Gas 0.9 305 2.3 100
Ethylene -104 Gas 0.97 425 2.3 36
Hydrogen -252 Gas 0.07 560 4 77
Methane -162 Gas 0.55 537 4.4 1.7
Propane -42 -104 1.56 470 1.7 10.9
Kerosene 40 +38 1.2 210 0.7 5

2.2.1 Notes on combustion

Once combustion is started, it will continue until either

  • the combustible material is all consumed
  • the oxidising agent concentration is changed
  • the gas concentration is lowered to below its LFL or increased above its UFL
  • the flames are chemically inhibited (oxygen starvation)
  • the mixture is sufficiently cooled to prevent further reaction
  • sufficient heat is removed or prevented from reaching the combustible material

The partial vapour pressure of a flammable gas will have a small influence on the flammable range but since the majority of consideration is for release into atmosphere then no account or correction of this is required.

Research has shown that oxygen enriched atmospheres will widen the flammable range, lower the ignition temperature and decrease the minimum ignition energy levels.

Raising the temperature of a gas does not decrease the amount of spark energy required to ignite it. Investigation has found that there is no link and that heat and energy are separate issues.

There are some 750 known flammable substances of which about 250 are in the most common industrial and commercial use. It is these materials which have been tested at various times in the past for other Standards and are now compiled into reference lists included in IEC60079-20. The important issue here is that the properties of gasses and vapours remain totally consistent.

The compilation and study of this information has enabled their categorisation into a system of classification accepted internationally that is applied both to the gasses and to the equipment that is to be used in their presence. This makes the task of defining a hazard and selecting equipment to operate in association with the hazard more effective and reliable.

2.2.2 Mist

A mist is formed when a liquid is released in droplet form under relatively high pressure. It is understood that if ejected at high pressure, the release can force the emission of vapour from the liquid at lower temperatures than that of the true Flashpoint. Research is being performed on this aspect at the time of writing and it is likely that additional guidance will be published in a proposed new part of the IEC60079 Standards.

The hazard is worsened if the liquid temperature is above the flashpoint because the mixing with atmosphere is made more efficient. In any case, a flammable liquid, having a higher mass than a gas is likely to travel further, increasing the area over which the hazard may be present.

It is also known as a result of tests that, at high-pressure releases, the surface temperature and the energy level required to cause ignition are lowered, and in some instances, quite significantly.

Testing may be necessary to ensure conditions are well understood and defined. Spray-painting using solvent-based paint is such an example.

2.3 Dusts

Dusts are defined as materials that comprise finely-divided particles. About 70% of dusts used in industrial processes are considered to be flammable.

Coal dust and saw-dust (from wood cutting) are obvious examples of matter that occurs by being processed. Other substances used in common processes are purposely utilised from a powder form, such as types of paint pigment, tablet manufacture in the pharmaceutical industry, grain husks that disintegrate and settle in silos and plastics in pellet form for moulding purposes.

2.3.1 Characteristics of dusts

The ignition and burning properties of any dust or particulate matter will be principally determined by:-

  • particle size
  • concentration in air of an airborne cloud, and
  • moisture content

Since these properties are inconsistent, unlike gases, then actual characteristic will vary and are unique to the circumstances of the method of processing and handling the dust. This can even vary throughout the handing train.

There are two further considerations for the form of dusts:-

  • Clouds, and
  • Layers

In ‘Cloud’ form, the air and dust are said to be mixed: in this form the dust is readily ignitable by sparks and heat.

In ‘Layer’ form, the dust is not subject to mixing with air. A layer may be easily converted into a dust cloud by air movement or shockwaves transmitted through the air.

Of greater concern is that a layer of dust over a heat-dissipating surface of equipment will act as thermal insulation. It is likely that the surface temperature underneath the layer will rise because it cannot now shed the excess heat. If the surface temperature goes too high, the inter-most particles of the layer will begin to decay at a point called the ‘smouldering temperature’. Soon after, the dust will begin to decompose and burn causing an expansion effect.

Figure 2.2
Flammability limits illustrated for temperature

The Fire Triangle needs to be expanded to a Fire Pentagon to explain the additional circumstances of dust ignition. The flame propagation mechanism within a dust causes a two phase effect. The first phase is the decomposition expansion. That causes the hot dust to be propelled outwards causing it to mix with air. This dislodged hot dust now forms a cloud with a lower ignition temperature. If the heated dust cloud is above the cloud ignition temperature then it ignites. This causes a more powerful pressure wave causing more dust in the layer to be converted into a cloud which ignites and so the process propagates the explosion with increasing destruction.

2.3.2 Dust ignition temperature

The ignition temperature of dusts in both Cloud and Layer form must be determined by experiment. The Standards now refer to common terminology as follows:-

TCL = Ignition temperature of a dust Cloud
T5mm = Ignition temperature of a layer of dust 5mm thick
T12.5mm = as above but 12.5mm thick
TMAX = maximum calculated temperature of equipment in a given dust atmosphere

2.3.3 Dust spark ignition

Tests are carried out to determine the spark ignition level for the dust. These are now done in accordance with methods specified in Standard IEC13821. Generally dust clouds require millijoules of energy rather than microjoules but if the value is less than 10mJ then a cloud is likely to be ignited by the static charge that can be generated by plant operators and so addition precautions need be put in place.

2.3.4 Dust KST and PMAX

Where prevention of ignition is complex and difficult then pressure relief of dust explosions in handling systems are used as additional personnel and plant protection. To design such a system, the maximum explosion pressure PMAX reached by an ignited dust cloud and the KST value must be determined.

The KST value depends on the material of the dust, the volume of the dust cloud, the volume of the containment area and the concentration levels. It is determined by a combination of calculation and experiment.

Obtaining the KST value of a dust is a measure of its explosion propagation rate. It is a mathematically derived constant calculated from measurement of pressure rise in a 1m3 container that is fully filled with an optimum concentration of a dust in cloud form according to ISO 6184/1:1985 and IEC14034 Parts 1 & 2. Dusts are divided into three groups of explosibility for which pressure relief systems are certified.

Two forms of Dust Practice are introduced in the IEC60079-14:2007 Installation Standard. These refer to the layering of dusts on equipment and the determination of the T rating of equipment. This Standard should be consulted for more detailed information including for dust layers of varying thicknesses and the de-rating factors that must be used.

2.3.5 Typical dust data

Table 2
Information on Dusts originally from IEC61241-20
Material Ignition Temperature of dust cloud (°C) Minimum Spark Energy required for ignition of cloud (mJ) Minimum Explosive Concentration (g/metre3)
Aluminium, milled 550 15 35
Zinc 600 650 385
Polystyrene 490 15 12
Urea resin 450 80 55
Cocoa 420 100 35
Coffee 410 160 70
Cotton seed 470 80 45
Grain dust 430 30 45
Sugar 350 30 30
Coal 610 60 45
Cork 470 45 30
Sulphur 190 15 30
Wood flour 430 20 35

The approach to determining explosion protection safety with a dust hazard was given in the International Standard IEC 1241. This Standard has now been fully integrated with the IEC 60079 Series.

2.3.6 Behaviour of dust

By its very nature, dust flammability characteristics are not as predictable as that of gases or vapours. When dust is released into the atmosphere it disperses in the air as a cloud and may become widely spread by air movement. A cloud of flammable dust within its flammable range can be ignited and flash-fire or, in a confined space, it will explode. A lower particle weight (light) dust remains in suspension longer than a heavier dust. Dust in still air eventually settles and can lie dormant in a layer on exposed horizontal surfaces. Should these surfaces attain an elevated temperature above ambient, or should another source of ignition be present, a layer of flammable dust is a constant risk and may be ignited.

Some flammable dusts in layer form when ignited have the ability to propagate combustion by flame or smouldering, the latter particularly when the dust is present in bulk. In some cases when the ignition source is removed, combustion of the dust layer ceases. In other cases, it continues and the dust layer is said to train fire.

A layer of flammable dust can be disturbed to form a dust cloud, which may spread and eventually settle again to form another layer. This cycle can be repeated from time to time. Should a small explosion occur layers of flammable dust over a large area could be disturbed to form a cloud. This, on ignition can create a secondary explosion and/or fire of considerably greater damage-potential than the small primary explosion. In a plant handling flammable dust, a high standard of ‘housekeeping’ is therefore essential.

The problems associated with dust layers and dust clouds are complex. The data that may be relevant in the case of ignition of dust layers and clouds are ignition temperature, minimum ignition energy and thermal stability and, in addition, in the case of dust clouds, lower flammable limit. For the latter typical values are in the range 0.01-0.06 kg of a flammable dust dispersed in each m3 of air and such a concentration is clearly visible. It should be noted, however, that these parameters do not provide a direct measurement of the sensitivity of the dust to ignition and are subject to certain limitations.

The moisture content of the dust will also affect its behaviour. This means that in different places its ignitability may vary because of the way it is involved in a process. Particle size, material and density affect the ignitable properties but also affect its ability to absorb moisture or any other material. Particle size and ease of ignition are not necessarily related. It does not appear a cloud of more finely divided material will ignite with less energy or with a lower temperature exposure.

In the case of dust layers, the data are affected by the thickness of the layer, the temperature of the surface on which the dust rests, and that of the immediate surroundings. For example, the ignition temperature may fall as the thickness of the layer increases. It will vary should the dust be exposed to hot surfaces for a period and although in many cases the ignition temperature will rise when the dust degrades there are other instances when it will fall.

In the case of dust clouds, the data may be affected by solvent content, presence of additives, and by the particle size distribution or clumping effect in the cloud. It is therefore essential to obtain expert advice to provide such data and to ensure that it is relevant to the particular dust in the form in which it will occur in the plant. The testing of samples is best practice.

2.4 Theory into practice

So far the theory of combustion has been explored and then quantified to a large extent. From this discussion emerges the need to identify what form a source of ignition can take in practical terms.

2.5 General sources of ignition

At this point it is appropriate to list some obvious sources of ignition which must be identified in order to ensure that adequate precautions are taken.

  • Hot surfaces
  • Arcs and sparks
  • Electrostatic discharge
  • Electromagnetic radiation
  • Atmospheric discharge (lightning)
  • Mechanical friction
  • Impact sparks
  • Ultrasonic energy
  • Adiabatic compression (shock waves)
  • Ionizing radiation
  • Optical radiation
  • Chemical reactions (Exothermic and Thermite)
  • Open flames
  • Welding
  • Grinding
  • Cutting

These can be divided in processes, actions and other categories but one common theme is where electricity is used.

2.6 Use of electricity and its sources of ignition

Electrical energy can be utilised to produce, light, heat, magnetic force and chemical changes. Such applications for the purposes of this discussion are either for power usage or for measurement and control:-

Power Applications

  • Motors: Fans and Pumps
  • Lifts, cranes, hoists, stirrers, conveyors
  • Chillers & Refrigeration
  • Air conditioning and ventilation
  • Lubrication
  • Valve positioning
  • Material handling
  • Lighting
  • Heating

Instrumentation and Control

  • Measurement of process values
  • Communication of these values
  • Programmable Logic Controllers
  • Distributed Control Systems
  • SCADA (supervisory control and data acquisition)
  • Emergency Shut-down Systems
  • Safe Guarding Systems
  • Communications equipment

In all cases there are various types of hazard associated with the use of electricity. As a result, most countries have laws and guidelines in place providing for the safety of electricity distribution and consumption. In the UK, BS7671 (the IEE’s 17th Edition rules) are the typical guidelines and are similar to many other countries. They cannot normally be harmonised because different voltages and frequencies are used in different countries historically.

User companies will normally write ‘Electrical Safety Rules’ for their plant, which are dependent on its design and operation. Personnel must be trained and supervised in the application of these rules for safety.

The types of hazards may be summarised as:-

  • shock
  • flash-over
  • heat
  • radiation
    and of particular interest here is:
  • fire
  • explosion

Ignition can be provided directly by spark or heat derived from the use of electricity by any of the following means:-

  • Resistance heating
  • Dielectric heating
  • Induction heating
  • Leakage current heating
  • Energy from arcing
  • Static discharge
  • Lightning current heating

Electricity may provide the ignition directly as in the overheating of a conductor or indirectly by providing the power to rotate a motor where the bearing has failed and the friction causes a rise in temperature. Either way, each potential source of ignition must be identified and controlled if it can occur in the presence of a flammable gas mixture.

It is important for equipment designers to understand the processes where operation or failure can cause ignition and design with ample precautions and adequate safety margins.

Incidentally, there are six possible ways of generating electricity in varying quantities:-

  • by friction - static
  • by pressure – piezoelectric
  • by heat – thermoelectric
  • by light – photo electric
  • by chemical action – battery
  • by magnetism – an electromagnetic generator

The latter two are useful for the generation or storage of higher capacity supplies. The other methods of generating electricity only produce low quantities but the principles are often used for measurement and therefore instrumentation and control purposes.

2.6.1 A simple circuit

Figure 2.2
Basic circuit operation to explain key electrical concepts for ignition

Consider a supply, a switch and a load as shown in Figure 2.2. When the switch is closed, a current will flow according to Ohm’s Law. This allows calculation of dissipated power, I2/R or V2/R or VxI in the ‘load’ and is related to the heating effect. The temperature of the load will rise depending on the quantity of dissipated power, the mass and the heat dissipation properties of the load.

In addition, the circuit dissipates some heat wherever a current passes through any resistance including that of the conductors. They are therefore sized as part of the design to be appropriate for the load current.

When the switch is open, no current flows and the voltage across the poles of the switch will be the ‘open circuit’ value, that of the supply voltage. When closed, an expected value of current (said to be the ‘load’ or the ‘operating’ current) will flow according to Ohm’s Law. It will flow from the supply, through the switch and then through the load back to the supply. The circuit is said to be a series circuit.

The action of breaking or opening and making or closing the contacts will change the current flow through the touching parts of the switch from zero to full load. The change at the mechanical interface of the contacts will cause sparking. The magnitude of the spark will depend on the values of the open circuit Voltage and the load Current experienced at the transition of the switch. The circuit is purely resistive.

Little is understood about the actual mechanism by which sparks or heat couple energy into flammable gas mixtures causing them to combust. The spark process is thought to be due to ionisation taking place through the gas as in:– ‘an electric discharge, usually accompanied by a popping sound, that occurs when air or gas between two charged conductors becomes highly ionized, enough to breakdown, conducting current through a distinct, luminous channel’. Nevertheless, the point at which ignition occurs can be demonstrated as consistent and can be measured.

Figure 2.2
Wheeler’s Spark Test Apparatus

Originally it was believed that any spark would ignite a flammable gas/air mixture. Wheeler and Thornton from Newcastle University, who were appointed to investigate the Senghenydd mining disaster of 1913, created experiments to demonstrate that the size of the spark was influenced by the magnitude of Voltage and Current. They developed a method of determining what currents and voltages would create incendive sparks. Measurements were subsequently made and recorded using the apparatus which is shown diagrammatically in Figure 2.2.

A modern ‘Spark Test Apparatus’ based on this design is included in the Standard IEC60079-4 used to determine both gas and equipment characteristics. A flammable mixture surrounds a revolving contact arrangement. This uses a tungsten wire brushing on a slotted cadmium disc to make and break a circuit 1600 times during the course of a test. The voltage across the contact is derived from a constant-voltage supply. A variable resistor placed in series with the supply limits the short circuit current, through the contact when it is closed. A range of voltages and currents are then applied to the switching contact to determine at what level ignition occurs. A graph is plotted, taking the curve below the lowest points. Different flammable mixtures reveal different curves. The curves and now tabulated figures for the more precise interpretation by apparatus design engineers and certifying authorities are included in the appropriate Standards. These are discussed later in this manual.

The material of the disc was found to influence the combustibility of the mixture. If Cadmium, Zinc, Aluminium or Magnesium were used then the spark produced for the same current/voltage was larger. These materials are often found in association with electrical circuits and therefore the standards publish curves and figures, which include these materials for the benefit of equipment circuit designers.

2.6.2 Stored energy

In any electrical circuit, energy can be stored in different forms. The size of the spark released at the switch contacts in Figure 2.2 will also be increased by the stored energy in the circuit accumulated in elements of capacitance or inductance. This stored energy must be released instantaneously as a spark when the circuit conditions change, i.e. from closed to open. The Spark Test Apparatus was used to define limits of Inductance for values of Current and Voltage for values of Capacitance for a range of gasses and vapours. These curves and tabulations are also included in the appropriate Standards.

Ignition from electrical sources is by four mechanisms:

  • Switch action releasing sparks in purely resistive circuits
  • Discharge of circuits releasing sparks with stored energy in capacitance
  • Interruption of circuits releasing sparks with stored energy in inductance
  • Power dissipation causing heat from current flow in conductors.

The ignition mechanisms may occur in a variety of desirable, or undesirable, ways: in relay or switch contacts which are deliberately designed to make and break circuits; fuses are designed to blow open-circuit but produce an arc in so doing. Short circuits from insulation failure, mechanical damage or component failure are all possible, and arc-over between components or conductors.

The components or circuits that present a potential ignition source may be designed in a variety of ways in order to prevent ignition of a hazardous atmosphere.

2.6.3 Energy release

Energy from electrical sparking is relatively easy to quantify and students may be aware of the calculation form for stored energy in Inductors and capacitors as in E=1/2LI2 or E=1/2CV2, respectively.

Other forms of energy release are not readily quantifiable, such as mechanical impact causing sparks. The mechanical energy to create the impact can be defined and measured but the energy absorption between the materials may or may not release sufficient energy to cause ignition.

Flame propagation itself is capable of being controlled and this fact is utilised by one type of protection, Ex d (Flameproof). The derivation of energy released measurements with this application will be described in that section of this manual.

2.7 Electrical protection (as opposed to explosion protection)

It is a requirement of the Laws and Regulation in all countries that the installation and operation of electrical equipment and circuits is protected by the correct use of suitable fuses and/or circuit breakers. This is known as ‘electrical protection’.

Fuses will operate under short-circuit conditions effectively but cannot be relied upon to “discriminate” between normal full load running current and an overload condition. Correct electrical protection involves the use of overload trips, which are to be properly sized and set up so that they switch out circuits that operate above their full load current.

Electrical protection alone cannot guarantee that any ignition capable effect be prevented from issuing excessive heating or sparking to prevent ignition of a flammable atmosphere. Additional measures are taken to ensure that under specific conditions, ignition capability is not possible with adequate reliability. This is referred to as “Explosion Protection”. There are nine internationally recognised types which will be explained later in the manual.

2.8 Toxicity hazard

In addition to the flammability of a material, it may be necessary to take into account its toxicity. Hydrogen Sulphide, H2S, is a prime example of a common gas found in the oil and gas industry that is highly toxic. A concentration of only 0.1 parts per million (PPM) can be detected by the human nose but a concentration of 10ppm is the highest level permitted for safe working. At 50ppm, it poses a serious health risk yet its flammable range is between 40,000ppm and 460,000ppm (i.e. 4% to 46%).

H2S Level PPM %
Smell detection level 0.1 0.000001
Safe working level 10 0.0001
Serious health danger level 50 0.0005
Lower flammable limit 40,000 4
Upper flammable limit 460,000 46

2.9 Conclusions

The chapter above has introduced terminology and definitions used in describing flammable materials and their ignition characteristics. The use of electricity has been explained to show how it can cause ignition which has, in turn, begun to expose what must be controlled in order to prevent it from doing so.

In subsequent chapters the practical application of this information and terminology will become evident.


3


Area Classification

This chapter explores the area classification requirements used to determine the degree of Hazard by defining the probability of presence, the extent and the nature of Hazardous Areas. It introduces the Risk Assessment recommended for industrial premises on which decisions about explosion protection safety must be based. The conclusions drawn must be documented and reviewed periodically so that correct Electrical Equipment can be selected and used in Hazardous Areas where Explosion Protection must be applied. The implications of the application of Hazardous Area Classification cross over into all systems of management from plant design to maintenance.

Learning objectives

  • To understand the purpose of area classification
  • To study the terminology used
  • To see how it is applied as a Risk Assessment
  • To study the ‘source of release’ method
  • To study the ‘industry’ recommended methods
  • To describe how to document the findings and decisions
  • To examine the responsibilities of Management called for by the Standards to which area classification is carried out.

3.1 General

The responsibility for all aspects of industrial plant safety rests with its owner. The owner must effectively co-ordinate the design, selection, installation, operation and maintenance of the plant through a management structure possessing the appropriate skills.

In the appendix, a number of notable disasters and situations are cited with comments on the outcome of investigations. The common theme in these and many other such incidents is at the heart of Hazardous Area Classification (HAC), that of inadequate definition and communication of the aspects of the hazard.

HAC can be introduced as a means to define, and at the same time, summed up by these principle stages:-

  • To identify the hazard and the associated risk
  • To influence plant design to minimise risk
  • To define and quantify the risk of release
  • To develop and implement rules for the control of the risk
  • To communicate to and to educate those who need the information for safety purposes and
  • To audit the process, as often as is necessary, to ensure appropriate levels of safety are attained and maintained

This assessment process should begin during initial plant design and be developed in parallel with it and as part of the ‘HAZOP’ study. Such assessment must be regarded as a mandatory and a critical safety management tool.

It must be applied in a diligent manner by personnel who have the appropriate background, training and experience. It is interesting to note that the harmonised International Standard IEC60079-10-1:2015 now emphasises this very important point which is as a result of the investigations of previous accidents.

The mnemonic HAC will be used in the following text.

3.2 Overview of the principles of safety

Laws, Regulations, Directives, and just plain common sense all dictate that engineering design must use the principles of a ‘Safety First’ philosophy. It is also enshrined in the IEC Standards:-

  • Avoid any hazard by design

This must be considered at various detailed stages starting with initial plant design:-

  • Use non-flammable materials where possible
  • Where this is not possible then plant design (layout AND operation) must minimise the possibility of release
  • If release occurs then keep ignition sources out of the vicinity
  • If unavoidable then use Explosion Protected (Ex) equipment.

At the stage where flammable materials must be used on a plant the following requirements are placed upon the management:-

  • Identify and record those materials that are deemed flammable
  • Identify the places in the plant where materials are stored, handled or processed
  • Identify where there is a possibility of an atmosphere conducive for explosion or fire to occur
  • Apply HAC to each separate area

The decision as to what constitutes a Hazardous Area now needs definition.

3.3 Definitions

As defined by the original version of IEC 60079-10, where there is no risk of flammable gasses being present, the area is referred to as a ‘Non-Hazardous Area’. Industry has quoined the name ‘Safe Area’ because it is possible for the prefix, ‘Non-’, to be left off the phrase. Whichever is used, it is defined as:-

  • “An area in which an explosive atmosphere is not expected to be present in quantities such as to require special precautions for the construction, installation and use of equipment”

It follows that a Hazardous Area is defined as:-

  • “An area in which an explosive gas atmosphere is present or may be expected to be present in quantities such as to require special precautions for the construction, installation and use of equipment”.

It is not possible to declare whether specific places on an industrial plant are hazardous or non-hazardous until an assessment has been made of whether flammable materials are used, how, when and where they might be in a form that could be ignited. It is therefore necessary to treat the examination of information as a ‘risk assessment’. The stages and processes involved will be explained in the following sections with definitions and discussion.

3.4 The process of assessment

Before the quantification part of the process of assessment can be started, consideration must be given to issues which will affect the integrity of the outcome. There are two Standards of which the plant designer/owner must use:
IEC 60079-10-1:2015 Classification of Hazardous Areas – Explosive gas atmospheres
IEC 60079-10-2:2015 Classification of Hazardous Areas – Explosive dust atmospheres
The intention of bringing these into the same Standard Series is so that they are considered together as some sites have both types of hazard on site. In some cases materials could give rise to mixtures of dust and vapour, dealt with as a ‘hybrid’ mixture.

There are two principle methods of approaching the subject HAC:-

  • The first to be described immediately below is known as the ‘Source of Release’ method. It should lead to accurate, repeatable, well-documented and high-quality assessment but requires considerable time and labour resources if it is to be performed diligently, as intended. This technique is described and recommended in the IEC60079-10-1 Standard.

and

  • The second acceptable method is described as ‘Use of industry codes and national standards’. This is a ‘best-fit’ approach where pre-defined classifications, published in industry standard documents, are compared to a user’s design and operating conditions. Where matches are recognisable and acceptable according to the user, then the pre-defined classifications are applied as being appropriate and adequately safe.

The later approach uses published external experience to make safety judgements on a user’s plant. It does, however, require considerably less engineering effort from the user and is faster to implement; factors which reduce the overall cost. A management must balance these issues is against each other in the decision as to which to implement.

In practical terms probably both will be used initially, as HAC recognises standard applications to be dealt with by best fit methods and the more difficult ones are applied from first principles. Retrospectively, the situations will be reviewed in the light of experience running the plant and adjustment made accordingly.

A third approach is now discussed in the Standard under ‘Simplified methods’. This is concerned with judgements made by referencing a set of criteria based on industry experience extended into what is appropriate for the particular plant.

Ultimately the final classification may be made on a combination of techniques that are judged to be appropriate.

The quality of the decisions made have an impact on overall plant safety. Over-specifying the level of the hazard may

  • restrict the choice of equipment
  • increase the cost of maintenance
  • constrain the operation of the plant
  • be generally more difficult to manage

Under-specifying the degree of Hazard may

  • Provide for lower levels of safety
  • Encourage operator shortcut mentality

The standards encourage accuracy in the implementation of classification to preclude either of these extremes.

3.4.1 Sources of release

In order to know where the gas may or may not be present a study is conducted which identifies all possible sources of release:-

  • ‘A point or location from which a flammable gas, vapour or liquid may be released into the atmosphere such that an explosive atmosphere may be formed.’

There are three grades of release defined:-

Table 3.1
Grade of Release:- Definition
Continuous A release which is continuous or expected to occur for long periods
Primary A release which can be expected to occur periodically or occasionally during normal operation
Secondary A release which is not expected to occur in normal operation and if it does occur, is likely to do so infrequently or for short periods of time

Assessing the grade of release only identifies a point from which a release can emerge but more information is required before the extent of the release and the hazard that it represents can be quantified.

The location of each source of release must be identified and recorded. This is often done by diagrammatic representation and plotting methods using Computer Aided Design systems (CAD). Where several sources of release are in close proximity then the cumulative effect must be considered for accurate assessment purposes.

Generating a simple database to record information for ease of future reference is highly recommended. Necessary additional detail from which the final decisions are made will comprise:-

  • what gas can be released
  • from where it can originate
  • how it is released
  • at what pressure
  • what size of aperture
  • in what general direction
  • at what temperature
  • into what ambient temperature
  • near to what other sources of release and
  • under what other influential conditions, i.e. ventilation

This information is eventually used to quantify the hazardous area comes from and then its rate of release and possible size. This is known as classification by ‘Source of Release’ methodology.

3.4.2 Examples of sources of release

Refer to Figure 3.1 for illustrations of examples. In each case additional information is required to assess the extent of the release.

Figure 3.1
Examples of Sources of release

The examples in Figure 3.1 are classic illustrations because they are easy to see how the principles are applied, given the additional information to consider. The welded pipe is an example of where a joint is made but the likelihood of leakage is very low.

It is equally unlikely that the wall of a pipe or vessel will leak. The risk must always be considered but can be realistically evaluated as an adequately low possibility.

Table 3.2
Point of release: Consider: Zone
A Open surface of liquid: Area of exposure, flashpoint, liquid temperature (ambient and extremes), ambient temperature, air movement effect. 0
B Flanged Pipe joint: State of material (liquid or gas), type of gasket (wound or spiral), pressure in pipe, fluid density, fluid temperature, ambient temperature 2
C Pump seal: frequency of maintenance, manufacturer’s recommendations, history of failure on given application 2
D Drum Filling: Rate of filling, fluid volatility, temperature, flashpoint 1
E Spray nozzle: Rate of ejection, delivery pressure, solvent characteristics, temperature, flashpoint. 1
F Welded pipe No need to consider: Likelihood of release is adequately low Safe

Inside vessels and pipes, flammable materials may be present but if no air or oxygen source is present then there is no risk.

The state of the released fluid must be known. Gasses may condense and liquids may evaporate. The rate at which this occurs needs to be known and taken into account. The virtually instantaneous evaporation of a liquid can occur when released if the temperature is well above Flashpoint. If so the release can be considered as behaving like a gas and is relatively well defined.

In the case of liquid being released at a temperature that is below its flashpoint, then no hazard is created, provided this is a reliable set of conditions. If the liquid is ejected on to a hot surface i.e. above the Flashpoint then this constitutes a hazard. Where liquid is sprayed, the droplets have increased surface are and the velocity is known to liberate vapour at below the flashpoint so that a flammable atmosphere may be formed.

In the above situations it will be realised that the geometry of the source of release and the pressure greatly influences the situation.

Examples of issues to be considered are listed below: -

Table 3.3
Grade Likely condition
Continuous The surface of a flammable liquid in a fixed roof tank, a permanent vent to the atmosphere, e.g. an oil/water separator
(expected to release flammable material into the atmosphere continuously or for long periods)
Primary Seals of pumps, compressors or valves
Water drainage points on vessels which contain flammable liquids
Manual sample points
Relief valves, vents and other openings
Fluid release points on instrumentation for calibration purposes
(expected to release flammable material into the atmosphere during normal operation)
Secondary Seals of pumps, compressors and valves
Flanges, connections and pipe fittings
Fluid capturing sample points
Relief valves, vents and other openings
(not expected to release flammable material into the atmosphere during normal operation, but only for a short time if it occurs)

Equipment will be designed to minimise the release of fluids and the manufacturer will guide the user on procedures on and frequencies with which preventative maintenance must be carried out. This will affect the expectation of the duration of release and must be considered in the stability of the grade of release awarded.

3.5 Normal and abnormal operation

One of the key issues with HAC and which has been mentioned in the definitions of ‘Sources of Release’ is the concept of Normal Operation. This relates to plant and equipment.

An industrial plant and the installed equipment is designed to perform functions in a certain way. Operators are trained to work within the limits of what is defined as ‘normal operation’. If operation occurs outside those design limits, then this may have many undesirable consequences, but critical to consideration here is that the action may defeat the safety parameters of the plant.

The term ‘normal operation’ and what is considered to be ‘abnormal’ require some definition.

‘Normal operation’ in this context means:

  • The actual standard of design used
  • The achieved state of maintenance
  • The expected limitations
  • The usual operations and operating practices employed and so on.

It is not intended to mean the ‘ideal’ or ‘perfect’ condition.

‘Abnormal operation’ can be considered as the unintended, the unpredictable but non-catastrophic events.

In the well-ordered modern plant examples of abnormal operation might be:-

  • the collapse of a pump gland
  • the failure of a pipe gasket
  • the loss of control of the manual draining operation of a tank
  • the fracture of a small branch pipe
  • the accidental spillage of small quantities of flammable liquid

Instances of this are perhaps foreseeable at the design stage and so precautions will have been taken. As a result of experience, secondary systems, such as gas detection may be put in place. When the failure occurs, the abnormality can be quickly detected and rectified by a procedure so that the duration of release is limited.

An example of a catastrophic event is the bursting of a process vessel or a large pipeline. HAC is not designed to cope with this potential scale of disaster. It must be realised, from previous accidents, that in the case of a large release of flammable material into the atmosphere, it is only a matter of time before it will meet with a source of ignition.

In modern plants handling flammable materials, it is of course the main objective of design, maintenance and operating philosophy to ensure that there are few ways in which a flammable atmosphere can occur. This is to be achieved by:

  • Safe design of process
  • Proper selection of process equipment
  • Safe disposal of vented products
  • Properly trained and experienced operators and maintenance personnel
  • Defined operating procedures
  • Clear safety procedures
  • Regular, high-quality inspection and maintenance
  • Good production supervision
  • Management commitment to safety

3.6 Classification into zones

Once each individual ‘source of release’ has been identified, the next stage of the process is to begin the classification into Zones. From the previous discussion, plant design and operation greatly influence the final classification outcome.

3.6.1 Authority

A broad range of specialist knowledge is necessary so that all relevant conditions can be considered properly during the process of HAC. In the past, this has been left to the electrical authority on a plant as it is the electrical equipment that has been the principle consideration in Ex safety. Recent changes in safety management thinking have been reflected in the Codes of Practices whereby all sources of release and ignition must be assessed. The plant senior management is encouraged to engage appropriate personnel from all technical and managerial departments to come together for the HAC task. It is recognised that the required knowledge is likely to be widely distributed amongst such personnel. No one individual has sufficient knowledge to perform this task alone when applied to a complex plant.

The required information on which to base high quality HAC is from:

  • The production process and its parameters
  • The process operating procedures
  • Process safety aspects
  • Process equipment used
  • Electrical equipment used
  • Ignition capability of equipment
  • Mechanical influences
  • Risk form additional services such as steam, air, hydraulic and electrical supplies
  • Maintenance issues
  • Operator training procedures
  • Building design
  • Ventillation and
  • extensive knowledge of the characteristics of ALL the flammable materials used on site for whatever purpose.

Another issue to emerge here that may affect the outcome is that operation of the plant may be changed during the course of time. Batch manufacture may use different flammable materials. Vessels may be opened for cleaning using flammable materials where the process is not hazardous. All of these situations must be considered. Changes in normal operation could then be under ‘maintenance’ or sometimes called ‘work-over’ situations. Caution should be given here against improper use of the work-permit system to allow activities which have not been appropriately risk assessed.

Provision for dealing with accidents and emergencies may also impact on HAC decisions. During such conditions consideration must be given to procedures concerned with:-

  • Safe shut-down of processes
  • Isolation of process vessels having ignitable material
  • Containment of spillage
  • Disconnection or isolation of defective equipment

It is important to identify the roles to be played by those who are involved in design and operation of the plant and machinery. For example, implementing additional ventilation may be needed to quickly reduce the intensity of a hazardous atmosphere.

3.6.2 Definitions for non-dust hazards

Areas where there is the likelihood of the presence of explosive gas-air mixtures are referred to as ‘zones’. Zones are defined in IEC60079-10 and are classified as shown in the table below. The higher the number in this classification the smaller is the risk of an explosive atmosphere being present. The application of these zones will be discussed in following sections.

Table 3.4
Zone Definition
Zone 0 An area in which an explosive gas/air mixture is continually present or present for long periods
Zone 1 An area in which a gas/air mixture is likely to occur in normal operation
Zone 2 An area in which a gas/air mixture is not likely to occur in normal operation, and if it occurs, it will exist only for a short time.

3.6.3 Dusts

Classification of areas where dusts are present uses the same criteria as for gasses and vapours but the characteristics are different. The definition of the classification is modified to take account of this. Layers are not classified because they do not combust. In conditions allowing clouds to form the following classification is used:

Table 3.5
Zone 20 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently
Zone 21 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur, occasionally, in normal operation
Zone 22 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only

3.6.4 Mists

IEC 60079-10-1:2015 now includes an Annex covering mists. It states that when a liquid is handled at or above its flash point, any release will be treated through the normal area classification process described in this standard. If it is released below the flash point, under certain conditions, it may form a flammable mist cloud. Even the liquids that can be considered as non-hazardous at process temperatures, in some situations may form a flammable mist which may then give rise to an explosion hazard. Examples of liquids that are commonly considered in this regard include high flash point liquid fuels, heat exchange oils and lubricating oils.

In practice, a liquid release will normally comprise of broad range of droplet sizes with larger droplets tending to fallout immediately, leaving only a small fraction of the release airborne in the form of an aerosol. The flammability of the mist depends upon the concentration in air of the droplets plus any vapour, a function of the volatility of the liquid and the droplet sizes within the cloud. The size of droplets depends upon the pressure at which the liquid is being released, the properties of the liquid (primarily density, surface tension and viscosity) and the size and shape of the release opening. Normally, higher pressures and smaller openings will contribute to the degree of atomization of the release jet thus giving the rise to an explosion hazard. On the other hand, smaller release openings imply smaller release rates thus reducing the hazard.

A release jet of mist impacting on a nearby flat surface can increase the fractions of particles that are flammable. The Annex suggests that careful assessment of the conditions are needed where mists are more likely to be formed. The assessment should look at the situations where mist could be formed and the possible conditions commensurate with the formation of a hazard.

3.6.5 Hybrid mixtures of vapour and dust

IEC60079-10-1:2015 also includes Annex I for where a combined mixture of a flammable gas with combustible dust or flyings.

3.7 Area classification process

The recommended process for the HAC analysis of a plant is outlined in Figure 3.2, which is taken from the IEC60079-10-1 Standard.

This Standard recommends the following stages where the data collected is held in a conveniently tabulated form:-

  • Establish what flammable materials are present on site. Record the flammable characteristics data for each form of the material.
  • Identify all locations at which releases of the above materials are possible noting the conditions temperatures, pressures, etc.

The detail suggested is included in the appendix.

Whilst information is being collected for new plants under design, the process may prompt consideration of how plant layout modification and re-design may help to minimise the likely release of flammable material and therefore reduce the level of risk.

The next stage might depend on the plant layout. Some initial calculations (discussed later) may be necessary to ascertain the perceived extent of the Hazardous Area, and therefore a process of examination to define regions of similar or overlapping hazardous areas might be useful. The alternative and, arguably, the more scientific approach is, firstly, to calculate every possible release into an equivalent Zone distance. This can be a very time consuming occupation and would not be necessary if worst case scenarios may be identified with the benefit of experience.

The process requires a meticulous approach, documenting each stage so that a traceable path of thinking is recorded for the benefit of future changes to the plant.

In summary, the information required for the calculations are based on the following assessments:-

  • frequency of release
  • duration of release
  • the release rate

Of these, the release rate is dependent on:-

  • opening size of the point release
  • it’s topography
  • the concentration likely to result from the release rate
  • the velocity at which it emerges

Once released the extent of the hazard will be greatly influenced by:-

  • ventilation
  • the proximity and propensity expected of other releases in the same vicinity.

In the following sections each of these issues will be discussed in more detail. Examples of the calculation methods used to support this information will follow. The calculations are typical and can be then performed using formulae available from the Standards and other textbooks as necessary to determine the quantity of hazard involved. The philosophy behind the calculations is shown in Figure 3.3.

Figure 3.2a
Classification process schematic, first stage from IEC 60079-10-1:2015 Annex F.1 (informative)

In the Schematic shown in Figure 3.2a, the series of initial prompts lead to the determination of the grade of release. In Figure 3.2b the questions help to refine the process taking in to account dilution and ventilation that acts in the areas of the plant. The final zoning is therefore dependent on the train of options.

Where the ventilation effect is so great that a source of release is diluted immediately below its lower flammable limit after release then the extent of the zone is said to be negligible (N.E.)

Figure 3.2b
Classification process schematic, subsequent stage from IEC 60079-10-1:2015 Annex F.2 (informative)
Only the stage for Continuous grade of release is shown here. Refer to the Standard for Annexes F.3 and F.4

The final outcome of HAC assessment is only concluded after an iterative and cyclic process of re-evaluation as more information is considered. The classification team may choose an agreed plant location to start with and from which the assessment will work outwards from so that considerations are made in an ordered fashion.

3.7.1 Calculation process

This mathematical approach is based upon a combination of fluid dynamics, kinetic theory of gases and practical measurements. In Figure 3.3, the stages are seen by which the extent of the hazard is calculated.

Figure 3.3
Summary of gas and vapour release calculations

In essence, to apply the calculation method, the rate of release the mass of the flammable material (M) must be assessed as it will determine the volume (V) that it occupies in the immediate vicinity of the source of release. The concentration is actually determined by the mixing effect with air and other factors which can only be assessed at a later stage. Initially it is necessary to determine the extent (Distance X) of the likely hazardous area where the gas will mix to the point of dilution below the lower flammable limit (LFL) for the specific gas. The distance X may not be final as ventilation and other factors need to be added into the assessment and are discussed below.

3.8 Openings

The initial consideration for the likely Zone is the type of opening through which the release of a gas or vapour occurs. This could be an inadvertent leak from a seal or a deliberate release from an operator opening a valve as part of a documented procedure. The reason for the release will determine other factors such as the frequency and duration but the size and location of the release must be assessed firstly.

3.8.1 Openings classification

Openings are classified as A, B, C, and D with the following characteristics:-

Table 3.6
Type Description Examples
A Openings not conforming to the characteristics specified for types B, C or D Open passages for access or utilities, for example ducts, pipes through walls, ceilings and floors:
Fixed ventilation outlets in rooms, buildings and similar openings of types B, C and D, which are opened frequently or, for long periods.
B Openings that are normally closed and infrequently opened and which are close fitting. Screw threaded joint
C Openings normally closed and infrequently opened, conforming to type B, which are also fitted with sealing devices along the whole perimeter; or two openings type B in series, having independent automatic closing devices. Flanged pipe joint with a gasket
D Openings normally closed conforming to type C, which can only be opened by special means or in an emergency. Type D openings are effectively sealed, such as in utility passages or can be a combination of one opening type C adjacent to a hazardous area and one opening type B in series. ducts, pipes

The following table describes the effect of openings on grade of release.

Table 3.7
Zone upstream of opening Opening type Grade of release of openings considered as sources of release
Zone 0 A
B
C
D
Continuous
(Continuous)/primary
Secondary
No release
Zone 1 A
B
C
D
Primary
(Primary)/Secondary
(Secondary)/no release
No release
Zone 2 A
B
C
D
Secondary
(Secondary)/no release
No release
No release

Note: For grades of release shown in brackets, the frequency of operation of the openings should be considered in the design.

Openings between areas should also be considered as possible sources of release. The grade of release will depend upon:-

  • The zone type of the adjoining area;
  • The frequency and duration of opening periods;
  • The effectiveness of seals or joints;
  • The difference in pressure between the areas involved.

Any inadvertent leak, say, from a seal is type C, but any deliberate release from a valve will be type B. These definitions help to identify sources of release.

3.8.2 Topography

The size and shape of the opening together with the pressure difference, the physical properties of the fluid (which may be liquid, vapour or gas) and the resulting escape velocity will have considerable influence on the rate of release as it leaves one space and enters an adjacent space in the form of a leak.

The distance travelled and the mixing effect now needs to be ascertained. This is the complex part of the assessment as there are many variables. The direction in which the hazard is formed may be of interest. A release bound by surfaces blocking or sheltering the progress of the fluid may assist or impede the dilution depending on other variables.

Release from an open source such as the surface of a liquid will be dependent on vapour pressure and air movement. It can be accelerated by movement in the liquid due to agitation caused by pumping action.

3.8.3 Concentration

A number of factors will affect the discharged concentration of the gas into a hazardous area.

Relative vapour density
If the gas or vapour is significantly lighter than air, it will tend to move upwards. If significantly heavier, it will tend to accumulate at ground level. The horizontal extent of the zone at ground level will increase with increasing relative density and the vertical extent above the source will increase with decreasing relative density. Consideration should always be given to the possibility that a gas which is heavier than air may flow into areas below ground level for example pits or depressions and that a gas which is lighter than air may be retained at high level, for example in a roof space.

Vapour pressure
The vapour pressure increases with temperature, thus increasing the release rate due to evaporation for a liquid above its Flashpoint.

Release into air movement
The release velocity of gas also plays a significant role in determining the extent of the volume. A low velocity release of gas in a poorly ventilated building or space may give rise to inefficient mixing with air and thus the gas/air mixture will vary from place to place giving rise to large and unpredictable hazardous areas.

A high velocity release will form a conical stream in a more defined direction yet will provide a more predictable zone of influence. The rate of dispersion is greater because the mixing effect with the air is more turbulent thus entraining air and actively being self-diluting.

Obstacles which impede the ventilation may increase the extent of the zone. On the other hand, some obstacles, for example bunds, walls or ceilings, may limit the extent.

Lower explosive limit (LEL)
For a given release volume, the lower the LEL, the greater will be the extent of the zone.

Volatility of a flammable liquid
This is related principally to the vapour pressure, and the heat of vaporization. An explosive atmosphere cannot exist if the Flashpoint is above the relevant maximum temperature of the flammable liquid. The lower the Flashpoint, the greater may be the extent of the zone. If a flammable material is released in a way that forms a mist (for example by spraying), an explosive atmosphere may be formed below ‘the flashpoint of the material’ for example.

The above issues will all affect the extent of the zone. The major external influence is that of ventilation.

3.9 Ventilation

Gas (or vapour) released into the atmospheric air will eventually be diluted below the lower explosion limit by dispersion or diffusion. The rate at which the mixing of the gas and air occurs is affected by their relative velocities and directions.

In still air, and with very little pressure behind the gas, the Relative Vapour Density will dominate and determine if the mixture will rise or fall and how fast. As the air moves faster and the gas release velocity increases the gas/air mixture will expand creating a larger volume that is likely to be ignitable.

This effect, known as ventilation, must be taken into account in determining the direction and size of a hazardous area. If the gas is quickly diluted by the movement of sufficient air then there may be no hazard or a very small one immediately around the point of the source of release. In this case the reliability of the dilution must be taken into account.

There are two recognized types of ventilation:-

  • Natural
  • Artificial

Local ventilation is applied to or near equipment whereas general ventilation is applied to a building. The concept remains the same.

3.9.1 Natural ventilation

This is a type of ventilation that is accomplished by the movement of air caused by the wind and/or by temperature gradients. For outdoor areas, which are open and unshielded, ventilation rate is based on a minimum wind speed of 0.5 m/s, (equivalent to waking slowly). The advantage here is that its availability continuous and reliable. Frequently the wind-speed will be above 2 m/s.

For indoor areas, natural ventilation used to assist dilution by can providing appropriately placed openings in walls and roof structures. Hot equipment mounted inside will create convection currents which may or may not increase the flowrate so measurements under different weather conditions may be necessary to obtain velocity data and availability.

3.9.2 Artificial ventilation

In this case the ventilation is forced or induced by mechanical means, for example by fans or extractors. It is only effective in enclosed spaces, bound by walls and a roof, so that inlet and outlet conditions can be well defined.

With the use of artificial ventilation, it is possible to achieve:

  • Prevention of the generation of an explosive atmosphere
  • Reduction in the extent of zones
  • Shortening of the time of persistence of an explosive atmosphere

An artificial ventilation system, which is designed for the control or reduction of Hazardous Areas, should meet the following requirements:

  • Its effectiveness should be controlled and monitored;
  • Consideration should be given to the classification immediately outside the extract system discharge point;
  • For ventilation of a hazardous area the ventilation air should usually be drawn from a non-hazardous area;
  • Before determining the dimensions and design of the ventilation system, the location, grade of release and release rate should be defined.

In addition, the following factors will influence the quality of an artificial ventilation system:

  • Flammable gases and vapours usually have densities other than that of air, thus they will tend to accumulate near to either the floor or ceiling of an enclosed area, where air movement is likely to be reduced
  • Changes in gas density with temperature
  • Impediments and obstacles may cause reduced, or even no air movement, i.e. no ventilation in certain parts of the area

3.9.3 Degree of ventilation

The effectiveness of ventilation influencing dispersion and persistence of the explosive atmosphere will depend upon the degree and availability.

The most important factor is that the degree or amount of ventilation is directly related to the types of sources of release and their corresponding release rates. This is irrespective of the type of ventilation, whether it is wind speed or the number of air changes per time unit. Thus optimal ventilation conditions in the hazardous area can be achieved and the higher the amount of ventilation in respect of the possible release rates, the smaller will be the extent of the zones (hazardous areas), in some cases reducing them to a negligible extent (non-hazardous area).

The methods developed allow the determination of the type of zone by:

  • Estimating the minimum ventilation rate required to prevent significant build-up of an explosive atmosphere and using this to calculate a hypothetical volume, which, with an estimated dispersion time, allows determination of the degree of ventilation.
  • Determining the type of zone from the degree and availability of ventilation and the grade of release.

3.9.4 Availability of ventilation

The availability of ventilation has an influence on the presence or formation of an explosive atmosphere. Thus the availability (as well as the degree) of ventilation needs to be taken into consideration when determining the type of zone.

Three levels of availability of the ventilation are defined:

  • Good: ventilation is present virtually continuously;
  • Fair: ventilation is expected to be present during normal operation. Discontinuities are permitted provided they occur infrequently and for short periods;
  • Poor: ventilation, which does not meet the standard of fair or good, but discontinuities are not expected to occur for long periods.

Ventilation that does not even meet the requirement for poor availability must not be considered to contribute to the ventilation of the area. Natural ventilation in outdoor areas will be present virtually continuously and so can be considered as ‘good’.

Artificial ventilation calls into question the reliability of the equipment and its availability so that a provision for both main and standby blowers need be considered. Interlocks, so that the plant is shut down on failure can also be considered in order to maintain safe conditions.

Combining the approach of the degree of ventilation and level of availability, results in a quantitative method for the evaluation of the type of zone.

3.10 HAC calculation

The original versions of IEC 79-10 was issued in 1995 and subsequent versions of the harmonised standard were in 2002, 2007 and 2015. The level of detail in each issue has increased dramatically and the latest version provides more practical and calculation-based information to assist in the process of classification. The general principles do not change but the current standard Part 10-1 exposes ventilation and dilution with much more background information helpful in the decision making processes.

In order to determine the Zone and its extent, a general mathematical approach can be taken to calculate the likely fluid flow rate from a point of release.

Once the release rate is known and the factors affecting ventilation and dilution are taken into account, some consistent assessment of the volume of gas and the space that it occupies until it is diluted to below its lower flammable limit may be calculated.

The mathematical approach published in IEC 60079-10:2007 is used here in these worked examples. They illustrate the three stages refer to in the diagrammatical explanation in Figure 3.3.

3.10.1 Release of gas and vapour

The formulae are given below:
(I) The mass release of gas from an orifice can be calculated as given below,
G = 0.006aP(M/T)0.5 kg /s
Where: G = mass release, kg/s
a = cross-sectional area of leak, m2
P = upstream pressure, N/ m2
M = molecular weight
T = absolute temperature of released gas, deg. K

(II) The equation as above is valid if upstream absolute pressure exceeds 2 x 105 N/ m2. Below this pressure, the effect of atmospheric pressure is significant and the diffusion effect needs to be considered.

The volume of released gas V,
V = Vo G T / To M
Where: Vo = molar volume, m3/kg
To = melting point of ice, 273°K
The formula then becomes,
V = 0.082GT / M

(III) The distance ‘X’ at which the gas or vapour due to mixing with air while traversing falls below LEL can be calculated as below:
X = 2.1x103[G/E2M1.5T0.5]0.5
Where: E = Lower explosive limit (LEL), %
Note that the above formula is valid as long as jet velocity is sufficiently high compared to wind speed. As the jet velocity decreases then the following alternative formula which is based on experimentation can be used:
X = 10.8[GT/ME]0.55

3.10.2 Typical progression of gas release

Figure 3.4
Geometry of gas release formed in hazardous area

A typical gas or vapour leak releases a plume of material having a probable envelope shape as shown in Figure 3.4. As the gas is released, an increase in velocity and a fall in pressure cause vortices to develop after the exit cone (bound by lines ‘a’). This promotes a more efficient mixing of the gas with the air and provides sufficient inertia to expand the envelope outwards in a further conical manner. The lines ‘b’, indicate the probable way that an outer envelope forms to the point where mixing occurs. The lines ‘b’ and the curve ‘c’ indicate the likely extent of the release to where the gas will be diluted below its lower flammable limit. The distance to ‘c’ is taken as being the extent of the hazardous area.

3.10.3 Examples of gas and vapour release

These worked examples are given as models for typical release calculations.

Example 1. Leakage from a pipe flange gasket:

Data table:

Type of gas Ethylene
Molar weight 28
Ambient temperature 22°C
Size of Orifice 4 x 10-5 m2
The pressure in pipe 4 x 105 N/ m2
LEL at the given Ambient 2.7%

The released mass will be:
G = 0,006.a.P.(M/T)0.5 kg /s = 0,03 kg/s

The distance will be:
X = 2,1 . 103[G/E2M1.5T0.5]0.5 = 2,7 m

The hazardous area will be a sphere extending 2,7 m from the source of release in all direction.

Obstruction to the gas and vapour release
In case the path of release of gas or vapour is obstructed by the presence of a barrier or an object before the distance X determined above then the volume need to be calculated as given below:-

B = 100 – L [(100 – LEL) / X] … %

Where: B = % gas / vapour in air
L = Distance to obstruction
X = Distance to LEL

Thus knowing the volume ratio the new effective molecular weight is calculated as given below:
M (mixture) = [M(gas) x % (gas)/100] + [M(air) x %(air)/100]
V (gas) = 0.082GT/M (gas)
V (mixture) = V (gas) x [100/% Gas in mixture] m3
G (mixture) = V (mixture) x 12.19 M (mixture) / T kg
LEL (mixture) = LEL (gas x [100/%gas in mixture) %

At the point of obstruction the new values are calculated of the mixture and new dispersion distance found out using the formula for X (see Figure 3.2).

Release of liquid below its atmospheric boiling point

As in the case of gas and vapour, in case of liquid below boiling point will take the form of jet or mist from an orifice like opening, depending on the pressure. The formula to calculate the release is:
G = 1,13.a [ σ1 (P – 105)]0,5

Where: G = mass release, kg/s
P = Upstream pressure, N/m2
σ1 = Liquid density at atmospheric conditions, kg/m2

The velocity of jet can be calculated based on:
v = v cos ◻σ.( 2gh + v2sin2◻◻σ)0.5 + v.sinσ]/g

The horizontal distance up to which extent of hazardous area will be present can be found out by:
X = (2,v2.h/g)0.5

Where: v = velocity, m/s
g = gravity acceleration (9.81 m/s2)
h = height of initial release, m

Release of liquid above its atmospheric boiling point

In this outdoor scenario the liquid at the point of release will evaporate partially after absorbing heat from atmosphere and the rest will fall on the ground based on the trajectory of jet, the pressure, etc.

However, after falling on ground the evaporation will take place very rapidly, almost immediately. The release quantity in such cases is calculated as given hereunder:
G = 0,8A[2◻σm (P1 – PC)]0.5

Where: A = cross-sectional area of leak , m2
σm = density of released mixture, kg/m3
P1 = containment pressure, kg/m2
PC = 0.55 x vapour pressure, kg/m2

Note that the vapour–pressure curve for the material in question is required for calculating σm with reference to PC.

The fraction of mass which vapourises on being released can be calculated as given below: Mg =(T1 – Tc) C1 /L

Where: Mg = fraction of mass release which is vapour
T1 = process temperature, K
Tc = temperature giving Pc, K
C1 = heat capacity of liquid, kj/kg/°C
L = latent heat of vapourisation, kj/kg

Now the density of the mixture can be calculated as below:
σm = 1 /[ (Mg / σv ) + {(1 – Mg) / σ1 ) } ]

Where: σm = density of released mixture, kg/m3
σv = density of vapour at T1
σ1 = density of liquid at T1

From above the velocity of release can be calculated and then the formulae given for release from jet in previous section can be applied.

Assessment and estimation of hypothetical volume ‘Vz’

This approach is based on IEC 79.

The ideal formula for estimating a hypothetical volume Vz of potential hazardous and explosive atmosphere around the source of release is,
Vz = (dV/dt)min
              C

Where: C = is the number of fresh air changes per unit time (s-1)

In an enclosed space the value of C is:

Where: dVtot / dt = the total flow rate of fresh air, and
             Vo = is the total volume being ventilated.

In open space this value is 0.03 / s based on an average wind speed of 0.5 m / sec.
(dV/dt)min = the minimum volumetric flow-rate of fresh air (volume per time, m3/s)

The theoretical minimum ventilation flow rate can be obtained as given hereunder,

Where: (dG/dt)rnax - is the maximum rate of release at source (mass per time, kg/s);
             LEL- is the lower explosive limit (mass per volume, kg/in3);
             k - is a safety factor applied to the LEL; typically:
                 k = 0,25 (continuous and primary grades of release); and
                 k = 0,5 (secondary grades of release).
             T - is the ambient temperature (in Kelvin).

The formula for Vz as given above would hold for an instantaneous and homogeneous mixing at the source of release, given ideal flow conditions of the fresh air. In practice, such ideal situations will generally not be found, for example because of possible impediments to the air flow, resulting in badly ventilated parts of the area. Thus, the effective air exchange at the source of release will be lower than that given by C in formula for enclosed space, leading to an increased volume V2. By introducing an additional correction (quality) factor, f to formula above, one obtains:

Where: f denotes the efficiency of the ventilation in terms of its effectiveness in diluting the explosive atmosphere, with f ranging from:

f = 1 (ideal situation) to, typically,
f = 5 (impeded air flow).

The volume Vz represents the volume over which the mean concentration of flammable gas or vapour will be either 0.25 or 0.5 times the LEL, depending on the value of the safety factor, k used in formula. This means that, at the extremities of the hypothetical volume estimated, the concentration of gas or vapour will be significantly below the LEL, i.e. the hypothetical volume where the concentration is above the LEL would be less than V2.

Estimation of persistence time t

The time (f) required for the average concentration to fall from an initial value X0 to the LEL times k after the release has stopped can be estimated from:

Where: X0 is the initial concentration of the flammable substance measured in the same units as the LEL, i.e. % vol or g/m3.

  • Somewhere in the explosive atmosphere, the concentration of the flammable matter may be 100% vol (in general only in the very close vicinity of the release source). However, when calculating t, the proper value for X0 to be taken depends on the particular case, considering among others the affected volume as well as the frequency and the duration of the release, and for most practical cases it seems reasonable to take a concentration above LEL for X0.

Examples of calculations to ascertain the degree of ventilation

The example of toluene gas for the three cases has been taken followed by an example for leakage from compressor seals. These are illustrative in nature and based on IEC 60079.

Example 1:

Characteristics of release Data
Flammable material toluene vapour
Source of release Flange
Lower explosion limit (LEL) 0.046 kg/m3 (1,2% vol.)
Grade of release continuous
Safety factor, k 0,25
Release rate, (dG/dt)max 2.8 x 10-10 kg/s
Ventilation characteristics  
Indoor situation  
Number of air changes, C 1/h, (2.8 x 10-4 / s)
Quality factor, f 5
Ambient temperature, T 200 C (293 K)
Temperature coefficient, (T/293 K) 1

Minimum volumetric flow rate of fresh air:

Evaluation of hypothetical volume Vz:

Time of persistence:

This is not applicable to a continuous release.

Outcome

The hypothetical volume Vz is reduced to a negligible value. The degree of ventilation is considered as high with regard to the source.

Example 2:

Characteristics of release Data
Flammable material toluene vapour
Source of release Failure of flange
Lower explosion limit (LEL) 0.046 kg/rn3 (1,2% vol.)
Grade of release Secondary
Safety factor, k 0.5
Release rate, (dG/dt)max 2.8 x 10-6 kg/s
Ventilation characteristics  
Indoor situation  
Number of air changes, C 1/h, (2.8 x 10-4 / s)
Quality factor, f 5
Ambient temperature, T 200C (293 K)
Temperature coefficient, (T/293 K) 1

Minimum volumetric flow rate of fresh air:

Evaluation of hypothetical volume Vz:

Time of persistence:

Outcome

The hypothetical volume Vz is significant but can be controlled. The degree of ventilation is considered medium with regard to the source on this basis. However any release would persist and the concept of zone 2 may not be met.

Example 3:

Characteristics of release Data
Flammable material Toluene vapour
Source of release Failure of flange
Lower explosion limit (LEL) 0.046 kg/m3 (1,2% vol.)
Grade of release secondary
Safety factor, k 0.5
Release rate, (dG/dt)max 6 x 10-4 kg/s
Ventilation characteristics  
Number of air changes, C 12/h, (3.33 x 10-3 / s)
Quality factor, f 2
Ambient temperature, T 200 C (293 K)
Temperature coefficient, (T/293 K) 1

Minimum volumetric flow rate of fresh air:

Evaluation of hypothetical volume Vz:

Time of persistence:

Outcome

The hypothetical volume VZ is significant but can be controlled. The degree of ventilation is considered medium with regard to the source on this basis. Based on the persistence time the concept of zone 2 would be met.

Example 4:

Characteristics of release  
    Flammable material Propane gas
    Source of release Compressor seals
    Lower explosion limit (LEL) 0.039 kg/m3 (2.1% vol.)
    Grade of release secondary
    Safety factor, k 0.5
    Release rate, (dG/dt)max 0.02 kg/s
Ventilation characteristics  
    Number of air changes, C 2/h, (5.6 x 10-4 / s)
    Quality factor, f 5
    Ambient temperature, T 20° C (293 K)
    Temperature coefficient, (T/293 K) 1

Minimum volumetric flow rate of fresh air:

Evaluation of hypothetical volume Vz:

Time of persistence:

Outcome

The hypothetical volume Vs. is significant and for a room (10 m x 15 m x 6 m) will extend beyond the physical boundaries of the room and will persist. The degree of ventilation is considered low with regard to the source.

3.11 Area classification of dust hazards

The principles involved in classifying plants with dust hazards into Zones 20, 21 or 22, are similar to those used to handle gases and vapours. In Chapter 2 the differences between gasses and dusts were examined.

3.11.1 Classification method

Because a cloud of flammable dust can arise not only as a result of release of dust from plant equipment but also from disturbances of deposits of dust around the plant, and because the area of spread of dust clouds cannot readily be quantified, it is recommended that a generalized method of classification based on judgement and experience should be used. Additionally, however, in certain cases it may be appropriate to use the techniques of the source of hazard method.

In carrying out an area classification, it is necessary to:

  • Identify those parts of the plant where flammable dust can exist including, where appropriate, the interior of process equipment
  • Assess the likelihood of occurrence of a flammable atmosphere (taking into account the general level of ‘housekeeping’ which will be maintained in the plant) thereby establishing the appropriate zone classification
  • Delineate the boundaries of the zones taking into account the effect of likely air movement

In assessing the area classification of a plant, the influence of the classification of adjacent plants must be taken into account. The classification should be carried out in accordance with the following criteria:

3.11.2 Non-hazardous areas

These areas are self-evident once the hazardous areas have been classified.

In exceptional circumstances, a non-hazardous area may be achieved by a very high level of ‘housekeeping’ and/or an efficient extraction system.

Areas, in which pipes or ducts containing flammable dusts are installed without joints, or with strong joints designed not to leak, may be considered non-hazardous provided there is a negligible risk of their being damaged.

3.11.3 Surface temperatures

Irrespective of the Zone of classification, the temperature of all surfaces on which a flammable dust can settle shall be below the ignition temperature of the dust in layer form. Similarly, the temperature of all surfaces with which a cloud of flammable dust can come into contact shall be below the ignition temperature of the dust in cloud form.

In some exceptional cases where, for process reasons, a surface temperature, which is higher than the ignition temperature of the dust concerned, is required, the apparatus shall be designed and maintained to prevent accumulation of dust on such hot surfaces and the plants shall be operated and maintained to prevent the formation of dust clouds.

3.12 Responsibility and personnel involved

The responsibility for HAC is taken by senior management. The usual approach is that they appoint a HAC team. It usually comprises expertise from the engineering disciplines of mechanical, electrical, civil, instrumentation, process control, chemical, scientific and production. They will need to agree on the risks. In the case of operating plants, responsibility for area classification maintenance rests with the Works Manager.

Once a plant has been classified and all necessary records made, it is important that no modification to equipment or operating procedures is made without discussion with those responsible. Unauthorized action may be unsafe. It is necessary to ensure that all equipment affecting the HAC, which has been subjected to maintenance, is carefully checked during and after re-assembly to ensure that the integrity of the original design, as it affects safety, has been maintained before it is returned to service.

Competency of those involved in performing Area Classification at an appropriate level for the input they provide is another key requirement of the Standards for HAC.

3.13 Documentation

The Standard IEC60079-10 for area classification requires the generation of adequate documentation.

3.13.1 Area classification drawings

Area Classification drawings are necessary to communicate the Hazard and its extent on any part of a plant. They will be needed by any individual who works on the plant so that appropriate precautions in those specific areas can be taken, according to the conditions that are stated on such drawings. The requirements for drawings are laid out in the Standard.

Although called an ‘Area’, the space taken up by the release of a gas is a three-dimensional region hence plant layout drawings in plan and elevation will be necessary and expected. The scale and the number of drawings of must be sufficient to communicate the detailed location of equipment within a hazardous area. This normally takes several drawings perhaps varying in scale to home in on an appropriate level of detail.

The method of showing the type and extent of the area classification is as shown in Figure 3.5. Examples of classification are shown later in this chapter.

Figure 3.5
Area Classification depiction on Drawings

In addition to the region of the hazard, the apparatus Group, Temperature rating and ambient rating should also be stated along with other critical information for safety purposes. Where changes in the classification are caused by additional materials or extended areas cause by changes in working practice albeit temporarily then addition drawings may need to be produced unless the separate conditions can be shown in an unambiguous way on the same drawings.

3.13.2 Verification dossier

The Standard IEC60079-14:2007 (Selection and Installation of Equipment in potentially explosive atmospheres) and subsequent editions require the generation of a “verification dossier”, mentioned previously. Whilst there is no prescribed format, the information it contains is used to communicate and control safety issues by providing verification that the plant and the equipment used is adequately safe. This can only be done by stating details about the hazard, the policy for making the hazard safe and demonstrating how this is achieved. It requires the following information:-

  • A statement of policy declaring what Standards have been used
  • Area Classification decisions and calculations
  • HAC Drawings and supporting documentation
  • Records of equipment used on site and identifying those used in Hazardous Areas
  • Equipment location drawings
  • Signage used to identify hazards
  • MSDS showing material characteristics
  • Equipment certificates
  • The results of the area classification study and any subsequent alterations to it shall be placed on record.
  • Plant operation procedures
  • Safe shut-down procedures
  • Employee training and competency records
  • and other relevant information

Such documentation forms part of the overall ‘Plant Safety Documentation’. It shall be maintained as accurate. Given below is a single line diagram of the general requirement of the ‘Hazardous Area Verification Dossier’ as required in IEC 60079-14 (see Figure 3.6).

Figure 3.6
Major documentation requirement for hazardous areas

3.14 Policy and guidelines for implementation

The plant owner is required to generate a safety policy which must include for the management of hazardous areas. The policy must include thinking on such subjects as:-

Structure of authority for management and engineering

  • How information is recorded
  • How it is communicated
  • Format of drawings presented
  • Location of information for reference
  • Equipment selection
  • Maintenance and inspection
  • Plant breakdown and recovery
  • Change control procedures
  • Periodic review of existing plant
  • Issue of information to contractors
  • Review of contractors’ safety procedures
  • Permit to work system operation in respect of hazard management
  • Electrical safety rules
  • Other safety rules

These are written statements in the form of procedures

3.15 General information

Some typical points to be considered and their implications are listed as a guide in the following table:

Table 3.8
Subject Consideration
Sources of release Any joint in pipe-work must be a potential source of release: minimise number of joints. Continuously welded pipe preferred.
Piping to instruments where there are drain-cocks: How often are drain cocks used?
Process pressures The distance from the source of release that vapour will travel if released. This governs the extent of the Zoning. Higher plant pressure will have larger zone areas.
Vapour density If low, then vapour will rise, making roof spaces a potentially hazardous area.
If high, then vapour will fall and may be trapped in pipe trenches.
Upper and Lower flammability limits What is the likelihood of dilution of release? Would installing fans reduce below the LFL? Dilution more likely than concentration.
Prevailing winds Use this to ease leaked vapour dispersion.
Plant layout Wider spacing of vessels may be preferred. How high should bund walls be?
Plant operating temperature What are Flashpoint and ignition temperatures of flammable materials used?
Environmental conditions Ambient temperature may affect T rating of apparatus
Grouping of vapours Suitability of Ex protection method

3.15.1 Review

A closer examination of just the joints used on a plant reveals some useful statistics and approaches. The quantity of leakage of gas or vapour from a flange joints depends on:

  • The type of joint face and
  • The gasket used

Gaskets can be:

  • A compressed-asbestos fibre (CAF) gasket. In this case the blow-out of a section of gasket between adjacent bolts is always a possibility. It is reasonable to assume an orifice 25 x 1 .6mm for such a blow-out.
  • A metal-clad or spirally supported gasket with backing ring. A blow-out is considered virtually impossible. The orifice through which a leak can occur is reduced to something like 0.05mm over a length of 50 mm (i.e. to about one tenth of the cross-sectional area possible with a simple joint and gasket).

In many situations the hazardous area arising from a joint leak from the use of metal clad gasket is insignificant in comparison with other likely leakage.

Any joints which are subjected to sharp changes in temperature are the most likely to leak. This would only be in abnormal conditions so zone 2 is created by the presence of the joint.

The liquid or gas or vapour leaking from a joint will be in the form of a jet and full area of influence need to be classified based on the exit velocity, pressure and time taken to shut-off the source.

Figure 3.7
Ground proximity effect causes distortion of Zone 2 exent

In Figure 3.7, a leak of flashing liquid from a joint in an elevated position is assumed to be circular but probably offset. If the pipe is close to the ground then the leak will diffuse more slowly extending the area by up the horizontal extent of the Zone 2 is only 70% of that for the same leak at or near ground level since the vapour diffuses more quickly at a height above ground than at ground level. All possible directions of travel need to be considered.

The use of guards round joints is said by some to reduce the extent of the hazardous area but the released product must still travel somewhere so the zone shape would most likely be distorted.

3.16 Area classification standards

The IEC60079-10 Standard, as discussed, gives clear guidance on the calculation models from which to perform HACS. Other document giving similar ‘best practice’ guidance are sometimes referred to as ‘standards’ are best described as ‘industry specific guidelines’ in that they are not Local, National or International Standards written and adopted by countries’ appointed standards-writing bodies.

Area classification outcome cannot be set rigidly into any standard because each industrial installation will be different in some respect and therefore each site must be examined on its individual merits. In contrast, petrol forecourts are all basically the same as are many oil refineries producing petrol and hydrocarbon by-products. So rules developed by the UK’s Institute of Petroleum are published in a set of documents known as the IP Code (not to be confused with IEC60529 on Ingress Protection!): Part 15 deals with HAC in the Petroleum Industry.

The Institute of Gas Engineers also has such a Code of Practice specific to the Methane gas Transmission and Distribution industry which is IGE/SR/25. The Aerosol manufacturing industry has an advisory body called BARMA who issue a CoP for their members and spray booth manufacturers s have similar industry-standard guidelines that are applied in pant shops in the automotive industry.

The guidelines are always open to interpretation and so the way they have been applied should be recorded.

3.17 HAC examples

This attempts to illustrate the typical situations for which common practice is applied. To provide a basis for assessing the extent of classified areas studies are commissioned by the factory owners based on the process parameters and the ways in which release of flammable gases, vapours and liquids can be expected to occur during month-in, month-out running of equipment handling such materials.

The organisations then develop guide lines based on the sample calculations discussed during this session for methods to calculate the size of gas or vapour clouds so formed and the distances necessary for their dispersal and dilution with air, in well-ventilated situations, to below their lower flammable limit. The object would be to establish a physical model of the escape from each source of hazard and to use it to derive figures for the extents of Zones 1 and 2 either from practical observations and/or calculations or, in the limit, on the basis of personal judgement.

Described below are some such case studies to illustrate typical installations and area classification principles based on various standards (like IEC 60079-10-1).

3.17.1 Tank area classification

In Figure 3.8, a fixed roof tank with vents is filled and emptied. If the liquid temperature was above the FLASHPOINT and the vapour given off was heavier than air then vapour would remain present above the surface of the liquid. When the tank is emptying, air is sucked in the vents and mixes with the vapour to form a flammable mixture. Inside the tank is normally classified as Zone 0. This is because the air/gas mixture may be continually present or present for long periods. Note that the allocation of Zones is not concerned with concentration levels at this stage although it may be possible to prove that the evaporation was so prolific as to exclude air from access. This would have to take into account the rate at which the liquid level is lowered. When the tank is filling, the vapour is pushed out of the vents and falls down the side of the tank. Turbulence in the tank during filling may force greater rates of vapour liberation.

Figure 3.8
Tank area classification

For a distance around the tank, vapour will appear, albeit diluted by the external air. At the base of the tank, vapour will collect up to the height of the bund wall around the tank, which helps to contain liquid spillage but tends to hamper the dispersal of any vapour. These areas are typically designated Zone 1 because the gas air mixture is likely to occur in normal operation.

For a further distance around the tank, at the base level and for a distance above ground, vapour is only likely to appear if there is a major spillage, which is considered abnormal conditions and so is designated Zone 2. The prevailing wind will influence the size of the surrounding Zone 2.

The above thinking is known as “Blanket Classification”. Such an approach is seen as a low cost solution which can be implemented by engineers with a lower level of skill. It tries to cover the worst case situation and often overstates the extent of the risk, increasing the cost of equipment and the complexity of maintenance and record keeping. It is discouraged by the Factory Inspectorate authorities.

It is always necessary to consider realistic or actual operational conditions on a plant. These may vary according what cycle the same plant is set up to operate. Dynamic conditions and maintenance not just static conditions must be considered. If the pumping rate into the tank was increased this may result in more vapour being pushed out and the Zone 1 area may be increased. Prevailing winds will help to dilute the released vapour whilst the proximity of other tanks may shield the vapour release from dispersion, affecting the concentration levels. Judgement needs to be made based on common practice on these issues.

Every area in a plant must be given a recognisable category of risk that is to be observed during all operations concerning electrical apparatus. Each area would have a Zone, Apparatus group and a T rating assigned to it. A typical simple example drawing of a tank farm with a pipe trench could look similar to that shown in Figure 3.9.

Figure 3.9
Tank farm area classification

The points to observe are the scale or dimensions of the drawing and the way that the areas become additive and are classified to take account of the installation. The pipe trench also has flow instruments installed on the pipe and valves to take the product off to the tank. This poses an increased risk. The vicinity is therefore designated Zone 1.

Figure 3.10 Inserting a thermocouple causes changes to the area classification

In Figure 3.10, a continuously welded pipe or a vessel has a thermocouple installed. Whereas before, no external gas hazard need have been considered, the mechanism of the thread is considered a source of potential weakness. It may result in the classification of the immediate area as a Zone 2 as it constitutes a secondary source of release. It is not expected to constitute a Zone 1 although many engineers assume that the zones may follow in descending order of hazard. This is not the case in the normal situation. The distance that a zone extends to is a function of the pressure in the pipe, the considered likelihood of failure of the thread and the assembly’s susceptibility to external damage, etc. Additional mechanical protection may reduce the zone considerations.

3.18 Dusts

The IEC 60079 Series of Standards were specific to gases and vapours but now include Dusts. These were previously dealt with in IEC61241. As both represent a hazard, albeit with different approaches, it is sensible to consider the entirety of explosion protection as one issue.

Where a dust is mixed with air (above its lower flammable limit) within the confines of a process plant as in Figure 3.11, then that is considered to form a Zone 20 within and sources of ignition must be prevented reliably. The extent of the dust travel around an air sealed joint is negligible and so no outer area classification need be considered. The dust collection drum is not sealed and therefore must be Zone 21 because in normal operation the dust is likely to be present however the surrounding area above the collection point adjacent to the Fan and the outlet would be Zone 22 as release is not expected.

The extent of the area is not calculable in exactly the same way as for gasses and vapours. Experience shows that the distance depends on the density of the dust and the expected movement of air in the building together with other house keeping activities. The extent may be initially confined by the walls of the building but eventually may be less because of an accumulation of factors mentioned above.

Figure 3.11
Dust Area Classification example

3.19 Classification in North America

“Hazardous Locations” is the term used in North America for ‘Hazardous Areas’ where Area Classification is stated in ‘Divisions’ as opposed to Zones. with the following equivalence:-

Table 3.9
Comparison of Zones and divisions
Zone Division Definition
0 1 (Zone 0) Hazard likely to be present in normal operation
1 1 Hazard likely to be present in normal operation
2 2 Hazard NOT likely to be present in normal operation

In recent years, and in line with IEC thinking, it has been accepted in North America that an increased risk is present when the hazard is continuously present. As a result, ‘Division 1 Zone 0’ conditions are now recognised. The simplistic approach of only having the two divisions is understandable as this forms the majority of outcomes of classification, but the greatly increased risk of continuous presence is considered to require a much higher integrity approach for protection under the IEC rules in the situations when it does occur. Confining the classification in this way to only two levels is perceived to be lowering levels of safety.

3.20 Conclusions

In this chapter the subject of Area Classification has been discussed in some detail but it will be evident that the actual process comprises extensive and iterative consideration through design and early operational phases of a process plant.

Documentation of the decisions taken and of the data which was considered in order to make the decisions is vital to the correct selection and proper installation of Ex equipment. At the end of this section the recommended tabulations for laying out hazards and sources of release on a plant are provided from the Standards discussed. Simply gathering the information to fill in these forms develops the disciplines needed to complete the Area Classification task.

The next stage is to consider how equipment is selected and protected for safe use.

Table 3.10: IEC60079-10-1:2015 Table A1: Hazardous Area Classification data sheet - Part I: Flammable substance list and characteristics
Plant:
Area: Compressor facility Example
Reference
Drawing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
  Flammable Substance Volatility a LFL Ex Characteristics  
No Name Composition Molar mass (kg/kmol) Relative density gas/air Polytropic Index of adiabatic expansion Y Flash Point (°C) Ignition Temp. (°C) Boiling Point (°C) Vapour pressure at 20°C kpa Vol (%) (kg/m3) Equipment Group Temp. class Any other relevant information and remarks
                             
                             
                             
                             
                             
                             
                             
                             
                             
                             
                             
                             
a) Normally the value of vapour pressure is given but in the absence of that, boiling point can be used (see 4.4.1d).
Table 3.11: IEC60079-10-1:2015 Table A2: Hazardous area classification data sheet - Part II: List of sources of release
Plant:
Area: Compressor facility Example
Reference
Drawing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
  Source of release Flammable Substance   Ventilation Hazardous Area  
  Description Location Grade of Release a) Rate of Release Release Characteristics Reference b) Operating Pressure and Temperature State c) Type d) Degree of Dilution Availability Zone type 0-1-2 e) Zone Extent (m) Reference f) Any other relevant information or remarks
              (C) (kPa)           Horizontal Vertical    
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
                                   
a) C - Continuous, P - Primary, S - Secondary b) Quote the number of list in Part I
c) G - Gas, L- Liquid, LG - Liquified Gas, S - Solid d) N - Natural A - Artificial e) See Annex C. f) Indicates code reference if used, or calculation reference

4


Explosion Protection Philosophy and Equipment Classification Systems

This chapter looks firstly at the classification systems that are applied to Ex protected equipment. The same notation is used for Area Classification drawings to assist the equipment selection process. Secondly, a brief summary of the principles of the nine main types of protection is given here before the detailed examination for each type in the following chapters.

Learning objectives

  • To understand the Equipment Classification systems
  • To study how it relates to Hazardous Areas
  • To examine an overview of Ex protection concepts
  • To apply basic selection criteria

4.1 General

In the preceding chapters of this manual the concepts of flammability, ignition by electricity and Area Classification have been studied in order to define a Hazardous Area.

In this chapter, the philosophy behind the engineering applied to the general design and construction of Explosion Protected equipment that is considered acceptably safe in a hazardous area is explained. The detailed theory of design features and implementation is discussed in subsequent chapters but the classification systems and selection process outlined here forms the important link.

Electrical equipment for use in Hazardous Areas of a specified nature must be:-

  • Designed
  • Constructed
  • Tested
  • Marked
  • Operated and
  • Maintained

so as not to cause ignition under defined parameters of operation in the given degree of hazard.

Explosion Protection is defined in IEC60079-0 as:-
“The measures applied in the construction of electrical equipment to prevent ignition of surrounding explosive atmosphere by such equipment”.

Explosion protected ‘equipment’ is, therefore, equipment incorporating these applied ‘measures’. Such equipment is identified as being Explosion Protected by labelling it with the letters ‘Ex’; this mark is now recognised formally and internationally. The marking is a formal requirement of the Standards and includes other information by which the suitability of equipment can be assessed. This is part of the selection process philosophy that will be examined here and is specified in the Standards.

It is agreed that the state of being completely safe is never achievable. Where equipment is designed and operated in line with the requirements and guidelines published in recognised Standards then equipment is deemed to be adequately, or acceptably, safe.

The Standards referred to are the IEC60079 Series upon which this course is based. The Standards are constantly being updated to reflecting changes in the state-of-the-art technology and experience. For the current situation the most recent Standard should always be consulted.

4.2 Classification concepts

The subject of Area Classification has already been discussed. In addition, there are three important additional classifications system to be explained related to the following concepts:

  • Apparatus Grouping
  • Temperature Classification and
  • Types of Ex protection

The Classification systems used with both Hazardous Areas and Ex Equipment Marking involve the specification of limit values based on ignition characteristics. To explain this, bear in mind that ignition of a flammable material is to be avoided. As a result then there are two important statements to make:

  1. The equipment must not emit more energy than the Minimum Ignition Energy (MIE) value of the flammable material
  2. The equipment must always remain cooler than the Ignition Temperature (IT) of the flammable material

(Note that MIE and IT were discussed in Chapter 2)

The ENERGY and TEMPERATURE values are, on the basis of research divided into specific and convenient ranges so that equipment limits are placed within a range after measurements have been made.

In this way the simplistic approach of comparison for acceptability is clear. If specific criteria are not met then it is considered unsafe and is not permitted under the terms of the Standards. These comparisons will be explained and form the basis of the equipment selection process for hazardous areas.

4.3 Introduction to equipment certification

The ‘measures’ to protect equipment, rendering it to be of an Ex design, are conceived by the manufacturer and built into the product. The product is then type-tested by a separate organisation (with suitable technical knowledge and experience) that will be referred to here as a Testing Authority (TA). This gives an independent assessment of applied safety.

The TA will issue a Certificate of Conformity with the Standard to which the equipment has been designed if the product meets the requirements. The certificate will include the Ex marking, mentioned above, will also provide information for safety purpose for the correct selection and operation of the equipment. This includes information on the applied technique of protection as well as Temperature and Energy emission.

4.4 Temperature classification

Equipment located in a hazardous area has to be considered as being surrounded by a flammable gas/air mixture. It also may be possible that the hazard may enter the enclosure depending on the protection type design. It is necessary to know that the surface temperature of the equipment enclosure (or parts inside if ingress is possible) will not get hot enough to ignite the gas.

Given that there are some 750 different flammable materials it would be impossible to label equipment with all the gasses in which it could or could not be used. The label would be larger than the equipment!

A best practice method has evolved of indicating the Maximum Surface Temperature of the equipment to which the gas has access using an Index number. This is known as the ‘Temperature Classification’. It is also referred to by other similar names or abbreviations: ‘T Rating’ and ‘T Class’ are quite common.

4.4.1 Maximum surface temperature values

The standards define temperature classification as:

  • One of six values of temperature allocated to electrical apparatus derived from a system of classification according to the maximum surface temperature of the apparatus.

The six values are defined in IEC60079-0:

T Class Maximum surface temperature
T1 450oC
T2 300oC
T3 200oC
T4 135oC
T5 100oC
T6 85oC

These values are based on an ambient temperature range of –20°C to + 40°C. If the ambient temperature is elevated or suppressed beyond these limits, equipment must be tested for the wider expected range.

4.4.2 Testing and assessment

Equipment is tested for temperature rating at normal full load operating conditions. Additional testing in prescribed conditions may apply for specific types of protection. There are safety margins built in to the testing;

  • 5oC at T6, T5, or T4 and
  • 10 oC at T3 or T2.

Suppose a lighting fitting was being tested and reached a maximum of 190°C. The T Rating assigned and marked would be T3. If the measured temperature was 191oC, then this is within the safety margin of 10°C and the T Rating would go up to T2.

The T Rating system can be confusing. Another way of understanding this is to realise that when a temperature is measured on equipment under test, the value must fall in between two T ratings in the above list. The lower T rating (representing the higher of the two temperatures) is taken as the rating given to the equipment.

In some cases, temperature ratings on equipment or parts of equipment cannot be easily and appropriately measured for assessment. They can only be awarded as a result of experience or justified calculation. Small surface areas are permitted to operate at a higher temperature than large ones and so some dispensation is given to small components in some circumstances. Precise and detailed guidance on this subject specific to apparatus designers is given in various relevant parts of the IEC60079 Standards, depending on the protection method employed. This is taken into account in the equipment T rating.

4.4.3 Application

From a user point of view, Standard IEC60079-20 lists common gasses stating the T rating of equipment that is permitted for each specific gas. It is derived from the Ignition Temperature. This Diethyl Ether is known to ignite at 170°C. Using equipment in its presence which could reach a surface temperature of 200oC (T3) would be unsafe. The Standard states a requirement of T4 which will not get hotter than 135oC and is acceptable. Other gasses’ ignition temperatures are compared with T ratings in the diagram in Figure 4.1.

Figure 4.1
Temperature rating system and Ignition Temperatures compared

To reiterate an important point previously made, the ignition temperature of gases and vapours are in no way related to the ease of ignition by energy. Ignition temperature has to be a completely separate consideration.

Note that Hydrogen, requiring Equipment Group IIC (discussed below) and being one of the most sensitive gasses to ignition by spark energy, has an extremely high ignition temperature. This gas is used for cooling in some industrial applications because of its high thermal conductivity. This demonstrates how diverse the properties of flammable materials can be.

4.4.4 Ambient

The ambient temperature in which the equipment is to be located in the hazardous area must be known and the equipment correctly chosen. If elevated or suppressed temperatures are to be encountered then equipment must be chosen having been tested at that ambient.

Equipment with only a T rating marked on the label, must be used within the default limit values of –20°C to +40°C. Where equipment is certified at different ambient temperature levels this will be indicated on the certification and on the equipment label in the form, for example, Ta or TAMB = -40°C to +60°C. If only marked Ta = 48°C then the lower value remains as the default. Manufacturers will normally ascertain if their Certified Equipment is likely to be mounted and operated where elevated ambient temperatures may be encountered. It will therefore be tested at a suitable range of temperatures during the certification phase. It is possible to get existing equipment reassessed but this can be expensive. Rating to a lower Temperature Class to attempt to compensate for a higher ambient is not permitted.

4.4.5 Comparison of other temperature classification systems

Before the International Standards were published and adopted, some countries had already developed their own temperature classification systems. The Japanese (and German) ‘G’ marking is no longer used for certification now but old equipment certified to their Standards may still be in service. Equipment still in service in the UK which is made to BS889 (lighting equipment) uses the letters X, Y and Z (160°C, 110°C and 85°C, respectively) and on even older BS229 equipment there was no Temperature marking at all. It was not considered necessary on mining equipment as methane in the form of firedamp has an IT of 595°C!

The following table will give and idea how this classification is applied worldwide;

IEC60079-0 EN50014 Temp °C.   JAPAN RIIS-TR-79-1
GERMANY VDE0171
Temp °C.   BS889 Temp °C.   BS229
T1 450 G1 360 X 160 NONE
T2 300 G2 240 Y 110
T3 200 G3 160 Z 85
T4 135 G4 110
T5 100 G5 80
T6 85 G6 70

4.4.6 Marking

In some cases manufacturers will mark actual temperatures on equipment or certify equipment for use with named gasses/vapours. This is an older practice which is discouraged but nevertheless appears on some types of equipment labels and on their certificates.

If equipment is certified for a dust hazard the actual temperature is marked on the label. This is because the Installation Standard now requires a calculation which includes a de-rating factor and the T rating may overstate the Temperature restricting the application.

4.5 Equipment grouping

The mining industry first recognised and took precautions against the risk of flammability but only had one gas to consider; firedamp (a form of methane).

On investigation, it was found that firedamp taken from different sources, ignited with different levels of spark energy. This was found to be owing to varying levels of other gasses mixed with the methane that formed the major constituent. Equipment for mines was only ever tested for the ignition of the most-easily-ignited form of firedamp on the basis that if the equipment could not ignite this gas then it could not ignite any other forms that were more difficult to ignite.

The “Grouping” concept originally emerged during the development of the surface industries. A greater range of flammable chemicals were being used, each with a different MIE. It was not practicable to manufacture and certify equipment that was safe for all gasses.

‘Grouping’ was originally referred to, in older standards, as ‘Gas Grouping’ because the gasses were initially tested and placed into Groups, according to the amount of energy required to ignite each of them. The range of values was very wide but over time and experience, energy bands emerged and were assigned to the equipment when it was tested.

It then became known more appropriately as ‘Apparatus Grouping’ because it was the apparatus that was subsequently tested to prove that it would not ignite gases with certain ranges of characteristics. The term ‘apparatus’ was used to indicate that there was something special about the equipment in that it was not ordinary but had ‘measures’ incorporated into the design and manufacture to allow it to be used in a hazardous area.

The Grouping system is now known as the Equipment Grouping system in the latest International Standard. It is believed that this is because equipment not mounted in the hazardous area but connected to it can become part of the consideration for safety. One simple example of this is overload protection. This requires more detailed explanation (to follow) but the term ‘Equipment Group’ is therefore considered more precise.

Three Equipment Groups are now defined based partly on energy emission banding and partly by the industry type in which the equipment is used. There is some correlation which will become apparent in due course.

The definitions were newly defined in the IEC60079 Part 0: 2007 document. This states:-

Group I Electrical equipment intended for use in mines susceptible to firedamp.
Group II Electrical equipment for use in places with an explosive gas atmosphere (other than mines susceptible to firedamp)
Group III Electrical equipment for use in places with ignitable dusts

4.5.1 Group I

The types of protection for this group, according to the original requirements in older Standards, take into account the ignition of both firedamp and coal dust along with enhanced physical protection for equipment used underground. Other specific requirements cover the avoidance of electrostatic charge build up, fastening of enclosures, materials of construction and a surface temperature limit of 150°F on surfaces which can be touched by personnel.

Updated Standards require a surface temperature limit of 150°C where coal dust can accumulate and 450°C where it cannot. Other constructional requirements regarding robustness of mechanical protection specific to this industry must also be met but are not detailed here.

Apparatus Group Representative Gas Energy Band
I Methane < 400μJ

4.5.2 Group II

Electrical equipment designed for use in Group II is principally for gases, vapours and mists. Assessment is made of the energy emission capability of this equipment by testing or by calculation. If no energy can be emitted by design then it will be safe to use in any gas without further consideration. The equipment is marked as (Group) II.

4.5.3 Group II subdivisions

Where energy emission is possible from installed Ex equipment (and the way this might occur will be discussed in more detail during the course), then during certification the level will be established. In the same way as Temperature uses a T rating number (T1 through T6) to declare a level, Grouping subdivisions use one of three letters, A, B or C, as a mark to inform what level of energy limited band the equipment meets. These relate to the value of maximum amount of energy available from the Ex Equipment.

The ‘representative gases’ for each Group are those used in equipment testing. The energy bands are expressed in micro-joules.

Equipment Group Representative Gas Energy Band
IIA Propane ≤180μJ
IIB Ethylene ≤ 60μJ
IIC Hydrogen ≤ 20μJ

If the energy emitted from equipment being tested proves to be less than a particular Group value then the equipment is acceptable for use in that group and is marked accordingly with the Group, i.e. IIB. It is also acceptable for use in any other group with a higher value.

Equipment Group Required for Hazardous Area Equipment marking suitable for use in that area
IIA IIA, IIB or IIC
IIB IIB or IIC
IIC IIC only
All gases II (A, B or C not added) or IIC

Thus, equipment deemed safe for Hydrogen is useable in any gas because Hydrogen is the most easily ignitable gas. However, apparatus suitable for Ethylene (IIB) is not considered safe in Hydrogen and is not permitted but can be used where Propane may be present.

Equipment sub-grouping can be based on one of three quantifiable energy representations according to the Standards. These are:-

  • Minimum Ignition Energy (MIE)
  • Maximum Experimental Safe Gap (MESG) and
  • Minimum Igniting Current (MIC)

These will be explained in the context of the type of protection in which the term is used later in this manual.

Electrical equipment may be specifically tested for a particular explosive atmosphere in which it is to be used and marked accordingly. This is unusual nowadays but still possible. It dates back to when unstable substances such as cellulose were introduced into the surface industry.

4.5.4 Group III

Dusts do not have consistent ignition properties, as previously discussed. Equipment designed for dusts must avoid flat surfaces to limit accumulation and must shed heat in such a way as to avoid the blanketing effect of layers of dust. Distinction between conducting and non-conducting dusts is included in this Group system. The enclosure IP rating must be raised when dusts must be prevented more reliably from entering enclosure with bare live conducting parts. More information is expected to emerge on Group III as the IEC60079 and IEC61241 Standards will merge completely in the near future and this concept will be developed.

4.5.5 Ingress protection of enclosures

Another classification system closely associated with enclosures for Ex protected equipment is the Standard for Ingress Protection to IEC60529:2013. The following tables are taken from this Standard which includes descriptions to aid the understanding of application.

Table I of IEC 60529: 2013
Degree of protection against access to hazardous parts indicated by the first characteristic numeral.
FIRST CHARACTER NUMERAL Degree of protection
Brief description Definition
0 Non-protected -
1 Protected against access to hazardous parts with a finger The access probe, sphere of 50 mm diameter, shall have adequate clearance from hazardous parts.
2 Protected against access to hazardous parts with a tool The access probe, sphere of 12 mm diameter, 80 mm length, shall have adequate clearance from hazardous parts
3 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts
4 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts
5 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts
6 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts

 

Table II of IEC 60529:2013
Degree of protection against solid foreign objects indicated by the first characteristic numeral.
FIRST CHARACTER NUMERAL Degree of protection
Brief description Definition
0 Non-protected -
1 Protected against solid foreign objects of 50 mm diameter and greater The object probe, sphere of 50 mm diameter, shall not fully penetrate a
2 Protected against solid foreign objects of 12,5 mm diameter and greater The object probe, sphere of 12,5 mm diameter, shall not fully penetrate a
3 Protected against solid foreign objects of 2,5 mm diameter and greater The object probe, sphere of 2,5 mm diameter, shall not penetrate at all a
4 Protected against solid foreign objects of 1 mm diameter and greater The object probe, sphere of 1,0 mm diameter, shall not penetrate at all a
5 Dust-protected Ingress of dust is not totally prevented, but dust shall not penetrate in a quantity to interfere with the satisfactory operation of the apparatus or impair safety
6 Dust-tight No ingress of dust
a The full diameter of the object probe shall not pass through an opening of the enclosure

 

Table III of IEC60529:2013
Degree of protection against water indicated by the second characteristic numeral.
FIRST CHARACTER NUMERAL Degree of protection
Brief description Definition
0 No protection -
1 Protection against vertically falling water drops Vertically falling drops shall have no harmful effects
2 Protection against vertically falling water drops when the enclosure tilted 15° from vertical Vertically falling drops shall have no harmful effects when the enclosure is tilted at any angle up to 15° on either side from the vertical
3 Protection against spraying water Water sprayed at an angle up to 60° on either side of the vertical shall have no harmful effects
4 Protection against splashing water Water splashed against the enclosure from any direction shall have no harmful effects
5 Protection against water jets Water projected in jets against the enclosure from any direction shall have no harmful effects
6 Protection against powerful water jets Water projected in powerful jets against the enclosure from any direction shall have no harmful effects
7 Protection against the effects of temporary immersion in water Ingress of water in quantities causing harmful effects shall not be possible when the enclosure is temporarily immersed in water under standardised conditions of pressure and time
8 Protection against the effects of continuous immersion in water Ingress of water in quantities causing harmful effects shall not be possible when the enclosure is temporarily immersed in water under conditions which shall be agreed between the manufacturer and the user but which are more severe than for numeral 7
9 Protection against high pressure and temperature water jets Water projected at high pressure and high temperature against an enclosure from any direction shall not have any harmful effects

Enclosures are not required to be gas-tight but may need to be dust-tight in a hazardous dust atmosphere. Dust can have a detrimental effect on electrical equipment if the dust is conducting and settles on live conducting parts. Similarly, moisture that can be absorbed by a dust and then enable conduction between parts to occur may also be unacceptable. Thus, in the discussion on types of protection to follow, some will specify minimum requirements for the prevention of ingress.

Table 1 above specifies 7 levels of protection against the possibility of contact by parts of the body.

Table 2 specifies 7 levels of protection from the ingress of solid foreign bodies with a physical size that varies in diameter down to the level of dust particles.

Table 3 specifies 9 levels of protection against the ingress of liquid.

The liquid used is water. If other liquids with properties different from water, such as oils, may be encountered which must be rejected by the seals of an enclosure then specific testing will be required. The seals of enclosures are only type tested when products are designed. The IP rating must be preserved when equipment is in service.

The code is a two-digit code prefixed by the letters ‘IP’. A ‘0’ indicates no protection. As the numerals raise in value the protection level increases. For example IP00 has no protection against human contact with live or moving parts and water has free access. Where one of the codes is not relevant, the letter ‘X’ is used to depict that the option is of no consequence. Additional letters are available but not in common use.

Terminals are generally IP20 in that they cannot be touched by fingers but terminal drivers and conductors can have access to their metallic conducting parts.

The seal of an enclosure that is designed to be submersed to a specified depth of water, IPX8 may leak profusely if used for an above water level application because the seal may rely on the pressure at the submersed depth to maintain the seal integrity. Choosing the right IP rating for the enclosure is therefore important for its correct operation.

The term ‘harmful deposit’ requires some explanation; if a small quantity of dust can enter an enclosure it may be acceptable depending on where the dust settles. If it can surround terminals or conducting parts then this would be deemed a harmful deposit.

4.5.6 Resistance to impact

The enclosures may also be subject to impact strength testing. A 7NM drop test on non-light-transmitting parts is performed to check that damage cannot invalidate the IP Rating awarded to an enclosure. Light transmitting parts are subject to a 4NM test. Where impacts are likely above these values addition mechanical protection is required where equipment is installed.

4.6 Types of explosion protection

There are 9 recognised ways in which electrical equipment can be Ex Protected. These are known by their type description and a correspondingly assigned letter. They are discussed in summary in the remainder of this chapter and in detail in the following chapters.

4.6.1 Table of types of protection

Standard IEC60079 Ex Code letter Permitted Zone of use Type description Application
3 ‘p’ 1,2 Pressurisation Switchgear and control cabinets, analysers, large motors
5 ‘q’ 1,2 Quartz filling Transformers, capacitors, terminal boxes for heating conductors
6 ‘o’ 1,2 Oil-immersion Transformers, starting resistors
18 ‘m’ 1,2 Encapsulation Switchgear with small capacity, control and signaling units, display units, sensors
7 ‘e’ 1,2 Increased Safety Terminal and connection boxes, control boxes for installing Ex-certified components, squirrel cage motors, light fit-tings
15 ‘n’ 2 Non-incendive All electrical apparatus for Zone 2, less suitable for switchgear and control gear
1 ‘d’ 1,2 Flameproof enclosure Switchgear and control gear and indicating equipment, control systems, motors, trans-formers, heating equipment, light fittings
11 ‘i’ 0,1,2 Intrinsically-safe apparatus and Systems Measurement and control technology, communication technology, sensors, actuators
33 ‘s’ 0,1 Special protection Wide range but more specialised

4.6.2 Observations

In general they can be described as best suited to either ‘heavy current’ or ‘light current’ applications. Another common way of putting this is ‘power’ or ‘instrumentation’ use though there is in this case some cross-over.

Not all types can be used for all applications. Part of the equipment selection process may be to determine which type of protection is the most suitable for a given application in a given set of circumstances.

Another issue is that they are not equally reliable and are therefore confined to use in certain Zones according to their level of integrity. These issues will become more apparent.

4.6.3 Marking

A picture now emerges of complete equipment marking. The syntax for the label of any Ex equipment is shown in the example below:

Figure 4.2
Equipment Marking

The type of protection, Group and T Rating form what is called the Safety Code. All certified equipment must be marked in this way so that the important Explosion Protection features are clearly recognisable.

The hazardous area classification, allocated into zones, must also include additional details of:-

  • The required Equipment Group
  • The required Temperature Rating of equipment and
  • The required Ambient Temperature

for each specific zone.

Thus, the selection of equipment for use in a given zone only needs to match the marking of equipment to give assurance that the equipment is adequately safe. This illustrates how the classification systems are designed to be clearly applicable. They consider:

  • The nature of the explosive atmosphere
  • The probability that an explosive mixture is present
  • The maximum energy that can be produced
  • The maximum surface temperature that can be achieved

4.7 Overview of explosion protection theory

In the following chapters of this manual, the theory behind and the practical aspects of Ex protection is explained in detail but to understand the basic differences between them it is helpful to realise that there are three basic approaches to providing practical explosion protection:-

  • Explosion confinement
  • Ignition source isolation
  • Energy-release limitation

Understanding these techniques can be explained by the use of the Fire Triangle analogy described in the first Chapter of this manual. This triangle illustrates the three basic ingredients necessary for combustion to occur. The suppression of any single one of either fuel, air and ignition source will prevent ignition.

4.7.1 Explosion confinement

As the description implies, this approach to protection permits a surrounding hazardous gas/air mixture to enter an enclosure and to be ignited but the explosion is contained by the strength of the enclosure. The internal explosion cannot propagate to the outside atmosphere.

4.7.2 Ignition source isolation

Where the elements of gas, air and ignition source cannot exist at the same point and at the same time then no ignition can occur. There are several techniques which use this idea but in different ways. Some use physical separation by the use of a medium which can be gas, liquid, semi-solid or solid. Other subtle mechanical means of spark and heat elimination can also be used.

4.7.3 Energy release limitation

This form of explosion protection permits electrical energy to be conducted by a circuit within fuel-air mixture but, by design, limits the amount of electrical and thermal energy, which could be released to that which is incapable of causing ignition. The energy supplied and stored in the circuit must be considered.

4.8 Brief comparison of types of protection

When electrical equipment is to be located in a hazardous area it must be designed, manufactured and certified for that purpose. No single method of explosion protection is suitable for all applications and there are some limitations on usage in some situations. Some techniques are more reliable, having a higher grade of integrity than others. Where the risk is higher owing to the likelihood of presence of a gas during the operation of the plant, then a more reliable type of protection must be used.

In the Standards, shown in the previous table, types of protection are specifically permitted in particular Zones and not in others. Current versions of the International Standards now permit separate assessment of the reliability which is expressed as an Equipment Protection Level. (This is equivalent to the European ATEX ‘Category’ concept formally introduced in 2003.)

Equipment Protection Level ATEX Equipment Category Definition Permitted Zone of use
EPL a 1 Very high level of protection 0, 1 or 2
EPL b 2 High level of protection 1 or 2
EPL c 3 Normal level of protection 2 only

The purpose of performing area classification into zones will eventually be related to what EPL level is necessary. This is to accommodate Non-electrical equipment which does not have so clearly defined reliability record.

4.8.1 Explosion confinement (containment)

The basis of safety for ‘Flameproof’ (Ex d) equipment is that ignition-capable electrical parts such as relays, switches, terminal blocks, etc., can be located inside a substantially strong enclosure. The enclosure itself is sufficiently robust to withstand the effects of a gas air explosion which could occur inside but without allowing the ignition energy in the form of a flame (propagating through the explosion) to exit the enclosure and ignite the surrounding flammable gas.

It must contain the effects of the ignition i.e. the burning gas (or flame) which will try to exit the enclosure and which is a separate consideration from merely containing the explosion pressure. These two facts of the technique must not be confused.

The flame is ‘cooled’ or de-energised by the careful design and manufacture of ‘penetrations’ between the inside and the outside of the enclosure. A penetration could be a screwed-cable gland or the flanged lid of an enclosure. In the latter case such a path or penetration will be present owing to the requirement to gain access to the inside of the enclosure. The penetrations are referred to as flamepaths because they represent a possible path through which the burning gas might try to travel. The design will have critical dimensions which must be preserved to prevent this unwanted transfer of flame energy.

The critical dimensions are derived from experimental work done to define the ‘maximum experimental safe gap’ (mesg). From this there is a safe working gap dimension which is in part determined by the internal volume of the enclosure and the explosion characteristics of the gasses for which it is being designed.

In addition, all external surfaces must be kept below the ignition temperature for the specific gas the enclosure will be exposed to.

This type of protection is known in North America as “Explosionproof”.

Although a popular and useful technique, this technology does pose many drawbacks. Since the enclosures must contain an explosion, they are bulky, heavy, expensive to machine and difficult to install. All cabling entering and exiting these enclosures must maintain the integrity of flameproof by withstanding the explosive forces and by not allowing the flames or hot gases to exit the enclosure. Certified glands must be installed. The enclosures must be inspected frequently to ensure integrity. Loose or missing bolts on the lids of enclosures effectively defeat any explosion protection for the enclosure. It is represented diagrammatically in Figure 4.3.

Figure 4.3
Principle of Ex d

4.8.2 Ignition source Isolation (exclusion)

This basis of safety relies on segregation of gas air and ignition source in a number of possible ways.

The following briefly outlined methods all satisfy this requirement in a number of different ways:-

Pressurised (Ex p)
Air or an inert gas blown into an enclosure (housing unprotected electrical equipment) in such a way as to maintain a positive pressure (above atmospheric) which will prevent a Hazardous Atmosphere outside from entering the enclosure. This is shown in Figure 4.4. The only concern is for the outside surface temperature and so T Rating is necessary but the equipment concept will not emit energy and so is just Group II.

If the system delivering pressure fails then action must be taken to eventually shut down the equipment inside.

Purged (Ex p)
An alternative is to maintain a flow through the enclosure and at the same time maintain a pressure inside. This means that a flammable gas cold be introduced but would be diluted below its lower flammable limit. Alternatively the air flow could be used for cooling equipment .

Figure 4.4
Principles of Purging Ex p

Ventilated (Ex v)
This technique is NOT embodied in the International Standards but is permitted to be used under local Standards such as in Australia. It is used in larger volume chambers to dilute flammable gas or vapour to well below L.E.L. and includes the intention to reduce the temperature of electrical equipment by the use of the air flow. In this system air is fed into a potentially hazardous area increasing the proportion of air in the atmosphere thus decreasing the gas / air mixture to less than the L.E.L.

Many variations on the main principle have been implemented in applications. One such example is in a Control Rooms where personnel are located. Standards are being written currently to guide sectors of industry which require this form of protection.

This type of protection has no EPL assigned as yet.

Oil immersion (Ex o)
The integrity of this arrangement depends upon the presence of liquid as opposed to gas. The same T rating arguments apply. The requirement is for a suitable depth of mineral insulating oil under which ignition capable uncertified equipment is submerged as depicted in Figure 4.5.

Figure 4.5
Principle of Oil immersion Ex o

Sand filled (Ex q)
This is also known as ‘sand’, or ‘powder’, ‘-filled’. After electrical parts are fixed in a position inside an equipment enclosure usually like a thin walled tin-can, it is filled with a fine powder or granulated material (semi-solid) that will exclude a gas/air atmosphere from any arcing or hot surfaces. It will conduct away heat from inside to the outside surface so that it is low enough to prevent ignition of the surrounding hazardous atmosphere on the outside the enclosure (see Figure 4.6).

Figure 4.6
Principle of Ex q

Encapsulation (Ex m)
The main requirement for encapsulation is that the apparatus to be protected is encapsulated in resin with at least 3 mm of resin between it and the surface. In this system, the flammable gas or vapour cannot come into contact with arcs or sparks due to the operation of the apparatus. Used mainly for items not readily repairable. This method is generally suitable for Zone 1 and 2 (see Figure 4.7).

Figure 4.7
Principle of Encapsulation Ex m

Increased safety (Ex e)
The design and manufacture of this type of equipment assures safety by ensuring that, inside an enclosure where a gas/air mixture may enter and surround electrical parts, they will not provide a source of ignition. These parts are designed or chosen so that, firstly, the incidence of arcs and sparks are completely and reliably prevented. Secondly, the temperature of these parts does not become excessive.

Some heat is expected but the design and construction helps to dissipate any heat. In this way insulation integrity remains very high and failure, to cause arcing, cannot happen. Since no energy is emitted then the equipment is Group II (no A, B or C) and the Temperature Rating is applied to anywhere inside or outside the enclosure. Sparking is further prevented by the use of high integrity, de-rated insulation with increased creepage and clearance distances applied and connections that cannot vibrate loose. Internal current-carrying parts and their insulation are made larger than for normal use and are rigorously tested subsequently becoming Component Certified to ensure reliability. Ingress Protection IP54 minimum and Impact strength test requirements for the enclosure are required to prevent the moisture or conductive dust paths across insulation between conductors.

This type of protection is designated EPLb suitable for Zone 1. It is represented diagrammatically in Figure 4.8. This protection type can incorporate other means of Ex protection described above under specific and control conditions. In this way it is conceived to provide a lower cost technique for some applications.

Figure 4.8
Principle of Increased Safety Ex e

Non-incendive (Ex n)
This type of protection is based on the use of good quality industrial grade equipment, housed in an enclosure. The enclosure is not gas tight, (this is similar to Ex e), but the parts inside the enclosure are chosen so that they do not produce incendive arcs, sparks or hot surfaces in normal operation. It is represented diagrammatically in Figure 4.9. To clarify this, sparking is permitted but must not be at incendive levels, hence the name, ‘non-incendive’. If incendive levels are likely, and excess sparking or heating can take place, then the rules permit additional secondary protection means (again using the other type principles described) to be incorporated into the design in a number of different ways with simpler testing applied to prove safety. Stringent additional testing (and component certification) is NOT applied to Ex n (as it would be in Ex e). It is considered as a lower integrity concept that is restricted in use to Zone 2.

Figure 4.9
Principle of Non Incendive ( Ex nA and Ex nR depicted)

Ex n application, particularly to instrumentation, is now seeing greater use. Early in its development it had only been applied to higher power equipment such as induction motors, junction boxes and light fittings. Recent changes to the Standards have relaxed and clarified the requirements.

It is still reliant on sound construction and an enclosure with a minimum of IP54 is needed. This type of protection is designated EPLc.

4.8.3 Energy limitation

This approach is referred to as Intrinsic Safety (often written as ‘I.S.’) and comes in three forms of varying integrity levels; Ex ia, ib and ic.

It is different from the other types in that it requires consideration of the complete circuit in the hazardous area, as shown in Figure 4.9. Voltage, current, stored energy and power are controlled and limited to safe values in each individual circuit entering the hazardous area by electrical design.

There are three parts to the circuit that can influence safety;

  • The safe area equipment (Interface)
  • The hazardous area mounted equipment (either certified or ‘simple’) and
  • The interconnecting cabling

and so a ‘System’ approach is required to verify circuit safety.

With intrinsic safety, there is always the need for a certified interface such as a Shunt Zener Diode Barrier or a Galvanic Isolator (sometimes built in to certified equipment) to couple the circuit in the Safe Area to the equipment in the Hazardous Area. Some types of equipment in the Hazardous Area are termed ‘simple’ apparatus which does not need assessment and certification. Simple apparatus is defined in IEC 60079-11. Interfaces perform their safety function by limiting supply under specific fault conditions.

The limiting of power dissipation and stored energy release is controlled with highly reliable safety components that are termed ‘infallible’.

An Ex ia system is the highest form of integrity usable in Zone 0 and is designated EPLa. Two safety component faults can occur yet the limiting function is still maintained for safety. Ex ib is safe with one fault and Ex ic only safe in normal operation and so these are designated EPL b and EPL c, respectively.

Figure 4.10 Principle of Intrinsic safety Ex i

In general, a circuit can only be made intrinsically safe if it operates on less than about ½ Watt. It is for this reason that intrinsic safety is limited to measurement and control circuits. Circuits so protected can tolerate live working which suits instrument operation for live calibration, etc.

4.8.4 Special situations

Protection by ‘Special’ means (Ex s) can be used to assess and certify equipment that does not comply with any of the recognised types described above, but which can be shown to be adequately safe for prescribed hazards. ‘Ex s’ is designated EPLb but if marked as ‘Ex s(Zone 0)’ it is EPLa.

The type of protection is not possible to describe except in the context of its application as each type device will be unique.

4.8.5 An important note:-

It should be realised that not all these techniques can be used in all applications. Some are best suited to low power uses such as instrumentation whilst others will be preferred for higher power handling situations. In addition some techniques used within some types of protection are best suited to certain functions whilst others cannot be used in such an application owing to the subtle differences which are yet to be explained.

The subtlety of approach is sometimes found confusing leading to the belief that some types of protection are similar to others, for example Ex e and Ex n. Some aspects may be similar but other detail is critically different and it is this detail that separates the types of protection. If aspects of the detail of the type protection are compromised because of incorrect installation or maintenance then safety is undermined.

Ex i and Ex nL (a subtype of Ex n discussed in a later Chapter) use similar principles which are implemented in different ways; the same design curves are used but different safety factors are applied.

4.9 Mixed techniques

Equipment manufacturers have always innovated ways of solving user problems and this can often be achieved by combining types of Ex protection to suit a particular application. The mixing of types often becomes a standard approach, for example as is seen with a fluorescent lighting fitting in Figure 4.10. Motors using Ex d for windings and Ex e for terminals are common practice. Instrument arrangements also use combinations for various purposes. This approach is used to optimise functionality whilst meeting the requirements of safety and yet keeping the cost of manufacture and ownership as low as possible. The issue of mixing types of protection is best explored having looked at the detail of how each individual type of protection is achieved and after understanding its limitations.

Figure 4.10
Typical fluorescent luminaire of the Ex e type includes Ex d and Ex m ‘components’

4.10 Dust explosion protection methods

The above techniques are useable in dust atmosphere with some limitations. Owing to the variable nature of dust and the fact that the primary weapon to guard against uncontrolled dust presence is good ‘house-keeping’, then other specific types of protection are emerging such as ‘tD’. Essentially this is protection by dust-tight enclosure where temperature rise is the focussed concern.

Generally three methods are used:-

  • Principle of operating under inert atmosphere
  • Principle of Containment
  • Principle of Ingress Protection

The method of operation of process under inert atmosphere implies that the concentration of oxygen is monitored and it is limited so as to prevent the explosion. Thus it is essential that the Limiting Oxygen Concentration (LOC) of the dust or powder be known to apply this protection method. The integrity of the inert atmosphere technique requires the installation of reliable oxygen measurement instrumentation to monitor the atmosphere.

In case the consequences of a dust explosion are excessively dire then prevention measures for explosion containment methods need to be deployed. This is achieved by manufacturing the process equipment in such a way that it is strong enough to withstand the maximum pressure of the explosion whilst pressure relief, designed to suit the application, is allowed to occur under controlled circumstance. This can be an expensive option as it requires expert advice based on experience to engineer and implement solutions. It is the main approach used on larger scale systems and is often the most practicable way of protecting complex manufacturing system using expensive processing equipment. The ‘pressure relief’ panel devices release the pressure if it builds up excessively so that other equipment does not suffer extensive damage.

4.11 Selection of the type of explosion protection

The process of selection of the type of Ex Protection for a given application must consider many aspects. Clearly the location will determine the Grouping, Temperature Rating and Ambient temperature requirements but the application itself will have a bearing on what type of protection is best suited to the task.

The principal consideration is which types are permitted to be used in a given Hazardous Area. The cost of purchase, installation and ongoing maintenance will need to be considered before a final choice is made. It is not possible to give a thoroughly worked example of the selection process as it must depend on many variable factors which may need a detailed analysis of given set of circumstances.

By way of an example, choosing a motor for use in Zone 1 will depend not only on the required power but also the duty. An induction motor could be Ex d, e, or p. A continuously running machine would be acceptable in an Ex e design whereas it is unsuitable for a stop-start duty that Ex d or Ex p would handle.

If an instrument, fault tolerance and Safety Integrity Levels to IEC61508 need to be considered separately as these are not part of the Explosion Protection assessment.

No one method of explosion-protection is ideal or can be described as better than another inherently. The suitability will be inextricably linked to the application. Each has their advantages and disadvantages; disadvantages may dissuade the use of certain techniques in certain situations.

The following table is taken from the IEC60079-14:2014 selection and installation Code of Practice and shows what type of protection is permitted for use in Gases and Dusts, using the EPL discussed above but pre-qualifying the EPL with a G or D respectively. Thus EPL Ga may be used for Zone 0.

Notice that many of the Codes include the lowercase letters a, b or c to depict the level of protection afforded. Thus Ex ma certified equipment will be specially certified for Zone 0 where as anything certified Ex m is restricted to Zone 1, as it has been certified to an old Standard which did not require or specify additional construction and testing requirements at the time of issue.

EPL Type of protection Code According to
‘Ga’ Intrinsically safe ‘ia’ IEC 60079-11
Encapsulation ‘ma’ IEC 60079-18
Two independent types of protection each meeting EPL ‘Gb’   IEC 60079-26
Protection of equipment and transmission systems using optical
radiation
‘op is’ IEC 60079-28
Special Protection ‘sa’ IEC 60079-33
‘Gb’ Flameproof enclosures ‘d’ IEC 60079 1
Increased safety ‘e’ IEC 60079-7
Intrinsically safe ‘ib’ IEC 60079-11
Encapsulation ‘m’ or ‘mb’ IEC 60079-18
Oil immersion ‘o’ IEC 60079-6
Pressurized enclosures ‘p’, ‘px’, ‘py’,
’pxb’ or ‘pyb’
IEC 60079-2
Powder filling ‘q’ IEC 60079-5
Fieldbus intrinsically safe concept (FISCO) Withdrawn see
-11
IEC 60079-27
Protection of equipment and transmission systems using optical
radiation
‘op is’, ‘op sh’,
’op pr’
IEC 60079-28
Special Protection ‘sb’ IEC 60079-33
‘Gc’ Intrinsically safe ‘ic’ IEC 60079-11
Encapsulation ‘mc’ IEC 60079-18
Non sparking ‘n’ or ‘nA’ IEC 60079-15
Restricted breathing ‘nR’ IEC 60079-15
Energy limitation ‘nL’ IEC 60079-15
Sparking equipment ‘nC’ IEC 60079-15
Pressurized enclosures ‘pz’ IEC 60079-2
Fieldbus non incendive concept (FNICO) Withdrawn see
-11
IEC 60079-27
Protection of equipment and transmission systems using optical
radiation
‘op is’
‘op sh’
’op pr’
IEC 60079-28
Special protection ‘sc’ IEC 60079-33
‘Da’ Encapsulation ‘ma’ IEC 60079-18
Protection by enclosure ‘ta’ IEC 60079-31
Intrinsically safe ‘ia’ or ‘iaD’ IEC 60079-11
Special protection ‘sa’ IEC 60079-33
‘Db’ Encapsulation ‘mb’ IEC 60079-18
Protection by enclosure ‘tb’ or ‘tD’ IEC 60079-31
Pressurized enclosures ‘pD’ IEC 61241-4
Intrinsically safe ‘ib’ or ‘ibD IEC 60079-11
Special protection ‘sb’ IEC 60079-33
‘Dc’ Encapsulation ‘mc’ IEC 60079-18
Protection by enclosure ‘ta’ or ‘tD’ IEC 60079-31
Pressurized enclosures ‘pD’ IEC 61241-4
Intrinsically safe ‘ic’ IEC 60079-11
Special protection ‘sc’ IEC 60079-33
New protection marking codes with identification of EPLs may be introduced in the future

EPLa

The current Selection and Installation Standard, IEC60079-14, does permit two techniques to be combined in parallel, i.e. one inside the other, to achieve a higher protection level. Thus using a flameproof enclosure in a pressurised enclosure (Ex dp), but NOT pressurising the Ex d enclosure, combines two EPL Gb systems to provide an EPL Ga solution. This gives the protection capability great flexibility but at some increased cost and inconvenience. It would not allow for one of the biggest advantages of using intrinsically safe Ex ia systems, being that maintenance can be carried out ‘live’ without the need to ensure that the area is ‘gas free’.

Combining EPL with a type of protection is now evident in the previous table. Ex ma may be used in Zone 20 but Ex mb is restricted to Zone 21. The use of additional lettering to qualify the EPL is now being applied consistently in the Ex Construction Standards where variations warrant different levels.

4.12 Conclusion

Equipment of a suitable Equipment Protection Level must be used in Hazardous Areas as defined by Area Classification having determined the Grouping, T rating and ambient temperature requirement and matching these to that offered by Manufacturers.

The certification achieved by the manufacturer will deem the equipment to be acceptably safe provided that it is used in compliance with the requirement of the Standards and Codes of Practice and the manufacturer’s instructions.

The Ex related Standards do not take into account other operational and functional safety issues which must be considered separately by the designer.

The plant duty-holder/owner/operator/user is responsible for the safety of the plant. It must be managed by those with an adequate skill-base and the use of good judgement to ensure that the equipment is installed in accordance with regulations and is tested before initial use. The equipment must be properly maintained and subject to regular inspection and maintenance.

Study so far includes the properties of gases, vapours and dusts and the classification systems that are used so that equipment can be chosen safely. An overview of explosion protection techniques has been received and it is now clear that the responsibility of safe plant will transfer to users/operators/owners after the selection and installation of equipment.

In next few chapters, greater detail about the types of Ex protection and considering the use to which they can be put will be revealed. Examination of:-

  • the Construction Standards
  • definitions and terminology
  • theory
  • key features
  • practice of equipment use
  • some typical applications
  • the variants available and
  • Inspection, maintenance and repair issues

for each type of protection.

Having completed this we will look at general installation and inspection/maintenance philosophies which will close the loop on the responsibility of the users.


5


Protection Concepts: Type of Protection; ‘d’

In this chapter we look at one of the oldest types of protection that is designed to contain an explosion and not let the resulting flame provide a source of ignition for the atmosphere on the outside of the enclosure. Strong enclosures are needed to withstand the explosion but the assembly must have joints allowing access to equipment but which do not permit the transmission of flame from inside to outside. The engineering of these joints and the preservation of their integrity is fundamental to the technique.

Learning objectives

  • To understand the theory of the ‘flameproof’ concept
  • To understand how it operates with typical applications
  • To explain the care needed to ensure compromise in safety does not occur

5.1 Name

Type of protection d is otherwise known as Flameproof Equipment. In North America it is referred to as Explosion-Proof. There is no difference in the way that the equipment affords protection to the Hazardous Area.

5.2 Standards

Current International Standard:

  • IEC60079-1:2014

Older Standards

  • EN50018
  • BS5501: Pt 5
  • BS4683: Pt 2: 1971
  • BS889: 1965 (Light Fittings)
  • BS229: 1957 (Originally 1928)
  • AS 2380.2
  • USA Approval to NEC Article 500 / UL698

5.3 Definition

IEC60079-1:2014 states:-

An enclosure in which the parts which can ignite an explosive gas atmosphere are placed and which can withstand the pressure developed during an internal explosion of an explosive mixture, and which prevents the transmission of the explosion to the explosive gas atmosphere surrounding the enclosure

To help the understanding of common terminology, here follows some definitions taken from the Standards.

5.3.1 Flameproof enclosure

The term FLP (mnemonic for Flameproof) originates from the Mining industry where it was first used. The term flameproof is applied to enclosures of electrical apparatus certified by a certified testing body as having been examined, type-tested where necessary, and found to comply with the SABS / IEC 60079-1 or or BS / EN 50018 (1995). The enclosure must be strong enough to withstand the stresses of internal ignition.

5.3.2 Flameproof joint or flamepath

A place where the corresponding surfaces of two parts of an enclosure, or the conjunction of enclosures, come together and which prevents the transmission of an internal explosion to the explosive gas atmosphere surrounding the enclosure

5.3.3 Length of flame path

The shortest unobstructed path traversed by a flame through a joint from the inside to the outside of an enclosure.

5.3.4 Gap

The distance between the corresponding surfaces of a flameproof joint after the electrical equipment has been assembled.

5.3.5 Definition of re-marking with EPL

In this Standard and in line with the Equipment Protection Level concept introduced into all Standards Ex d protection is re-designated ‘db’ owing to its permissible use in Zone 1 (or Zone 2) but NOT Zone 0.

5.4 Principle of operation

This type of protection houses electrical parts in an enclosure. The hazardous atmosphere surrounding the enclosure is not prevented from entering inside and the resulting gas/air mixture could be ignited by the electrical parts inside. If ignition occurs then there will be a pressure rise inside the enclosure which is what the enclosure is designed to withstand.

The pressure front exerts large forces on the internal surfaces of the enclosure. The pressure front is followed by the flame as it travels outwards from the point of ignition as a progression of combustion. The internal flame is extinguished when all the fuel/air mixture inside the enclosure is burnt. So far there is no threat to the surrounding gas atmosphere. There are some secondary effects to this mechanism which must be considered as in Figure 5.1.

Figure 5.1
Principle of operation of a flameproof enclosure

The force on the lid of the enclosure will cause the securing bolts to stretch, thus there will be some small separation between the flange surfaces of the joint. The flame will try to pass from the inside of the enclosure to the outside through the temporary separation. The fact that the two surfaces are not permitted to part any great distance is important as they form what is called a Flame-path. This has critical properties. It serves to cool the flame so that by the time the flame-front reaches the hazardous atmosphere outside the enclosure there is insufficient flame energy to ignite the surrounding gas.

It is important to realize that the separation, often referred to as a ‘gap’ is a consequence of the ignition inside the enclosure: in the example shown it is because of the resulting force. It is believed by many that the presence of this ‘gap’ is an integral design feature and is somehow necessary to relieve the pressure inside the enclosure. This is simply not true and is a potentially dangerous mis-interpretation of the principles. Further explanation will help to dispel this myth.

5.4.1 History

This type of protection was first used in the mining industry. It is not known precisely when it first came into use but it was developed in parallel in the UK and Germany. German engineer Dr. Ing. Carl Beyling is credited with the development of the concept in 1908 and was honoured by the Institution of Mining Engineers, in the UK in 1938. It is understood that Beyling initially believed that the technique worked by simply containing the pressure inside the enclosure. The original German phrase used was “druckfest encapsulung” which means ‘pressure containing’. It was for this reason that the letter d was chosen to depict the type of protection. For reasons which will become apparent in this section of the manual, the principle is something of a misnomer and is considered by some to be misleading because it is now realised that it must be ‘flame containing’ not pressure containing. It is possible to allow sufficient flame energy to be released even though the pressure is contained. Thus the important criteria is containing the flame.

The development of the ‘Flameproof Enclosure’ followed the beginnings of use of electricity in coal mines in Europe. At the beginning of 20th century, motors and equipment, operated from Direct Current (d.c.), were in use. Ignition-capable arcs and sparks were produced by this equipment. This was the only considered ignition source because the Ignition Temperature of Firedamp (form of Methane) was 595°C. This explains why the early equipment tested and certified (FLP to BS229) was only tested for flame transmission. There was no consideration of the likelihood of reaching the ignition temperature of firedamp and so Temperature Rating or marking was not applied. The concern was only that equipment could get too hot to touch and so in the UK a limit of 150°F was implemented.

The ideal way to protect equipment from causing ignition would have been to completely exclude the explosive gases from coming in contact with this sparking. This was not possible from a practical point of view owing to the unreliability of the equipment at that time. It needed to be opened frequently for maintenance. If enclosures were to be sealed, the seal would have to be broken to gain entry for repair and then resealed reliably. This would give rise to the practical problem of establishing the quality of the seal by testing which would have been difficult to do with the equipment installed underground. Added to this, rotating shafts, spindles and rods also make sealing permanently and under all conditions a near impossible task.

On accepting the fact that an explosive atmosphere cannot be permanently kept out of such an enclosure, it became imperative to look for the means to ‘contain’ the internal explosions as and when they occurred. Thus, an enclosure would need to be designed with sufficient strength without incurring any damage to itself and without transmitting the flame to the outside atmosphere.

Where a flameproof enclosure was required to allow moving parts to go through from the inside to the outside, such as in a motor design, the shaft and the end-shield could not be sealed and there must be a clearance between them. It was realised and subsequently demonstrated that if the separation was too large then the flame could travel through. Work began to quantify the maximum permitted size of any clearance.

5.4.2 Maximum experimental safe gap

In seeking to make measurements it was discovered that the flame front caused by the explosion would effectively be extinguished as it travelled through the separation between the faces of a flanged joint. There was also found to be a relationship between the minimum length of the flanges and its maximum separation distance to guarantee to extinguish the flame passing through it.

Any path, which the flame or hot gases may take, needs to be of sufficient length and constriction to cool the products of the explosion so as to prevent ignition of a flammable atmosphere external to the enclosure.

A flame travelling through any flamepath is said to loose its ignition capability in three ways; firstly, by molecular bouncing; secondly, by adiabatic expansion and, thirdly, by the cooling effect of mixing with the outside atmosphere. It is therefore important that these three mechanisms are not compromised by the design, installation and operation of the enclosure. These will be discussed later in the chapter.

Originally, the test devised to define what became known as the maximum experimental safe gap (MESG) was developed using an eight-litre bronze sphere, which is made of two halves and joined by a flange of one-inch path length. The sphere is enclosed in a tank of 8 cubic feet in capacity and filled with the most easily ignitable mixture of a test gas. The distance between the flanges of the two halves of the sphere, known as the gap, is varied in a series of tests with each gas. A substantial spark is introduced into the inside of the sphere. The size of the gap is increased until the point is found at which sufficient flame energy is released from the sphere, through the gap, to ignite the surrounding gas. The object is to find the maximum gap size of a one inch flame path that will not transmit ignition. This is said to be the ‘maximum experimental safe gap’. To add a margin of safety, the MESG was reduced to a Safe Working Gap that was incorporated into the Standards that were written as a result of this definition, issued in the UK in 1928.

Subsequent work revealed that the larger the volume of the enclosure, the longer the length of the flame path was required to be for a given Safe Working Gap. Conversely if the flamepath was made shorter in length then the Safe Working Gap had to be reduced. Thus equipment could be designed to be safe for a given gas by controlling the design and the dimensions of an enclosure.

It was realized that the MESG and the Minimum Ignition Energy, MIE values for any given gas correlated. Therefore the same marking for Grouping could be used IIA, B or C. This is illustrated in the table below:

Representative Gas Ignition.Temp °C MESG Safe Working Gap Minimum Ignition Energy Required Equipment Group
Propane 466 0.016” 0.008” 180μJ IIA
Ethylene 425 0.008” 0.004” 60μJ IIB
Hydrogen 560 0.004” 0.001” 20μJ IIC

Note that the Ignition Temperature is included as a reminder that it must be considered separately!

5.5 Types of flamepath joint and uses

From the above table it is noted that a joint suitable for a Hydrogen hazard must remain within very tight limits and so it is not practicable to manufacture and certify flange joints for such an application. Other types of joints devised were considered more suitable for certain applications and constructional requirements. The basic and most common types found are illustrated in Figure 5.2.

Figure 5.2
Types of flamepath construction permitted in Ex d design

5.5.1 Threaded joints

Threaded joints provide for the most controlled flamepath and so are best suited to Group IIC applications, such as for Hydrogen. The requirement is that a minimum of 5 threads must be engaged when assembled; this gives an effective flamepath length of 5 x Π x the diameter of the thread. Care must be taken that the thread is not damaged otherwise the flame may try to traverse across the threads.

5.5.2 Flanged joints

Flanged joints are where two parallel surfaces, the box and its lid, meet and are held together with bolts. The design is tested to ensure flame transmission does not occur with all bolts in place. Surfaces do have to be parallel and undistorted but do not need to be ground flat. A maximum surface roughness of 6.3 microns is specified in the Construction Standards.

The length of a flame path must be unobstructed but where a fixing bolt is present the length of the path one side ‘L’ constitutes the main flame path, whereas the other path ‘l’ is not critical. A manufacturer will agree this with the Certifying Authority. The user should be cautious not to compromise either as they will not know which is critical.

The penetration of the bolt into the enclosure fixing has minimum dimensions so if thread damage is caused care must be taken when performing repairs that the strength of these fixings are not compromised.

Bolts are required to be of sufficient strength, amply threaded, have adequate land on the head and to secure the lid during an internal explosion. All bolts of the correct type must be in place to hold the lid secure; removal of any one is likely to allow sufficient distortion of the lid during the explosion to allow the flame path to widen beyond safe limits. The aspects of flanged joints and fixings are illustrated in Figures 5.3 and 5.4.

When assembling the enclosure and lid, care must be taken not to allow any protective grease to build up in the blind hole such that the bolt will not locate fully and allow the lid to rise, increasing the size of the flamepath.

Shrouds around the bolt head force the use of a tool to remove or secure it. Older Standards required the use of special tools i.e., triangular or 5-sided head bolts and Allen screws were common. Current Standards now require the use of ordinary tools so socket spanners are acceptable.

Figure 5.3
Features of a flanged joint
Figure 5.4
Aspects of flange fixing bolts

5.5.3 Spigot joint

A Spigot joint is where parts mate together, one inside the other, as a clearance fit and are of the type used for the end-shield of a motor as seen in Figure 5.5.

Figure 5.5
Cross-section of typical motor showing key flamepath joints

5.5.4 Dimensions

Whilst threaded joints have no specific clearances to be measured, all the other types of joint have maximum safe working dimensions to which the equipment must be designed. It is necessary for these dimensions to be checked periodically throughout the operating life of the equipment, as discussed later.

Figure 5.6
Flamepath joint data from IEC60079-1

In Figure 5.6 note that the type of joint, the flamepath width, the internal volume of the enclosure and the Equipment Group all have influence over the Maximum Permitted Gap allowed. Measurement must be made using a suitable method.

5.5.5 Checking the dimensions

When first designed, constructed, certified and supplied, the dimensions must comply with the Standard for the types of joints employed. It is not necessary to check these are correct on new installations unless it is suspected that damage has been sustained.

During the service life of the equipment it will be necessary to check the dimensions and quality of the flame paths which can only be done by dis-assembly. In the case of a flange joint, feeler gauges can be used to ensure that flange surfaces are flat and mating. But in the case of a spigot joint, measurement is necessary.

Figure 5.7
Flamepath joint data from IEC60079-1

To illustrate how the information is used, in the case of the spigot joint in Figure 5.7, the Standard for inspection requires a series of measurements to be made to ensure that any tendency to ovality is allowed for. The largest female diameter measured minus the smallest male diameter will give the largest possible gap. This value must be compared with the maximum permitted gap from the table and if larger then the equipment is no longer acceptable and should be replaced.

Suppose the male o/d was damaged by scoring and an attempt was made to remove the damage, the flamepath would be widened at that point and the integrity of the enclosure compromised.

5.5.6 Small Volume d

Where the internal volume of an enclosure is 10cc or less, it was found that a contained gas/air mixture of that volume could not be ignited. The European Standard EN50018 allowed devices to be designed and incorporated into equipment considerable enhancing the flexibility of Ex d to allow mating parts as connections which could be live. This revolutionised and revitalised the Ex d design breadth considerable with economic benefits.

5.5.7 Flamepath impairment

The process by which the energy and heat is taken out of the flame travelling through the flamepath must not be affected by the way the equipment is installed. The flame travelling through the separation (caused by the explosion force) is cooled and de-energised partly by mixing with the outside atmosphere. Impeding the exit path with excess external influence such as paint, grease, tape and objects could cause local heating where the flame velocity is slowed down.

Figure 5.8
Impedance of flamepath

As a result the clearance specified between the joint exit and any obstruction as shown in Figure 5.8 must not be less than:-

  • 10mm for Group IIA,
  • 30mm for Group IIB
  • 40mm for Group IIC

unless otherwise tested for and included in the certification.

Guidance Rules for the use of other influences are discussed in the notes on Installation at the end of this chapter.

5.6 Explosion pressure

The Ex ‘d’ method of protection relies on the mechanical strength of an enclosure to withstand an internal explosion. It must not allow the explosion to propagate to a flammable atmosphere surrounding the enclosure. The pressure rise inside an Ex d enclosure is measured as part of the certification process. In an empty enclosure the pressure consistently reaches approximately 7-9bar, with any gas or vapour ignition. It is the rise-time TR of the explosion pressure which varies between different types of gas as shown in Figure 5.9.

Figure 5.9
Rise time of Explosion pressure in an Ex d enclosure

After combustion has occurred, the pressure in the enclosure is sometimes dissipated but may even form a vacuum as the products of combustion take up the space occupied by the fuel/air mixture. The pressure will not necessarily be contained by the enclosure as some engineers and technicians believe. The vacuum is why it is sometimes more difficult to gain entry to and enclosure.

5.6.1 Pressure piling

Where an enclosure can become divided into two separate areas, by the placement of internal equipment mounted within, a phenomenon known as “pressure piling” will occur. With reference to Figure 5.10, if ignition occurs in area A, the combustion causes an explosion pressure of 7-9 bar which impacts on the adjacent walls of the enclosure in that area. The pressure wave continues around the obstruction then pre-pressurises the un-burnt gas in area B. The pressure wave front is followed by the flame-front which then ignites the un-burnt gas at the higher initial pressure to raise the explosion pressure yet again to 14-18 bar. It is this higher explosion pressure that is exerted on the adjacent walls of the enclosure in area B. The enclosure must then be manufactured and tested to withstand the higher pressure or the pressure piling effect.

This cascade effect can multiply if the design of the Ex d equipment is based on a train of chambers created within an enclosure.

Pressure piling is defined as:-
A condition of rise in pressure resulting from ignition of pre-compressed gases in compartments or subdivisions other than those in which ignition was initiated and which may lead to a higher maximum pressure than would otherwise be expected.

Figure 5.10
Rise time of Explosion pressure in an Ex d enclosure

5.7 Certification

There are some important aspect of Certification

5.7.1 Component certification

A Flameproof enclosure without any electrical equipment mounted inside it is tested and certified as compliant with the Construction Standards for Ex d. The Certification issued is known as ‘Component Certification’. It is depicted by the use of the letter U after the certificate number, i.e., BAS01ATEX1234U. This theme of component certification re-occurs with many Ex equipment.

5.7.2 Equipment certification

It is not until electrical equipment is positioned in an enclosure that two issues affecting the application of the enclosure will be discernable.

Firstly, the Temperature Rating (T number) can only by assigned when the internal electrical equipment is being operated at full load and under the specified test conditions in the Construction Standard.

Secondly, the layout of equipment inside the enclosure may cause pressure piling, so tests must be carried out to discover by testing if this is the case and that the enclosure is adequately designed to withstand the pressures involved.

If these are satisfactory and the results of other tests and measurements are acceptable, then Apparatus Certification (now called Equipment Certification) is granted. A Certificate is issued with a Certificate Number without the U, for example PTB05ATEX1357. The Certificate Number and Safety Code would be marked on the dataplate as specified in the Certificate together with any ambient temperature if different from the basic range, for example, “Ex d IIB T5 TAMB = -30°C to +60°C”.

If the equipment inside the enclosure is subsequently altered or the position of the internal components are changed, then the effect of this would need to be assessed by testing before it could be acceptable as safe. No changes to the enclosure or contents are permitted unless specifically allowed as variations on a Equipment Certificate. Variations are stated throughout certificates and supplementary certificates are denoted by /1, /2 etc, after the original certificate number on new issues of the document.

5.7.3 Special conditions

Where there are special conditions imposed by the certification authority for the safe use of the equipment then this requirement is denoted by the addition of the letter X after the certificate number. These conditions must obviously be followed.

5.8 Cabling requirements for Ex d

Ex d enclosures will require connection to electrical circuits. The manufacturer, during the certification process, will agree with the certification authority what is permitted. On a smaller enclosure likely to be designed for specific equipment such as a motor termination this will be limited to single or double entry and predrilled. On larger enclosures such as those designed as junction boxes, more combinations may be permitted. Where and how many holes may be drilled/tapped and to what sizes will be options that the user must specify and the Manufacturer must check that these are permitted by the certification before they are machine-cut (by only the manufacturer).

In general ‘Cable Entry’ requirements must:-

  • preserve the flameproof integrity of the enclosure
  • prevent transmission of the explosion
  • prevent transmission of any process fluids
  • provide necessary mechanical protection for the cable
  • maintain the earth integrity

There are two principle methods of cabling that satisfy the requirements of the standards.

  • Conduit
    Tubular metal sheath into which insulated conductors may be drawn as necessary; the requirement originates from the old British Mining Standards (BS229 / BS889) but which are currently still preferred in North America.
There are two main forms: ‘Solid Drawn’ or ‘Continuous Welded’
  • Steel Wire Armour (or SWA)
    • provides an equivalent degree of mechanical protection to conduit
    • preferred method of cabling in the other IEC member countries including Europe.

5.8.1 Requirements for conduit

The original British Standard, BS229 defined the difference between direct and indirect entry. Installations of equipment to this standard require the following use of cable in conduit as the method of connection.

5.8.2 Indirect entry

Where a chamber containing ignition capable equipment, hot surfaces of sparking contacts, is mounted within an enclosure and a separate enclosure is mounted adjacent to the first which only contains terminations (not considered ignition capable) for the purpose of connecting the equipment.

Figure 5.11
Rise time of Explosion pressure in an Ex d enclosure

In Figure 5.11 the ‘terminal’ (indirect entry) chamber is connected to the main chamber with lead through bushes or similar tested arrangements. This permits the use of solid-drawn conduit of 1 inch outside diameter (OD) or less to be used.

For a conduit diameter larger than 1 inch, the installation method is shown in Figure 5.12 which includes a “stopper box”. This is an additional independent chamber terminating the conduit and is filled with a compound to ensure that an explosion could not be transmitted up the conduit. After Amendment Number 8 of this Standard, direct-entry is permitted using welded seam conduit provided a stopper-box is fitted. This is shown in Figure 5.13.

Figure 5.12
One inch OD or larger conduit

5.8.3 Direct entry

Direct entry is where the conduit enters directly into the main part of an enclosure where there are ignition capable electrical parts.

Figure 5.13
Direct entry using a ‘stopper box’ for Welded Seam Conduit

No restriction is placed on the diameter of the conduit.

Whilst making installations with conduit, stopper boxes are required in the hazardous and safe area ends to prevent transmission of process fluids and explosions from hazardous to safe area.

Conduit threads engagement requirements are set out in the following table:-

Figure 5.14
Table: Conduit Thread Requirements

5.8.4 Requirements for cabling

Cable must be properly rated for the electrical and environmental duty by other regulations. In addition to these, the Standards for Ex equipment require that cables should not be subject to mechanical damage. If this is considered likely then cables that are adequately mechanically protected must be used. The alternative to the use of conduit is by steel wire armouring (SWA).

Cable entering Ex d enclosures must maintain the Ex d integrity of the enclosure and so must withstand any explosion pressure without allowing the cable to be pushed out or any flame to exit into the surrounding hazardous atmosphere. The use of suitable glands which are tested to be adequately safe must be used to prevent the transmission of:-

  • flame
  • explosion pressure
  • process fluids
  • moisture
  • dust

In addition, any gland must:-

  • provide anchorage for cable via armour (2 tonnes)
  • protect cable cores and armour against corrosion from weathering
  • solidly earth the armour

Parallel threaded glands require 5 full threads to be engaged or 8mm whichever is the greater. They should also be chosen to suit the type of cable used.

5.8.5 Cable entry

In the Standard, the following alternative arrangements are permitted:-

  1. Cable and glands of types that are certified for the equipment
  2. Glands designed for ‘thermoplastic’, ‘thermosetting’ or ‘elastomeric’ cable; which is substantially compact and circular, using an extruded bedding. Any ‘fillers’ must be non-hydroscopic. The gland must incorporate a sealing ring.
  3. MICC with appropriate flameproof cable entry device.
  4. a flameproof sealing device designed for and certified where the cable gland incorporates compound or other appropriate seal. The design must permit stopping around individual cores and must be fitted at the point of entry to Ex d enclosure.
  5. devices incorporating compound filled seals around the individual cores or other equivalent sealing arrangements (a ‘lead-through’)
  6. other means which maintain the integrity of the flameproof enclosure.

There are different types of glands provided by cable gland manufacturers. Not all are suitable for use on Ex d installations. The three principle types suitable are:-

  • Stuffing Glands (Used where no mechanical cable protection is required: uncommon and mainly in Germany)
  • Sealing Glands (ordinary type certified Ex d)
  • Barrier Glands (used to prevent transmission of flame through the cable)

5.8.6 Sealing gland

A Sealing Gland maintains the flameproof properties of the enclosure at a cable entry hole. These are fitted where a “filled” cable is used, (i.e. a cable which has a filling material between the interstitial gaps of the circular insulation of the conductors and will not permit the conduction of an explosion). If the cable is unfilled then their use is permitted in lower risk situations. See the selection flowchart in Figure 5.17. An example of a sealing gland is given in Figure 5.14. Its features are that it provides a pressure seal and anchor to prevent the cable being blown out of the gland. In addition it must seal the cable sheath to prevent access to moisture that can corrode the armouring. If fitted incorrectly this can be a weak part of the installation.

Figure 5.14
Example of a Sealing Gland

5.8.7 Barrier gland

A Barrier gland should be fitted where a non-filled cable is used and where an explosion or process fluid must be prevented from moving through the cable. In addition to the sealing gland requirements the Barrier gland cannot allow explosions to travel up the cable because the cores are sealed using a hardened epoxy resin plug to stop transmission through the cores.

Figure 5.15
Example of a Barrier Gland

Care is needed to assemble the glands correctly. The use of gland sheaths are not recommended as they can hold moisture which can, over time, corrode the armour and de-zinkify the gland, causing it to weaken.

5.8.8 MIC cable and glands

Mineral Insulated Copper Cable (MICC) is permitted for Ex d installation since the sheath gives adequate mechanical protection. The requirements of the gland are that it is a component certified device with a suitable compression ring. There are no special requirements for the pot seal as it is effectively inside the Ex d enclosure.

Figure 5.16
MICC Glands

5.8.9 Unused holes in Ex d enclosures

Unused holes in Ex d enclosures must be filled with “blanking plugs”. These must be proprietary component certified devices or supplied with the enclosure. They are part of the flame containing arrangement and so must be fitted according to the certifying authority requirements and the manufacturer’s instructions. A blanking plug should not be fitted into where an adapter has also been used but should be sized for the original hole in the enclosure.

5.8.10 Gland selection guidance

The correct selection of cable gland types are given in IEC 60079-14:2013 Clause 10. The rules have changed from the 2007 Standard and are now to be compliant with the following Clause 10.6.2 entitled ‘Selection of cable glands’

The cable entry system shall comply with one of the following:

a) Cable glands sealed with setting compound (barrier cable glands) in compliance with IEC 60079-1 and certified as equipment;

b) Cables and glands meeting all of the following:
– cable glands comply with IEC 60079-1 and are certified as equipment
– cables used comply with 9.3.2(a)
– the connected cable is at least 3 m in length;

c) indirect cable entry using combination of flameproof enclosure with a bushing and increased safety terminal box;

d) mineral-insulated metal-sheathed cable with or without plastic outer covering with appropriate flameproof cable gland complying with IEC 60079-1;

e) flameproof sealing device (for example a sealing chamber) specified in the equipment documentation or complying with IEC 60079-1 and employing a cable gland appropriate to the cables used. The sealing device shall incorporate compound or other appropriate seals which permit stopping around individual cores. The sealing device shall be fitted at the point of entry of cables to the equipment.

NOTE 1 The minimum length of cable is to minimize the potential for flame transmission through the cable (see also Annex E);

NOTE 2 If the cable gland and actual cable are certified as a part of the equipment (enclosures) then compliance to 10.6.2 is not necessary.

The UK did not wish to adopt the practice stated in Clause b) above, namely the 3 m distance. The inclusion of Annex NA (National Annex) which provides the original flowchart that enables an installer to decide what type of gland to use. This is shown in Figure 5.17.

Cable glands shall be installed in a manner that after installation they are only capable of being released or dismantled by means of a tool.

Figure 5.17
Cable gland selection to IEC60079-14: 10.3

5.9 Design and type-testing

A summary of ‘type tests’ carried out on all Ex ‘d’ equipment before it can be certified is given in this section as background information. Familiarity with the Standards is necessary for a more detailed understanding of this information which is given here only as a guide to the approach.

5.9.1 Pressure test

Tests of the ability of the enclosure to withstand pressure, and the rate of rise of pressure, developed inside the enclosure during an explosion - the “Reference Pressure”. The enclosure is then overpressure tested to 1.5 times the “Reference Pressure” usually with compressed air or water, to ensure a factor of safety. During manufacture all equipment that is of a steel fabricated construction must be routine tested at a static pressure of 1.5 times the “Reference Pressure” to ensure the integrity of the welding. If equipment is of cast or moulded construction, it can be exempted from the requirement for Routine Pressure Testing if the sample passes a four times overpressure test.

5.9.2 Flame test

These tests are done to ensure that the enclosure is flameproof, i.e. that an internal explosion does not create an external explosion.

5.9.3 Methodology

To determine the explosion “Reference Pressure” at least three explosions are performed inside the enclosure using the appropriate gas to suit the Equipment Group the enclosure is designed for. For electric motors, these tests are performed both while the motor is running and also while it is stationary.

The equipment is placed in a test chamber, which is filled with the same explosive gas mixture as is used in the equipment under test. The gas inside the equipment is exploded at least five times to prove that the internal explosions do not ignite the external gas.

The equipment is considered to have passed the tests if no flame transmission has occurred, and the enclosure has not suffered any damage or permanent deformation that may affect its flameproof properties.

Before any explosion testing is performed, copies of the approved drawings are carefully checked to ensure that the design details meet the requirements of the Construction Standards. After explosion testing is completed the samples (prototypes) are disassembled and all the component parts are carefully measured to ensure that they have been manufactured to the dimensions and tolerances specified on the Manufacturer’s certification submission drawings.

5.10 Installation and conditions of use

In this section, issues relevant to the selection, installation and maintenance of Ex d equipment is outlined. These are implied from the above descriptions and principles.

5.10.1 Use in zones

Ex d equipment is only permitted for use in Zone 1 and Zone 2 (gas and vapour risks). When used in Zone 2 risks no relaxation of the application, installation or maintenance requirements is be permitted.

5.10.2 Ambient temperature effects

In line with all types of Certification, the default ambient temperature is -20°C to + 40°C unless otherwise tested. The effect of raising the ambient temperature, causing expansion, may be to change the flamepath characteristics, rendering the equipment unsafe. It is therefore important that the ambient temperature in which the equipment will be operated is known and tested for that level of heat. It is inappropriate for the installer/user to estimate a change in T rating for any adjustment in ambient temperature as it will not be known whether changing the ambient temperature affects the equipment adversely unless properly tested.

At ambient temperatures below the certified ambient temperature range, aluminium could suffer from embrittlement. At such low temperatures, aluminium Ex d enclosures could be shattered by the internal explosion. Aluminium has a large coefficient of expansion, so at ambient temperatures above the certified ambient temperature range, the flamepath gaps may become sufficiently large to permit the internal explosion to spread into the hazardous area. The expansion of the aluminium may cause the flamepath gap to buckle, leaving the flameproof enclosure permanently ignition capable.

Cyclic heating of bolts can loosen them, and flameproof bolts are rarely locked. It only takes one loose cover bolt to by-pass the method of protection.

5.10.3 Electrical protection

It should be noted that a flameproof enclosure is not tested for its ability to withstand the effects of an internal electrical fault explosion. Correct overload and short circuit protection must be implemented in the safe area on the circuits going out to Ex d equipment located in the hazardous area.

5.10.4 Weatherproofing

Ex d is not inherently weatherproof; moisture and dust can enter by the flamepaths. The construction Standard permits O-rings or gaskets to be included in the design. The equipment must remain Flameproof with or without the gasket present and tests to prove this will be performed prior to Certification being issued. A gasket is additional to but separate from the flame path. Such a seal may help to reduce corrosion owing to partial ingress of moisture but this must not be viewed as gas-tight seal.

Enclosures used not to carry an IP rating but were declared as ‘flameproof-weatherproof’ by the manufacturer. Now some enclosures do provide IP ratings which can be used to place the enclosure where such a precaution is needed.

5.10.5 Gaskets fitted after certification

Gaskets must not be inserted into flame paths between flanged joints as an attempt to provide weatherproofing. If an existing gasket is to be replaced on a device to which one is fitted as part of its original construction (and certification) then it must be of the same material, size, thickness and hardness as the original.

5.10.6 Dust Risk

Flameproof enclosures may be used in dust risks and combined dust and gas/vapour risks if additional precautions specified in the Construction Standards are complied with. An enclosure that is specifically designed as flameproof/weatherproof is normally required.

5.10.7 Use of ‘grease’

The recommend partial protection against ingress of moisture causing corrosion and assisting with weatherproofing on joint surfaces is the application of ‘a light smear of non-setting compound’. Several greases were tested of other flameproof enclosures may be achieved by the use of a suitable grease or flexible non-setting compound in the flame path provided that chemicals with which they may come into contact do not adversely affect these.

5.10.8 Use of ‘Tape’ (grease impregnated linen)

‘Denso’ tape may be applied to the outside of the joint provided the requirements laid down in standards are observed. One layer only is permitted on apparatus installed for IIA applications. It is not permitted for IIB or IIC locations. Where permitted, an overlap of only 25mm is allowed. The tape can become brittle after a period of time and should be replaced before this happens. Tape shall not be applied to any spindle or shaft gap.

5.10.9 Paint

Where a flameproof enclosure is exposed to any corrosive conditions including weathering, suitable additional precautions should be taken such as the use of a suitable paint. Care should be taken not to allow the build-up of paint across the flamepath. Only one layer of paint is advised. Aluminium-based paint should not be used because of the potential danger from ‘Thermite’ reaction occurring when aluminium and rust and impacted together.

5.10.10 Cement erosion

Light Transmitting Parts such as sight glasses are usually held in with an acceptable form of cement but this can be eroded over time. Where the erosion is above figures recommended by the Certifying authority then replacement or repair by the manufacturer is required.

5.10.11 Materials of manufacture

Most Ex d enclosures are made from cast iron. Precautions are therefore necessary to inhibit rusting.

Very small enclosures for specialist applications such as switches (using ‘small volume d’ enclosure rules) can be made out of plastics without undergoing spark erosion tests. Larger plastic enclosures can be used if subject to this type of testing but this is not popular.

Aluminium may be used for Ex d enclosures but are generally restricted to lower power applications because of the ease with which released electrical energy can burn through this material. The use of aluminium conductors inside enclosures are not encouraged as the metal has a tendency to emit hot particles when overloaded. These could be ejected from the enclosure or cause damage weakening the enclosure. Additional rules must be followed which now make the design uneconomic.

Inspection and maintenance regimes must be appropriate to the operating conditions and materials used; consideration shall also be given to increasing the frequency of maintenance where particularly onerous conditions are encountered as stated in IEC60079-17 to be discussed later.

5.10.12 Research results

The Electrical Research Association (ERA) Reports DT 129 and DT 131 show that the tape-wrapping of flanged joints and other openings, or the presence of obstacles near the edges of flanged joints and other openings, may impair the protection afforded by a flameproof enclosure.

ERA Report 5191 describes experiments which showed that the presence in a flanged joint of grease or a non-setting jointing compound caused no deterioration in the flameproof qualities of the joint.

ERA Report 70-32 considers the risks which can arise when using aluminium in Ex d applications. Aluminium enclosures can eject hot aluminium particles under fault conditions, and because of the danger of arcs burning through the enclosure, the use of such enclosures is restricted to circuits protected by a 15-ampere or smaller fuse. Also cables with aluminium conductors shall not be used in such enclosures unless the possibility of ejecting hot aluminium particles from the enclosures has been minimized by certain design features.

5.11 Regional variations in Ex d implementation

Although IEC60079-1:2013 and the installation Code of Practice IEC60079-14:2014 are now ‘harmonised’ almost internationally, there are some countries that superimpose their own local Regulations and Standards on installations performed in that locality. Some comment is made here on known variations that apply although the situation is changing so this should not be taken as definitive; local expertise always should be sought.

Australian Standard AS.2380.2 table 1.1 (refer clause 4) is used for Ex d dimensions in Australia. Cable glands have to comply with AS 1828 until it is withdrawn. Users will have to follow it in addition to what is mentioned in AS/NZS 60079.0

South Africa requires all cable glands to comply with SABS808 or SABS1213. These requirements are not the same as 60079-0 and 60079-1.

North America still prefers Conduit over SWA cable but NEC or CEC rules guide on installation practice.

5.12 Illustrations of mechanical construction

The following figures illustrate various aspects of the mechanical construction.

Figure 5.18
Ex d motor fixed bearing gland assembly (Roller bearings)

In Figure 18 a traditional bearing cartridge cross-section shows the flamepaths associated with the assembly. If a bearing fails in this arrangement the shaft will rub on the cartridge’s flamepath causing frictional heating. This will obviously damage that flamepath and wear the shaft. It is vital that bearing failure is avoided by condition monitoring or timely bearing changes during service life. The rotor may also rub on the stator causing damage to laminations which will provide hotspots if repair is not carried out diligently.

In Figure 19, three examples of small-volume ‘d’ are illustrated. A is a simple push button with a two-pole change-over switch action. Note the terminals on the outside would usually be to Ex e design and so this device would be component certified as Ex de. The TLX single pin tube can spark across the contact in the holder with no risk of causing ignition. Such an arrangement is not designed to be disconnected but in the case of the plug and socket arrangement, with additional shrouding the system can be disconnected whilst energised. This permits ‘hot swapping’ on slide-in connectors as part of an instrument system, provided the mating components are handed to prevent misconnection.

Figure 5.19
Small Volume d examples
Figure 5.20
Typical example for Ex ‘d’ enclosure

Figure 5.20 shows typical examples of Ex d certified equipment. Illustration a) shows a flameproof switch built to a comparatively old Standard that requires larger flamepath widths. In the more modern standards, taking account of the improved casting and machine accuracies available, flange joints are thinner because flamepaths are of better quality construction. Illustration b) is of a large aluminium component certified enclosure showing how the o-ring seal is offset to give adequate flamepath length, given the size of the enclosure.

5.13 Summary

This section has explained how Ex d relies on an enclosure into which a hazardous atmosphere may enter. The enclosure has the design feature of flamepaths included in various forms which are engineered to prevent transmission of the flame, as a result of an expected internal explosion. The flame will try to traverse from inside to outside through any structural opening. The enclosure itself will withstand the explosion pressure but the weaknesses are the structural openings necessary to access equipment inside an enclosure. Maintenance is required to keep these flamepaths in good condition for the service life of the equipment.

It is the responsibility of the user to install, use and maintain flameproof apparatus in such a way that its safety is preserved:

  • Keep all joint surfaces clean and free from corrosion
  • Protect with light smear of a non-setting compound
  • Ensure that all bolts, screws, studs and nuts are present and are secured
  • Cable glands are correctly selected, are correctly assembled and are undamaged
  • Inspect at periodic intervals to assure correct conditions are met for safety

6


Protection Concept ‘e’

In this chapter we look at a type of protection that uses the carefully-engineered design of equipment to prevent electricity becoming a source of ignition by controlling heat dissipation and preventing the emission of spark energy.

Learning objectives

  • To understand how this technique effectively controls the electrical characteristics that could otherwise be ignition capable
  • To explain the care needed to ensure compromise in safety does not occur
  • To understand the marking of equipment

6.1 Name

Ex e protection comprises electrical parts inside an enclosure which is not designed to be gas-tight. It must be expected, therefore, that a flammable atmosphere can form around an enclosure and, indeed, inside the enclosure, where it can come into contact with those live electrically conductive parts. These parts could be either insulated or uninsulated.

The basis of protection is, firstly, that nothing permitted inside the enclosure will be allowed to cause arcs or sparks. Secondly, that no surface temperature on equipment inside the enclosure, or built up on the outside of the enclosure itself, will be capable of attaining an excessively high temperature. By maintaining these conditions during the operation of equipment, as a result of careful design, the protection concept of ‘increased safety’ is reliable enough to be permitted for use in Zone 1 or Zone 2.

6.2 Standards

Current
IEC60079-7:2015

Old
BS4683 Part 4
BS5501 Part 6 / EN50019

6.3 Definitions

Type of protection ‘e’ is defined in Standard IEC60079-7:2015 as:

  • ‘A method of protection by which additional measures are applied to electrical equipment so as to give increased security against the possibility of excessive temperatures and of the occurrence of arcs and sparks during the service life of the apparatus.

In this Standard and in line with the Equipment Protection Level concept introduced into all Standards Ex e protection is re-designated ‘eb’ owing to its permissible use in Zone 1 (or Zone 2) but NOT Zone 0.

In the Chapter in this manual discussing type of protection ‘n’, the type of protection Ex ‘nA’ is being re-designated as Ex ‘ec‘, where the ‘ec’ mark only permits its use in Zone 2 as it refers to EPLc. The principles of safety are not changed. The ‘nA’ requirements now fit more appropriately within the current edition of the Ex e Standard and this reduces confusion about the difference between Ex e and Ex n application.

6.3.1 Limiting temperature

The ‘Limiting Temperature’ concept is important to understand. It is the ‘Maximum permissible temperature’ for electrical equipment or a part of electrical equipment and is the lower of the following two temperatures determined by:

  • The danger of ignition of an explosive gas atmosphere (T1 to T6).
  • The thermal stability of the materials used.

The importance here is that if insulation fails, it will give rise to the occurrence of arcs and sparks which are expressly designed out of the protection type.

6.3.2 Comparative tracking index (CTI)

A measure of the reliability of insulation material used to separate conducting parts of a terminal.

6.4 Principles of design for increased safety

By contrast with Ex d, Ex e cannot provide a source of ignition.

Historically, the Germans initially formed the view that Ex d, the earliest type of Ex protection, could be improved upon by the elimination of any source of ignition. Their pioneering work realised that this could be achieved as a result of good mechanical design and construction. The name given to the concept was “ erhochte sicherheit”, which means ‘increased safety’ and so the letter ‘e’ was used to depict the approach. The enclosure would not then need to withstand an explosion; its function would only be to house electrical parts. The parts themselves must have enhanced and reliable insulation and high quality conducting parts with greater separation and improved cooling arrangements that cannot get excessively hot and cannot spark. The enclosure is important as it must keep out moisture and dust reliably to prevent degradation of the insulation properties of internal components.

It was originally conceived for fixed installed high power equipment. It evolved, with the benefit of research and experience, to using good quality materials having well defined insulation properties combined with adequately de-rated mechanical and electrical design specifications.

In order to prevent any arcs or sparks occurring within an enclosure, there were two main considerations.

6.4.1 Sparking

Prohibiting the use of unprotected and discontinuous contacts, such as in switches, meant that there is no obvious source of ignition. In addition, the possibility that joints in conducting paths, such as terminations, may become intermittent and produce sparks must be prevented. The mechanism for this to happen would be loosening occurring due to vibration or heat cycling. Prevention is achieved by designing a terminal arrangement to minimise, or better, to eliminate this possibility.

Figure 6.1
Terminal Clamp: Conventional and ‘Ex e component certified’ compared

In Figure 6.1, a standard screw-clamp terminal is compared to the enhanced design of a type suitable for Ex e. The Screw Clamp leaves jack up onto the thread to lock it more securely against loosening. The Shoe that accepts the conductor is shaped and grooved to maximise grip.

Figure 6.2
Terminal Block: Enhanced Creepage and Clearance

In a second example in Figure 6.2, a terminal block has a threaded termination around which the conductor wraps and a system of washers are used to hold it in place. The correct assembly of this arrangement is necessary to guarantee the capture so that the terminal will not work loose and sparking cannot occur. Such an arrangement is tested as a Component Certified assembly, to be explained below.

6.4.2 Heating

Naturally, any conducting material will dissipate some heat. The increase surface area for contact in both Figure 6.1 and 6.2 lower the contact resistance by deforming and spreading the contact area so reduce the possibility of over-heating occurring by design. The use of the correct size conductor in a deformable copper form is only acceptable for this design otherwise the point contact made may increase resistance defeating the object of the arrangement.

The main concern of Ex e equipment is that of the generation of excess heat. The same precautions taken to eliminate the possibility of sparking also help to reduce the temperature rise in current carrying parts. They are made larger than necessary or may be good quality standard parts, which are suitably de-rated. In this way where it occurs, heat generation is minimised and dissipation is increased, so that temperature rise is reduced.

6.4.3 Insulation quality

Sparking and arcing (in the form of tracking) across insulation could occur if the insulation failed. This could be caused by overheating or by tracking effects owing to collection of moisture and dust between conducting parts. The insulators chosen for Ex e parts are subject to Comparative Tracking Index (CTI) testing across the surface of the insulation between conducting parts. The CTI must be greater than 500 to be acceptable. The enclosure IP rating assists in preserving insulation integrity.

6.4.4 Creepage and clearance

In Figure 6.2, the creepage and clearance distances are enhanced by adding insulation between conducting parts placed on insulators to further increase the integrity and lower the possibility of surface tracking. Minimum requirements for levels depended on operating voltages are specified in the Standards.

6.4.5 Enclosure integrity

Enclosures are designed in such a way that the entry of moisture and dust in detrimental quantities is prevented. There is no attempt to make the enclosure gas-tight. The minimum Ingress Protection level is IP54 for enclosures. In addition they must meet an impact strength test requirement of 7NM for ‘non-light-transmitting’ parts and 4NM for ‘light-transmitting’ parts.

The integrity of the enclosure must not be degraded by other influences, for example, the weight of cables entering could distort the sealing arrangements. Forcing an enclosure open with inappropriate tools could also cause damage to the seal itself or the sealing arrangements.

Tests to plastic enclosures are extensive under the present Standard. Subjected to certain conditions, ageing and deterioration can occur owing to weather and plant conditions. This includes testing for the effects of:-
High temperature
Low temperature
Thermal cycling
UV light exposure
Solvent vapour withstand

Where particularly harsh operating conditions are encountered on an industrial plant additional testing may be necessary to prove that enclosures are adequately protected. Where there is an enhanced possibility that damage may be sustained, additional precautions may be necessary to protect equipment installed.

6.5 Component certification

Component parts to be included in larger arrangements will be ‘component certified’ for some flexibility. In an Ex ‘e’ junction box for example, the enclosure will be impact tested. The terminals to be used within will be component approved.

The main uses of this technique are found in higher power circuits such as induction motors, fluorescent lighting fittings, junction boxes, and terminal housings. The German standards from which this came promote the use of toughened plastic cable sheaths on permanent installations as opposed to the more expensive steel wire armoured cable used elsewhere.

When applied to junction boxes, an Ex ‘e’ enclosure is given an ‘enclosure factor’ when certified. This represents the highest number of ‘terminal-amps’ permitted in the box. Terminals mounted in the box must be component approved. The total of terminal-amps must be calculated and must be equal to or less than the enclosure factor.

6.6 Internal requirements of Ex e

Only components which have been tested and certified are permitted inside Ex e equipment. Terminals or terminal blocks must meet specified requirements. Devices such as switches and relays are deemed to be ‘discontinuous contacts’ and are not permitted. Any other form of connection must meet specified reliability criteria. Current carrying parts are suitably de-rated from normal design or enlarged to assist with lowering contact resistance and increase heat dissipation in order to keep temperature excursions as low as possible.

6.6.1 Internal wiring

Whereas crimps as shown in Figure 6.3 may not be permitted for use on conductors during installation of equipment on site, the manufacturer is permitted to use crimps if they are applied under controlled assembly conditions as part of the assessed manufacturing method. If incorrectly fitted to the conductor without adequate control over area and pressure of contact, crimps could be loose or present a higher resistance connection which could give rise to excessive heat or sparking.

Figure 6.3
Typical crimped connection

6.6.2 Sparking devices

Whilst sparking devices are not permitted under Ex e design, it is permitted to use component certified devices that have ‘additional protection’ included. Such protection can include types, such as Ex d, m, q, or s. In the previous Chapter the ‘Small Volume d’ enclosure was explained. The external terminals were built and certified to Ex e requirements and so this may be included in a suitable Ex e enclosure. The complete arrangement must then be apparatus certified and used according to the requirements of the certification.

6.6.3 Special conditions

Where Ex e certified equipment, comprising the enclosures containing component-certified parts are to be installed, it is highly likely, given the nature of care required for this type of protection, that ‘Conditions’ or ‘Special Conditions’ will be imposed by the Equipment Certificate. These must be met for the safe use of the equipment when it is also operated and maintained. The nature of these conditions may vary but is dependent on the application of the equipment. The certificate must be read so that the installer and user can comply. Failure to do so may lower the integrity of the type of protection.

6.7 Ex e enclosures

Ex ‘e’ enclosures with no internal equipment fitted are only ‘component certified’. The temperature for T rating purposes cannot be assessed until the internal equipment has been selected and tested as one entity.

6.7.1 Junction boxes

In the case of an enclosure configured as a junction box, as in Figure 6.4, it is only when terminals are mounted within and the total current carrying capacity is calculable, tests can be done at the rated current to determine the likely heating effect that will occur in the enclosure. There are two accepted methods for specifying an Ex e enclosure terminal capacity which are described below.

In addition other requirements are specified which must be observed during the installation of a junction box. These issues apply to wherever Ex e terminals are used and are necessary to maintain the integrity of the type of protection by helping to reduce possible heating effects. These are:-

  • Looming of insulated conductors from different sheaths is prohibited
  • Only looming of conductors from the same sheath permitted
  • Loose cabling (i.e separated conductors) is encouraged (to allow air circulation for cooling)
  • Correct size of conductors for terminals
  • Only one wire per terminal
  • The Installer must not crimp cables (dependent on design)
  • All terminal screws tightened (used or unused)
  • Others may apply that are more specific to a given terminal design.
Figure 6.4
Typical enclosure

6.7.2 ‘Enclosure Factor’

The rating for an enclosure may state the total value of ‘terminal-amperes’ permitted. Terminals are also awarded a value depending on their size and current carrying capacity. The terminal would be component certified and usually by the same manufacturer who holds the apparatus certificate for the assembly.

The enclosure factor system offers flexibility in configuration to meet the user’s needs. It allows the manufacturer to place the appropriate number of terminals in the enclosure provided that it does not exceed the rating. For example, an enclosure with a factor of 820 may house 41 terminals each with a factor of 20. The same enclosure will be permitted to house 51 terminals if the later was rated at only 16. In practice a manufacturer would have several capacities of terminals certified so that a combination of terminal sizes could be housed up to the maximum value. The manufacturer would record the version or arrangement supplied. If subsequent changes were required the manufacturer could advise on a safe combination.

6.7.3 Wattage rating

The alternative and current method uses an enclosure maximum ‘wattage’ rating which would be marked on the enclosure, say, 10.3 Watts. The dissipation of the proposed terminal arrangement to be mounted into the enclosure is calculated to ensure the total is below the maximum permitted.

This is achieved by test and measurement of the individual terminals. Their dissipation at full Ex e load capacity is added to an allowance for the length and number of conductors connected to the terminals in the enclosure. This approach is said to be a more accurate representation of the heating effect experienced under operating conditions.

There are variations on this approach in which the manufacturer and testing authority permit the installer to place certified terminals in certified enclosures and do the calculations to determine that the arrangement meets the required criteria. In some cases the T rating has to be calculated and retrospectively marked on the enclosure. Extreme care must be taken to ensure the correct application of such approaches as the calculations are sometimes not completed and assumptions are made that any arrangement is safe because the certificate has not been read properly.

6.8 Rotating electrical machines

Only induction-type a.c. motors can be protected by the application of Ex e because this design does not include sparking devices such as commutators and slip rings. Adequate heat dissipation is the main consideration and so physically larger frame motors than for an equivalent non-hazardous application would be necessary as are improved cooling arrangements.

The main points of design may be summarised in the following list:-
Ex e type terminations for incoming supply cables
Ex e terminal box to house the terminals
Defined types of internal conductor connections
Increased clearances between conducting parts
Losses designed to be lower
Non-hydroscopic insulation with high tracking resistance used
High quality windings, insulation and varnish
Adequate radial clearances for rotor and fan
High quality bearings
Operation with correctly selected overload protection
Duty limited to non-arduous: continuous running at constant load
Operation with selected over-current trip

6.8.1 Construction

The construction standards lay out specific requirements to be met in respect of testing as discussed below. The risk of localised heating due to power dissipation in conductors is considered and cables and conductors require space and circulation of air to assist in cooling.

6.8.2 Overload protection

The Standard requires that where an Ex e motor is installed an overload relay, chosen for its suitability, must be included in the supply and it must be set correctly as part of the requirements.

The motor will have a ‘T rating’ and an insulation grade specification. These values determine the ‘limiting temperature’, the lower of which must not be exceeded.
The overload relay must trip if the current drawn by the motor exceeds the full load value within the tE time specified for the motor. Referring to Figure 6.6, the tE time is that taken to reach the limiting value (C) when stalled from the temperature that the motor runs at under full load (B).

Figure 6.6
Typical enclosure

The motor data will state an “IA/IN” Ratio with a tE Time. These are defined below:-
IA is the starting current
The highest r.m.s. value of current drawn by an a.c. motor while starting from rest (inrush current at zero speed), when supplied at rated voltage and rated frequency
IN is the normal full load current
“IA/IN” is the Starting Current Ratio
The ratio between the initial starting current IA and rated current IN

tE is the time taken to reach the limiting temperature
It is the time taken for an a.c. winding, when carrying the initial starting current IA, to be heated from the temperature reached in rated service (B) after running up from the maximum ambient temperature (A) up to the limiting temperature (C).

The characteristics for a trip will show a curve of “IA/IN” versus “tE” Time. The starting current ratio is found and the time value is read off the curve. The value must be greater than a prescribed minimum of 5 seconds but must not exceed the tE time for the motor. Under these conditions the trip is acceptable and should be set for the full load current in service.

Figure 6.7
Typical enclosure

As an example of the application of this requirement, suppose the IA/IN Ratio of a motor is 6 and its tE is 8.5. On the Characteristic for an overload relay shown in Figure 6.7 the time to trip would be 6 seconds at that ratio and so this would be an acceptable combination of motor and trip.

6.9 Cable and glanding requirements

As with Ex d protection, there is no such thing as Ex e cable. It is cable that meets the minimum requirements of the type of protection. Any substantial cable is acceptable for Ex e provided that it is suitably rated, protected from adverse environmental conditions where used and is substantial in size as stated in the installations Code of Practice IEC60079-14, clause 9.

The original concept did not require the use of steel-wire armoured (swa) cable but recommended a toughened sheath cable as acceptable. In Germany it is common practice to use such cable but the rest of the world tends towards swa. The cost savings recognised by the use of the German practice are not making the industry reconsider.

Glands only need to maintain the IP integrity of the enclosure at IP54 and so most industrial grade metal glands will achieve this without the need for testing. Manufacturers produce Ex e component certified glands suggesting that their use is mandatory but this is not the case. The issue has been further confused by some glands being Equipment Certified as equipment where a T rating value is added.

Plastic glands are quite common and acceptable in certain circumstances but the use of plastic for this purpose means that testing for degradation of the sealing properties is mandatory.

Figure 6.8
Typical Ex e Gland
Figure 6.9
Typical Ex e enclosure with Earth strip (‘earth plane’)

A typical metal gland as shown in Figure 6.8, requires a sealing washer to maintain the IP rating as there is no stipulation for the engagement of any number of threads as in Ex d. The walls of Ex e component certified enclosures are relatively thin so any thread is likely to allow water and dust though. The gland must be earthed if used with swa cable. In some junction boxes additional metal earth plates are wrapped around the inside of the enclosure specifically to assist in providing earth planes. Locknuts are recommended on the inside to make good contact. Even if plastic glands are used, locknuts will assist with IP rating integrity. A diagram of an enclosure with an earth plane is shown in Figure 6.9.

Where Mineral Insulated Copper Cable is used on some applications, the ordinary industrial gland meets the IP rating but there are enhanced requirements for the pot seal where the arrangement is shown in Figure 6.10.

Figure 6.10
Typical Ex e Gland for MICC

6.10 Periodic inspection requirements

The main points for the periodic inspection of Ex e equipment by the user are based on the possible reduction of integrity of the type of protection.

The enclosure integrity should be checked initially externally before opening under controlled safe conditions is required at a more detailed level of inspection.

6.10.1 Exterior condition of the enclosure:

The enclosure must be visually checked externally for signs of corrosion, holes, cracks and distortion that could undermine the IP rating of the enclosure.

Cable glands must be present, of the correct type, suitable for the IP rating chosen (if it is higher than the required IP 54). The positions of the cable entries must be within the allowed area for entry according to the manufacturer’s instructions and any certification conditions. Checks may be necessary to ensure that additional cables have not been added that are not permitted.

Unused entry holes must be filled to maintain the IP rating of the enclosure.

The enclosure should be clean, undamaged, correctly labelled and identified according to drawings. Metal and plastic boxes are common for Ex e so correct earthing of metal enclosures and caution against static generation on plastic enclosures must be considered.

6.10.2 Interior condition of the enclosure:

Internal inspection must verify that the enclosure is clean and dry inside. It must be examined to see if moisture and dirt has managed to penetrate to the interior via the sealing system. If so then gaskets or seals will require replacement. Brittleness in the gasket or visible mechanical damage to the lip of the gasket is not permissible.

The electrical wiring and connections inside the apparatus must be regularly examined for traces of discolouration or any other signs of possible overheating. Cable insulation integrity need be examined. Any such signs may require further investigation under controlled safety conditions using proprietary methods suitable to permit safe working.

The terminals must be checked periodically for damage reducing creepage and clearance distances and also tightness. Signs of disturbance or modification from the original conditions must be further examined.

6.10.3 Cable glanding

Correct assembly and tightness of the cable glands must be examined. The armour or brading of cable, where used, must be correctly and securely captured with the correct design and size of gland for the type and size of cable used. The cable sheath must be sealed against ingress of moisture to prevent deterioration.

6.10.4 End of Life considerations

More recently, experience has shown that a fluorescent light tube will draw a greater current as the tube depletes its electrodes during the aging process. Additional heat is dissipated around the endcaps of the tube and also in the ballast. This has caused serious deformation in the case of some plastic enclosures. The temperatures experienced have been in excess of the fittings’ T rating and therefore pose a danger to the installation. The requirement to ensure that tubes are changed within the manufacturers’ recommended lifetime are incorporated into the conditions of certification.

Light-emitting diodes (LED) luminaires are now commonplace and make economic sense in view of their lower running costs. As a result concern has been raised for the optical radiation density as a source of ignition. Guidance is given in Standards discussed in Chapter 10.

6.11 Marking

Ex e equipment is marked with ‘Group II’ but no A, B or C is applied as subgrouping because energy is not emitted from Ex e arrangements. The T rating will be stated and there value is dependent on the design of equipment and its application.

It is quite common to see Ex e equipment using other methods of protection such as Ex d for where the Grouping will be stated, i.e. Ex de IIC T4.

No modification, addition or deletion shall be made to Ex e equipment. Replacement of parts such as seals may be necessary but only supplied by the manufacturer or his distributor. Any other modification will invalidate the certification.

Ex e can be used with dusts where the enclosure is dust-tight to IP66. T ratings must be selected for a suitably assessed dust risk where the temperature rating factors applicable to the layer thickness has been calculated.

Enclosures marked with a letter U after the certificate number are not equipment certified, only component certified, and must not be installed in a hazardous area without being part of an equipment certified arrangement. T rating marking should be done by the manufacturer.

6.12 Applications

Some typical and common lighting arrangements are shown in Figures 6.11 and 6.12 showing fixed equipment. Ex e protection is often used on instruments and systems where there are higher power requirements than can be provided by Ex i or other techniques in a convenient manner. Live working is not normally permitted but special arrangements in place to guard against contact or short circuit this is sometimes permitted in lower voltage applications such as with instruments.

Figure 6.11
Sheet metal emergency light fittings type 6018; Explosion protection marking: II 2 G; EEx e d m IIC T4
Figure 6.12
Ex e fluorescent fittings

Instrumentation loop systems are not usefully accommodated within this method owing to the inclusion of circuits with variable resistances or switches which are regarded as discontinuous contacts. The enclosures are often purchased as junction boxes for use on IS circuits because they are robust and reliable as proven by the Ex ‘e’ testing. The Ex ‘e’ certification is not used and should be removed from the box because it can cause confusion over how circuits are protected.

6.13 Summary

Ex e equipment does not permit sparking to occur by design (unless additionally protected means are included) and so heat is the major concern. From the descrption above it will be realised that great care is taken in the design, construction and testing of equipment. The stringency of testing and attention to detail design of anti-vibration arrangements to eliminate sparking renders the equipment suitable for Zone 1 or Zone 2 and is termed to grade EPLb.


7


Protection Concept ‘n’

In this chapter we look at a type of protection that is restricted to use in Zone 2 (EPLc or ATEX Category 3) because it is a lower grade of integrity than the other types of protection. It allows the use of industrial grade equipment that is not considered ignition capable in normal operation. It is a versatile concept and can be applied to a wide range of equipment types from instrumentation to higher power motors. It must not be confused with Ex e, although there are some obvious similarities.

Learning objectives

  • To explore type of protection n
  • To understand its similarities and differences to Ex e
  • To explain the additional protection techniques permitted

7.1 Name

Ex n takes the letter ‘n’ from the term ‘non-incendive’. The term ‘non-sparking’ only considered emitted energy and would not consider heat dissipation whereas ‘non-incendive’ considers all aspects of ignition capability.

7.2 Standards

Current

  • IEC 60079-15:2010

Old

  • DIN EN 50 021
  • VDE 0170/0171 T16
  • BS 4683: Part 3 (Type N)
  • BS 6941 (Type n)
  • AS 2380.9

The evolution of this type of protection requires some explanation before the definition is considered because it was developed over an extended period of time with new approaches added as more formal Standards were adopted. Initially confined to UK use, the later European and then International input has clarified and developed this technique which has helped to bring about subtle changes in the certification process in Europe under the ATEX Directives which will be discussed later.

7.2.1 ‘Division 2 approved apparatus’

Prior to hazardous areas in the UK being described in the three Zones, they were referred to as Divisions as discussed in Chapter 4. At that time, equipment was submitted to Her Majesty’s Fire-service Inspectorate (HMFI) for examination and assessed as generally in accordance with BS 4137. Where the apparatus was found satisfactory HMFI issued a ‘letter-of-no-objection’ to the use of the apparatus in Division 2 areas. The apparatus was therefore commonly referred to as ‘Division 2 approved’. No temperature classification was stipulated. The responsibility for making such a judgement on this aspect would have fallen on the user.

7.2.2 History of progression

The Standard BS 4683: Part 3 eventually emerged to formalise the concept where it was first referred to as Type ‘N’ equipment. This Standard clarified that equipment could produce sparks or permit surface temperatures to rise but neither could be at levels that could cause ignition. This Standard was only written for the certification of lighting fittings.

Re-designated Type ‘n’ under BS 6941, this Standard allowed much more flexibility to include induction motors and instrumentation. Insulation requirements were relaxed for circuits with less than 75Vdc. Discontinuous contacts were then permitted by this Standard provided the resultant spark could be shown to be non-incendive.

The Standard stated that such apparatus shall not, in normal operation:

Produce an arc or spark unless:

  • The operational arc or spark has insufficient energy to cause ignition of a flammable atmosphere; or
  • The operational arc or spark occurs in an enclosed break device; or
  • The operational arc or spark occurs in a sealed device.

Considering the above as options; in first case, the voltage and current applied to switching contacts could be assessed using the same curves as for Ex i equipment (see chapter 8).

In the second case, it was known that small volume enclosures met the Ex d criteria (see chapter 6).

In the third case, it was obvious that adequate sealing would not permit the entry of a gas/air mixture to a spark (see chapter 9).

The second and third cases stated above use an ‘additional means of protection’. The contact is deliberately configured within a protected environment.

Some further clarification is needed in that sliding contacts are considered to be sparking in normal operation. Where contacts mated and could not slide, provided that contact pressure was adequate then these were considered to be non-arcing.

7.2.3 Category 3 equipment

As will be explained in due course, Ex n is considered ATEX Category 3 equipment (Equivalent to EPLc) which means it is restricted to use in Zone 2. This type of equipment is permitted to be certified compliant with the construction Standards by the Manufacturer according to ATEX 94/9/EC Directive. Thus the cost of this equipment is generally lower.

This raises the concern that area classification may be inappropriately influenced in order to accommodate Ex ‘n’ apparatus.

7.3 Definitions

Type of protection ‘n’ is defined in the current International Standard as:

“A type of protection applied to electrical apparatus’ such that, in normal operation and in certain specified regular occurrences, it is not capable of igniting a surrounding explosive atmosphere.”

Other definitions referred to in this Standard and the previous edition are given below in order to assist in the explanation of this concept.

Normal operation
Operation of equipment conforming electrically and mechanically to its design specification and used within the limits specified by the manufacturer

Non-sparking device ‘nA’
A device constructed to minimize the risk of occurrence of arcs, sparks or hot spots capable of creating an ignition hazard during normal use. Normal use excludes the removal or insertion of components with the circuit energized.

Enclosed break device ‘nC’
A device incorporating electrical contacts that are made and broken and that will withstand an internal explosion of the flammable gas or vapour which may enter it without suffering damage and without communicating the internal explosion to the external flammable gas or vapour

Non-incendive component ‘nC’
A component with contacts for making and breaking a potentially incendive circuit where either the contacting mechanism or the enclosure in which the contacts are housed is so constructed that ignition of the prescribed flammable gas or vapour is prevented under specified operating conditions

Hermetically sealed device ‘nC’
A device which is so constructed that the external atmosphere cannot gain access to the interior and in which the seal is made by fusion, e.g. by soldering, brazing, welding or the fusion of glass to metal

Sealed device ‘nC’
A device which is so constructed that it cannot be opened during normal service and is sealed effectively to prevent entry of an external atmosphere

Restricted breathing enclosure ‘nR’
An enclosure that is designed to restrict the entry of gases, vapours and mists

Energy limitation
A concept applicable to circuits in which no spark or any thermal effect produced in the test conditions prescribed in this standard is capable of causing ignition of a given flammable gas or vapour

Energy-limited equipment
Electrical equipment in which the circuits and components are constructed according to the concept of energy limitation

Associated energy-limited equipment
Electrical equipment which contains both energy-limited and non-energy-limited circuits and is constructed so that the non-energy-limited cannot adversely affect the energy-limited circuits. Associated energy-limited equipment may be either

  • electrical equipment which has an alternative type of protection listed in this standard for use in the appropriate explosive gas atmosphere; or
  • electrical equipment which has an alternative type of protection listed in [the Standards] for use in the appropriate explosive gas atmosphere; or
  • electrical equipment not so protected and which therefore shall not be used within an explosive gas atmosphere, e.g. a recorder which is not of itself in an explosive atmosphere but is connected to a thermocouple situated within an explosive atmosphere where only the recorder input circuit is energy-limited.

Self-protected energy-limited equipment
Equipment which contains energy-limited sparking contacts, the circuit (including energy-limited components or devices) supplying energy-limited power to these contacts, as well as the non-energy-limited source of supply to the circuit.

7.4 Principles of design

The ‘non-incendive’ concept of protection was originally accepted as safe on the basis that manufacturers used good quality and well-designed industrial-grade equipment with little or no additional requirements. It was conceived to be installed in areas where there was only a low risk of a potentially flammable atmosphere being present (Zone 2). The equipment is considered safe in normal operation and is to be used well within its ratings.

In normal operation, parts of equipment may rise in temperature but will not exceed the T rating of the assessed equipment. Sparks are permitted but must not be at levels which can cause ignition. Where incendive levels of heating or sparking can occur, the Standard does permit ‘additional means of protection’ to be incorporated into the design. A number of well-established principles can be used. In some cases tests to confirm the integrity of the arrangement are included in the Standards.

7.4.1 Failure

The definition for Ex n is qualified by the statement that it is not capable of providing a source of ignition; ‘… in normal operation and in certain specified regular occurrences, …. ’.

Suppose a lamp was to fail in a luminaire. The lamp could be replaced and the equipment could continue to operate. The failure of the lamp would not constitute a failure of the Ex protection of the equipment and so is permitted by the Standard.

Thus protection may be An illustration of this principle can be given by considering a circuit which includes a fuse. If the fuse rating was correctly designed and chosen then upon failure conditions in the circuit the fuse might blow. The conditions under which the fuse might blow are not examined to determine why it might do so and additional measures included to prevent this. In this way normal circuit design principles are acceptable.

The ‘additional measures’ taken to prevent ignition with other types of protection are subject to testing and examination of failure, or the degree of failure. This does not apply in such a stringent form to Ex n, where only normal operation is considered.

7.4.2 Temperature classification

The temperature classification of the equipment would be based on its operational full load rating and not under any overload condition. Since the enclosure is not gastight it needs to relate to any hot surface inside or outside the enclosure of the equipment. Where additional protection is applied to reduce a surface temperature, the higher temperature will not be considered.

7.4.3 Grouping

Equipment grouping depends on the inclusion of any additional means of protection for incendive sparking parts and the levels at which this can occur. The native Ex n protection principle is Group II (suitable for all gasses) as energy levels are ‘non-incendive’. Subgroups will be assigned depending on energy release criteria additional protection methods.

7.5 Construction

Ex n equipment can comprise conductors, terminals and other current carrying parts which normally will carry electrical currents at high power levels which would be considered ‘incendive’.

7.5.1 Enclosure

Such parts are surrounded by an enclosure that requires a minimum of IP54 where there are bare live conducting parts within. This may be reduced to IP44 or 20 under certain circumstances. The Impact Strength of the enclosure is required to meet 7NM or non-light transmitting-parts and 4NM for light-transmitting-parts.

The enclosure is therefore tested to prove its acceptability and so is Component Certified as Ex n.

So far then, the requirements are similar to that for Ex e and this fact alone causes some to make direct comparison between the types of protection conceiving them to be similar. This must be discouraged.

The internal parts are not certified but chosen for their application. The complete arrangement comprising the parts inside the component certified enclosure is then “Equipment Certified”.

7.5.2 Wiring and connections

The internal and external wiring need only follow common practice for typical industrial equipment. Some additional care to avoid cable damage by rubbing against metal parts or sharp edges would be necessary during manufacture. The construction method, for example, would include the fitting of grommets (as would be expected in industrial grade equipment) but a more substantial one may be used in this case.

The wiring and connections, internal or external would also be of a good grade as they would not want to emit sparking or excessive heating in normal operation. These connections could be:-

  • Screwed
  • Bolted
  • Crimped
  • Soldered
  • Brazed
  • Welded
  • Pressure by spring loading
  • Mechanically wrapped by machine
  • Plugs and sockets

7.5.3 Conductor insulation and separation

There are no particular requirements for conductor insulation and separation other than what would be expected in general electrical equipment depending on its function. Some care is taken with the following conditions:-

  • Separation of uninsulated conductors in air (clearance),
  • Uninsulated conductors across a surface of insulating material (creepage in air),
  • Separation of uninsulated conductors across varnished surfaces (creepage under coating) and
  • Separation distances through encapsulation.

7.5.4 Internal parts

Parts inside the enclosure are chosen for their ability to perform adequately given the job they must do and operate at acceptably low temperatures and without sparking but their design is not normally enhanced as with Ex e. As a result, these selected devices are not Component Certified (U). Only the enclosure carries component certification.

7.5.5 Equipment certification

The assembly of the Component Certified enclosure with its internal or associated parts are equipment certified as one entity. Replacement parts can be fitted but modification is not permitted without reference to the manufacturer.

7.6 ‘Additional’ protection

IEC 60079-0:2011 is entitled: ‘Explosive atmospheres: General Requirements’. In this Standard it uses the terms, ‘additional protection’ or ‘additional techniques’, to describe permitted extra facilities included in the consideration of Ex protection.

Equipment must be assessed for temperature rise and duly Temperature rated or classified accordingly. A design that helps to dissipate power to achieve lower temperatures is a natural requirement of Ex protection; increasing the available surface area of heat dissipating parts. Where specific technique of reducing surface temperature is incorporated then this is deemed to be ‘additional’ protection. There are recognised ways in which this can be applied to Ex n, such as a ‘restricted breathing’ design, Ex nR explained below.

The same is true for devices that can emit energy in the form of arcs or sparks. In the older standards names were given to these techniques but it is only in recent times that a letter has been added to the safety code to alert the user and maintainer of the appropriate technique incorporated.

These additional means were originally known as:-

  • Enclosed-break device.
  • Non-incendive component.
  • Hermetically sealed device, which in normal operating conditions can not be opened and have a free internal volume not exceeding 100 cm 3, and be provided with external connections, e.g. flying leads or external terminals.
  • Energy-limited apparatus and circuits.
  • Restricted breathing enclosure.

Ex n was originally conceived as a convenient approach to a type of protection for handling high power applications. Over the years it has been adopted to many other applications. This explains why the additional protection techniques have been accepted into the Standard; to maintain its versatility whilst only aiming at a lower integrity concept of protection. Thus the testing associated with the additional protection is not so stringent.

The Standard requires that the equipment be marked to show the additional method of protection used but it leaves the interpretation of the applied technique to the Certifying Authority and the manufacturer to agree. Clause 4 of the previous edition of the Standard specifically covers the requirements that are summarised below.

7.6.1 Ex nA

Type nA is applied to equipment that will not produce incendive sparking and is known as ‘non-sparking’. The following description will illustrate this issue:-

Where two mating parts of a connecting arrangement are incorporated into a design but these do not separate in normal operation, they are said to be ‘non-sparking’ provided they are designed with sufficient spring pressure to maintain good contact. Separating the two parts could cause a spark which may be incendive.

In practical terms it depends on what the functions of the mating parts are within the equipment and how the operation of the equipment is conceived. If these parts which constitute electrical contacts were permitted to slide then the intermittency could produce an incendive spark. If, however, the electrical supply was switched off whilst the contacts slid or were separated then this would be safe. If the contacts were required to be operated whilst the equipment was live, the Ex nA approach could not be used.

Figure 7.1 a
Ex nA I/O system module showing spring connectors for backplane bus system

7.6.2 Type nC

Type nC is used for sparking equipment in which the sparking parts are suitably protected other than by restricted breathing, nR, energy limiting, nL, or simple pressurisation, nP as described below. Contacts which are sealed or encapsulated cannot dissipate energy into a surrounding gas atmosphere and are deemed Ex nC.

Hermetic Sealing is where a contact arrangement with metal lead connections is placed inside a glass envelope. The molten glass solidifies and bonds with the metal forming an hermetic seal which is incapable of passing gas. The contact can therefore produce incendive sparks but is adequately separated from the gas atmosphere. Unfortunately, such an arrangement cannot withstand undue shock without shattering the glass and exposing the conductors. A Sealing Component can now use a moulded plastic or epoxy resin to house the same contact arrangement with an effective seal but a greater resistance to impact. Such an arrangement is shown in Figure 7.1 b.

Figure 7.1 b
Ex nA Reed relay arrangement showing hermetic sealing

Another method is the use of a contact arrangement that is placed within a small volume enclosure (less than 10cc), as discussed in the Chapter on Ex d. The contacts may spark and gas may be present within the chamber formed but the ignition cannot be transmitted to any gas atmosphere outside the small enclosure as the mating parts form the flamepath.

This system could be used for static mating contacts or plug-in arrangements to permit live disconnection or “hot swapping” of instrument modules.

7.6.3 Ex nL

Type nL can be applied to a circuit in which the values of current, voltage and stored energy available are adequately low such that any spark produced is non-incendive. This is based on the concept and the electrical value limits used for Intrinsic Safety (Ex i). The primary difference between Ex nL and Ex i is that Ex i principles require the addition of duplicated ‘safety components’ to preserve safety (under certain specified failure conditions) whereas Ex nL does not need to consider such failure conditions. Safety factors applied to Ex i are also not applied to Ex nL. This method will become more easily understood after studying the section on Ex i where it will be realised that Ex nL is equivalent to Ex ic as a concept.

Figure 7.1 c
Ex nL simple circuit showing resistance limiting

A resistor may be added to limit the current as the only precaution if the voltage derived from a supply is already adequately low and there is insufficient stored energy as shown in Figure 7.1 c.

The Open Circuit Voltage and the Short Circuit Current are now defined by the circuit. Provided these are within the published curves in the IEC60079-11 Standard for Intrinsic safety then the circuit supply capability is demonstrably Ex nL.

The use of square brackets to define a source of limited energy as in Intrinsically Safe Associated Equipment is common with the Ex i usage i.e. EEx nC[L] would indicate that the device is Zone 2 mounted and feeds a circuit in which the other equipment must meet EEx nL.

The UK’s BS1259 Standard was a simple application of Intrinsic Safety and is somewhat similar in approach to that eventually permitted under ‘energy limiting’ type n concept or Ex nL as it is now known.

7.6.4 Ex nR

Type nR is the term for “restricted breathing” and is applied to the enclosures of Ex n equipment. An example of this is a luminaire where the lamp normally reaches, say, 300°C. The T rating applied to the surface area of the lamp would be T2. By enclosing the lamp in a well-glass that has a sealing arrangement to the main body of the original enclosure, the surface temperature in contact with the surrounding potentially flammable atmosphere would be reduced to only, say, 150°C. This would qualify for T3.

Figure 7.1 d
Ex nR arrangement where T Rating is made T3 not T2 by the addition of the Well Glass

The up-rating of the T Rating only holds true provided that the seal is adequate to prevent ingress of a flammable atmosphere to the inside of well-glass owing to breathing, for example, when the lamp heats up and cools.

Such an arrangement requires a test of the seal to ensure that the seal does not allow a gas/air mixture to breath to the inside space where it would encounter the elevated temperature of the lamp. The test that the seal must pass is where a pressure difference of 12 inches water gauge is applied across the seal and the pressure difference must not fall to below one-half of that within three minutes.

(In some countries, additional requirements are placed on such arrangements: In Australia, restricted breathing enclosures are limited to use with gases and vapours whose restricted breathing factor is less than 20, see table A1 in AS 2380.9 Appendix A for typical breathing factors; acetylene, hydrogen and isoprene have restricted breathing factors higher than 20).

7.6.5 Pressurisation

Type ‘n-pressurisation’ proposed the use of a simplified pressurisation technique in the previous edition of the IEC Standard, part 15.

This might comprise the pressurisation of an enclosure outside the hazardous atmosphere with a known clean gas via a manually operated pump as depicted in Figure 7.1e. If the equipment seals are good enough then the pressurisation shall last for long enough for the function of the device to be used in the hazardous area without fear that the ingress of hazardous atmosphere will occur.

Figure 7.1 e
A simple ‘n-pressurisation’ arrangement where Air is pumped in in the Safe Area before equipment entry in to The Hazardous Area

7.6.5 Removals from the Ex n Standard

Techniques of protection previously permitted under Ex n have now been rationalised and included into the most appropriate Construction Standard. This has caused some confusion during the transition period but overall sees the unification of some Standards. Here are the current re-assignments:-

Ex nL is now Ex ic
Ex nA is now Ex ec
n-pressurisation is now Ex pcz
Ex nC ‘small volume enclosure’ will become an Ex dc component. (Note: at the time of writing)

The last reassignment (to Ex dc) is owing to the fact that the principle difference between enclosed break devices ‘nC’ and flameproof ‘d’ is that the dimensions are not controlled and safety factors have not been added.

It is highly likely that other elements of this Standard will be superseded by inclusion in other construction standards as time goes by.

7.7 Applications

Applications are many and varied. Motors, light-fittings, junction boxes and instruments are amongst the many types of equipment suitable for this type of protection.

  • Motors can only be of the induction type, i.e. without brush-gear. Frequent starting is not recommended and the use of variable speed drives requires careful consideration
  • Fluorescent light fittings may use bi-pin tubes but the starters will be encapsulated or of the electronic type
  • Other types of luminaires are available in an Ex n design

Instrumentation is the one type of equipment that appears to have benefited most from the updating of this Standard. Signal processing techniques permit a central point of collection/distribution of instrument signals through what has become known as an “i/o” block or module (input and/or output signal). The i/o block is mounted in Zone 2 and carries power supply and signalling facilities to instruments in the local area. Communication to the safe area is normally provided by using an appropriate protocol running under a field-bus like transmission system.

7.7.1 Conditions of use

Whilst type of protection ‘n’ has features in common with other types of protection, it is, in many respects, more lenient.

No modification, addition or deletion shall be made to apparatus with type of protection ‘n’ without the written permission of the certifying authority (such permission shall be obtained through the manufacturer of the apparatus) unless it can be verified that such change does not invalidate the certification.

Apparatus with type of protection ‘n’ is suitable for use in dust risks and in combined dust and gas/vapour risks if the additional precautions specified for protection against dust ingress are complied with.

When selecting apparatus special care shall be taken to ensure that the apparatus and its component parts are constructed to guard against electrical and mechanical failure in the intended conditions of use. Particular attention shall be given to the need for weatherproofing and protection against corrosion.

7.8 Installation

The diagram in Figure 7.2 outlines the basic requirements for any Ex n installation.

Figure 7.2
Non-incendive installation

The Standard recognizes that components may be wholly within the enclosure or mounted through the enclosure, such as an indicator lamp, in which case testing must be appropriate to prove suitability. The following summary gives the Ex n general requirements and attributes:-

  • Safe area wiring to any Standard
  • Adequate overload and short circuit protection required
  • Safe area fitted circuit isolation of the Hazardous Area Circuit (2 Pole)
  • Mechanically protected cable required
  • Ex n component certified enclosure
  • Only permitted in Zone 2
  • Adequate environmental protection
  • IP54 minimum
  • Impact strength tested

Additional requirements:-

  • Blank off any unused holes to meet the enclosure’s IP rating.
  • Cable glands must be appropriate for the cable, maintain the IP rating
  • Glands for restricted breathing equipment must prevent breathing down the cable

7.9 Inspection

Whereas with Ex d, the flamepaths are critical to safety and can be easily verified as acceptable or unacceptable by following the guidelines of the Standards, Ex e and Ex n are more difficult to assess because the exact mechanism for achieving safety is not apparent in an obvious physical and observable form. Inspection of Ex n, in a similar way to Ex e equipment, requires a broad and sound knowledge of principles and an observant approach because there is no individual application of a ‘safety principle’ in the same way as Ex d.

Signs of deterioration, corrosion, over-heating, arcing and other physical attributes must be looked for inside equipment where current carrying parts are present. The enclosure condition, cable entry devices and seals are also considered for integrity.

Standard IEC60079-17:2012 Inspection and maintenance of electrical installations in hazardous areas (other than mines).” provides an inspection schedule for Ex n installations in Table 1, at a detailed, close and visual grade. This should be consulted by maintenance planners.

7.10 Live working

Live working on Ex n equipment was always permitted if the user could prove the safety of doing so. Previous inspection and maintenance Standards as well as the current one, IEC60079-17:2013 (under Clause 4.8) gives the conditions under which work may be carried out in the clearest terms. This section of the Standard is reproduced for clarity and discussion:-

4.8 Isolation of equipment
4.8.1 Installations other than intrinsically safe circuits
Electrical equipment containing live parts, which is located in a hazardous area, shall not be opened except as described in a), b) or c).
  1. Work, for which the exposure of live parts is necessary, may be carried out subject to the precautions which would be applied in a non-hazardous area, under a safe work procedure (see IEC 60079-14).
This may require isolating of all incoming and outgoing connections including the neutral conductor. “isolation” in this context means withdrawal of fuses and links or the locking off of an isolator or switch.
Sufficient time may need to be allowed to permit any surface temperature or stored electrical energy to decay to a level below which it is incapable of causing ignition.
NOTE 1 The protective capabilities of an Ex d enclosure are always compromised by opening it, whereas Ex “e” and Ex “n” enclosures may be of lesser concern if moisture ingress is unlikely while they are opened.
  1. A relaxation of the requirements for increased safety ”e” equipment which also contains intrinsically safe apparatus is permitted, if all bare live parts not protected by the type of protection “i” have a separate internal cover providing at least the degree of protection IP30 when the enclosure of the apparatus is open.
This equipment should be provided with an external label stating:
“WARNING – DO NOT OPEN WHEN NON-INTRINSICALLY SAFE CIRCUITS ARE ENERGIZED”. Technically equivalent text may be used and multiple warnings may be combined.
NOTE 2 The purpose of the internal cover, when fitted, is to provide a minimum acceptable degree of protection against the access to energized non-intrinsically-safe circuits when the enclosure is opened for short periods to permit live maintenance of intrinsically-safe circuits. The cover is not intended to provide protection from electrical shock.
  1. In locations requiring EPL Gc or Dc, the work may be carried out subject to the precautions which would be applied in a non-hazardous area, if a safety assessment shows that the following conditions are satisfied:
    1. the proposed work on energized equipment does not produce sparks capable of ignition;
    2. the circuits are of such a design as to preclude the production of such sparks;
    3. the equipment and any associated circuits within the hazardous area do not include any hot surfaces capable of producing ignition.
If these conditions can be met, then work may be carried out subject only to the precautions which would be applied in a non-hazardous area.
The results of the safety assessment shall be recorded in documents which shall contain:
• the form(s) which the proposed work on energized equipment may take;
• the results of the assessment, including the results of any testing carried out during the
assessment;
• any conditions in association with the maintenance of energized equipment which the assessment has shown to be necessary.
The assessors of the equipment shall:
• be familiar with the requirements of any relevant standards, the recommendations of any codes of practice, and any current interpretation;
• have access to all information necessary to carry out the assessment.
4.8.2 Intrinsically safe installations live maintenance
Maintenance work may be carried out on energized intrinsically safe equipment provided additional care is taken to prevent violation of circuits where more than one circuit is in the equipment, subject to the conditions detailed below.
  1. Maintenance work in hazardous areas
Any maintenance work shall be restricted to:
1) disconnection of, and removal or replacement of, items of electrical equipment and cabling;
2) adjustment of any controls necessary for the calibration of the electrical equipment or system;
3) removal and replacement of any plug-in components or assemblies;
4) any other maintenance activity specifically permitted by the relevant documentation;
5) use of any test instruments specified in the relevant documentation.
Where test instruments are not specified in the relevant documentation, only those instruments which do not affect the intrinsic safety of the circuit under test may be used.
The person carrying out any of the functions described above shall ensure that the intrinsically safe system or self-contained intrinsically safe equipment meets the requirements of the relevant documentation after completion of any of those functions.
  1. Maintenance work on intrinsically safe circuits and equipment located in a non-hazardous area Maintenance of associated electrical apparatus and parts of intrinsically safe circuits located in non-hazardous areas shall be restricted to that described in a) whilst such electrical apparatus or parts of circuits remain interconnected with parts of intrinsically safe systems located in hazardous areas.
Safety barrier earth connections shall not be removed without first disconnecting the hazardous area circuits, except where duplicate earth connections are provided, in this case a single earth may be removed to facilitate earth resistance checking.
Other maintenance work on associated apparatus or parts of an intrinsically safe circuit mounted in a non-hazardous area shall be carried out only if the electrical apparatus or part of a circuit is disconnected from the part of the circuit located in a hazardous area.

It is important to realise that this is no longer confined to Ex n but covers other types of protection but it will be observed that with energy limiting techniques the requirements are less stringent as is shown in the discussion under the Intrinsic Safety maintenance situations.

7.11 Conclusions

The vast majority of hazardous areas are probably designated Zone 2 and so Ex n, being a lower equipment cost option, offers a wide range of equipment types and applications. Industrial grade equipment can be made to meet Ex n with little or no additional design work and so it is in manufacturers’ interests to offer it to a wider market which includes for use in Zone 2. It is also in the users’ interest to have the option of equipment in both safe and hazardous areas as maintenance personnel will be familiar with the operation of the equipment.


8


Protection Concept ‘i’ Principles

In this chapter, the subject of Intrinsic Safety is examined. Where energy and power that are available in a circuit entering a Hazardous Area are reliably limited to levels proved to be incapable of causing ignition, then the circuit is said to Intrinsically Safe. The type of protection is very different to the others and involves the concept of an all-important ‘system’ approach that is not otherwise required.

Learning objectives

  • To study the basics of Intrinsic Safety
  • To understand fault and infallibility definitions
  • To understand energy limiting
  • To compare Ex i with other techniques
  • To consider Ex i Systems

8.1 Name

Type of protection ‘i’ is otherwise known as Intrinsic Safety, or Ex i. Intrinsic Safety is one of the more common methods by which equipment associated with instrumentation is protected for use in hazardous areas. The term is often misused and can cause confusion, as in describing equipment as ‘intrinsically safe’ when it should be described as ‘explosion protected’. Even worse is the use of the term to describe a ‘hazardous area’, saying that it is an ‘intrinsically safe area’.

8.2 Standards

Current International Standard:

IEC60079-11:2011

 

Older Standards

EN50020
BS1259
SFA3012
USA Approval to NEC Article 500 / UL613

8.3 Definition

Intrinsic safety is defined in Standard IEC60079-11:2011 as:

‘A type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects’

An Ex i circuit is defined in Standard IEC60079-11:2011 as:

“A circuit in which any spark or thermal effect produced in the condition specified in this International Standard, which include normal operation and specified fault conditions is not capable of causing ignition in a given explosive gas atmosphere.”

8.4 Origins of intrinsic safety

The origins of this type of protection date back to the Senghenydd Coal Mine Disaster on 13th October 1913 that was introduced in Chapter 1.

During the subsequent investigation, Wheeler and Thornton found evidence that prior to the explosion occurring the power supply to the electric bell signalling circuit shown in Figure 8.1 used ‘wet’ cells making up a battery. These had been replaced with more modern ‘dry’ (Leclanché) cells which had recently become available at that time. In order to find out if this could have been the cause of the incident they set up a test, using equipment they had devised called ‘Spark Test Apparatus’, to reproduce the conditions in which the circuit of the signalling system would have worked underground.

The test apparatus, introduced in Chapter 2, comprises a contact, making and breaking, in an envelope of methane or firedamp, to represent the switching effect of the spade across the signalling wires. The test revealed that the new cells were capable of producing an incendive spark, whereas the old cells could not. The internal resistance of the dry cell was lower than that of the wet cell and so had a greater current delivery capacity.

Wheeler and Thornton had effectively stumbled upon the technique of energy limiting that is now known as Intrinsic Safety.

Figure 8.1
The mine signalling system circa 1900

8.5 Principles

The Fire Triangle (Figure 8.2) analogy applied to explosion and fire prevention could now be clarified as a result of this discovery about the cell resistance. In the presence of the “most easily ignitable mixture” of a given flammable gas with air, ignition cannot occur if the level of spark energy is insufficient.

Figure 8.2
The fire triangle

The principle of Ex i protection is to ensure that levels of heat or size of sparks that occur in an electrical circuit which comes into contact with a flammable gas, are specifically limited to below those which will cause ignition.

In order to achieve this, the circuit must satisfy three key requirements:

Design to limit the power dissipation
Design to limit the energy emission
Prevent the circuit from invasion of additional power and energy

The use of the term ‘circuit’ in the definition has important implications. Electricity can only produce heat when flowing (power dissipation) and sparking when starting or stopping the flow (energy emission). Since it can only flow in a complete circuit, it is the safety of the circuit that is of concern.

Levels of heat and sparks emitted from the circuit can be assessed for compliance with the Construction Standards. Once this has been correctly designed, the installation must ensure that these levels are never elevated and it is the Codes of Practice for installation that provide guidance on how this is to be achieved.

A ‘circuit’ can mean any of the following arrangements, increasing in complexity:

A single cable looped through a hazardous area.
An ‘assembly’ of electrical components working together as an electronic device (such as an instrument).
A number of ‘assemblies’ can be interconnected in the same circuit.

The circuit may operate with energy levels that are quite safe under normal conditions. Under probable fault conditions acting within or onto the circuit, the circuit must still not be able to emit heat or sparks in sufficient quantities to cause ignition where it encounters a hazardous area. Internal failures and certain external fault conditions that can arise must be anticipated and adequately protected against.

The possible ‘faults’ are predicted by careful examination of what failure mechanisms could occur. Components are built into the design of the complete circuit in order to maintain energy to known safe levels under these fault conditions. These components are termed, ‘safety components’ and are in-built into the operation of the circuit. To further enhance the integrity, the failures of the ‘in-built’ components are separately assessed to ensure that if they fail in a specified manner, safety is still maintained.

The dictionary definition of the word, ‘Intrinsic’, is given as, ‘of its nature’ or, ‘in-built’. Safety, which is achieved by the inclusion of specific ‘safety’ components into a circuit, is part of the equipment design and is therefore said to be ‘intrinsic’ to the equipment.

8.6 Electrical theory

The understanding of the principles and practice of Ex i requires a working knowledge of simple electrical theory. The principles are based around the application of Ohm’s Law.

Suppose that a conventional (constant-voltage) power supply provides an open circuit output voltage, VOUT. In theory, it would be able to deliver from zero up to an infinitely large current. The current, IL, supplied to the load will be controlled by the load resistance, RL, such that:

VOUT / RL = IL, according to Ohm’s Law.

8.6.1 Power dissipation

The power, PL, (measured in Watts), dissipated in the load resistor RL is:

VOUT x IL = PL, again, according to Ohms Law.

Figure 8.3
Power dissipation in a load RL

In a simple circuit, Figure 8.3, as a result of the power dissipated in the resistance, RL, the temperature rise experienced will depend on its physical properties such as mass, surface area, heat-dissipation properties, the ambient temperature and external cooling effects.

Now consider the effect of taking values between:

RL = an infinitely high value (where no current is drawn from the supply)
through to
RL = an infinitely low value (where the highest current is drawn and there is no volt-drop)

Power dissipation in RL is inversely proportional to the value of resistance (when the voltage stays constant). In this case, the power dissipated in the RL increases by the square of the current. (V2/R= Power). This is another aspect of Ohms Law.

The current delivered from any source of supply is dependent on its ‘internal’ resistance. A battery has a low internal resistance when charged but this rises as the battery becomes exhausted. The short-circuit current capability of a supply is governed by its internal resistance. (A power supply using an electronic regulator is designed to maintain artificially low internal impedance throughout its current supply range.)

In order to limit the total power supply capability of a constant voltage source, a ‘current limiting resistor’ (CLR) will be included in the circuit (as artificial internal resistance). This will provide a minimum circuit resistance. The circuit is shown in Figure 8.4.

The effect of changing the value of RL (in series with a constant RCLR) may now be considered. Taking values between an infinitely high value, open-circuit, through to an infinitely low value, short-circuit, the power dissipated in RL can be examined.

Figure 8.4
Power dissipation in a load with current limiting resistor

When RL is either at either extreme of high values or low values then the power dissipated in RL tends towards zero. This is because:

when RL is very high (near open-circuit), the circuit current is zero and so I2RL will be zero, or
when RL is low (near short-circuit), the current is at a maximum, limited by the value of RCLR. As RL is zero, I2RL will be zero.

 

It follows that there must be a value for RL where the power dissipated in it reaches a maximum value. This is when RL = RCLR and can be proved with a few simple calculations.

Artificially setting the minimum circuit resistance therefore enables control of the maximum power that can be dissipated in loads connected to the supply. This is known as the ‘Matched Power’ theory.

The maximum amount of power dissipated in an Ex i circuit must not allow a surface temperature rise to occur in conductors or resistive components that come into contact with the gas/air mixture, such that ignition occurs. The amounts of power involved will be discussed in more detail later.

8.6.2 Energy

The power dissipation in RL has been examined during the transition from a low to a high resistance through variable intermediate values.

When the transition is made to happen instantaneously, for example, when a circuit is broken, the rates of change of voltage and current are extremely high. The transition from short- to open-circuit, as in the opening of a switch, is known to emit a spark (see Figure 8.5).

A spark dissipates electrical energy. The size of the spark produced at the breaking of a circuit is a function of VO-C and IS-C. The speed of transition between the two states precludes any ‘power’ being dissipated.

Figure 8.5
Switch action dissipating energy as a spark

It is known that sufficient spark energy will cause the ignition of a flammable atmosphere, but the physics of spark ignition are still not fully understood. The size of the spark is determined empirically and the method of testing a gas for its spark ignition energy is described in the Standard IEC60079-4.

There are then two techniques that may be used for controlling the size of the spark to below that which will cause ignition:

Reducing the Voltage, VOUT; and
Reducing the Current, IS/C, by raising the value of RCLR .

8.6.3 Stored energy

In any electrical circuit, the natural effect of inductance, capacitance and resistance will be present.
Stored electrical energy is measured in Joules and is represented by the letter ‘E’.
Stored energy may be calculated by the formulae as follows:

for inductance, E = ½LI2 (where L is the Inductance in Henrys, I is the current flowing through in Amperes)
for capacitance, E = ½CV2 (where C is the Capacitance in Farads, V is the charged on the plates in Volts)

Energy in the form of heat is dissipated by power in the resistance of the circuit and is time dependent.

Each and every source of stored energy in an Ex i circuit must be identified and assessed to ensure that it cannot provide sufficient energy to pose a danger. The stored energy must not be released in an uncontrolled fashion otherwise incentive heat or sparks may be produced. Clearly, where there are many sources in the same circuit then the likelihood of the energy becoming cumulative must also be assessed. It may be necessary to take additional precautions such that the risk of this is minimized to an acceptable level.

The circuit designer must therefore include components, which act to limit power dissipation, energy release and stored energy levels in the circuits of electrical equipment. This must be done in a way, which meets the requirements of the Standard but does not inhibit or affect the operation or indeed performance of the equipment. In this way the safety is ‘in-built’ or intrinsic to the equipment. There are many techniques that may be used in the internal design of electronic apparatus, but these are closely protected secrets within the Ex i design and manufacturing industry.

8.6.4 Energy limits

The work of Wheeler and Thornton began by investigating the size of voltages and currents (limited by resistance) causing ignition. Empirical studies have published results which quantify the maximum values. These studies also state voltage and current levels for the limits of capacitance and inductance, respectively and will be discussed later in this chapter.

As an example, by consulting the resistance (voltage-current) curves, shown in Figure 8.6, it will be realised that if a supply of 50 V was connected through a 1000 Ω resistor to allow 50 mA to flow, switching action in the presence of the most easily ignitable mixture of hydrogen with air would not cause ignition. However, by increasing the current to 60mA (reducing the resistance to 830 Ω) ignition could occur since the point at which these values co-exist is into the upper region of the graph where ignition is expected. The graphs were plotted to take the worst-case points.

The use of the curves for the design of a Ex i supply to circuits requires safety factors to be included. This can be demonstrated by the example set out below, referring to Figure 8.6):

Determine the highest open circuit voltage required: 28 V
Apply a safety factor of 1.1 (i.e. add 10%)
Find the intersection of the Voltage and Curve
Read off the Current at the intersection.
Apply a factor of safety of 1.5 (i.e. reduce current by 33.3%)
The value found is the highest short circuit Current.

For circuits operating in IIB and IIA gases or vapours, readings are taken from the appropriate curves.

It will be realized that in order to achieve a short circuit current of not more than 93 mA from a source of voltage not greater than 28 V, the minimum series resistance, RCLR, value can be calculated by:

RCLR = VO/C ÷ IS/C = 28/0.093 = 300 Ω

Figure 8.6
The method of determining the permitted current from a given Safety Description Voltage

In some interpretations of the standards the 10% additional factor has not been considered necessary and the extra available power from the circuit has been useful in some applications.

Note that the graphs specifically include the use of aluminium, magnesium, zinc and cadmium, which are known to exacerbate the sparking effect. Since these elements are in common use in modern electronic circuits, their inclusion in the standards forces the design to use the most onerous conditions thereby increasing safety margins.

8.6.5 Stored energy limits

There are also maximum permitted values for stored Inductance and Capacitance against given current and voltage values, respectively. Storage of energy in circuits must not be allowed to accumulate above incendive levels. These levels must be realistic such that circuits can operate and be made safe.

Typical circuit values found on a powered instrument loop can be of the order of 500 μH and 0.1 μF. The working limits of current and voltage due to these values are shown in relation to the IIC (Hydrogen) curve in Figure 8.7.

Factors of safety of 1.5 are added to current values and 1.1 added to voltage values as determined by the design standards for Ex i apparatus and systems.

The inductance curve for hydrogen, IIC, is usefully approximated to a straight-line value of 40 μJ, which makes calculations for stored energy possible. The same approximation technique does not apply to capacitance values.

8.6.6 Power limits

Returning to the subject of power dissipation, some experimentation was needed to quantify the amount of power permitted in a circuit as heat. If the power in an Ex i circuit is limited to 1.3 Watts or less, the temperature of conducting parts of the circuit have been found by measurement not to rise above 130°C starting from an ambient of 40°C. The Standard states that T4 may be awarded to such a circuit without further testing.

This provides a load line on the voltage/current graph in Figure 8.7. The combined general limits of power and energy can be visualised in this representation. It can be seen that the choice of voltages and currents revolve around the operating point, which gives the most flexibility to the design of circuits.

These are not absolute limits. Power, energy, voltage and current levels may be increased or varied to suit special applications. Trading off one characteristic for another may do this. High voltage systems are permitted by introducing higher resistance, thereby limiting the current, and vice versa.

Using the example in 8.3.4, a 28V 93 mA 300 Ω Safety Description may be analysed for maximum power dissipation in order to assess the temperature rise experienced.

The power dissipated in a matched resistance (Matched Power discussed in Section 8.3.1), that will appear connected across the source of supply is given by the formula:
(VO/C . IS/C ) /4 = 28 x 0.093 / 4 = 0.6503 mW

Since this value is below the maximum value of 1.3 W dissipation, this satisfies the T4 rating requirement.

Figure 8.7
The useable area of intrinsic safety

8.6.7 Component failure and infallibility

Limiting the voltage and current to maximum values is achieved by the specific inclusion of certain electrical components into the design of the complete Ex i circuit. Such components are carefully chosen for their enhanced reliability. When designed into a circuit, they are said to be “infallible”.

The definition of an infallible component or an assembly of components is given as:

Components or assemblies of components that are considered as not subject to certain fault modes as specified in this standard. The probability of such a fault mode occurring in service or storage is considered to be so low that they are not to be considered.

The Ex i integrity may be influenced by the reliability of any (whether designated ‘infallible’ or not) of the components used and the way they have been designed into the circuit. For example, some heating effects caused by other parts of the circuit having an influence on these components is necessary during the design of the apparatus. It may be that their location and mounting is adjusted to minimize the effect. These components may otherwise affect the reliability of the infallible components. There are three fault definitions which must be understood as defined in Standard:

A fault

Any defect of any component, separation, insulation or connection between components, not defined as infallible by this Standard, upon which intrinsic safety of a circuit depends

A countable fault:

Fault which occurs in parts of electrical apparatus conforming to the constructional requirements of this Standard

A non-countable fault

Fault which occurs in parts of electrical apparatus NOT conforming to the constructional requirements of this Standard

The countable fault applies to a component, which is so placed in the design of a circuit to provide a limiting function in that circuit. The component is often referred to as a ‘safety component’. The Standard gives the criteria that these components must meet. These are stringent conditions of acceptable type and de-rating factors that the circuit designer must demonstrate have been met.

The safety components, which are added to the circuit in order to limit voltages and currents, are liable to failure during storage or service. The likelihood of failure must be reduced to an acceptable risk in order to say that it is safe. If the components failed, the circuit could become unsafe. Voltages and currents could rise to incentive levels.

The function of certification of apparatus is an affirmation that the circuit with its components has complied with the requirements of the standard as assessed by a competent authority. The built-in safety of Ex i forces the examination of all possible faults and their effect on the failure of the components. An Ex i category is awarded to the apparatus when it has been certified.

8.6.8 Categories of apparatus

The category of Ex i apparatus refers to the integrity of the preservation of the limiting properties in the event of certain specified faults occurring in the components on which safety depends. There are now four recognised categories:-

Simple Apparatus

This category needs not consider any fault condition because no fault condition however caused could render the circuit in which it was placed in an unsafe condition. This concept is discussed in detail later on.

Ex ‘ic’

Apparatus that is safe only in normal operation.

Ex ‘ib’

Apparatus that may have one countable fault. Category ‘ib’ equipment permits one countable fault to occur yet the circuit will still remain safe. A second countable fault on an ‘ib’ circuit may render it unsafe.

Ex ‘ia’

Apparatus that may have two countable faults. If two countable faults can occur but the circuit still limits the design voltage and current then it is awarded category ‘ia’. A third countable fault occurring could then make the circuit unsafe.

Examples of fault counts will be more easily understood when the implementation of Ex i is examined in Section 8.7.

8.7 Implementation of IS

To explain the theory of Ex i and how it is implemented, a simple example will be used to demonstrate what has to be considered. Figure 8.8 shows a basic status-input loop to a control system.

The object of such a loop would be to transfer the status of the switch from one circuit to another. It would not be good engineering practice to try to switch high power electrical circuits directly from field contacts. On conventional relay-logic control systems interposing relays are used between the field and the contactors to link the circuits. These handle the interfacing to higher power devices such as motor starters, etc. in a more convenient manner. PLCs have now replaced many of these relay logic systems but the same concepts apply. The simple example will be used to show the thinking behind the electrical safety aspects.

Figure 8.8
A typical application

The circuit in Figure 8.8 shows a constant voltage supply (normally 24 Vdc) often located in equipment cubicles associated with a control room. It is connected in series with a switch located on the plant and back to a relay positioned near the supply. The supply is derived from ac mains, rectified, smoothed and regulated. The regulator is shown in simple form for ease of explanation. The relay would be chosen according to its function. It must operate adequately with the voltage and current levels in the field circuit. It must provide the correct contact rating to drive other external circuits in the equipment cubicles. These may be other low power relays for logic and/or interlocking purposes or may be contactors for higher power purposes.

Examination of the circuit as a whole is required to consider how it could cause ignition. It must involve all the electrical devices and connecting cables in the circuit whether mounted in the safe or the hazardous area.

The making and breaking of the contact will cause a spark. The voltage and current fed to the circuit will influence the size of the spark. Ignoring the inductance of the relay coil at this stage, the resistance of the relay coil will limit the supply current value. Suppose that the supply was 24 Vdc and the coil resistance was 100 Ω. The current would be 240 mA and the power in the coil would be 5.76 Watts. These are not unrealistic values if the relay was a motor contactor.

Comparing these values (24V dc, 240 mA) with the permitted voltage and current values, they are below the IIC curve and therefore would not cause ignition. The resulting spark would not be incendive. Additional safety factors, discussed above, to be applied where a loop is signed to Ex i would not permit this circuit to be used.

The inductance of the relay would increase the size of the spark dramatically. The field switch changing to open circuit causes the current to fall to zero. The collapsing magnetic field of the relay coil cuts its own turns and produces a high reverse voltage called a ‘Back-E.M.F.’. The voltage is so high that it jumps the breaking contacts of the switch, producing a large spark. The stored energy held in the coil is therefore dissipated in that spark. It could well be large enough to cause ignition. The value of inductance of any coil is complex in nature and depends primarily on the number of turns, the cross-sectional area covered and the magnetic properties of the medium within the area formed by the turns.

Other mechanisms that could influence the energy levels in any part of the circuit, which enters the hazardous area (in no particular order), are:

The coil could become a short circuit or shorted turns may decrease its resistance. In this case the current would rise.
The supply could fail in two ways as seen in Figure 8.9. Fault 1 shows the collapse of the regulation circuit in a conventional series regulation system. Fault 2 is the worst-case failure mode, where the mains become connected to the output terminals of the supply by some internal catastrophic failure. This is more serious in that the instrument loop could have the mains supply directly across it.
The insulation of the relay could break down such that voltages applied to the contacts may become connected to the coil circuit.
The cable or connections forming the circuit in the hazardous area may be broken by external mechanical influences. A spark could occur at other places in the circuit other than at the switch.
External energy could enter this system owing to mechanical faults in cabling running in parallel with this cabling.
Figure 8.9
Mains faults in typical application.

Of the greatest concern would be the fault mode where the live mains terminal was to become connected to the supply circuit (fault 2). This would elevate the whole circuit with respect to earth. Under this single fault condition, the mains voltage superimposed on the low voltage circuit may not be detected until a connection in the hazardous area is inadvertently made. This fault could be internal to the supply or external invasion causing the elevation. These faults, however unlikely they may seem, need to be considered if safety is to be assured at the highest level. Taking the following additional precautions may protect against all the above situations:

Limiting the voltage across the external circuit into the hazardous area.
Limiting the current in the external circuit.
Limiting the fault energy from supply failure systems.
Limiting the stored energy in the hazardous area circuit
Limiting the likelihood of invasion from external sources.

These are the basic requirements of any Ex i circuit. The last point can only be controlled by care taken during installation. This aspect will be discussed in Chapter 12.

8.7.1 Energy limiting ‘system’

The arrangement shown in Figure 8.10 is a simple assembly of components that operate together to limit energy flow into the hazardous area. Firstly, it limits the voltage in a circuit by the use of the Zener Diode and, secondly, it limits the current that can be sourced from that voltage by the inclusion of a Current Limiting Resistor (CLR).

Figure 8.10
Voltage and current limiting arrangement

The Zener Diode is connected across, in parallel or in shunt with the circuit to be protected. The operation of the Zener Diode can best be explained graphically as shown in Figure 8.11

Figure 8.11
Zener diode characteristics

Used in the ‘reverse bias’ direction, the Zener Diode does not conduct appreciably whilst the reverse voltage is well below the Zener voltage or the ‘Avalanche Voltage’ as it is sometimes called. In the graph, Line A represents the leakage current of the order of pico- or nano-amperes at low voltages. This is considered insignificant until the Working Voltage (VWORKING) is encountered. Line B (almost perpendicular to Line A) represents the constant voltage developed across the Zener Diode when appreciable current flows through it. Where lines A and B meet is often referred to as the ‘knee’.

The Zener Diode junction ‘senses’ the voltage across it and reduces its resistance from infinity to a lower value. It maintains a constant voltage across it by reducing its resistance. It may be thought of as acting like a voltage-dependant resistor when an applied voltage exceeds the voltage, VWORKING. The supply is assumed to continue to provide current, which will dissipate power in the form of heat in the Zener Diode. The highest voltage that can be developed across the Zener Diode is known as the VMAX.

Whilst the voltage across the Zener Diode is “clamped” at the maximum value, VMAX, the highest current IMAX that can be sourced from the circuit into the hazardous area is limited by the current limiting resistance (CLR) according to VMAX divided by RCLR as discussed in the theory above. This is said to be the “Safety Description” of this network of components.

The Safety Description defines the Open Circuit voltage and the Short Circuit current that are available to the hazardous area. These values must remain below the values permitted by the Standards when safety factors have been correctly applied. They may be expressed as a Voltage, Current and/or Resistance.

A typical example is 28 V, 300 Ohms, 93 mA: 28 V divided by 93 mA gives 300 Ohms.

This voltage and current will only persist until the safety fuse ruptures and clears the fault. Thus, the fuse has a most important role to play (see Figure 8.12(a)).

Figure 8.12(a)
Energy limiting arrangement in associated apparatus

This arrangement will not let through enough energy to cause a spark in the hazardous area during the normal operation of the circuit. The effect of the back-EMF produced in the relay coil is also clamped by the diode in its forward-biased condition. The design of the safe area electrical circuit must accommodate the characteristics of the added components.

Note that the original relay specification may not be useable at this stage because of the extra circuit resistance introduced by the inclusion of the current-limiting resistor. This will be discussed more fully in the section on applications.

Clearly the Zener Diode(s) will dissipate power from the source of supply during a fault where an over-voltage condition from the safe area supply exists. Given sufficient time under overload conditions, it would eventually overheat and fail if no other precautions were taken.

A suitably rated fuse is therefore interposed between the Zener Diode and the source of supply such that when excess current is drawn by the Zener diode, the fuse will act to disconnect the whole circuit before any damage is done to the diodes. The breaking capacity of the fuse is specified in the Standard at 1500 A for a maximum supply voltage of 250 Vrms. The fuse characteristics are carefully chosen such that it will operate reliably with the known parameters of the Zener Diode combination.

The circuit must be referenced to earth. This is to ensure that the hazardous area circuit cannot float at mains potential.

The mechanical layout of the safe area arrangement must now be such that if faults 1 or 2 occur then there is adequate segregation within the design to ensure the limiting circuits cannot be bypassed. It is for this reason that the whole safe area device including the shunt-diode network must be assessed for safety. This would become known as certified ‘Associated Apparatus’. Refer to section 8.6.

8.8 The shunt diode safety barrier

The Shunt Diode Safety Barrier (hereafter referred to as a ‘Barrier’) comprises the component arrangement discussed above but in a self-contained package. It may be interposed easily between safe and hazardous area circuits.

The arrangement shown in Figure 8.12b will provide energy limiting to the hazardous area under all supply failure conditions or sources of energy introduced into the safe area.

Note also that there is an extra connection to the barrier, which is referenced to the earth of the supply. It is this that provides the return path for fault current routing such that mains faults on the supply are detected and the fuse(s) act to clear the fault. With the introduction of the safety barrier, no failure modes of the power supply now need be further considered. Earthing is discussed in detail in chapter 11.

The great benefit of using this type of external interface is that the safe area equipment does not need to be assessed for safety, provided that it meets certain simple criteria discussed in chapter 12, on installation requirements.

Figure 8.12 (b)
Barrier protected Ex i circuit

8.8.1 Component infallibility

The components that provide voltage and current limiting are therefore critical to the safety of any circuit to which they are applied. The consequences of their failure would render the circuit unsafe.

Understanding how they may fail will allow adequate precautions to be taken to ensure that certain failure modes do not compromise the explosion protection integrity. If higher reliability devices are chosen, this reduces the initial risk. Such components are referred to as ‘infallible components’. If these components are then backed up by duplication or even triplication where necessary then the likelihood of failure is reduced to such an acceptable level that the technique is considered to be the safest and may be used in a continuous hazard situation (Zone 0).

The Standards therefore specify some conditions that must be applied to these components. These will be discussed now.

8.8.2 Zener diodes

Zener Diodes are treated in IEC60079-11:2011 under section 9.1, which deals with the specific requirements of shunt-safety assemblies. Examining the failure modes of shunt Zener Diode assemblies show that they may fail to open circuit or short circuit. Zener Diodes used in barriers are normally 5 Watt dissipation types of a special construction. The standard describes a temperature cycling test and a pulse testing technique necessary to ensure that the Zener voltage is consistent and will pick up any substandard Zener Diodes.

8.8.3 Failure to short circuit

The voltage developed across the Zener Diode would collapse to zero and so actually the more usual failure mode because the diode junction simply fuses together.

8.8.4 Failure to open circuit

The effect of the Zener Diode blowing open circuit would remove its shunt path from the supply and allow the full fault voltage in the safe area through to hazardous area. The current would still remain limited by the circuit resistance but is likely to be higher than permitted due to the higher voltage. The open circuit condition is considered unsafe.

It is for this latter condition that Zener Diodes are duplicated or triplicated to achieve the category of safety previously described.

Figure 8.13
Barrier configurations

Figure 8.13 shows a triplicated Diode arrangement where the failure of any two diodes to open circuit still allows the third to perform the limiting. The circuit meets the criteria for safe in two countable faults of the infallible components: Ex ‘ia’. The diodes used in this configuration are subjected to less stringent testing.

Figure 8.14
Duplicated Zener barrier configurations

Alternatively, only two diodes as shown in Figure 8.14 may be used in an Ex ‘ia’ barrier if a specific construction of the Zener is used and the Zener diodes are pulse-tested in accordance with specific requirements in the IEC60079-11 Standards. The representation shown in Figure 8.15 may well be used on the barrier and the user will not necessarily know which configuration is used inside.

Figure 8.15
Barrier representation

The safety description voltage requires selection of the Zener diodes within bands of 0.1V to achieve consistency and accuracy in the maintenance of the safety description. Zener diodes may be “totem-poled” or stacked to give different safety description voltages. The connection of a Zener diode across the barrier is said to form a chain so it is the chain that must be duplicated or triplicated according to the type of barrier.

8.8.5 Resistor (Current limiting)

The resistor closest to the hazardous area terminals is required to be wire-wound or metal film. This is so that any failure of the resistance cannot reduce its value, which would increase the available current to the hazardous area circuit. Wire wound resistors are preferred because their characteristics show that all failure mechanisms act to increase its resistance. This is the required condition. Some resistance elements such as carbon can fuse when stressed to give a lower resistive value, which is clearly unacceptable. These types are specifically excluded from use.

8.8.6 Safety fuses

The fuse types are required to be high breaking capacity (HBC) and are generally ceramic powder-filled. This design of fuse does not allow the wire to vaporize and leave a metal trace that could encourage tracking and arcing inside the fuse. There is no other failure mode to consider. The Standard generally specifies the fuse design.

The resistance of the fuse is not insignificant. Its temperature coefficient is sometimes a problem in applications.

Taking the combination of the CLR, Zener Diodes and Safety Fuse if any two components might fail then the circuit will remain safe. All these components are deemed infallible types and thus such an assembly is given the category Ex ‘ia’.

The likelihood of failure of these individual infallible components has been estimated as approximately 1 in 1016 per annum. This is accepted as extremely reliable.

Having chosen components to manufacture the barrier the construction itself must be reliable and of high integrity. The internal connections must not fail in any dangerous way and so specifications are included in the Standard to which a barrier must conform. These include power dissipation and temperature rise under safe area fault conditions prior to the fuse rupture. The termination sizes and separation are detailed, governing but not specifying the arrangement of the barrier.

8.8.7 Barrier characteristics

There are two distinct sets of information that describe the characteristics of barriers: These are referred to as Safety and Operational characteristics. It is important that they are not confused. Figure 8.16 summarizes these differences. Illustrations of the use of these figures are discussed in the Applications section.

Safety
The safety characteristics provide the Barrier’s Safety Description, which is the highest output voltage under safe area over-voltage-fault conditions. These figures are necessary in order to safely connect the barrier into an Ex i circuit in the hazardous area. The safety aspects of barriers have been discussed but their use in the operation of an instrument loop must now be considered. This concept is introduced here. There are a number of common industry standard safety descriptions used. It is the safety description, which is of the utmost importance from the point of view of safety.

Figure 8.16
Safety and operational characteristics

Operational
The operational characteristics are that necessary for the loop design in order for the instrument loop to work properly. The barrier must pass signals without distorting the signal’s electrical properties. Careful design and selection of barriers must be used to ensure correct operation but the system as a whole must be safe and must work properly.

One important point is that the Safety Descriptions are deliberately used with particular fuse ratings. This is so that a hazardous area short circuit, which is safe from the point of view of explosion protection, will not blow the fuse if the state persists for a medium term. After a longer term short the fuse may well age and eventually blow.

Electronic over-volt protection techniques are built in to the design of some barriers. These are commonly referred to as Semi-active barriers (as they are no longer the simple passive types described so far) and are more expensive than passive types. Their use is primarily for battery backed systems where the float charge over-voltage may exceed the VMAX and so the barrier fuse does not blow. They are only used on a few applications.

Other electronics are incorporated into the barrier in order to provide a floating supply for 4/20 mA transmitter applications.

8.8.8 Polarity of barriers

In Figure 8.17, a single barrier ‘channel’ is shown with different diode configurations to allow the channel to be operated +ve, -ve or ac (unpolarized) w.r.t. the earthy or return channel.

Figure 8.17
Barrier polarities

The need for polarization is best illustrated during discussions using applications to explain the theory. It is particularly important when using a number of barrier channels together. The primary reason that ac barriers are not used universally is that cable parameters values would be restrictive.

8.8.9 Multi-channel barriers

It is convenient to use two barrier channels together in an Ex i loop. The circuit in the hazardous area is therefore not directly connected but is still referenced to earth. This allows greater flexibility in the connection to the safe area circuit.

Note in Figure 8.18, that the circuit is referenced to earth via the chains of Zener diodes. It may take up any potential between 0V (earth potential) and ±VMAX.

Figure 8.18
Two channel barriers

2-Channel barriers are usually of the same polarity but not always of the same safety description. They do provide for a simpler installation, reducing cost and space taken up. Two, three and four channel types are now common. Multi-channel types for specific applications such as strain gauge bridges are also popular.

8.8.10 Typical barrier channel data

Some typical barrier types are listed by Safety Description in the table below. Safety and operational information is provided for comparison and to assist in the understanding of the operation. Table 8.3 gives the essential data needed.

Table 8.3
Common safety and operational barrier characteristics
Safety Description1
V
Ma Ω
Max end-to-end resistance2 Ω VWKG at 10(1)μA3
V
VMAX4
V
Fuse rating5
Ma
1 100 10 120 0.3 2.0 250
3 300 10 318 (0.6) 3.6 250
10 50 200 85 6.0 6.9 50
15 100 150 155 12.0 13.0 100
22 150 147 185 19.0 20.2 50
28 300 93 340 25.5 26.6 50

8.8.11 Barrier characteristics descriptions

Definitions of operational information are not described in the Standard. Manufacturers and users have evolved common terminology to describe non-safety related characteristic knowledge of which is necessary to ensure the correct operation when designed into instrument loops. To understand the function of the values given in table 8.3, definitions are given in table 8.4.

Table 8.4
Terminology definitions
Name Description
1. Safety description The safety description of a barrier, e.g. ‘10V 50 Ω 200 mA’, refers to the maximum voltage of the terminating Zener or forward diode while the fuse is blowing, the minimum value of the terminating resistor, and the corresponding maximum short-circuit current. It is an indication of the fault energy that can be developed in the hazardous area, and not of the working voltage or end-to-end resistance.
Barriers may be polarized + or -, or non-polarized (‘ac’). Polarized barriers accept and/or deliver safe-area voltages of the specified polarity only. Non-polarized barriers support voltages of either polarity applied at either end.
2. End-to-end resistance The resistance between the two ends of a barrier channel at 20oC, i.e. of the resistors and the fuse.
3. Working voltage (VWKG) The greatest steady voltage, of appropriate polarity, that can be applied between the safe-area terminal of a ‘basic’ barrier channel and earth at 20oC for the specified leakage current, with the hazardous-area terminal open circuit.
4. Maximum voltage (VMAX) The greatest steady voltage, of appropriate polarity, that can be applied continuously between the safe-area terminal of any barrier channel and earth at 20oC without blowing the fuse. For ‘basic’ barriers, it is specified with the hazardous-area terminal open circuit; if current is drawn in the hazardous area, the maximum voltage for these barriers is reduced.
5. Fuse rating The greatest current that can be passed continuously (for 1000 hours at 35oC) through the fuse.

8.8.12 Component arrangements

Physical barrier construction varies from one manufacturer to another but generally speaking all follow the same component arrangement shown in Figure 8.19. The infallible components are encapsulated such that they cannot be interfered with and the safety fuse cannot be replaced incorrectly. This is a requirement of the standards. If the safety fuse were to be replaced by a higher value, then the safety of the circuit may be compromised. Some manufacturers provide external fuses in the form of replaceable types that can be used to protect the internal safety fuse and/or can act as loop disconnect features. The UMAX is normally 250 V rms. Barriers may be two or three-diode construction types provided that they are Ex ‘ia’ rated for Zone 0 use. Connection to an Ex i Earth is mandatory.

Figure 8.19
Barrier construction / connection

8.8.13 Combinations of barrier channels

Where more than one source of energy and power supplies the same Ex i circuit into a hazardous area, the combination of these safety descriptions must be assessed. In Figure 8.19, two independent channels, each with its own separate safety description can provide increased energy and power into the circuit (under simultaneous fault conditions). The total current and voltage from all possible configurations of that arrangement must be examined. This is to ensure that power and energy levels from the combination are still within safe values for the given application and hazard.

8.8.14 Shunt diode safety barrier earthing

The requirement to provide an Ex i earth, as shown in Figure 8.20, is fundamental to the safe use of shunt diode safety barriers. The general principles of earthing and the reason for the Ex i will be discussed fully in the section on Earthing and Bonding.

Figure 8.20
Barrier Ex i earth

The previous discussion on the principles of barriers showed a single barrier with its “earth” connected to the mains earth of the safe area supply. This is acceptable only in order to demonstrate the principle. In practice, the earth connection on the barriers must be taken back to the plant earth associated with the instrumentation system’s source of supply. This is normally defined as the point where the neutral of the distribution transformer for the instrument system is connected to the main earth point of the plant. Note that it may be single phase (as shown here for simplicity) or three phase. It is labelled as the Ex i Earth and connects back to the Star-Point/Neutral-Earth or (“SPNE”).

8.8.15 Galvanic Isolation

The inherent disadvantage of a Shunt Diode Safety Barrier is that its installation requires connection to an Ex i earth. This forces the safe area and hazardous area instrument loop circuits to be commoned at the same earth. The commoning effect is not itself detrimental, provided that the Ex i Earth connection is correctly made and with adequate integrity.

If it were to become disconnected then the potential of the circuit in the hazardous area may become elevated w.r.t. the structural earth. A fault between the circuit and earth might cause incendive sparks or heat dissipation at the point of the fault. Where many barrier circuits are used together sharing a common earth, the effect of losing the Ex i earth would be to increase the chance that stray paths would pose a danger. Clearly there are many possible fault scenarios, which could then occur.

The earthy connection of a barrier therefore may be considered as a direct route for current to flow from Safe to Hazardous area. No voltage or current limiting can be applied to this connection. It is a weakness in the arrangement.

The earth path, shared by the safe area circuit and the hazardous area circuit in a barrier, is broken by the electrically isolating arrangements in a “galvanic isolator”. In this way, current cannot flow between the two parts of the circuit. A potential difference can occur which can be tolerated across the isolator, but this cannot jeopardize the safety, as a current cannot be injected into the hazardous area. The hazardous area circuit may be earthed at one point without causing currents to flow in that earth. It may be referenced to earth.

The internal arrangement for a typical isolator is shown in block schematic format in Figure 8.21.

Figure 8.21
The galvanic isolator

Galvanic isolation achieves this by placing a method of signal isolation (compatible with the required application) between the safe and hazardous area circuits. Electronics in the safe area and the hazardous area circuits are necessary to communicate the signal over whatever isolating system is chosen. In the above case, optical isolation is used.

The system must also provide power to the hazardous area circuits and so a double-wound power transformer is used. It may be sourced from ac mains. 24 Vdc powered versions using inverters at higher frequency (reducing the size of the transformer and increasing the supply efficiency) are now more popular.

The design of the isolating components must be in accordance with the requirements of the Standard. Manufacturers use specially constructed isolating devices. These have to be “component approved” for use in the circuit. They are not “infallible components”, but are said to be ones “on which safety depends”. Their design and construction provides guaranteed isolation to prescribed levels. Faults are considered but other mechanisms may be required to protect component approved devices because they cannot be duplicated in a circuit. For example, thermal trips are embedded into mains transformers to protect against shorted turns causing overheating. Some ‘component approved’ devices are designed and constructed by manufacturers in such a way as will allow their use in a range of isolating devices. Transformers, relays and opto-couplers are good examples of this.

The fault condition in the safe area side cannot permeate the insulating properties providing the isolation. The failure of the safe area supply becoming connected to the hazardous area terminals does not need to be considered.

The worst case scenario is the failure of the hazardous area circuit, where the secondary of the isolating transformer inadvertently becomes directly connected to the hazardous area terminals. The same energy-limiting network used on a barrier is necessary at the hazardous area terminals of an isolator to protect against this fault. No connection to earth is necessary. This is because no invasion can occur onto the hazardous area circuit from any other supply owing to the isolation provided. This can be observed in Figure 8.22. The energy in the hazardous area circuit is self-contained.

The isolator will have a safety description in the same way that a barrier does.

Figure 8.22
Effect of invasion on isolated circuits

8.8.16 Application of galvanic isolators

Galvanic isolators may be for analogue, status or digital applications.

The technique requires sophisticated electronics in both the safe and hazardous area circuits. The circuit must accept the incoming signal and suitably change its form to be communicated over some appropriate means of isolation. The other circuit must receive the signal from the isolating medium and convert it to a form that can be re-transmitted out.

In some cases the signal path must be bi-directional. In other cases the signal that goes from safe to hazardous are may not be the same as that which goes from hazardous to safe area. Two-wire systems using this format of powering and signalling to devices in the hazardous area will be discussed in the section on applications.

The safe area circuit can take up any potential completely independently of the hazardous area circuit. The hazardous area circuit may float or may be earthed as required depending on the circuit conditions. An example of this useful feature is the use of earth leakage detection, which can be applied in order to raise the integrity of the signal. This cannot be done with barriers.

The design of isolators is usually specific for their intended application. Other useful features are often designed into the isolator, as it is a convenient place to perform signal manipulation in the signal path.

The operational characteristics of isolators are different to their safety parameters. As with barriers, the safety description gives the highest possible values that will appear in the hazardous area under any fault conditions. The operational characteristics are dependent on the application and will be declared by the manufacturer. The use of these figures will be discussed in the section on applications.

Originally isolators were expensive, but as demand has increased the cost of manufacture has reduced. The applications are broadening to encompass custom design for special signal and power requirements.

8.8.17 Barriers versus isolators

There is no difference between the levels of safety achieved by these two techniques. The differences are in the application and the requirement for safety earthing.

8.9 Associated apparatus

The function of Barriers and Isolators, described above, limit or preclude excess energy from entering the hazardous area from the safe area. These devices are known as ‘Associated Apparatus’ and this term is defined below.

The IEC60079-11:2010 Standard defines “Associated Apparatus” as ‘electrical apparatus which contains both Ex i circuits and Non-Ex i circuits and is constructed so that the Non-Ex i circuit cannot adversely affect the Ex i circuits’.

8.9.1 The certified interface

The block diagram in Figure 8.23 shows a typical system with Certified Apparatus in the hazardous area connected through an Ex i interface to uncertified safe area apparatus.

Figure 8.23
Modern Ex i system with associated apparatus (Certified Interface)

Such apparatus has the effect of electrically interfacing between the safe and hazardous areas. Barriers and Isolators are known as “Ex i interfaces” and are self-contained devices that interpose between the two circuits. Signals may pass in either direction and a limited amount of power is allowed to enter the hazardous area Ex i circuit. Certified interfaces usually pose industry standard Safety Descriptions and are easy to match with certified apparatus. Certified apparatus itself follows a standard form.

8.9.2 Ex i ‘front-end’ interfaces

The energy limiting arrangement used for barriers and isolators can be built into certified safe area mounted equipment. A data logger or a DCS I/O card, for example, may have Ex i inputs directly onto its terminals and may not need an external interface. The complete arrangement containing the Ex i input and all associated equipment i.e. the entire data logger or the whole I/O rack for the DCS must be certified. Clearly this is convenient in many respects but is also very expensive to certify. The equipment is still Associated Apparatus by definition. It is said to have an Ex i “front-end” or have ‘internal interfacing’.

The electronic design of the front-end circuits will be the same as for barriers or isolators. Where galvanic isolation is employed, an Ex i earth will not be required. Where shunt diode barrier techniques are used then the Ex i earth is necessary. It may be integrated with other earthing arrangements and it may not be easy to understand how the earthing is actually arranged.

Figure 8.24
Older system concept

In the early days of Ex i (Figure 8.24 shows a typical arrangement) two pieces of apparatus were designed and built to work together. The electronic designer could choose operating voltages and currents in the circuit to suit the design provided they did not exceed the maximum values permitted for safe use in Ex i circuits. The range of Safety Descriptions now available have been chosen to accommodate many standard applications thus avoiding the high cost of development and certification of this old arrangement and providing a greater choice of interface arrangements (Barrier or Isolator).

The use of internal Ex i interfacing is still encountered where there are particular supply/signal needs for specialist applications. However there are commercial implications in which a manufacturer will use a non-industrial standard safety description so that his equipment in the hazardous area may only be used with his interface or associated apparatus.

8.9.3 Electronic current and voltage limiting

Associated Apparatus discussed so far has used Zener diodes and current limiting resistors to perform the limiting functions. Most standard interfaces are built this way. Other methods of achieving this are by electronic means. Some equipment uses elaborate supply systems to source power for a hazardous area apparatus where resistive limiting is operationally unacceptable.

This is achieved by the design of separate regulation circuits that are placed in series as shown in Figure 8.25.

Figure 8.25
Electronic limiting

If a semiconductor junction fails then it is more likely to fail to a short circuit. The junction fuses into a low resistance. In this case the low resistance would allow a higher voltage at greater current into the hazardous area. The preferred regulator design includes current limiting or ‘re-entrant’ characteristics for the over-current condition.

Three series regulators are required by the IEC60079 Standard but this still only permits certification to Ex ‘ib’ and are therefore restricted to use in Zone 1. In most cases the supply systems using this technique are for apparatus such as Operator terminals that would never be mounted in Zone 0 and so this is an acceptable limitation.

8.10 Electrical apparatus in the hazardous area

Electrical equipment mounted and operated in the hazardous area must be designed such that it cannot cause ignition.

Electrical equipment surrounded by a potentially flammable atmosphere in a hazardous area must not be allowed to accumulate energy such that if released in an uncontrolled way, it could generate a spark or heat at sufficient levels to be incentive.

Some electrical devices such as a switch or contact in the hazardous area cannot store or generate any energy. A switch contains no element of inductance or capacitance. It cannot therefore store electrical energy. There is also no mechanism to generate electrical energy in a simple switch.

The switch operated relay loop discussed previously, uses a switch mounted in the hazardous area connected to an interface. In this arrangement, the only possible source of power and energy into the loop will come from the safe area via the associated apparatus (interface, barrier or isolator).

The Ex i circuit can only receive excess energy from one other source i.e. invasion from some external source. This is clearly undesirable and is a risk that must be reduced to acceptable levels. Such a risk is outside the scope of any individual assessment of safety and can only be controlled by rules governing the integrity of installation. These rules will be discussed in the section on Installations.

Figure 8.26
Apparatus permitted in hazardous areas

Ex i apparatus mounted in the hazardous area and connected to equipment in the safe area must be classified as one of two types as indicated in Figure 8.26.

8.10.1 Simple apparatus

The simple apparatus concept is of the greatest importance to Ex i systems.

There are many instrument devices used in measurement and monitoring applications, which conform to this class of equipment. The simple apparatus class was conceived for this purpose. Clarifications over the original wording in the European standard have been welcomed to provide even greater flexibility. The extract in table 8.5 is taken from IEC60079-11:2011 Clause 5.7 and its significance is explained subsequently.

Table 8.5
IEC60079-11:2011 Clause 5.7 on Simple Apparatus
“The following apparatus shall be considered to be simple apparatus:
5.7 Simple apparatus
The following shall be considered to be simple apparatus:
a) passive components, for example switches, junction boxes, resistors and simple semiconductor devices;
b) sources of stored energy consisting of single components in simple circuits with welldefined parameters, for example capacitors or inductors, whose values shall be considered when determining the overall safety of the system;
c) sources of generated energy, for example thermocouples and photocells, which do not generate more than 1,5 V, 100 mA and 25 mW.

Simple apparatus shall conform to all relevant requirements of this standard with the exception of Clause 12. The manufacturer or intrinsically safe system designer shall demonstrate compliance with this clause, including material data sheets and test reports, if applicable.
The following aspects shall always be considered:
• simple apparatus shall not achieve safety by the inclusion of voltage and/or current-limiting and/or suppression devices;
• simple apparatus shall not contain any means of increasing the available voltage or current, for example DC-DC converters;
• where it is necessary that the simple apparatus maintains the integrity of the isolation from earth of the intrinsically safe circuit, it shall be capable of withstanding the test voltage to earth in accordance with 6.3.13. Its terminals shall conform to 6.2.1;
• non-metallic enclosures and enclosures containing light metals when located in the explosive atmosphere shall conform to the electrostatic charges on external non-metallic materials requirements and metallic enclosures and parts of enclosures requirements of IEC 60079-0;
• when simple apparatus is located in the explosive atmosphere, the maximum surface temperature shall be assessed. When used in an intrinsically safe circuit within their normal rating and at a maximum ambient temperature of 40 °C, switches, plugs, sockets and terminals will have a maximum surface temperature of less than 85 °C, so they can be allocated a T6 temperature classification for Group II applications and are also suitable for Group I and Group III applications. For other types of simple apparatus the maximum temperature shall be assessed in accordance with 5.6 of this standard.

Where simple apparatus forms part of an apparatus containing other electrical circuits, the whole shall be assessed according to the requirements of this standard.
NOTE 1 Sensors which utilize catalytic reaction or other electro-chemical mechanisms are not normally simple apparatus. Specialist advice on their application should be sought.
NOTE 2 It is not a requirement of this standard that the conformity of the manufacturer’s specification of the simple apparatus needs to be verified.

Examples of the first of the three categories above are switches, resistors, temperature dependent resistors, LEDs Terminal blocks, Connectors which are totally passive devices, as seen diagrammatically in Figure 8.27.

Figure 8.27
Passive simple apparatus components

In order to use the device for its intended purpose, power may need to be applied to Simple Apparatus from an Ex i source. This should not be confused with the devices ability to generate or store energy of its own volition. A switch requires an applied voltage to sense if it is conducting. These devices cannot in any way contribute to the energy in the circuit. There is therefore no limit to the number of simple apparatus devices included into any Ex i circuit. Terminals and connectors are also included here.

Figure 8.28 (a)
Energy storing simple apparatus

The second category of “sources of stored energy within well-defined parameters” illustrated in Figure 8.28 a covers any device containing inductive or capacitive elements where the total value can be examined under the L & C curves published in the standards. This clarification is the most useful but raises some questions. It implies that circuits which contain capacitance or inductance can be classed as Simple Apparatus if the total cumulative capacitance does not exceed the maximum permitted by the Safety Description of the feeding circuit (the associated apparatus). The values of L &/or C must be added to the actual cable parameters (which will be discussed in the section on Installation). Since the cable parameter concept is concerned with systems certification (See Systems), the effect of this is that even Simple Apparatus with significant values of L & C should be covered by a formal analysis of the safety parameters. This means that a systems ‘certificate’ or ‘descriptive document’ is necessary.

For example, the inductance of a magnetic pick-up coil may now be assessed to be within the limits of this Simple Apparatus clause, provided its total inductance together with the system cable matches the cable parameters of the interface.

Figure 8.28 (b)
Generating simple apparatus

In the third category, as shown in Figure 8.28 b, a thermocouple is an example of a device, which will generate energy. It is well below the maximum values stated and is therefore unquestionably Simple Apparatus. Photovoltaic Cells can generate in excess of 1.5 Volts in strong sunlight conditions and therefore may not always be considered Simple Apparatus unless use in low light levels is guaranteed. The separation of sources of stored energy from that of generated energy allows separate treatment of some apparatus to the advantage of the user. Piezzo-electric devices used in ultrasonics and in vibration monitoring were previously precluded. There is no specific maximum energy limit in this international standard other than that represented by the curves. Older Standards quoted 20 µJ (in IIC gases).

The standard states that the above devices need not be “certified” or marked in accordance with the standards. It is recommended that the user carefully documents simple apparatus with energy storing capability.

Simple Apparatus is considered as one of the categories of Ex i apparatus because failure need not be considered. In other words if simple apparatus devices fail then there is no risk that explosion protection integrity will be compromised.

Some devices to be mounted in the hazardous area are unquestionably simple apparatus as in the above examples but they may still be certified and marked if it is in the manufacturers’ best interest to do so. This will be for commercial reasons rather than technical ones. Examples of products dealt with in this way are resistive strain-gauge bridges and some Linear Variable Differential Transformers (LVDTs).

Clause 2 clarifies the use of integrated circuits with an on-board ability to generate voltages. This has been the source of dispute for some time. Where an integrated circuit is provided with a 5V supply and can generate + and - 12V rails, then this is specifically excluded from being classed as simple apparatus due to the complexity of analysis.

Simple apparatus will usually maintain a 500Vac insulation (for 1 Minute) test to earth in line with other certified apparatus unless it is designed to work with respect to earth (in which case it should be used with isolating interfaces).

The standard suggests that specialist advice be sought on sensors, which use catalytic reaction or other electro-chemical mechanisms.

8.10.2 Analysis of devices to assess compliance with IS

The analysis of field- or hazardous area-operated devices in circuits can be performed to determine the aspects of safety. There are three main situations.

The conditions for Simple Apparatus are met—A device may be analysed to ascertain if it may be connected and used as part of an Ex i circuit under the Simple Apparatus clause.
The conditions for Simple Apparatus are not met—If however stored or generated energy is above that permitted by Simple Apparatus, other clauses in Standards may permit the use of such a device under either specific or limited conditions.
If the above criteria cannot be met, then the device or equipment must be designed to include specific safety components that limit the energy and power aspects. The assembly then requires formal Apparatus Certification.

8.10.3 Analysis application methods

Techniques for analysing the energy and power dissipation of devices are relatively simple.
Generated mechanical energy can be determined by the formula:

             E = ½mv2
Where: E is energy in Joules
             m is the mass
             v is the velocity

This is directly related to electrical properties where stored energy is determined by:

             E = ½CV2 or ½LI2
Where: C is capacitance in Farads
             V is the applied Voltage or
             L is inductance in Henrys
             I is the current in Amperes

A Piezo-electric device used for an accelerometer may generate a high voltage when subjected to shock. It would be necessary to calculate the capacity to store energy and to compare the values with those permitted in the Standards.

In such cases some knowledge of practical values and other limitations will be necessary. Some assumptions and justifications will need to be stated in documentation of the analysis in order to prove whether the device complies with the requirements.

Circuits containing inductance and capacitance may be analysed by simply adding together the total value of the like properties. This assumes the worst case failure condition.

8.10.4 Certified hazardous area apparatus

If apparatus cannot be classified as ‘Simple’, then it must be assessed for conformance with a given standard. This enables a certificate to be issued (or “approval” to be given) by a testing authority. The Apparatus is termed ‘Certified Apparatus’ or ‘Certified Hazardous Area Apparatus’ and may be selected for use in a hazardous area. It must be installed and used in accordance with requirements discussed in chapter 13.

Design of apparatus
The design of each type of Ex i apparatus is unique and depends upon its function and specification. The circuit of the apparatus can range from being uncomplicated, as in inductive sensors, to very sophisticated, for example, 4/20mA process transmitters, multiplexers and display systems.

In each case, the designer must demonstrate that the voltages, currents and energy storage within the circuits and sub-circuits are securely controlled and adequately limited from causing ignition in the hazardous area. The requirements to be met are published in the Standards. It is not the intention to discuss in detail all the possible techniques that can be used to design Ex i apparatus in this manual. Some general and widely accepted techniques are shown for illustration.

Higher values of energy and power may be used in circuits provided that adequate precautions are taken (to the satisfaction of relevant parts of the standards). Specific components will have been included in the design and are analysed for failure in the same way as for a barrier circuit. Some common techniques to protect large reservoirs of energy are shown in Figure 8.29, to illustrate how energy storage and charge limiting is achieved. Resistors, diodes and Zener diodes are used in various ways, as shown.

Figure 8.29
Treatment of energy storing components in certified apparatus

The use of large value capacitors in a circuit design is generally avoided where possible. Where necessary, they are often combined with Zener diodes to limit the voltage applied under failure conditions to other parts of the circuit. Diodes or Zener diodes may be single, duplicated or triplicated depending on the application.

Using a series resistor will limit the charge/discharge rate and so eliminate incentive sparks. A shunt resistance across a capacitor will allow the capacitor to discharge such that parts of the circuit become de-energized after a controlled time. The component with its safety component(s) may be encapsulated to raise the reliability and this will also help to dissipate heat, prevent the ingress of moisture and generally increase mechanical robustness from an operational point of view.

Where parts of circuits are isolated from each other, component approved devices are used. Complex devices, such as process terminals, can be built up in separate sections in order to use more overall power. Within Ex i apparatus, the areas of design that need special attention are:

The selection of components, rating, size, characteristics
The layout of the PCB and physical spacing of components
Arrangement of terminations
Heat dissipation
Energy capture
Integrity of joints and connection
Documentation / drawings

Segregation
The design and construction of the circuits must be of high integrity and so there is a great emphasis on the layout. Since each piece of apparatus is unique it is not possible to specify the detail of design in any document. Adequate separation distances are published in the standards and the minimum quality of PCBs and insulating materials are specified against the voltages used on the apparatus. Figure 8.30a illustrates how minimum separation requirements affect the layout of PCBs.

Figure 8.30a
Treatment of energy storing components in certified apparatus

Creepage and clearance distances
Creepage and clearance distances are specified in the Standards as shown in table 8.6. and applied as indicated in Figures 8.30 (a) and (b). These figures apply both internally and externally to Ex i Circuits and systems.

Table 8.6
Creepage and clearance
Peak Voltage 10 30 60 90 190 375 550 750 1 000 1 300 1 575
  Distances in mm
Creepage distance 1.5 2 3 4 8 10 15 18 25 36 40
Creepage distance under coating 0.5 0.7 1 1.3 2.6 3.3 5 6 8.3 12 13.3
Minimum CTI 90
90
90
90
90
90
90
90
175
175
175
175
175
175
175
175
175
175
175
175
300
175
Clearance 1.5 2 3 4 5 6 7 8 10 14 16
Distance through casting compound 0.5 0.7 1 1.3 1.7 2 2.4 2.7 3.3 4.6 5.3
Distance through insulation 0.5 0.5 0.5 0.7 0.8 1 1.2 1.4 1.7 2.3 2.7

Clearance is the shortest distance in air between two conductors. Creepage is the shortest distance over a solid surface.

In Figure 8.30 (b) - diagram (a) these measurements would be the same. In diagram (b) a separating partition effectively increases the creepage value at higher voltages.

Figure 8.30 (b)
Treatment of energy storing components in certified apparatus

Where a circuit has two adjacent tracks, the voltage difference gives the appropriate figures of creepage and clearance. As an example, associated apparatus with a 240 Vac supply track adjacent to a 12 V track on a flat PCB would require 6mm clearance and 10mm creepage. In this case the 10 mm figure would prevail. Other internal apparatus requirements would be determined during the design of apparatus. There are many permutations in any individual design of apparatus. The requirements of different standards do vary slightly; this should be understood before apparatus design layout is finalized before certification.

Two independent Ex i circuit termination systems, for example, on interfaces or in junction boxes would require minimum creepage and clearance distances of 6mm between circuits and 3mm to earth because this is stated as a requirement in the IEC Standard.

Comparative tracking index (CTI)

The insulation quality of materials must also meet minimum requirements as stated in table 8.6. The Comparative Tracking Index (CTI) is used to measure the effectiveness of a given material to resist surface breakdown and allow tracking from one conductor to another over the surface of the material. The test is detailed in IEC112 and is used in the testing throughout the electrical equipment industry and particularly with Ex e components (see Chapter 6).

Two electrodes are positioned on the surface of an insulating material at a fixed distance apart. A voltage is applied and a salt-water solution is allowed to fall between the electrodes to encourage the track effect to start. The voltage and the number of drops used to cause breakdown are recorded as a measure of the index value.

This test is only of paramount importance to apparatus designers but not to those who select apparatus for use in hazardous areas, since the requirement will have been met at the apparatus design and manufacturing stage.

8.11 Enclosures

Ex i apparatus does not need any enclosure to be part of the method of protection. The Standard requires that:

“where intrinsic safety can be impaired by access to conducting parts, an enclosure of at least IP20 shall be provided as part of the apparatus under test”.

The function of an enclosure is not therefore to provide mechanical strength, but to prevent inadvertent shorting or earthing of the current carrying conductors of circuits. Occurrences of this on the same circuit will not cause concern but where two or more circuits share an enclosure, the combination of the circuits could pose a greater risk.

The IP20 requirement is equivalent to allowing access of a terminal with a small screwdriver but not allowing the termination to be touched by the human finger. Thus termination must be deliberate.

Impact testing is not required for Ex i equipment. In the cases of the more mechanical methods of protection, degradation could be caused by the ingress of solids or water if an enclosure suffered a damaged seal. There is no concern for Ex i equipment.

8.12 Temperature

The maximum temperature reached by components (safety or otherwise) within any piece of hazardous area apparatus requires detailed assessment.

During the operation of a circuit, heat will be developed from all components in which power is dissipated. The rise in temperature of the surface of the components, which may come into contact with a flammable mixture, will be measured to ensure that the T rating required (or to be achieved) is not exceeded.

This condition must be true under normal operation and under specified fault conditions for the apparatus under test. All components are normally examined. Heating from one component must not be permitted to affect another, particularly if it is a safety component. This assessment requires the judgement of skilled and experienced test personnel to interpret and apply the standards. Testing may reveal areas where apparatus needs to be redesigned or the layout changed in order to improve the acceptability of the design.

There are clearer guidelines for the definition of temperature ratings in the IEC60079-11:2011 standard than there have been in previous or other standards. This was accepted from revisions to the EN50020: 1994. It clarifies a previously difficult area. Methods of assessing the ratings of conductors and copper track-work on PCBs used in certified apparatus are now provided. Tables give ratings that the designer must work to when aiming for T4, T5 or T6 ratings.

There is a ‘relaxation’ of the rules for ‘small components’, which requires some explanation. It is recognized that where the surface area of a component is small, even though the surface area may reach the ignition temperature of a gas/air mixture, tests have shown that the heat transfer from the component to the gas is insufficient to allow ignition to take place. The standards give options under which high temperatures in small components are permitted.

For apparatus to be rated T4, the total surface area of a component, excluding lead wires must be less than 20mm2 for a permitted temperature rise up to 275°C. Above 20 mm2 the power dissipation must be a maximum of 1.3 Watts. Greater than 20 mm2 but less than 10 cm2, the limit of 200°C is given. This assumes 40°C ambient and further de-rating factors apply with higher ambient temperatures. T5 and T6 rating guidance are also given.

8.13 The Ex i systems concept

In addition to the Construction Standard IEC60079-11:2011, another Standard IEC60079-25:2010, Intrinsically Safe Systems, must be complied with. In this Standard it states the definition of a system:

“An assembly of interconnected items of electrical apparatus described in a descriptive system document in which the circuits or parts of circuits intended to be used in a potentially explosive atmosphere are intrinsically safe circuits.”

This definition was taken originally from European Standard EN50039 which first formalised the need to consider the safety of a system in 1980.

8.14 An Ex i ‘system’

A ‘system’ may comprise two or more pieces of Ex i equipment connected together in some way as the part of an instrument loop into the hazardous area. Where two Ex i devices become connected together, then the combined capability of those individual devices to cause ignition must be assessed to ensure that the system complies with certain installation requirements.

This includes the interconnecting wiring, which has, in itself, the ability to store energy. An illustration of a system is shown in Figure 8.32 for the purposes of discussion.

8.14.1 System certification

Many industry standard ‘systems’ are covered by formal ‘system’ certification, i.e. process transmitters working with specific interfaces. This is where a manufacturer has Ex i equipment tested for safety compatibility with other specific pieces of Ex i Certified Equipment (of his manufacture or of other manufacturers). A Testing Authority formally certifies the combination of each set of equipment on a ‘Systems Certificate’.

This will stipulate any special conditions that are to be met for safety purposes but will include maximum permitted cable parameters. The installation can only be considered safe if it has complied with all the requirements of Intrinsic Safety Standards.

The acquisition of System Certificates is not mandatory; some manufacturers do not provide system certification for a number of reasons which would require explanations outside the bounds of this Course and Manual.

8.14.2 System descriptive document (DSD)

The onus is placed ultimately on the user to ensure that the installation conditions have been met and that adequate proof, including any calculations and measurements necessary, to demonstrate safety are recorded on a DSD according to the requirements of Standard IEC60079-25:2010.

This can only be done on the completed installation where all the variables are known. The purpose of the DSD is to demonstrate how an Ex i system, in which a number of pieces of certified Ex i equipment are interconnected including the cabling, conforms to the system requirements and is therefore considered to be safe. The document is vital for any personnel commissioning, inspecting or maintaining such equipment.

8.15 Assessment of safety

The assessment of safety is performed by a comparison of the Inputs and Outputs of the apparatus. The electrical connections to a piece of certified apparatus in the hazardous area may expect to receive energy. The certification of the apparatus will dictate how much energy can be received for the apparatus to remain safe.

8.15.1 Safety description

Associated apparatus, such as an interface, to which the certified apparatus is to be connected, may expect to give out electrical energy. The certification of this apparatus will dictate what the highest values will be under fault conditions. Since this device will provide the source of energy to the circuit, it will specify what the maximum energy storage parameters can be. These values are compared to the values seen at the power-receiving end added to the values, which are introduced into the circuit by the interconnecting cable (see Figure 8.31).

If, according to the appropriate certification, the interface cannot provide higher output values than the equipment in the hazardous area is permitted to receive and the energy storage and cable parameters fall within the limits set by the associated equipment, then the combination is considered safe.

The Standard terms that describe the outputs & inputs parameters are as follows:

Output values: (from a ‘power source’, i.e. Associated Equipment): ‘Safety Description’:-

Where UO, IO, PO, is the maximum output of voltage, current and power respectively.
Where CO, LO, is the maximum permitted values of capacitance and inductance respectively that may be safely connected to the output of the Associated Equipment comprising other apparatus and cabling.

Note: UMAX, This specifies the highest safe area supply voltage to the associated apparatus, normally 250 Vac rms.

Input values: ( to a ‘power receiver’, i.e. Certified Equipment): ‘Entity Parameters’:-

Where VI, II, PI, is the highest acceptable input value for V, I & P respectively.
Where C i, L i,i is the effective value of input C & L, respectively, that appears at the input terminals to this apparatus input.
Figure 8.31
System concept

The ‘input C & L’ is sometimes written as C eq, L eq, Where C eq, L eq, is the effective equivalent value of input C & L, respectively, that appears at the input terminals. It is equivalent in that it must be added to the cable parameters. The Installation Standard and the System Standard explains how this must be done and the variants according to system complexity.

‘Entity’ parameters are sometimes referred to as ‘Modular’ parameters in North America.

8.16 Simple apparatus

Simple apparatus can appear in a system in one of three forms. These are introduced here but discussed more fully in the context of applications, subsequently:

8.16.1 Passive simple apparatus

The passive form requires no further explanation because no energy contribution whatever can be made from this type. Most system certificates expressly permit the system to comprise Simple Apparatus without restriction.

8.16.2 Other (non-passive) simple apparatus

Where there can be stored energy in Simple Apparatus, IEC 60079-14:2013 Clause 16 requires some assessment of the total cumulative effect. The treatment of this in a system depends on the type of device. Generally, if there is only one device in a system, then it is not considered significant.

8.16.3 Simple apparatus equivalence

Some apparatus is designed to present a ‘non-energy storing’ connection into an Ex i Circuit. It is said to be ‘like simple apparatus’. This useful facility is discussed in the section on Applications. It is sometimes referred to as having simple apparatus equivalent inputs or outputs. The apparatus is certified and the apparatus certificate will dictate what the maximum figures presented to the circuit will be These are normally 1.5V, 15Ω, 100mA with any L & C contribution stated energy storage. If a number of devices are used in the same circuit then the combination of devices will require assessment.

8.17 Safety parameters

It is possible that terminals on a piece of apparatus can both receive and source electrical values under different conditions. It is usual for these to be quoted; for example a temperature transmitter connection to a thermocouple will have both input Entity Parameter and an output Safety Description. The use of these requires some expertise to correctly apply the figures (see Figure 8.32).

Figure 8.32
System analysis

In the scheme shown in Figure 8.32, an Ex i System is shown comprising, say, a 4/20 mA Temperature Transmitter and its associated equipment. Some detailed analysis is required to understand the safety and operational implications.
The Ex i concept generally only looks at the safety aspects of electrical apparatus and is not concerned with its operation. The user must satisfy him that the circuit will perform the task for which it is designed.

8.18 Temperature classification of systems

With many intrinsically safe applications T4 may be awarded automatically where the matched power dissipation is less than 1.3 watts. Simple apparatus devices, which cannot dissipate power (because resistance is zero or infinite), are automatically awarded T6.

The system takes up the worst case temperature classification of all the apparatus used in the hazardous area loop. The interface does not possess a T rating because it is safe area mounted but the system certificate or documentation must consider the power dissipation.

Normally, T4 ratings for systems are adequate and almost invariably offered. There are very few vapours with ignition temperatures below 135°C.

8.19 Systems concepts in other standards

The systems concept in IEC60079-25 requires the generation of a DSD described above. Many other countries’ Standards did not publish separate ‘systems’ guidance, but it is often, to some extent, included in the Apparatus Standard. A typical example of this is Australian standard AS2380.7 clause 2.9, which is entitled ‘Ex i electrical systems’. It requires the “electrical parameters and all characteristics of the interconnecting wiring …to be specified in a descriptive system document or ‘Block diagram’.” It goes on to clearly specify that circuits in Zone 0 must use multicore cables with individual pair screens. Other minor differences are stated.

This, like many other standards, acknowledges the need to consider system phenomena but not with as much clarity as does the harmonised and adopted IEC standard.

8.19.1 Systems approach in North America

The approach in the US and Canada is similar in approach with a different name.

The ‘entity concept’ provides the rules for the permitted interconnection of approved apparatus. Apparatus is approved with entity parameters, which must be matched in the same way as for the IEC requirements. However, the values for FM/UL/CSA approved apparatus are slightly different from the CENELEC specification owing to the different interpretation of the V/I characteristics. The same graphs are used but, for example, reversed voltages are considered as additional to the main barrier channel voltage.

In other ways the same rules apply. All apparatus must have defined parameters even if they are non-energy storing. Simple Apparatus is now accepted.

The safety of a system requires description on a Control Drawing issued by the manufacturer.

8.20 Conclusion

In this chapter Ex i equipment and systems have been introduced and explained. The idea is relatively simple but the implementation requires a considerably more detailed explanation owing to the subtle concept of fault and infallibility analysis which gives this protection the highest grade of integrity.

The equipment design is done by the manufacturer but the System concept is of greater importance to the user and, as it involves operational consideration, it is the subject of a training course in its own right due to complexity.


9


Protection Concept Ex p

In this chapter we look at the type of protection that uses an enclosure to contain a protective gas which surrounds electrical equipment such that it is separated from contact with any hazardous gas outside the enclosure. This is known as a ‘segregation’ technique. This type of Ex protection has the great benefit that it can accommodate virtually any electrical equipment with little or no requirements placed on the equipment inside an enclosure. Maintaining the condition inside the enclosure requires a monitoring system.

Learning objectives

  • To examine the versatility of this protection type
  • To understand the necessity for additional equipment
  • To study the required interlock system
  • To examine operator action on failure

9.1 Name

There are many names associated with the concept of Ex p. Sometimes referred to (rightly or wrongly) as Pressurisation, Purging, Leakage-Compensation, Ventilation, Continuous-Flow and Inert-gas-blanketing. The basic concept itself is quite simple but the implementation requires a more detailed study because the technique requires additional equipment integrated into the overall design to provide a system that is safe for a given set of conditions. There are a number of variations of this type of protection to cope with different conditions. The most common of these approaches which are compliant with the Standards will be explained in this section, but others implementations are possible.

9.2 Standards

Current

  • IEC60079-2

Old

  • BS5501: Part 3
  • EN50016

9.2.1 Definitions

The IEC Standards define Ex p as:

  • The technique of applying a protective gas to an enclosure in order to prevent the formation of an explosive atmosphere inside the enclosure by maintaining an overpressure against the surrounding atmosphere, and where necessary using dilution.

The older European Standard stated:-

  • A type of protection in which the entry of a surrounding atmosphere into the enclosure of the electrical equipment is prevented by maintaining, inside the said enclosure, a protective gas at a higher pressure than that of the surrounding atmosphere. The overpressure is maintained with or without a continuous flow of the protective gas.

A ‘protective gas’ in this definition is described as any gas which cannot contain any element of flammable material. Air is preferred owing to ease of availability and handling, but an inert gas such as nitrogen or any non-reactive gas may be used.

‘Over-pressure’ (inside the enclosure) is a condition of the internal pressure to be established and maintained. This is the pressure inside the enclosure that is above that of the atmosphere surrounding the enclosure. It is not describing an un-desirable condition in this context.

The introduction to the subject in the Standard adds to the understanding by qualifying the above definition:-

This part of IEC 60079 gives requirements for the design, construction, testing and marking of electrical equipment for use in explosive atmospheres in which
  1. a protective gas maintained at a pressure above that of the external atmosphere is used to guard against the formation of an explosive gas atmosphere within enclosures which do not contain an internal source of release of flammable gas or vapour;
  2. a protective gas maintained at a pressure above that of the external atmosphere is used to guard against the formation of an explosive gas atmosphere within enclosures and is supplied to an enclosure containing one or more internal sources of release in order to guard against the formation of an explosive gas atmosphere; or
  3. a protective gas maintained at a pressure above that of the external atmosphere, is used to prevent the entry of combustible dust which might otherwise lead to the formation of an explosive dust atmosphere within enclosures, but only where there is no internal source of release of combustible dust.
This standard includes requirements for the equipment and its associated equipment including the inlet and exhaust ducts, and also for the auxiliary control equipment necessary to ensure that pressurization and/or dilution is established and maintained.

In the current IEC60079-2, the placement of an Ex p enclosure determines the designation of the type of protection and the necessary design criteria. Such arrangements are designated Ex px, Ex py or Ex pz and are defined below. The precise safety requirements depend on the properties and functions of the equipment and specify pressure and flow rate monitoring needs. This is expanded in further tables later in the section.

Type of Pressurisation Definition
px pressurization that reduces the classification within the pressurised enclosure from Zone 1 to non-hazardous or Group I to non-hazardous
py pressurization that reduces the classification within the pressurised enclosure from Zone 1 to Zone 2
Pz pressurization that reduces the classification within the pressurised enclosure from Zone 2 to non-hazardous

9.3 Principle of operation

The basic principle of operation is shown in Figure 9.1.

Figure 9.1
Ex p concept

The Hazardous Atmosphere surrounding ignition-capable electrical equipment is reliably replaced by the protective gas, contained in an enclosure of appropriate size and construction, thereby creating and maintaining an artificial safe area, as depicted in Figure 9.1.

The word ‘maintaining’ in the definition is key to the integrity of the technique. It has to be established before it can be maintained. To achieve this condition there must be a mechanism, such as a fan, to provide a low-pressure protective gas supply to the enclosure. In addition, there must be a means of ensuring that the overpressure condition is established whilst ignition-capable equipment inside is energised. Failure of the protective gas supply could mean that the safety of the protection technique was compromised.

9.3.1 Protective gas

Air, as the most common protective gas used in practice as it is the most readily available media. It must be drawn from a safe area in which there is no likelihood that flammable gasses can be present and that the air is always available. Such a place of air availability is known as a ‘controlled safe area’.

The air must be blown into the enclosure in such a way as to provide adequate overpressure. The degree of overpressure depends on a number of conditions in the enclosure.

The protective gas may not be induced to flow or sucked through otherwise a lower internal pressure will result. This would enable the surrounding atmosphere to enter into the enclosure through any point of access.

9.3.2 Overpressure level

The pressure inside an enclosure is specified in the Standards as a minimum of 50 Pascals (Pa) which is approximately 0.5mbar or ¼ inch water gauge (w.g.). The pressure need only be relatively low.

There is no realistic limit to the size of the enclosure in the Standards. It must be born in mind that the larger the enclosure the higher the forces owing to the increased surface area upon which the overpressure is acting. The strength of the enclosure must be assessed as safe if the outlet or vent becomes blocked, so pressure regulation in the form of relief valves or panels may be necessary to incorporate into the enclosure design.

There are no special requirements for the material of manufacture of the enclosure. Steel, stainless-steel, fibre glass, plastic sheet etc are all acceptable and would depend on the proposed location and service for the enclosure. Clearly there is a risk of static generation with non-metallic materials so due precaution is to be taken by the avoidance of the use of solvents and the rubbing of cleaning materials.

9.3.3 Operation

There are three separate stages to the operation of an Ex p arrangement that will be described during this discussion. These are generally referred to as:-

  • The Initial Purge
  • System Operation
  • Failure Shutdown

The Initial ‘Purge’
This starting phase is necessary because with this type of protection it must be assumed that worst case conditions are catered for the in the design and implementation. When the enclosure containing the unprotected electrical equipment is first secured shut, the content is assumed to be of 100% of the gas that is expected to be in the hazardous area. Before equipment inside is energised an initial purge is performed to ensure that the atmosphere inside the enclosure is not flammable. This can be achieved by a complete purge or a partial purge to dilute the gas below its LFL.

The length of time this process takes depends on:

  • the rate at which the protective gas is supplied
  • the volume of the enclosure
  • the capacity of the ducting, inlet and outlet
  • the positions of inlet and outlet in the enclosure
  • the relative vapour density of the hazardous gas in which the enclosure will be mounted
  • the internal layout of equipment providing obstacles to flow
  • the pressure re-distribution owing to moving parts

There is a minimum requirement of 5 volume changes before the internal equipment is energised. The actual amount time taken to purge the enclosure must be determined by calculation and testing.

The initial purging routine must be carefully examined such that no pockets of gas are left in the enclosure. The volume and shape of the enclosure and the properties of the hazardous gas must be taken into consideration when determining where to site the inlet and outlets.

There will be additional equipment monitoring the status of the Ex p enclosure. It will be located in the hazardous area, normally, and so will require appropriate Ex protection. A timer which must be set up as a result of the type testing to establish the purge time is included in the control and interlock system so that internal equipment may not be energised until time out has been reached.

Operation
Only when safe conditions are established inside the enclosure, the electrical supply to the internal equipment may be energised and operation may be started. The Ex p system and the equipment so protected now relies on the maintenance of the non-flammable atmosphere presence in the enclosure. The control system will monitor:-

  • the status of the Flow-switch
  • access doors/panels for indication of opening
  • pressure relief arrangement status, and
  • any other criteria determined by the testing authority

Shutdown
Should the monitoring equipment detect failure of air supply then it must take appropriate and specified action. These will be discussed in due course

9.4 Purging

Where a flow of protective gas is pumped through a enclosure, this is referred to as Purging, Continuous Purge or Continuous Flow.

9.4.1 Purging system

Basic purging is arranged as shown in the diagram Figure 9.2. A flow switch, set to a predetermined minimum flow-rate, establishes that an adequate movement of the protective gas is continually present. The switch is part of the interlock system which performs the required monitoring functions. It will detect when the flow falls below an acceptable level which will trigger an alarm. Response to this alarm will be discussed for all Ex p systems later in this section.

The equipment inside the enclosure will emit some heat. The Temperature Rating of the system is based on testing of the outside surfaces of the enclosure and the inlet/exhaust ducting with the equipment inside operating under its normal conditions. Group II would be assigned as there is no emission of energy on the outside.

Figure 9.2
Ex p purging – Continuous Flow

The movement of air through the Ex p arrangement could be relied upon to cool the equipment. Should the flow fail, the temperature inside and by implication on the outside of the enclosure will rise. It must not exceed the T rating which must be chosen to take account of the range of operational conditions of the internal equipment.

The internal (Ex protected) equipment must be subject to correct eternal overload and short circuit protection. Electrical faults and associated equipment failure conditions must be considered as part of the process integrity assessment and affect plant operational safety. Over-temperature monitoring may be linked into the Ex p Interlock system under specified conditions.

9.4.2 Containment and limited release

Another common example of a purging or continuous-flow arrangement is where a gas analyser mounted in an Ex p enclosure is sampling flammable gas. Such a scheme is shown in Figure 9.3. If released into the enclosure, the vapour must be adequately diluted to below its lower flammable limit before discharge.

The arrangement through which the flammable gas passes (i.e. the pipelines and analyser) is described as ‘containment’ system. Depending on the technical design of this containment system and gas feed system, it is described as one of three possible options

  • infallible containment system (no release)
  • a limited release system with a predictable maximum release rate
  • an unlimited release system

With limited release, a sufficiently large volume of air is used to dilute the combustible gas outside a small “dilution area” so that an explosive atmosphere is unable to form. If the exhaust were to become blocked then the build up of gas inside the enclosure would eventually rise above its LFL and could be ignited by a source of ignition from the unprotected equipment. A limited release is one which is predictable under all conditions.

With an unlimited release (where it is unpredictable in quantity), overpressure must be created by an inert protective gas, which prevents oxygen from penetrating the enclosure.

Figure 9.3
Ex p purging – Continuous Flow

The flow of protective gas could be used for cooling in addition. Where this is the case, other conditions may determine minimum flow rates otherwise the Temperature Rating of the enclosure may be affected.

9.5 Pressurisation

An alternative approach, referred to as ‘Pressurisation’ or ‘Leakage Compensation’ is shown in figure 9.4. In this mode of operation, the initial purge is still required, but, on completion of the start-up stage, the exhaust outlet is closed so there is no movement of the protective gas. It will only make up sufficient air that has been lost to leakage; hence the name.

In this situation, more often used with inert gasses which are expensive and pose a risk in other ways, the protective gas supply may be reduced in pressure from the original purge pressure to a lower operating state pressure. This is usually controlled by the Interlock system.

It is also used where a common air supply is available for a number of applications and requires pressure reduction before entry into the enclosure. Pressure relief panels/valves are required for certain supply pressure levels to guard against regulator failure upstream.

Figure 9.4
Ex p Pressurisation – Leakage Compensation

9.6 Variations

There are many variations on this technique. Documentation should provide detailed information on the specific requirements for each application in order that maintenance can be effectively carried out without compromise to the safety limits designed and certified for the specific enclosure. Procedures for work on Ex p equipment must bear in mind the design constraints and the application of equipment in the cabinet before being drawn up.

There are applications requiring the sitting of unprotected electrical apparatus in a hazardous area where the Ex p method is the only viable option. It can be easily adapted for an infinite range of enclosure shapes and sizes. Certification permits application dependent variations provided they conform to the general principles laid out in the standard.

The use of inert gases such as nitrogen may be employed but there are other safety implications in using this. The uncontrolled discharge of nitrogen can pose a risk to personnel in that if the nitrogen content of breathable air is raised then it becomes toxic.

9.6.1 Static pressurisation

A variant of pressurisation is also known as. In this case the overpressure is created BEFORE the enclosure enters the hazardous area. The system is commissioned by charging the enclosure with protective gas and maintained solely by the sealing of the enclosure. No further quantity of protective gas will be supplied in the hazardous area. The protective gas must be inert. A maximum oxygen concentration of 1 per cent by volume is permitted. Measuring equipment should be used to check this on every charging process. Its main use is for test and measurement equipment which does not need to be or is not wanted to be left in the hazardous area longterm.

9.6.2 Rotating machines

In the case of Ex p housing rotating equipment, the rotation will cause a redistribution of pressure within the enclosure. The low pressure point (LP) will be close to the shaft whereas the higher pressure (HP) will be at the extreme edges of a centrifugal fan. The LP point must be raised above the atmospheric pressure on the outside of the enclosure by increasing the static pressure inside the enclosure as a whole. Testing must be performed to ensure that suction does not occur such that the hazardous gas can seep through the bearing. The bearing grease must not be sucked or blown out otherwise bearing failure will result rendering the Ex p arrangement useless as the shaft and casing attains a high temperature very quickly.

Figure 9.5
Ex p with rotating equipment

9.6.3 Examples of using pressurisation

Where a ‘protective gas’ through-flow is used, arrangements must be made to dispose of the exhaust gas and so may complicate an application or increase the costs. In the following cases examples are given of where this is not considered necessary:-

  • A totally enclosed motor situated in a hazardous area is sealed as far as possible and pressurised with air or inert gas.
  • An instrument cubicle (into which flammable materials are not introduced) situated in a hazardous area is sealed as far as possible and pressurised with air or inert gas.
  • The interior of a motor of the force-ventilated type situated in a hazardous area is maintained at a pressure above that outside the motor, the pressure being provided by the separate, fan supplying the motor cooling air which is drawn from a non-hazardous area.

9.6.4 Rooms and buildings

Although the same principle can be contemplated for rooms and buildings, the implementation to this Standard does not cover such applications. IEC60079-13:2010, Equipment protection by pressurised room’ covers this requirement. The problems are of human occupancy must be taken into account as regards air quality and comfort.

This part of IEC 60079 gives requirements for the design, construction, assessment and testing and marking of rooms protected by pressurization in:
a room located in an explosive gas atmosphere or explosive dust atmosphere hazardous area that does not include an internal source of a flammable substance;
a room located in an explosive gas atmosphere or explosive dust atmosphere hazardous area that includes an internal source of a flammable substance;
a room located in a non-hazardous area that includes an internal source of a flammable substance.
NOTE
If ventilation is used and pressurization is not used, then this part of IEC 60079 does not apply. The situation is covered by the requirements of IEC 60079-10-1.
A room may be a single room, multiple rooms, a complete building or a room contained within a building and includes inlet and outlet ducts. This part of IEC 60079 also includes requirements for associated equipment, safety devices and controls necessary to ensure that pressurization is established and maintained.
This part of IEC 60079 covers rooms or buildings that are constructed or assembled on site, which may be either on land or off-shore, designed to facilitate the entry of personnel and primarily intended for installation by an end-user and verification on site. The room may be located in an explosive gas atmosphere or a explosive dust atmosphere requiring equipment protection levels (EPL) Gb, Db, Gc or Dc.
This part of IEC 60079 does not specify the methods that may be required to ensure adequate air quality for personnel with regard to toxicity and temperature within the room.

The external surface temperature of a building is not normally a problem but ancillary equipment outside the building will require ex protection.

Access and exit is needed to variable degrees in a room or building so air loss will affect the internal pressure which must be compensated for otherwise the atmosphere becomes uncomfortable. Interlocking doors and HVAC systems must be developed that are not ignition capable.

  • Air-lock doors
  • Sealed windows
  • Sealed cable ducts, pipe-racks and other entry/exit points
  • Controlled air outlets
  • Adequate heating, ventilation and humidity control

Pressurized or purged rooms shall be suitably labelled to draw attention to their special nature.

9.6.5 Pressurising / Purging

This is a method of protection employing both pressurising and purging simultaneously. It would be necessary to monitor and regulate both pressure and flow for independent reasons. Two examples are given here.

A room in a hazardous area containing instruments analyzing flammable gas is provided with a flow of air sufficient to prevent a hazard arising from leakage inside the room. The air pressure inside the room, being above that of the surrounding atmosphere, also prevents the ingress of a flammable atmosphere from outside.

An instrument analyzing flammable gas in a hazardous area is enclosed and provided with a flow of inert gas sufficient to prevent a hazard within the instrument arising from internal leakage. The inert gas pressure, being above that of the surrounding atmosphere, also prevents the ingress of a flammable atmosphere from outside the instrument.

9.6.6 Variations not covered by the Standard

Forced ventilation is normally applied to a paint-spray booth to draw off all flammable vapours in order to reduce or prevent a hazard arising in the booth space. It also provides breathable air for the operators.

A coating machine head applying a flammable liquid is fitted with an extract hood to draw off all flammable vapours in order to reduce or prevent a hazard in the surrounding area.

Fume cupboards use induced ventilation to remove excess gas liberated from chemicals of which some may be flammable

A hopper handling a flammable dust is equipped with an extract hood to draw off airborne dust in order to reduce its accumulation in the surrounding area.

The above situations described are certainly where the use of moving air helps to provide protection but these cannot be described as Ex p because they cannot comply with the requirements of the Standard. Unfortunately, the term Ex p is wrongly associated with such installations out of ignorance.

9.6.7 Dusts

Pressurisation and purging in the context of this Standard is not considered to be a suitable type of protection for dust hazards. A layer of dust will be converted into the more easily ignitable cloud form as a result of movement of air from such an enclosure. Even using an inert gas could assist in the dust mixing with atmospheric air on the outside of an enclosure. Blown air systems are used for powder transport (dense phase) where no source of ignition must be present within the blown air system. Static electricity discharge can also become a threat where dry powder and low humidity conditions are encountered so precautions must extend to cater for the individual application conditions. Greater guidance is expected to emerge as a result of the current amalgamation of the IEC60079 and IEC61241 Standards.

9.7 Application notes

Some general points on applications are included here.

9.7.1 Protective gas medium

Air should be used in preference to inert gas because of the asphyxiation risk with the latter. An inert gas, however, will usually provide a higher degree of safeguarding than purging with air where a continuous dilution is used and so acceptable if used for small volume enclosures.

9.7.2 Sourcing of protective gas

The source of supply of air or inert gas must be reliable; if necessary standby systems should be provided. Duplication of facilities is possible but the use of bottled gas for standby purposes is normally acceptable for small to medium sizes of enclosure.

The source of air or inert gas shall at all times be free from all traces of flammable contaminant. Consideration should also be given to the need for drying or cleaning the air or inert gas.

9.7.3 Pressure relief

In pressurized rooms, the pressure should be of the order of 6mm water gauge. This is equivalent in force to a wind of 30km/h on a vertical face. The force necessary to push a door open or shut is considerable and access doors may need to be power assisted. The door assistance mechanisms may need to be protected.

The construction standards require the strength of the enclosure to be designed such that it will withstand certain over-pressure limits. The clean gas supply must be assessed for its ability to maintain pressure without over-pressure.

9.7.4 Live working

Live working in the presence of a flammable atmosphere is not permitted. If entry into an enclosure is required whilst live then Gas-free or hot-work certificates/permits must be obtained.

9.7.5 Isolation

The requirement to isolate the electrical supply under failure conditions has been stated Owing to the complexity of equipment protected by Ex p, there may be a number of sources of supply entering an enclosure. Each electrical source, be it a secondary supply or instrument circuit will require disconnection when pressure is lost. This should be by isolation provided in the safe area. Suitable arrangements for isolation may be mounted in the hazardous area if appropriately Ex protected.

9.7.6 Sparking devices

This type of protection can be applied to almost any equipment where sparks could be created during normal operation, such as a motor with slip rings or a commutator.

On a continuous-flow application, if internal equipment is being protected, which produces sparks during operation with an operating current of more than 10 A and a rated operational voltage of more than 275 V a.c., or 60 V d.c., then ‘spark and particle barriers’ must be provided for the air exhaust if into a hazardous area.

Inspection windows, if they are fitted in the enclosure of the Ex “p” cabinet, be subjected to thermal endurance test to cold and heat followed by an impact test. In some cases additional mechanical protection may be required over a window in the form of a grating to shield the window from excessive impact.

9.7.7 Ex p sub-types

The supply of clean air must be from a controlled safe area. The discharge during the purge has to be carefully considered on each installation because it is likely that contaminated gas put into a safe area will create a hazard, thereby complicating the HAC. Discharging will be into a similar controlled safe area where any gas that does escape will be diluted below its flammable limit and cannot accumulate so that concentration might rise. Under conditions that require vented into a hazardous area, it may require the fitting of a ‘flame trap’ or a ‘spark and particle barrier’.

9.7.8 MiniPurge’ systems

Traditional Ex p systems were originally manufactured as a suitable enclosure plus all the ancillary devices. These comprise Flow/Pressure switches, timers, contactors and interlock circuits pre mounted, as one complete assembly that is tested and certified for a particular application. Manufacturers now make available a bolt-on (or bolt-in) device, best described as chassis, as shown in Figure 9.6, on which all the localized equipment needed for purge control and monitoring is mounted in a convenient package. In the case of the unit shown it is effectively. The industry recognizes this by the name ‘MiniPurge’ which is proprietary to Expo Safety Systems Ltd in the UK. The certification states the Safety Codes as shown which can be interpreted using rules discussed in the certification chapter of this manual.

Figure 9.6
Ex p MiniPurge System

The important point is the statement on the Certificate which says: “This MiniPurge Control Unit shall be incorporated into equipment and the appropriate Conformity Assessment Procedures applied to the combination, as defined in 94/9/EC. This Certificate is not a certificate that covers the combination.”

It goes on to say that a list of routine (verification) tests shall be performed by the ‘manufacturer’. In this case the ‘manufacturer’ is the person who combines the MiniPurge unit with an enclosure to form an Ex p system, not the certificate holder. Many organisations purchase equipment and do not read or understand the conditions placed on the safety by the Apparatus Certification. When all the tests are done, including purge time, pressure testing and T Rating assessment, then the system can be set up properly at which point the Certificate requires the labelling of the complete enclosure with the proper safety code as he equipment would then be deemed Apparatus Certified.

9.8 Certification and documentation

The manufacturer must provide the operator with all the information necessary for safe operation in the form of state diagrams, flow diagrams etc. in a detailed instruction manual. The upper and lower limits and their tolerances for safety requirements should be specified by the manufacturer and noticed to the user. The functional sequences of the safety devices and the required response in case of malfunctions or faults should also be described. Procedures must be written to define how each installation may be operated, serviced and maintained.

The user will be responsible for alarm response procedures should pressure fail.

The manufacturer’s information will be subject to scrutiny by the Testing Authority who will eventually issue the Ex protection conformity certificate. Conditions on the certificate may be specified and if deemed ‘special’ then an X is placed after the certificate number. These conditions must be followed by the installer and user.

9.9 Pressure / flow failure

During operation of an Ex p system, if the pressure or flow fails, the Standard requires action to be taken dependent on specific conditions given in the table and notes below:

Classification of the area in which the pressurized room or enclosure is situated (Note a) Action required on pressure failure
  Non-sparking electrical apparatus in the room or enclosure (Note b) Normally-sparking electrical apparatus in the room or enclosure (Note c)
Zone 1 Pressure-failure alarm Pressure-failure interlock (Note d) and alarm
Zone 2 Nil (pressurisation unnecessary) Pressure-failure alarm

Notes:
(a) Pressurising is not considered to provide a sufficient safeguard in Zone 0.
(b) ‘Non-sparking’ is used here in the general sense and includes non-incendive equipment.
(c) ‘Normally-sparking’ denotes equipment which in normal is ignition capable.
(d) If Automatic switch-off would introduce a more dangerous condition, other precautionary measures should be taken, for example duplication of protective gas supply.

Action on pressure failure must be incorporated into a procedure followed by the plant operators and depends on the level of risk prevalent when the failure occurs. Where an Ex p system located in Zone 1 houses ignition-capable equipment, failure of the pressure or flow must raise an alarm and should de-energise the equipment immediately. In practice, however, the Standard, in note d, requires a risk assessment to determine if it would be safe to de-energise the internal equipment at the same time. If switch-off would place the plant in greater jeopardy then the action taken must be to bring the plant to a safe condition i.e., under a controlled shut-down, within a set period of time.
If the enclosure houses ignition-capable equipment in Zone 2 then it is only necessary to raise an alarm as is the case where internal equipment is made non-incendive in its own right. Anti condensation Ex n heaters on Ex p motors are a good example.

The following methods are acceptable under general considerations for monitoring pressurised and purged systems:

  • A pressure-sensing device detecting the pressure in the room or enclosure.
  • A flow-sensing device (where appropriate) located in the outlet piping detecting the flow through the room or enclosure.
  • A pressure-sensing device detecting the pressure in the inlet piping.
  • A rotation-sensing device (where appropriate) on the pressurizing or extraction fan.
  • A fan contactor auxiliary switch (where appropriate).

Combinations of these must be considered where it is possible for them to indicate inappropriate status: the principles of fail safety apply in design.

Where more than one enclosure is pressurised or purged from a common header special care shall be taken in the selection and positioning of the monitoring devices.

9.9.1 Special applications

Whilst many of the principles outlined above apply to apparatus such as instruments analyzing flammable gas and the rooms which contain them, further reference should be made to standards and to specialist codes of practice on the subject to arrive at most economical way of protection in combination with other methods.

A typical example of this technique as applied to a control panel is illustrated in Figure 9.4.

Figure 9.4
Switch rack being protected by Ex ‘p’

9.9.2 Testing

All the requirements of the type tests on which the manufacturer’s declaration of conformity is based are as follows:

  • Verification of maximum overpressure
  • Tightness test
  • Purging test and timing
  • Verification of minimum overpressure
  • Verification of internal pressure limitation
  • Tests on the actuators and inspection windows
  • The temperature test

The routine tests to be performed by the manufacturer essentially comprise the functional and tightness tests.

9.10 Ex p protection type px, py and pz

The determination of the pressurization type is made from the table below:-

IEC60079-2 (Table 1)
Determination of protection type
Flammable substance in the containment system External zone classification Enclosure contains Ignition-capable equipment Enclosure does not contain ignition capable
No containment system 1 Type pxa Type py
No containment system 2 Type pz No pressurization required
Gas/vapour 1 Type pxa Type py
Gas/vapour 2 Type px (and Ignition- capable equipment is not located in the dilution area) Type pyb
Liquid 1 Type pxa (inert)c Type py
Liquid 2 Type pz (inert)c No pressurization requiredd
NOTE If the flammable substance is a liquid, normal release is never permitted.
a Protection type px also applies to Group I.
b If no normal release; see annex E.
cThe protective gas shall be inert if *(inert)’ is shown after the pressurization type; see clause 13.
d Protection by pressurisation is not required since it is considered unlikely that a fault causing a release of liquid will simultaneously occur with a fault in the equipment that would provide an ignition source.

As a result, the design requirements are laid out in the following table:-

IEC60079-2
(Table 2)
Design criteria based upon protection type
Design criteria Type px Type py Type pz with Indicator Type pz with alarm
Degree of enclosure protection according to IEC 60529 or
IEC 60034-5
IP4X minimum IP4X minimum IP4X minimum IP3X minimum
Resistance of enclosure to impact IEC 60079-0, table 4 IEC 60079-0, table 4 IEC 60079-0, table 4 IEC 60079-0, half the value in table 4
Verifying purge period Requires a timing device and monitoring of pressure and flow Timing and flow marked Timing and flow marked Timing and flow marked
Preventing incandescent particles from exiting a normally closed relief vent into a zone 1 area Spark and particle barrier required, see 5.8 unless incandescent particles not normally produced No requirement (note 1) Spark and particle barrier required, see 5.8 unless incandescent particles not normally produced Spark and particle barrier required, see 5.8 unless incandescent particles not normally produced
Preventing incandescent particles from exiting a normally closed relief vent into a zone 2 area No requirement (note 2) No requirement (note 2) No requirement (note 2) No requirement (note 2)
Preventing incandescent particles from exiting a vent open to a zone 1 area in normal operation Spark and particle barrier required, see 5.8 Spark and particle barrier required, see 5.8 Spark and particle barrier required, see 5.8 Spark and particle barrier required, see 5.8
Preventing incandescent particles from exiting a vent open to a zone 2 area in normal operation Spark and particle barrier required, see 5.8 unless incandescent particles not normally produced No requirement (note 1) Spark and particle barrier required, see 5.8 unless incandescent particles not normally produced Spark and particle barrier required, see 5.8 unless incandescent particles not normally produced
Door / Cover requiring a tool to open Warning, see 5.3 and 6.2 b) ii) Warning, see 5.3 (note 1) Warning, see 5.3 and 6.2 b) ii) Warning, see 5.3 and 6.2 b) ii)
Door / Cover not requiring a tool to open Interlock, see 7.12 (no internal hot parts) Warning, see 5.3 (note 1) No requirement (note 3) No requirement (note 3)
Internal hot parts that require a cool-down period before opening enclosure Warning, see 5.3 and 6.2 b) ii) Not applicable Warning, see 5.3 and 6.2 b) ii) Warning, see 5.3 and 6.2 b) ii)
NOTE 1 Subclause 6.2 b) ii) is not applicable for type py since neither hot internal parts nor normally created incandescent particles are permitted.
NOTE 2 There is no requirement for spark and particle barriers since in abnormal operation, where the relief vent opens, it Is unlikely that the external atmosphere is within the explosive limits.
NOTE 3 There is no requirement for marking or tool accessibility on a pz enclosure since in normal operation the enclosure is pressurized with all covers and doors in place. If a cover or door is removed, it is unlikely that the atmosphere is within the explosive limits.

9.11 Conclusion

The type of protection-pressurized apparatus p provides an internationally recognized method of explosion protection, which has proved its worth for special tasks over many years.

With the completion of international standards and the precisely defined requirements for Zone 2 applications – including, where appropriate, dust explosion protection – this type of protection will retain and increase its significance. The great advantage of this type of protection is that it places no (or only a few) additional requirements on the equipment installed inside the enclosure. It is therefore extremely versatile in use, including also special applications.

Lastly, it will be appropriate to state that,
‘Type of protection pressurized apparatus “p” is a practicable solution for explosion protection for more complex electrical equipment.’

This technique has been used in the past to place VDUs in hazardous areas. Access for maintenance may require equipment to be taken into a safe area, as the rules do not permit live working. The technique is expensive to install, operate and maintain. Air must be cleaned and pumped and there is a cost associated with this.

The system is expensive to operate and maintain because the clean air must be pumped and controlled by other equipment exposed to the hazardous area. The major benefit is that it can be used on very small enclosures up to complete control rooms. The problems associated with this tend to complicate area classification. A guaranteed gas free air supply must be maintained so it must be piped in from a safe area. Disposal of used air, if it is likely to contain gas during the initial purge, must be handled such that it does not convert a safe area to a hazardous area.


10


Other types of Ex Protection and their use in Combination

In this chapter we look at the other types of protection that are not so common yet can be used in certain applications where they provide a technical or practical advantage. After that we look at the combination of uses of the protection types to solve further application problems.

Learning objectives

  • To complete the study of all types of protection
  • To be able to compare the subtle differences between the types
  • To look at combinations and how this helps the user

10.1 General

Types of protection, Ex d, e, i, p and n, already described, are probably the principle techniques used for Ex equipment. There are additional types; o, q, m and s, for which Local, National and International construction Standards have been written and so find their use in specific applications. These, when used in conjunction with one or more of the popular methods of protection, can really lead to economically safe solutions to some specialist applications for which they are best suited.

These will be discussed further in the following pages.

10.2 Ex o

This is one of the ‘separation’ methods where liquid is used as medium to exclude a flammable atmosphere from a source of ignition. The Ex o method, shown in Figure 10.1, was originally conceived for high power equipment. It provides explosion protection on a similar basis to Ex p where liquid replaces the gas as a separation medium.

10.2.1 Name

Ex o derives its name from the oil that was originally used as the filling medium in the 2007 issue of the Standard. The name used in the current Standard is ‘liquid filling’ which permits use of a greater range of liquid types.

10.2.2 Standards

IEC: IEC 60079-6:2015
European Union: EN 50 015
Germany (old) VDE 0170/0171 T. 2

10.2.3 Definition

The definition of Ex o protection concept is:

‘A type of protection in which the electrical apparatus or parts of the electrical apparatus are immersed in a protective liquid in such a way that an explosive atmosphere which may be above the liquid or outside the enclosure cannot be ignited.’

 

10.2.4 Principles

As the definition suggests, a suitable container houses electrical equipment which is then immersed in a liquid. The presence of the liquid permits moving parts such as the contacts of a circuit breaker and if appropriately chosen, will quench any arc produced. The liquid may also assist in the cooling of the equipment by convection.

In line with the addition of Equipment Protection Levels the marking must state Ex ob as EPLb applies but certification may only permit equipment to be Ex oc limited to Zone 2.

10.2.5 Applications

Applications were originally conceived for higher power equipment such as power transformers where it would help to dissipate heat as well as enhance the insulation properties of the windings. Switch applications were also devised where the arcs produced were quenched, preserving the life of the contact breakers.

The choice of the liquid and its properties is critical because it must not give off toxic or hazardous fumes when subjected to heat and arcing conditions; it must be matched to the application in its capacity to cool, quench, insulate and inhibit corrosion. Unfortunately, low cost oils have silicon-based additives which are highly detrimental to platinum gas detection elements. This liquid filling type is therefore to be avoided on sites handling hydrocarbons where pellistor-type gas detection is common.

Its use in instrumentation is minimal with only one known application; that of the protection of circuit boards in a PLC system.

Figure 10.1
Ex o: Liquid immersion diagram

10.2.6 Requirements

As shown in Figure 10.1, the enclosure must provide a way of detecting the liquid level has not fallen below a predetermined position and a simple dip-stick arrangement, periodically read sufficed. More sophisticated level detection is acceptable on the current issue of the Standard.

Cable glands were made off below the level of the liquid and had to be oil leak resistant so expertly made off as this was the primary reason for loss of liquid.

The technique was confined to Zone 2 but if designed to the latest IEC Standards it is permitted in Zone 1 with constraints on maximum electrical values.

10.3 Ex q

This type of protection uses a semi-solid or particle-based filler to achieve separation of the source of ignition from a flammable atmosphere.

10.3.1 Name

Ex q takes its name from quartz which was used as the early filling material. It is also known as sand-filling as that has been used in some applications, mainly mining.

10.3.2 Standards

  • IEC: IEC 60079-5
  • European Union: EN 50 017
  • Germany: (old) VDE 0170/0171 T.4

10.3.3 Definition

The Standard defines this concept as:

‘A type of protection in which the parts capable of igniting an explosive atmosphere are fixed in position and completely surrounded by filling material to prevent the ignition of an external explosive atmosphere.’

 

10.3.4 Principles

Electrical equipment is placed inside a container which is filled with a semisolid medium. Sand or silica-based (quartz) beads were used in some early mining applications such as the termination boxes of telephones. More recently a mineral insulating powder such as that used in Mineral Insulated Copper Cable (MICC) is used. Unlike pressurisation and oil immersion, this type of protection does not permit moving parts.

The technique ensures that if a conductor breaks within the enclosure, the resulting spark cannot be surrounded by a flammable atmosphere. Similarly, separation from conducting surfaces which become excessively hot is also achieved.

Where a gas/air mixture can get between the particles of the filling, testing prior to the issue of the Standards proved that the small quantity available would not allow sufficient energy transfer to take place to allow combustion. The filling can also help to conduct away some excess heat. The Temperature Rating is assessed on the outside surface of the enclosure containing the filling.

10.3.5 Application

Such a type of protection is rarely used on its own because it will be necessary to terminate electrical cabling, passing conductors between inside and outside the enclosure. This is often achieved by the use of Ex e terminals. Ex e fluorescent luminaries design often employs running gear comprising chokes, starters and power factor correction capacitors in an Ex q protected enclosure. These application require Ex e component certification and would be Ex eq and the issued certificate would be numbered, “/U”.

Figure 10.2
Ex q: Sand filled

10.3.6 Requirements

When a design is required to meet EPLb (for mounting in Zone 1) the most recent issue of the Standard restricts the use of this technique to 16 A, 1000 VA and 1000 V. In previous issues the type of protection was limited to use in Zone 2.

More modern uses of the technique using powder employ a metal canister with a lid that is crimped on to provide a good seal. This is to prevent moisture being absorbed by the powder which might otherwise provide a low resistance path across electrical circuits when dampness ingresses.

10.4 Ex m

Encapsulation uses a solid medium to prevent flammable gases from reaching a potential source of ignition as opposed to separation by gas, liquid and semi-solid as in Ex p, o and q respectively.

10.4.1 Name

Ex m originally took its name from moulding which was formalised when Epoxy Resin technology became more helpful. It then became known more as ‘Encapsulation’ in line with language used to describe the application of the resins.

10.4.2 Standards

  • IEC 60079-18
  • European Union: EN 50 028
  • Germany: (old) DIN VDE 0170/0171 T.9
  • Australia: AS 2431

10.4.3 Definition

The Standard defines this concept as:

‘Protection of electrical components by enclosure in a resin in such a way that an explosive atmosphere cannot be ignited during operation by either sparking or overheating which may occur within the encapsulation.’

10.4.4 Principles

Apparatus certified Ex m was formerly treated as ‘Special’ Ex s owing to the lack of a suitable Standard. IEC 60079-18 permits apparatus to be encapsulated in compounds of various types in order to prevent a flammable gas atmosphere coming into contact with excessive heat. Sparks cannot normally occur in such circumstances and some rules for prevention must be obeyed. The technique can be of great benefit in providing robustness and reliability. Encapsulating components provides for greater shock resistance, reduced environmental effects and better rejection of chemical attack.

Repair of Ex m apparatus is precluded since it is almost impossible to reclaim encapsulated components without damage.

The method is not often used on its own but combined with other methods to solve application problems. The technique is often used with Ex i where apparatus is certified Ex m, but a distinction should be made between the potting of components to comply with Ex i requirements and certified encapsulation of apparatus in its own right. This is a subtle difference and requires detailed knowledge of a certification application to determine why it has been necessary to certify Ex m. ‘Potting’ is often used as a way of raising the integrity of limiting components fitted across energy storing components in Ex i circuits.

Figure 10.3
Ex m: Encapsulation

10.4.5 Requirements

There are two ways by which the Standard permits its manufacture. Where cavities are formed to allow moving parts then the use of an outside enclosure/container into which the resin is poured is normally required. Alternatively, the resin itself can form the outer surface of the equipment by allowing the resin to cure and then releasing the hardened assembly from the mold.

The T Rating applies on the outside surface. Group II (without A, B or C) is assigned by default as the equipment is suitable for all gasses as no energy is emitted.

The choice of resins available having different mechanical, thermal, chemical and elasticity properties is very wide. Thermally conducting resins aid heat dissipation from the encapsulated equipment.

Electrical connection is normally by flying leads which are anchored within the moulded assembly. The other end of the cable is usually kept short, additionally mechanically protected using flexible conduit and terminated in an Ex e enclosure if the application is in Zone 1.

10.5 Ex s

Where electrical equipment can be shown to be safe in a given hazardous atmosphere but does not conform to any of the other recognised techniques of protection, then it was ‘approved’ for use and was said to be ‘Special’ protection, Ex ‘s’.

10.5.1 Name

The term ‘approved’ was applied to ‘apparatus’ which had previously been assessed by Her Majesty’s Factory Inspectorate (HMFI) in the UK for use in specified flammable atmospheres. This was prior to 30th September 1969. There were no British Standards at that time to which the equipment could be assessed.

Manufacturers were developing instrumentation equipment in the mid 1960s for which there was no safety Standard covering the particular principle on which the argument for safety was based. Any assessment was subsequently performed by the British Approvals Service for Electrical Equipment in Flammable Atmospheres (BASEEFA) which operated under the auspices of the HMFI.

The original ‘special’ term subsequently fell into line with the developing regime of the use of the letters ‘Ex’ to depict explosion protection type followed by the letter to identified the specific type and so it became known as Ex ‘s’.

10.5.2 Standards

IEC 60079-33:2012: Equipment protection by special protection ‘s’

Previously:
BASEEFA: SFA (meaning: Standard For Assessment) 3009

10.5.3 Definition

SFA3009 defines this type of protection as:

‘A concept which has been adopted to permit the certification of those types of electrical apparatus which, by their nature, do not comply with the constructional or other requirements specified for apparatus with established types of protection, but which nevertheless can be shown, where necessary by test, to be suitable for use in prescribed Zones or hazardous areas.’

The Current IEC Standard Part 33 defines it as:-

A concept to allow design, assessment and testing of equipment that cannot be fully assessed within a recognized type of protection or combination of recognized types of protection because of functional or operational limitations, but which can be demonstrated to provide the necessary equipment protection level (EPL)

10.5.4 Principles

By its very nature, testing and assessment to special protection type ‘s’ cannot be as prescriptive as for other techniques. It is anticipated that considerable dialogue is required between the manufacturer and an ‘independent verifier’, i.e., an authority competent to provide verification (which may be an individual or an organisation). Additional tests may be required by an independent verifier to ensure the relevant level of safety is achieved.

It is recognised that Explosion Protection of electrical equipment is generally achieved through one or more of the following methods of protection:

  • containment of internal explosion;
  • exclusion of explosive atmosphere;
  • avoidance of ignition source;
  • energy limitation, both sparking and thermal;
  • dilution.

Special protection, ‘s’, uses one or a combination of these methods, and the verification shall identify the methods of protection used and how the implementation for each has been achieved. The independent verifier/s shall ensure all applicable requirements of IEC 60079-0 and those other parts of the IEC 60079 series relating to the recognized types of protection identified for the equipment are met (except as varied in specific Clauses 8, 9, 10 and 11).

There are very few pieces of equipment certified to the new IEC Standard at the time of writing.

10.5.5 Marking

The SFA3009 document stated that equipment was suitable for Zone 1 or Zone 2 use when marked ‘Ex s’. Where specifically tested for Zone 0 use it would be marked ‘Ex s (Zone 0)’

The current IEC Standard, Part 33, uses the Equipment Protection Level designations, ‘sa’, ‘sb’ or ‘sc’ to declare permissible location of use.

10.5.6 Application

The SFA3009 document permitted a range of equipment to be approved or certified such as:-

  • a battery hand lamp (torch)
  • a combined compressed air driven generator and lamp
  • a factory-sealed fluorescent hand lamp with flexible cable
  • a potted solenoid for valve operation complete with cable
  • ‘Pellistor’ type (platinum element) gas detector

It shall be noted that care is taken to obtain the correct dry cells for torches as the use of high-power cells may invalidate the approval or certification.

When selecting apparatus special care shall be taken to ensure that the apparatus and its component parts are constructed to guard against electrical and mechanical failure in the intended condition of use.

Particular attention shall be given to the need for weatherproofing and protection against corrosion.

An example of Ex s, complying with the SFA3009, written by BASEEFA in 1970, is described in figure 10.2. This arrangement forms a hydrocarbon gas detector element. It comprises a platinum coil through which a heating current passes that is housed in a small chamber. The chamber is open at one end and covered with a sintered metal disc. Sintering is the process of fusing small spheres together. The space between the spheres does allow the hydrocarbon gas and air to pass into the chamber. The hydrocarbon then reacts with the surface of the heated platinum and, by a process known as dis-association, raises the temperature of the platinum. The increase in temperature provides an increase in electrical resistance proportional to the increase in concentration for which gas the detector head has been calibrated. If the heat is excessive and the gas ignites then the sintered disk acts as a flame-trap and prevents the propagation of the flame to the outside atmosphere. This is in the same way as the Davy Lamp’s ‘gauze’ effect works. Thus, the technique but can be shown to be safe for the duty it is designed for.

The selection and installation Standard, IEC 60079-14:2013 now permits the use of equipment certified as Ex s with the added EPL letter ‘a’, ‘b’ or ‘c’ used for the appropriate Zone.

The outcome of more recent thinking is that some of the techniques previously certified or approved as Ex s are in fact sub-types of existing Standards. These have been modified to accommodate the origin technical concept and so Ex d now includes for gas detection using sintering and encapsulation is now dealt with under the Ex m Standard.

10.5.6 Standards for Ex s

Many countries had adopted the SFA3009 document, or at least the concept outlined in the document, for example, Australian Standard AS 1826 originally dealt with this protection concept but it was replaced by AS/NZS 2380 Series.

Figure 10.4
Ex s gas detection arrangement

10.6 Multiple-certification

From the discussion on the principles of the various types of protection, it will be realized that not all techniques are suitable in all circumstances. Indeed, not all techniques are available in all circumstances. It is becoming more commonplace to certify apparatus with more than one type where advantages are provided or where difficulties are overcome.

In Figure 10.5 the arrangement of a gas detector using three types of protection is shown to illustrate how the designer has employed these to allow the flexibility of construction, operation and ease of maintenance.

Figure 10.5
Gas detector with multiple certification

The gas detector is usually mounted in Zone 1 or Zone 2.

The detector head normally requires a relatively high current of 300-500 mA. The supply needed to the electronics at this current does not permit the system to be made completely Ex i.

A permanent Ex e supply is acceptable for mounting in Zone 1 or 2. The Ex e terminations are used to provide the supply to an Ex m encapsulated module containing the electronics for ranging and communication. The electronic module has an associated apparatus function to provide power and signalling to the indicator unit. This includes switches to function test the module. The indicator may be locally mounted as an integral part of the apparatus or may be remotely mounted from the detector head.

The high current detector head has a barrier-like arrangement, specially designed for the current required (at a low voltage). The advantage of this configuration is that, using the live working capability of IS, the detector head (and the indicator) may be removed and replaced with minimum down-time and without risk of causing ignition if a flammable atmosphere were to be present.

The most common type of protection for switchgear is Ex d where contacts will produce arcs and sparks. Many manufacturers have certified a motor frame as Ex d but the attached terminal housing is rated Ex e simply because the cost of manufacture (and the cost of installation and maintenance) would be lower. The motor would be certified Ex de.

10.6.1 Order of letters

In the case of an Ex e enclosure housing a small volume Ex d switch the certification might state that it was Ex ed or Ex de. Originally, Certifying Authorities would specify the order of the letters for the type of protection used in order of importance given the design of the certified equipment. There is some sensible logic to this approach except that this sometimes led to different opinions as to what was important. The Standards now require that the types of protection are listed in alphabetical order. Ultimately, the Testing Authority will declare the order to be used on the marking of equipment as it may be logical and less confusing if the letters are sequenced to help the application. Where there is an Ex i connection into the hazardous area, with a Safety Description, then the bracketed form of ‘[ia]’ will be the final arrangement in the chain and might serve best to go last in the list.

10.6.2 Other examples of equipment

Modern Ex protected luminaries use a combination of several types of protection because it can improve the equipment performance and flexibility together with reducing the cost. Ex d, e, q and m are often found together.

Instrumentation equipment often uses Ex i for signal circuits but where higher power for the supply then Ex e is used. The internal electronics that cannot be made Ex i or e are protected using Ex m.

Ex p is never used on its own because it requires additional external equipment to monitor the status of the purging or pressurising protective gas within the enclosure. Such equipment is mounted adjacent to the Ex p enclosure so it is in the Hazardous Area and must therefore be Ex protected Thus Ex d or Ex i is often found. If the enclosure contains Ex i equipment then the marking would show Ex pd[ia] as shown in Figure 10.3

Figure 10.3
Combination of techniques giving Ex pd[ia]

10.7 Selection of certification method

The choice of the type(s) of protection available to a system designer when selecting apparatus to use in a hazardous area is based on a number of criteria. These are:

  • Apparatus group required
  • Zone of use
  • Power requirements of the equipment
  • Live working / calibration needs
  • Environmental consideration
  • Risk of damage

More criteria are discussed in the Chapter on installations. The decision may be exemplified by the following situation:

An instrument loop may be protected by Ex i or Ex d.

A status switch application is to be located in a position where there is considered to be an increased risk of mechanical damage (loading arms and jibs are examples of possible situations). In this case the preference would be for Ex i. This is because if damage occurred to an Ex d switch using mains voltage levels there would be an increased risk of explosion if the conductors became exposed and shorted. Personnel shock risk may also be increased under these circumstances. However, a proximity switch uses the properties of ordinary encapsulation to enhance its mechanical performance whilst being compatible with Ex i techniques. Against this approach is the fact that a proximity switch is more difficult to provide with mechanical protection. If the switch was damaged there is no risk of ignition but the switch would not be available to the control system. The so called ‘availability’ of the signal is reduced. However the choice of using a more robust design such a switch designed and certified as Ex d would not get damaged so easily. Thus it would provide a more reliable availability. The Ex d properties would not be part of the protection. It would be important to identify this fact when documenting and explaining why that type of switch was chosen. This reinforces the view that Function and Ex applications must be considered in parallel during plant design.

10.8 Equipment certified for dust

Equipment for use in a dust atmosphere often simply requires the protection provided by a suitable enclosure to prevent the ingress of dusts to potential sources of ignition within. The issue then is that dust settling on the outside of an enclosure will force the dissipated heat from within to accumulate thereby reaching a higher surface temperature.

Existing techniques discussed so far may be used for dust applications (see selection section in IEC60079-14:2013) but with some reservations. The possible release of pressure with Ex p and the consequential explosion pressure release of some Ex d flamepaths could convert a layer of dust into a flammable dust cloud thereby increasing the risk. The properties of dusts have been discussed in the section on flammable materials. It is now accepted that it is not sufficient to just use an enclosure with what is believed to be an adequate IP rating according to IEC60529 as the external layer thickness could have some significant effect on surface temperature elevation.

10.8.1 Name

The Ex t technique of protection is concerned with, and takes its marking letter from the temperature rise and the control of the surface temperature of the enclosure.

10.8.2 Standards for Ex t

IEC60079-31:2012 describes equipment in enclosures which are marked Ex t. This replaces Part 4 of Standard IEC 61241, now withdrawn, in which the marking Ex tD was first used.

10.8.3 Definition

The Standard defines Ex t in this way:

In the a type of protection for explosive dust atmospheres where electrical equipment is provided with an enclosure providing dust ingress protection and a means to limit surface temperatures

There are three EPLs for Group III applications:-

  • Level of Protection “ta” (EPL “Da”), or
  • Level of Protection “tb” (EPL “Db”), or
  • Level of Protection “tc” (EPL “Dc”).

10.8.4 Application

In the table below, the relationship between ‘Levels of protection’ and the Dust Groups are provided to show the required IP rating.

Level of protection IIIC IIIB IIIA
“ta” IP6X IP6X IP6X
“tb” IP6X IP6X IP5X
“tc” IP6X IP5X IP5X

Gas and vapour hazards have been considered extensively so far. Many of the types of protection can be used in dust hazards but with further considerations made. Equipment certified for dusts is now given EPL Da, b or c as discussed in the section on Classification systems. The use of zones do not correlate exactly for example Ex m is limited to Zone 1 but can be used in Zone 20.

Many enclosures are component certified for Ex e, Ex n and Ex t. The Standard, Part 31 does not call for component certification for Ex t because the only thing that determines acceptability is the surface temperature rise owing to the dissipation of the equipment that is placed inside.

Providing the seals are adequate and meet the general requirements of IEC60079-0:2012 then there is no issue. But care with installation must be taken and IEC60079-14:2014 must now be applied as it covers selection and installation in dusts.

10.9 Conclusion

The recognised types of protection have now been examined in varying degrees of detail and the reader should now understand the major and the sometimes quite subtle differences between them.

It is now important to reinforce that the type(s) of protection used on equipment must be known and understood by personnel working with the equipment so that the integrity of protection is never compromised.

The use of combined protection types by equipment manufacturers is set to increase as equipment becomes more complex and includes other facilities such as diagnostic capabilities which the user industry seeks.

There is, as yet, no guidance on dealing with equipment protected by multiple techniques. Each use of a type of protection must be considered on its own merits but within the context of the application.


11


Earthing and Bonding

In this chapter, the subject of Earthing and Bonding is examined. Ex protected electrical equipment installed in Hazardous Areas must be provided with power in a way that cannot compromise the type of protection. This includes appropriate earthing arrangements. Where current flows through earthing paths it must not cause heating or sparking effects interaction

Learning objectives

  • To study the reasons for earthing
  • To be aware of the difference between earthing and bonding
  • To understand when earth currents flow
  • To consider the potential problems in Hazardous Areas
  • To examine how systems must be controlled and documented

11.1 Earthing

Correct “Earthing” is primarily required for the assurance of general electrical safety, reducing the risks to both human life and installations. The relevance to Explosion Protection is covered thoroughly from the explosion point of view with some initial general discussion to ensure that the intentions of the earthing philosophy are fully understood. The principles of electrical earthing are agreed upon internationally, though there are differences as to how these principles are best achieved in practice.

Electricity supply regulations may state specific requirements but these do not conflict with the intent of the standards and codes of practice for IS and other explosion protected installations. This is a common misunderstanding of requirements.

There are many terms for ‘earth’ used in related electrical industries. The terms are imprecise and sometimes lead to confusion thereby causing questionable levels of safety. In this discussion it is necessary to clarify the terms by defining the intention of the specific electrical connections used.

Electrical “earthing” is required for five main purposes:

  • To reduce the risk of personnel shock
  • To operate electrical protective devices
  • To guard against lightning surges
  • To control electrostatic discharge
  • To minimise electrical interference

These aspects will be discussed in detail in the following text.

11.2 Personnel safety

The effects of electricity on humans depend on the level of current and where it enters and leaves the body. Research shows that the limbs have a resistance of about 500 Ohms. The central torso has a very low resistance value owing to the high water content. The effect of electricity penetrating the skin can be liked to the characteristics of a Zener diode with a reverse breakdown voltage of between 5 to 10 V. This depends on the individual’s skin characteristics and the tendency to dry or greasy skin.

Hence the equivalent circuit of a human can be drawn as in Figure 11.1.

Figure 11.1
Human electric circuit

Under worst-case conditions, the threshold of sensation is at about 1mA according to Health and Safety experts. At about 5 mA the resulting shock is said to be disturbing causing surprise in the recipient. This can cause the recipient to sustain injury in other ways.

Currents in the range 6-30 mA can cause temporary paralysis. This explains why humans cannot sometimes let go of conductors under shock conditions.

Between 1-5 A, ventricular fibrillation can occur. This means that the heart muscles may become out of synchronization and so the heart cannot pump blood around the body efficiently. A current of 10 A, through the heart, causes cardiac arrest.

The likelihood of high current density through the heart mostly occurs when the left hand and the right foot become the connection points. Hand to hand connection can still be fatal.

Comparing these current values with the circuit for the body, and taking the worst case of a mild shock being permissible (with a current of 4 mA), then the maximum voltage desirable to be encountered by humans is of the order of

10 V + 10 V + ( (500+500) x 4 mA ) V = 25 V

This explains and reinforces the popularity of 24 V supplies for instrument systems. Under these conditions it is permissible to work live on such low voltage equipment without fear of injury due to shock. In practice, most people do not feel anything below about 40 V. Instrument systems sometimes use 110 Vac configured as 55-0-55 with the secondary centre tapped to earth. This practice is American in origin but has not been internationally followed due to the popularity of 220/240 V equipment.

However, there must always be the concern that heat generated from low voltage systems at high currents can cause injury in the form of burns. So even low voltage systems must be treated with due respect.

This means that under normal or fault conditions on a plant, the resistance of return paths must be sufficiently low so as not to produce voltages greater than say 25 V peak. Prospective fault currents may be calculated with a target earth loop impedance to prove personnel safety aspects are satisfied.

11.3 Hazardous area considerations

Structural or fault currents arising from electrical equipment operating in hazardous areas must not become a source of heat or sparks. Equipment must be adequately earthed to ensure that connections are of high integrity and low impedance. This must be adequately maintained throughout the life of the installation and equipment.

Fault paths are well defined and it is vital that correct fusing and over-current detection / protection is applied to power circuits such that any fault is cleared in an appropriately short time.

There is equally the concern that high fault currents flowing, even during the brief time between the occurrence of the fault and the operation of the electrical protection, can cause elevated voltages to appear at other places in the hazardous area. Adequate isolation between circuits prevents this posing a risk. The isolation may need to be proved adequate periodically during the life of the plant.

From the curves and tabulations for gas ignition it will be realized that at less than 10 V, ignition by sparking becomes significantly less likely. If earth fault path impedance is kept below values, which will generate this voltage, then it follows that the power dissipated, and therefore the heating effect, must also be relatively low. Maintaining low values can be considered an acceptable risk. The quality of the earthing and bonding conductors on a plant must be a regular requirement for inspection.

Figure 11.2 shows a prospective short circuit current of 4000 A through a 0.005 Ohm resistance generating a 20 V elevation across conductors in the hazardous area. The power dissipated in the s/c resistance is 80 kW.

If the conductors in the hazardous area were also terminated in 0.005 Ohms, i.e. a short circuit path perhaps through the earth structure of the plant, the dissipation would be 40 kW. The heating effect would be dramatic if the fuse did not operate.

Figure 11.2
Hazardous area fault, voltage elevation

The 10 V level is used as a rule of thumb to judge whether the quality of a loop bond is adequate. In practice very low values can be achieved with good cross bonding. This is however often taken to the extreme and an excess of copper is used in unnecessary places. This has the effect of increasing the time and cost of maintenance.

11.4 The difference between ‘earthing’ and ‘bonding’

There is a subtle but essential difference between earthing and bonding, which must be understood.

Earthing” is where a low impedance path is provided in order for return currents to operate electrical protection devices such as fuses and over-current trips in an appropriately short time. This is shown in Figure 11.3.

Bonding” is where voltage differences between electrical conducting parts are eliminated. This is shown in Figure 11.4.

 

Figure 11.3
Earth path

The bonding of all the cases to the structure and the structure to the SPNE forms the bonding paths.

Figure 11.4
Bonding

International Electrical Supply Regulations (ESRs) that cover fixed electrical equipment and installations require that there be an earth return that is backed up by a physical connection to “terrestrial” earth. In this way there are two return paths acting in parallel, which enhances the integrity of an earthing system. One path is an earth path because its primary function is to conduct fault currents. The other connection is to ensure that significant voltage differences do not appear between devices. Refer to Figure 11.5.

Figure 11.5
Earthing and bonding in parallel

Terrestrial earth, in this context, is taken as the art of providing a large surface area of a conducting material (usually copper) that is buried beneath the earth’s surface and is in contact with soil such that the impedance between the conducting material and the soil is sufficiently low for the intended purpose of the connection. The structure of a plant is required to be bonded to the terrestrial earth. During the course of this discussion, these terms will retain these definitions.

The “Earth” and “Bond” conductors act in parallel. This is an advantage in that the two paths reduce the impedance. One path may be viewed as backing up the other lest it should fail.

Consider a fault where the live conductor to the primary of the load transformer touches the metal (conducting) case, as shown in Figure 11.4. The resultant current in the live must be returned to the Neutral of the supply transformer. This is in order that the circuit protection device works to clear the fault. The impedance of the distribution transformer Neutral to Structural earth is critical for safety. The connection from Structural earth to terrestrial earth is not critical when considering these faults.

11.4.1 Power distribution

Transmission of electrical power from generator to consumer is normally done at relatively high voltage levels in order to minimize losses. Typically, the supply to an industrial plant would be from a high voltage substation owned by the distribution company. This would feed a local low-voltage transformer on site or adjacent to it. Here the line voltage would be reduced to 220/230 Vac for general services such as lighting, office equipment and instrument systems. High voltages three-phase supplies for high power motors may be derived at the substation. 11 kV three-phase “Incomers” may be transformed down to 6.6 kV and 3.3 kV to 440 V depending on the plant power requirements.

Since the ground’s resistance through which earth fault currents may pass is uncontrollable, and may be relatively high, then the fault conditions cannot be adequately protected by fuses alone and “out-of-balance” current detection is used.

Transformers used in distribution systems are inevitably “double wound” which means that they have full electrical isolation between primary windings and secondary windings. The level of insulation will be tested to well above the working voltage of the transformer. With these high voltage transformers, the design and testing must be such that the occurrence of breakdown of the primary (passing on the high voltage to the secondary) is negligible. In this way the fault protection for each section is contained and there is no need to consider the effects of high voltage fault currents occurring in a lower voltage section. See Figure 11.6.

Figure 11.6
Distribution system supply

Electricity supply regulations (ESRs) require distribution transformers’ secondary Neutral (of a single phase system) or the Star-Point Neutral (of a three-phase system) to be bonded to a “substantial” earth mat. The way this is done will be defined in the specification of the local ES authority applicable. Conventionally the Neutral and the Earth Mat are connected together at a substantial un-insulated busbar in a main switch-room and is referred to as the Star-Point-Neutral/Earth busbar (SPN-E). This requirement dates back to the ESR in 1905, 1937 and 1944. It acts as a well-defined reference for the purposes of both earthing and bonding as discussed.

The accessibility and distance from the plant it is serving is sometimes a problem for which some solutions are offered later.

11.5 Static electricity

Static electrical charge is caused by the forced separation of molecules of non-conducting materials. Movement of the material or friction against the material can cause the charging effect. When wrenched apart, a surface charge builds up a high potential difference between the separated materials. The size of the charge depends on the violence with which this separation occurs. The non-conducting materials act as a dielectric and allow the accumulation and storage of the charge as in a capacitor.

11.5.1 Generation of static

This poses a risk when handling certain material such as hydrocarbons because they naturally have a low conductivity and static charges are easily generated. Since the vapour is flammable the sudden discharge of static can produce ignition.

Explosions and fires on Oil and Gas Tankers have occurred where static has been blamed as the cause. After a tank has been emptied, vapour may be present having been mixed with air that has been sucked into the tank as a result of emptying. This provides almost ideal conditions for static to cause ignition during the start of cleaning operations.

Drum filling is a good example to illustrate the principle, see Figure 11.7. Where the liquid enters the drum leaving the supply-pipe nozzle, the flow profile deposits a charge on the nozzle and the opposite charge accumulates in the drum depositing itself on the metal of the drum. The charge appears between the nozzle and the filling cap rim. To equalise the charge it is necessary to bond the drum to the nozzle such that the charge is dissipated. As the nozzle is moved towards the drum at the start of the filling cycle and away at the end this is where any initial PD may cause a spark. Bonding must be in place before and after the filling event to ensure safety.

Figure 11.7
Drum filling static risk

The connection to terrestrial earth is not fundamental to safety as is often misunderstood. It is important to identify where the charge will accumulate. It is preferable to bond the charge collecting conductors as close to the accumulation point as possible. This is not always practical as in the case of Road Tankers. Specific information on this is published by road tanker legislative organizations.

Techniques to control static involve the use of chemical additives to raise the conductivity of liquids. Static combs are used to collect and dissipate charges. Locally induced ionization and artificially raised humidity can assist in some circumstances.

Effective bonding is essential, but reducing pumping rates will also reduce static build up.

11.5.2 Lightning

Lightning is generated by the movement of water vapour inside clouds. Up-draughts of warm air in contact with down-draughts of cooler air in large volumes and close proximity generate a vast static charge that build up to an extremely high voltage. It cannot be dissipated because of the low conductivity of pure water vapour. When the accumulated charge becomes so great that it breaks down the dielectric strength of the vapour, the result is seen as lightning. In circumstances where the charge is built up between the cloud and the ground, the discharge occurs when a lightning “leader” ionizes the air between cloud and ground providing a lower resistance path for the majority of the charge to pass. The resultant current can be in the order of 20-100 KA peaks.

The current pulse shape has a very sharp leading edge and a much slower decay. Because the rate of rise of current is so high and it contains a high frequency element, the impedance of conductors becomes significant.

The current from lightning rises from 10% to 90% of peak in typically 8 µS and decays to about 50% after 20 µS from the 10% rise point. The pulse shape is said to be an 8/20 wave and this is used as the model for designing lightning/surge protection systems and calculating the effect of the surges on circuits.

Tall constructions on any plant are susceptible to direct strikes. Lightning protection conductors are installed, not in order to handle the vast currents that flow but to conduct substantial but smaller currents that discharge clouds and prevent higher voltages from developing to the point of breakdown. In these cases the impedance connection to earth is critical. The current multiplied by the impedance will cause a rise in potential above ground for the duration of the strike. The lower the impedance, the lower the potential. Clearly if other paths act in parallel with the structure then current will be shared and may travel through the plant to get to earth.

Figure 11.8
Static form of lightning

In Figure 11.8, showing a simplified plant diagram, a strike to building A will be conducted to earth but some current will be diverted through resistance r to the earth of building B. The majority of current will be encouraged to flow to earth via REA, which should be maintained as low as possible. The remaining current will flow via the distributed resistance in the interconnecting bonding of the building. A potential gradient will exist across the structure that must be kept low by good bonding.

An oil refinery is a good example of where there is an ignition hazard combined with susceptibility to lightning strikes. Tall refraction towers act as charge collectors as they approach the clouds. The plant areas are spread out with a network of interconnecting steel pipes. A strategy of good bonding is essential to minimise the risks if lightening is commonplace in the plant’s location.

There are two situations that cause concern for instrumentation systems.

  • Firstly, the effect of a lightning strike on a plant will undoubtedly induce surges in other metals in the vicinity of the building through inherent coupling capacitance and inductance. This includes cables. Instrument cables can be particularly susceptible to these surges. High potentials are liable to breakdown insulation and/or damage delicate instrumentation unless adequate precautions are taken.
  • Secondly, an instrument cable C can carry the full potential rise of the strike to building A across to building B and so the danger and the damage may not just be local but may be distributed.

11.6 Clean and dirty earthing

In any electrical system using ac supplies current will flow in earthing and bonding paths. These are unavoidable and have to be coped with in the course of devising strategies. There are two reasons for this.

11.6.1 Parasitic capacitance

The capacitance that exists between an electrical conductor operating at some voltage above earth and the surrounding conducting materials of structural metalwork is referred to as “Parasitic Capacitance”. In the example of the electric motor, given in Figure 11.9, the capacitance between the field windings and the metal frame would allow the motor frame to attain an elevated potential if the earth returns back to the SPNE remained unconnected.

Bonding the metalwork to the neutral of the distribution transformer eliminates any voltage elevation. However, it allows a small ac current to flow through the structure depending on the size of the capacitance, the applied voltage and the construction of the motor.

Any electrical equipment such as motors and transformers having this conduction path will exhibit these characteristics and will require bonding to earth. When the equipment is switched on the capacitance will charge up and an increased current will initially flow as a result. In steady state conditions the currents flowing in the structure of equipment may be substantial and they will become additive in bonding and earthing conductors.

Figure 11.9
Stray currents from parasitic capacitance effects

11.6.2 Fault currents

Where a “phase to frame” fault occurs, as in Figure 11.4, very high current will flow in the earth path in order to operate the feeder’s protective device. A high current dumped to earth during such a fault will cause a dramatic elevation in the potential at the point where the current enters the earthing system. This situation must be cleared quickly by the fuse or breaker.

The concern for equipment operating in hazardous areas is that the currents flowing in earth returns are not interrupted such that sparks can occur or terminations allowed deteriorating such that heat can be generated at that point. Earthing must be substantial and proven.

11.7 Electrical interference

Electrical noise can be generated by the same mechanisms as discussed earlier. Capacitive and inductive coupling effects are responsible for noise that is induced into sensitive measurement and instrument circuits in many cases. This is where there is inadequate separation of signals from outside interference.

In Figure 11.10, the parasitic capacitance between the primary and secondary of the mains transformer couples the mains voltage to the secondary circuit. The resultant voltage appears on the input to the signal amplifier. When the field wires are connected, the capacitive coupling to other structural earths provides a current that develops a series mode voltage on the amplifier. This manifests itself as noise on the input.

Figure 11.10
Internal parasitic capacitance effects

The cure for this is to take the following precautions, seen in Figure 11.11.

  • Earthing the electrostatic screen of the transformer will reduce the coupling capacitance effect and conduct stray currents back to the source of supply.
  • Referencing the 0 V rail of the input system to a common potential reference point such as the return of the source of supply will conduct any coupling currents through the power supply circuit back to that point.
  • The use of screened cable should be used in the field to connect the sensor to the input amplifier. The screen must be earthed at the same reference point to where all the stray currents are connected, as they are all then returned to the source of supply.
  • The screen of the signal cable must only be earthed at one point. If it is earthed at the field end, then currents derived from the parasitic capacitance in field equipment may try to use this as an additional return path.
  • The screen will conduct other capacitive-coupled currents away thereby not permitting them to invade the sensitive signal circuit.
Figure 11.11
Correct reference strategy for instrumentation.

The use of twin twisted cable for balanced systems will also improve the rejection of interference. If interference becomes coupled to one of the conductors then it will also be coupled to the other conductor by the same amount. The signal appears in series mode but the interference appears as common mode. It is presented to an amplifier across an inverting and a non-inverting input, which rejects the common mode and passes the series mode signal almost perfectly.

11.7.1 Interference rejection with barriers

The same criteria apply when preceding the input of a safe area mounted device with a barrier. The barrier busbar conveniently becomes the focal point for all earths associated with the signal circuits. Care must be taken to ensure that only signal paths are connected into this arrangement. Adopting the strict philosophy of earthing screens at one point, that being the barrier busbar, ensures that coupling currents converge and are returned to the SPNE consistently (see Figure 11.12).

Figure 11.12
Correct earthing of safe area systems

It must be realized that there is still current flowing in the IS earth albeit relatively small. Disconnection of the IS earth may not be indicated by noise appearing at the amplifier inputs but is likely to cause elevations in the common mode voltage seen in the circuit. This may be acceptable to the input amplifiers and the circuits generally, but on a large system it could become dangerous. This is because if the system became inadvertently earthed by a fault in the hazardous area, the o/c voltage and the sum of the s/c currents, due to leakage, may then be enough to cause heat or sparks.

Return currents that are allowed to invade signal paths may result in the signal becoming distorted or swamped by the noise. Clear segregation between earth arrangements and then connection at a common point will assist in de-coupling interaction.

11.8 Earthing terminology

Two common and more meaningful terms are of some help in distinguishing the different earth paths.

The term “dirty earth” is applied to the return path conductor that can carry the prospective fault currents from electrical equipment. As a result the conductor is likely to shift in potential by a greater amount (with respect to the neutral of the supply system which is the ultimate reference). Fault currents are inseparable from the parasitic capacitance currents, which will flow along the same route although they will be at a much lower in level.

The term “clean earth can only apply to return path conductors, which cannot carry fault currents from electrical equipment. This, by definition, is impossible to control or guarantee since any circuit to earth will conduct fault currents back to the return of the source of supply.

It is more correct and important to recognize that the name of the “Earth” path tends to be defined by what is connected to it. It is therefore more logical to describe the Earth by the circuit types from which it is gathering “stray” currents. Hence power, instrument, computer, structural and terrestrial earths are more meaningful names but their function requires clear definition in plant safety documentation so that their use is not misunderstood. There is little concern that the risk of ignition will be increased by incorrect earthing but the operational aspects of systems may be severely hampered.

11.8.1 Sneak paths

One of the problems encountered with carelessly referenced systems is the devious route that unsuspecting currents can take. These are difficult to find but essential to solve as they will cause interference and may pose a danger in the hazardous area. An example of this is where screens have been connected at both ends during plant commissioning. A new large motor was subsequently installed adjacent to the field mounted instruments and severe interference is seen on the instrumentation. The problem is that structural earth currents have been introduced into the instrument and IS circuits. These situations manifest themselves in curious way.

Attempts at solving sneak paths must be done with extreme caution as disconnecting unsuspecting earths may have unpredictable results. Ideally, the plant should be made safe before attempts are made. Otherwise a carefully thought-out strategy of approach is required. This means isolating and disconnecting each circuit in turn whilst noting any improvement seen until the problem is solved. Progressive improvement suggests there is a cumulative effect because a common point to that group of circuits is at the centre of the problem. Such situations are difficult to generalize.

11.8.2 Power faults with IS apparatus

IS apparatus in the hazardous area normally requires to meet an isolation test requirement of 500Vac applied to the internal circuits with respect to the case (which is normally connected to the Structural Earth). The insulation must not breakdown within 1minute. The purpose of this test is to ensure adequate isolation during power fault conditions whilst elevated voltages may appear across this insulation. Invasion of power must not occur on one IS circuit which could elevate voltages and pose an increased risk on other circuits. This can happen in a number of circumstances discussed as follows.

Figure 11.13
Power fault to safe area channel

In the situation shown in Figure 11.13, where a power fault touches the signal line in a simple installation, the fuse in the barrier is the weakest link and will undoubtedly blow before the distribution fuse. Assume a current of 1 A flows before the fuse ruptures. If the resistance of the IS earth is 1 Ohm then the voltage elevation (Potential Difference, PD) seen at the instrument earthy terminal will be 10 V.

The PD would exist only for the duration between the fault occurring and the fuse rupturing to clear it. In this example there would be no serious risk due to the margins built in to the safety characteristics.

If no isolation were found in the hazardous area instrument then the fault current would share through the Structural return path. This would not be unsafe because the current is still limited by the CLR in the Barrier.

Figure 11.14
Power fault to earthy terminals

In Figure 11.14, the fault touches the earthy conductor of the safe area barrier circuit. The current is not limited by the barrier in any way and is dependent on the loop resistance it encounters. The distribution fuse may be 5 A and the peak current may be say 100 A before fuse rupture. The elevation on the busbar could peak at 100 V if the IS earth was 1 Ohm. Clearly the PD generated could be higher or reaching close to the limit of the supply momentarily.

In a third situation, shown in Figure 11.15, an earth fault may occur in the hazardous area as shown:

Figure 11.15
Hazardous area power fault

It could be argued that this failure is more likely to occur than the other two situations. This is because the mechanical construction and reliability of low power instrumentation in cabinets is higher than plant mounted high voltage and power equipment such as motors which run at higher temperatures where breakdown of insulation is more likely to occur. The fault current flowing can be much higher in this situation and the structural earth resistance through which it must flow will determine the PD experienced across the earthy line and structural earth. 500 V is internationally accepted as being a realistic peak value needed to protect the integrity of the IS circuit under this condition.

11.9 Connection of earthing systems

The points discussed so far can be summarized into current earthing practice where it can be seen why the philosophy of earthing has evolved to that required in the Standards and Codes of Practice.

11.9.1 Typical earthing arrangement

The diagram in Figure 11.16, illustrates a simple arrangement showing effectively 5 separate types of “earth” connection converging at the SPNE.

Figure 11.16
Earthing strategy
  • The structural earth may not be a physical cable but nevertheless requires representation in this diagram, as it is a part of the bonding system.
  • The dirty earth bar will collect all the earth returns from armoured cables to power equipment and circuits in the hazardous area. The fan out to other equipment may use sub-busbars but all eventually converge on one dirty earth bar that is ultimately connected back to the SPNE.
  • The chassis or cabinets of instrumentation equipment will inevitably be connected to structural earth via the steel-work of the plant construction. Cross-bonding between equipment is necessary to ensure that harmful potentials cannot develop. It happens as a natural process when equipment is mounted and bolted to frames that support other equipment. Where there is a risk that bonding may be inadequate devices are strapped with bonding conductors but this is sometimes taken to unnecessary extremes. It is therefore quite natural that the “dirty” and “structural” earths appear very similar. This will be discussed more in detail later.
  • The IS earth goes directly to the SPNE and must not be connected to any other earth system.
  • The Instrument system mains earth in this example is used to terminate all the supply earths, which would normally be commoned together as part of the interference control strategy. This bar should not be used for any signal earths.
  • The instrument cabinet earth connects the chassis of the instrument cabinet(s) to the chassis and metalwork of all the equipment as part of the bonding required for personnel protection. This is distinct from the instrument system mains earth because some equipment manufacturers specifically used not to internally connect the chassis to the mains earth. This practise is unfortunately changing but its disadvantage is that it reduces the control over interference. As the quality of insulation and components used in modern industrial equipment rises then this does become less important and these two earths may be more conveniently treated as one.

Switch mode power supplies are becoming more common in safe area use but their great disadvantage is that they are considerably noisier than analogue types. It is most important that they are correctly earthed to the mains supply earth for safety and noise control reasons as the filter capacitor network on the mains terminals must have a return path to earth.

Double insulated equipment mounted in plastic cases is rarely used in safe area industrial applications because it cannot meet EMC requirements. Referencing of the supply systems is still important.

11.9.2 IS earth specification

The IEC 60079-14 in line with most Codes of Practice requires that the barrier busbar earth is connected back to the earth reference (usually the SPNE) busbar in not less than 4 mm 2 copper (or equivalent) cable with a resistance of less than 1 Ohm. The IS earth must not be connected to any other earth system and must be identified as an IS earth. See section 7 of this manual.

11.9.3 IS earth installation

It has become an almost industry standard approach to use a pair of 10 mm 2 cables for the IS earth connection. The resistance of this is extremely low at about 0.005 Ohms for a 100 metre run. The method of identification is not specified and no one way of doing it has emerged as preferred. One suggested way was to wrap the cable pair with turns of blue insulating tape every half metre. This is seen on some plants but whatever method is chosen should be clearly identified by the plant safety documentation. See the Installation section.

11.9.4 IS earth testing

It is becoming common industrial practice and is highly recommended that earth circuits be connected using twin parallel conductors.

Figure 11.17
IS earth testing

In Figure 11.17, the IS earth is linked to the SPNE by two conductors. In this way the loop integrity can be easily checked by breaking one conductor and measuring or monitoring the loop resistance. Separate connections for each of the two conductors onto the busbars at each end effectively duplicates the connection thereby lowering the overall resistance and raising the integrity by including redundancy. This same strategy can be followed for all the busbar interconnections.

IS earths from other cabinets may be connected by looping through. This is acceptable provided that the integrity of the system as a whole can be proven. If connection using a ‘tree’ structure is used then each interconnecting branch must be proved to the next one in the chain and the furthest branch must still meet the requirement of connection back to the SPNE with a resistance of less than 1 Ohm.

11.9.5 500 V isolation test

The ability to withstand a 500 Vrms test for an IS circuit conductor has become internationally accepted as the fundamental requirement for circuit isolation. The conductors of IS circuits in a cable must be separated from Earth and from any other IS Circuit by the ability to withstand a 500 Vrms test for one minute. Where this requirement cannot be met, the installation conditions may require a special approach under certain countries’ codes of practice. This is discussed more in the application section where examples are given of the accepted techniques for this approach.

11.10 Power supply systems

Where supply systems other than from national mains distribution are associated with Intrinsic Safety then due regard must be given to the earthing integration. Locally generated supplies will be treated in a similar manner to ac mains distributed power and will be subject to the ESR discussed.

11.10.1 Offshore earthing

Offshore regulations permit the IS earth to be local to the barriers because the all-steel structure on which the whole installation is mounted is accepted as having such a predictably low earth resistance throughout the platform that no significant voltage rises will occur during fault conditions.

Figure 11.18
Earthing strategy offshore

The regulations stipulate that an IS earth cannot be connected to the deck of the platform closer than 2 m from the point where the generator Neutral-Earth connection is made, as shown in Figure 11.18.

11.10.2 Earthing skid packages

Self-contained skids built for installations must provide electrical systems that are compatible with the installation into which it is to be placed. If barriers are used then the IS earth should be made available for connection to existing IS earthing systems. In some cases the manufacturers may have earthed the barriers to the steel-work structure of the skid. If the application is for offshore then this may be acceptable but for onshore situations the IS earth must be kept separate as indeed should mains earths and signal screens so that they may be terminated in the proper place at the safe area end (see Figure 11.19).

Figure 11.19
Earthing on skids

11.10.3 Earthing of battery systems

Battery power supplies of the various types are usually required to be fully floating. The prospective short circuit current from batteries is extremely high and a great deal of damage can be done to batteries and the installation under unchecked short circuit conditions. Including protection devices is desirable but reduces the supply integrity. Battery backing is normally specified for reliability and so fault mechanisms are considered and designed for in order to give the highest supply integrity.

By operating batteries without reference to earth (floating), a single fault to earth of one terminal would not cause disruption of the supply. Earthing the batteries on one side would mean that single fault of a second connection to earth (of the other polarity) would provide short circuit conditions.

Figure 11.20
Battery supplied systems

Operation of barriers requires the referencing to earth and this may not suit the supply system being battery backed. If the batteries supply is used via an inverter system that has full isolation then there is no problem. If the batteries are used directly then it is possible to use barriers but some compromises have to be made. Figure 11.20 shows some possible options.

Galvanic isolators are therefore preferred for use with battery supplies for the obvious advantage that no earth referencing is required for safety purposes.

11.10.4 IS earthing on ships

The UK’s IEE (Institution of Electrical Engineers) issues ‘The Blue Book’, currently the sixth edition, 1990, entitled, “Regulations for Electrical and electronic equipment of ships with recommended practice for their implementation”. Section 23 contains the regulations for power generation and distribution on tankers carrying flammable materials. They do not permit the hull to be used as a return path for fault currents. The generation and distribution systems are fully floating and earth leakage monitoring is used on every section to assist in maintaining the integrity of supply. If, on one phase, a single fault to earth occurs, then fuses or circuit breakers do not disrupt the supply. Also, the fault can sometimes be cleared without disruption to supplies.

IS earthing is permitted provided that it does not defeat the supply arrangements required. This is acceptable as adequate isolation is normally included upstream of barrier systems between the distribution secondary, the neutral of which is permitted to be ‘referenced’ to earth.

Lloyds Register of Shipping and other internationally accepted organizations produce documents, which think along the same lines. Such documents are not internationally agreed upon and in line with IEC thinking. This may come in time.

11.11 Portable equipment using batteries

The risk of mains invasion is completely removed from portable equipment that operates solely on internal batteries. Measuring and calibration equipment need only be treated as having Safety Parameters that require examination when connecting into a system. This is dealt with in the section on applications and systems.

11.12 Earthing arrangement standard solutions

Provided that the simple strategies explained above are used, problems encountered in designing and specifying IS earthing systems on installation can be overcome. Difficulties often arise when existing plants are extended. In the following section various philosophies are discussed to bring together some of the principles into practical use.

Figure 11.21
Typical system distribution showing earthing arrangements

The simplest and most usual case, seen in Figure 11.21, on small installations is where the local transformer is relatively near to the control equipment and the cabinet earth can be conveniently taken back to the SPNE via the supply cable armouring. The IS earth goes back via two dedicated conductors.

The cabinet’s internal distribution system has all the mains earths collected together onto a Mains Earth busbar. Note that in the hazardous area, the armour of the cable is connected, at both ends, to structural earths. The screens are only earthed at one point, being the barrier busbar.

Figure 11.22
IS earth direct to local ground

In Figure 11.22, the same arrangement is used but the IS earth is direct to a buried electrode. This demonstrates that soil resistance becomes part of the 1 Ohm earth resistance argument. It is difficult to justify the acceptance of this because the resistance between the ground rods is no longer under the direct control of the user.

Figure 11.23
The intermediate transformer situation

The diagram shown in Figure 11.23 gives the arrangement used for the case where an intermediate transformer is used to supply the panel. If the transformer is merely to step down the voltage for the instrument system then it may not have the neutral earth connection made. In which case the upstream feeder transformer must still be the point to which the IS earth must be taken. This is normally only encountered on older installation, which have not been modified.

Figure 11.24
Local distribution transformer included with local IS earth

In Figure 11.24, the distance between the main feeder transformer and the panel is too long to tie to the IS earth. In another instance the supply may be delta and not star, perhaps because it is the only convenient feeder for the panel location.

The creation of a localized IS earth can be justified by the inclusion of an isolating transformer at which the secondary neutral is connected to an earth mat. Under these circumstances it is preferable to use out-of-balance current detection techniques to clear faults on the feeder because low impedance fault paths to earth may not be seen by long distance supplies fuse. The Out-of-Balance current detection would detect that the line current was not equal to the return neutral current and would therefore break the circuit.

This solution is recommended when ships such as tankers are to be fitted with IS barriers and equipment. It is also very useful in many other remote situations such as RTU outstations.

11.13 Earth loops

In Figure 11.15, showing a typical earthing arrangement, the connection between the structural earth and dirty earth busbar, which then meet at the SPNE bar, do form a closed loop. This loop will not introduce interference unless there were to be a strong magnetic field somewhere in the centre of it.

The mechanism causing interference in a loop is one of induction from a changing magnetic field that falls within the area of the loop. Currents are induced into the loop by magnetic coupling in series mode. A loop formed around a device that possesses a varying magnetic field, such as a transformer or motor, will cause induction in that loop. Higher frequencies will couple to a loop more efficiently than lower ones.

It is unlikely that sufficient energy can be coupled into a looped circuit such that it can become a source of ignition. The coupling would need to be optimised to do this as in RF heating systems. It is more likely that interference will be injected into loops that form part of sensitive measuring circuits.

Provided that the loop area between the conductors is kept as small as possible then no ill effects should be observed as in the use of twin-twisted cables. Thus induction to one conductor is most likely to induce equally on to the other conductor and the series mode induction is dealt with as common mode at the input to a differential or balanced-input amplifier. This is in the same way as capacitive coupled circuits can be arranged to reject noise.

Screening also helps to de-couple inductive pick up because eddy currents are induced into the screen and are conducted away. The eddy currents also oppose the magnetic effect that is inducing them, and this effect also helps to reduce the coupling to the signal on the wire.

11.14 Computer earthing

The electrical noise generated by the fast switching digital electronics in computer systems must kept away from sensitive analogue circuits. The provision for earthing these systems in order to assist in the noise control is fundamental to their successful operation. Computer control system manufacturers often specify a computer earth as yet another dedicated earth path that requires integration into the strategy.

Reconciling the earthing arrangements for computers can become complicated and a detailed knowledge of the internal supply and signal routing is necessary before the best arrangements can be made. If the computer system requires IS interfaces then the choice of devices may be influenced by the ease of accommodation of the earthing arrangements.

Generally there are two situations. Firstly if the computer I/O is fully and properly isolated then there should be no conflict in the earthing arrangements and barriers may be used with ease. This is often the cheapest solution.

If the computer I/O is not isolated then IS isolators are preferable because no safety earth is required and the isolators provide some flexibility for extending the capability of the computer system.

PLCs are not greatly affected by this discussion since they are relatively small and self-contained. They are normally installed in close proximity to the barriers and other associated supplies and equipment. DCS are more spread out and require more careful rationalisation.

Figure 11.25
Computer referenced to IS system earth

The connection XY, in Figure 11.25, may carry substantial noise and return currents from Switch and Analogue circuits to its power supplies. Its resistance must be kept as low as possible, preferably less than 0.1 Ω. The IS Earth will not carry the instrumentation currents but is likely to have some larger noise currents flowing due to the presence of the computer. This is the preferred connection way from an IS point of view.

If the manufacturer specifies that the computer system must be connected directly to earth then the arrangement below is acceptable with known limitations.

Figure 11.26
IS system earth referenced to computer system

In Figure 11.26 the computer earth becomes part of the protective earth route. It will probably carry substantial return currents (from X to Y) for the instrumentation. The resistance of the barrier busbar to SPNE link is therefore difficult to monitor because of these impressed currents on the measurement. It is an accommodated compromise from the recommended practice.

Figure 11.27(a)
DCS earthing commoned

Figure 11.27(a) above shows the practical implementation of the requirements. The exact arrangement depends on the physical distances involved and the nature of the supply circuits to the computer for both main power and instrument loop power. If the computer has its own mains supply transformer that is also used to supply the I/O loops then this is the most desirable situation.

Figure 11.27(b)
DCS earthing (separate)

Figure 11.27(b) depicts direct connection to local earth mats for each of the systems. This is acceptable from an IS point of view provided that the 1 Ohm specification is maintained for the protective earth path between the busbar and the SPNE. This arrangement is ill-defined and liable to cause all sorts of problems.

Figure 11.28
DCS earthing: cross-connection.

Figure 11.28 is thought of as the solution to all the above situations. Its downfall is that because the loop formed can be over a wide area then it is more susceptible to induced noise. Another problem is that power fault conditions in associated equipment may cause transients that can adversely affect the computer or instrument systems.

Other arrangements are seen on installations. Detailed analysis would be necessary before comment could be made on the acceptability and suitability from safety and operational viewpoints.

In the section on applications, examination of individual IS instrument loops, used as I/O for computer control systems, will describe how return currents impinge on the safety earth in more detail to allow a better understanding of the above.

11.15 Surge protection systems

Surge protection devices (SPDs) may be inserted into instrument cables in order to protect the instrument systems where there is a risk of invasion from induced secondary surges. An SPD works by limiting the energy passing through it into a protected circuit. In this way there are many similarities between an SPD and a shunt diode safety barrier. The main difference is in the magnitude of voltages and currents that devices must handle. Diverting large currents in a very short period of time require multistage protection techniques using a combination of Zener Diodes, varistors and gas discharge tubes to clamp the applied voltage yet shift the current.

Figure 11.29
Surge protection

The excess energy is diverted to earth in the same way, as shown in Figure 11.29. The principle difference in the technique is the levels of energy that must be handled by SPDs. The quality of the connection to earth is at the heart of their success. Connection to terrestrial earth via the most direct and shortest route is essential for good performance. The local earth connection becomes the reference point for the system. The intention is that the electronics are all transiently shifted from one potential to another and back. If they all move together simultaneously then no potential difference can appear across the protected electronics to its detriment.

The use of SPDs with hazardous area circuits where shunt diode safety barriers are used requires careful planning of the earthing systems. The IS barrier and the SPD must inevitably be referenced to a common potential. On small installations it is common to move the IS earth arrangement towards the SPD earth as shown in Figure 11.30.

Figure 11.30
SPU with barriers
Figure 11.31
Isolators with SPUs

Bonding to the common busbar fulfils the criteria for both barriers and SPDs but requires the structural earth and mains earth to be commoned at this point also. The earthing of screens and armouring becomes more important in this situation and will be discussed later in this chapter. Clearly in the larger installations, this is not always possible to do in such an elegant way and compromises have to be made.

This is where the use of galvanic isolating interfaces are preferred and there is no contention in the earthing requirements because no IS earth is required (see Figure 11.31).

The isolator can withstand the potential difference created across it by the surge conditions.

11.16 Summary

In this chapter the reasons for correct earthing and bonding have been explored to ensure that the students are aware of the implications when conducting currents through hazardous areas. Since the Safe and Hazardous Areas are inextricably connected by continuity through the earth then failure of equipment in the safe area could jeopardise the hazardous area and care must be taken to control conditions to minimise risk.


12


Installations

In this chapter we examine the general requirements for the design, selection and erection of electrical installation in hazardous areas, as detailed in the Standard IEC60079-14:2013, ‘Electrical installations design, selection and erection’. Some requirements, specific to the types of protection, have already been included in the appropriate previous chapters of this manual. Installers of equipment must be familiar with the contents of this Standard. Activities involving Inspection and Maintenance must also followed the full requirements of installation requirements and information generated by the verification dossier that must be created as a result of the completed installation.

Learning objectives

  • General introduction to the subject
  • The structure of the Standard
  • Design and selection criteria for installations
  • General requirements for all types of protection
  • Additional issues affecting the continued safety of equipment
  • The approach to wiring and cable requirements
  • The generation of the verification dossier

12.1 Introduction

The manufacturer of Ex protected electrical equipment for use in Hazardous Areas will certify it to comply with specific ‘construction’ Standards depending on what type of explosion protection has been applied. It might involve more than one type of protection. Once in the hands of the plant owner/user, the manufacturer has no control over the safety aspects of the way in which it was selected, installed, commissioned, operated, inspected and maintained.

The responsibility is placed on the owner of a plant, (often referred to as the ‘Duty-holder’) to ensure that throughout the life cycle of the plant, the installation is safe, right through from construction to operation. Hence there are separate requirements for the manufacturer of explosion protected equipment and for the installation designer and/or the plant user/owner.

12.1.1 A Code of Practice versus a Standard

International Standard IEC60079-14 issued after 2007 is entitled “Electrical installation design, selection and erection”, and provides requirements on these aspects. The document has the status of a Standard and is no longer considered to be merely a ‘Code of Practice’ (‘CoP’), (similar to Part 10 for Area Classification). The term ‘CoP’ implies that it provides guidance based on experience and technical knowledge and it is therefore not necessary to fully comply with. This Standard must be followed in all its detail but where non-compliance occurs the justification may be stated in a document referred to as a verification dossier (discussed later in this chapter) which explains how the intent of the Standard has been met.

The relationship between the documents required to demonstrate plant safety is shown in figure 12.1.

Figure 12.1
Construction Standards and Codes of Practice in Plant Safety Documentation including the ‘verification dossier’.

12.1.2 Brief history of the Code of Practice and Standard

Prior to the IEC Standard being issued, some countries had their own local Part 14s to suit their general conditions. Where no Part 14 existed, BS5345 Part 4 was often used or accepted, even outside the UK but it was made obsolete in the year 2000. IEC60079-14:1996 ‘Selection and installation’ was written to try to combine all the various national requirements for Ex protected equipment and installations into one internationally accepted set of documents. It was generally recognized that the early issue in 1996 did not provide sufficient depth of guidance as it tried to encompass the many different approaches that are accepted in different countries. In the UK, PD60079-14 was created to guide installation. The document was offered to the IEC and largely accepted, being issued as IEC60079-14:2002 and having the status of a Standard and not a Part 14. The 2007 edition introduced the requirement for the generation of a verification dossier and included Equipment Protection Levels (EPL) stating explosion protection integrity for use with specific Zones. It also covered selection and installation of equipment in dusts in a much more comprehensive form.

The 2013 edition lists a number of subject areas which, compared to the 2007 edition, are stated as either ‘minor editorial changes’ (very few), extensions (covering the majority of changes), or ‘major technical changes’, of which there are only two.

This is important because the plant safety documentation must retain a copy of the CoP/Standard to which the installation was completed for reference purposes. There may also be a need to re-assess existing installations in the light of new editions of the Standards published.

The “duty-holder” for the Ex electrical equipment should declare what Standards have been used as part of its safety documentation.

12.2 General

An installation will have been designed to ensure correct functional operation and adequate levels of safety. It is helpful if we break down a typical electrical installation into suitable parts for consideration at the design, selection and erection stages, giving a general case. The following diagram looks at a diagrammatic representation of the key issues which must be considered according to the Part 14.

Figure 12.2
Issues of installation

Electrical equipment, whatever its function, requires a supply and associated cabling arrangements to suit the application. For the sake of simplicity each circuit can be broken down in the same way as shown in figure 12.2. This section of the manual will explain how the Part 14 is applied in general terms with reference to the above arrangement.

12.2.1 Application of the code of practice

The Part 14 should be read to guide the designer on the requirements of the equipment that must be used, what types of Ex Protection ratings and the minimum requirements of the marking for Apparatus Group, Temperature and Ambient Temperature.

The designer must specify ratings, operating functionality, operational characteristics and safety requirements. Equipment will be selected on the basis of the designer’s specification. The selection process must deal with equipment at a detailed level because there may be conditions of use placed on equipment by the nature of it’s certification to comply with the Ex Protection Standards. An iterative process will begin until a solution is found that meets all the operational and safety requirements.

Thereafter, the process of erection will begin where the equipment is mounted, cabled and supplied in the Hazardous Area ready for use. It is at this point that an inspection (to IEC60079-17) must be performed to ensure that the complete installation meets the requirements laid out by the designer. Many of the requirements for the types of Ex Protection will have been discussed under the topic for each type of protection in previous chapters of this manual.

It is therefore appropriate to look at the Part 14 in some detail. It sets out what is to be achieved when the installation is complete.

12.3 IEC 60079-14: 2013 contents

The contents of the IEC 60079-14:2013 Standard are divided into sections. These are listed in table 12.1 for the ease of reference.

Table 12.1
Clauses of IEC60079-14:2013
Clause Title
1 Scope and object
2 Normative references
3 Definitions and terms
4 General requirements
5 Selection of equipment
6 Protection from dangerous (incentive) sparking
7 Electrical protection
8 Switch-off and electrical isolation
9 Cables and wiring systems
10 Cable entry systems and blanking elements
11 Rotating electrical machines
12 Luminaires
13 Electric heating systems
14 Additional requirements for type of protection ‘d’
15 Additional requirements for type of protection ‘e’
16 Additional requirements for type of protection ‘i’
17 Additional requirements for type of protection ‘p’
18 Additional requirements for type of protection ‘n’
19 Additional requirements for type of protection ‘o’
20 Additional requirements for type of protection ‘q’
21 Additional requirements for type of protection ‘m’
22 Additional requirements for type of protection ‘op’
23 Additional requirements for type of protection ‘t’

Clauses 1 to 3 give the conventional scope of the Standard, cross-referenced to other Standards cited in the document, and definitions of terms used where they are not defined in other documents.

Clauses 4 to 10 give general requirements that apply to all types of Ex Protection.

Clauses 11 to 13 cover specific types of applications

Clauses 14 to 23 are specific to each type of protection recognized by this Standard.

12.4 Verification Dossier

It is internationally accepted practice, if not actually construed in Law in most countries, that the owner of an installation possesses documentation relating to safety issues and safe working procedures for its employees. Such documents shall describe, and may be used as a source of reference for, all possible safety criteria affecting a plant and the changes that may occur throughout the life of the plant. This is in order to ensure the safe installation, operation and maintenance of a plant. This safety documentation has been introduced in earlier chapters in this manual. It is referred to in this Standard as a ‘Verification Dossier’

It will generally comprise the same information being amassed for the purposes of selection of equipment, design of the plant and operation of the process. The documentation and drawings produced by a design contractor, as part of the design, very often forms the basis of this information. Equipment suppliers’ documentation where it describes safety related information is added.

Decisions to be made about the running of the plant, such as maintenance and periodic inspection (as required by IEC 60079-17) can be made on the basis of information available from such safety documentation. The recording of philosophies and decisions should be maintained in this plant safety documentation.

A safety audit will include examination of a company’s safety documentation which will include their operating procedures. An assessment of how diligently the employees apply the procedures is a good measure of the management’s safety attitude. It is also a valuable tool to satisfy the quality assurance and insurance requirements placed upon plant operators.

The documentation can also be used to formulate the basis of training for employees and contractors working on a site. It is similar in concept to the requirements of the European ATEX Directives to produce an Explosion Protection Document.

There is no prescribed way of presenting the Verification Dossier. It can be generated using existing documentation and systems established to satisfy other industry requirements of other legislation.

12.5 General requirements of the standard

The general requirements of IEC 60079-14 state that electrical installations in hazardous areas shall also comply with the appropriate requirements for installations in non-hazardous (safe) areas. The Standard should be viewed as working alongside other documented requirements for electrical systems. There are no requirements in this Standard that defeat or conflict with the object of other general electrical installation practices. It is the intention of this statement to reinforce the user’s awareness of the need for personnel safety (shock and injury), fire prevention and overload protection.

12.5.1 Considerations

An initial list of points is made in Part 14’s introduction:

  • Classification is required into Zones before equipment can be selected
  • Location of equipment should be placed in the safe area if possible
  • If in a hazardous area, install in the least hazardous area
  • Design should be for ease of access for inspection and maintenance
  • Installation shall be according to a manufacturer’s documentation
  • Operation must be within the permitted electrical ratings of the equipment

The Standard also states that equipment and systems used in exceptional circumstances, for example in research, development and pilot plant where explosion protected equipment is not available, need not meet the requirements of the Standard, provided that the installation is under the supervision of a competent body and one or more of the following conditions, as appropriate, are met:

  • measures are taken to ensure that an explosive atmosphere does not occur; or
  • measures are taken to ensure that this equipment is disconnected on the occurrence of an explosive atmosphere, in which case ignition after disconnection, e.g. due to heated parts, shall be prevented also; or
  • measures are taken to ensure that persons and the environment are not endangered by fires or explosions.

In addition, the measures to be taken are laid down in writing by a competent body who:

  • is familiar with the requirements for this, and any other relevant standards and code of practice concerning the use of electrical equipment and systems for use in hazardous areas,
  • have access to all information necessary to carry out the assessment.

On completion of an installation, an initial inspection (prior to the operation of the equipment) is required to be carried out in accordance with IEC 60079-17: “Inspection and maintenance of electrical installations in hazardous areas”. This will be discussed in the specific section on Inspection.

12.5.2 Documentation

IEC 60079-14 states that in order to install, or extend an existing installation, the following documentation is required, where applicable. Note that references in parentheses refer to those in the Standard

  • area classification documents (see IEC 60079-10) with plans showing the classification and extent of the hazardous areas including the zoning (and maximum permissible dust layer thickness if the hazard is due to combustible dust);
  • optional assessment of consequences of ignition (see 5.3);
  • instructions for erection and connection;
  • documents for electrical equipment with conditions of use, e.g. for equipment with certificate numbers which have the suffix X;
  • descriptive system document for the intrinsically safe system (see 12.2.5);
  • manufacturer‘s/qualified person’s declaration;
  • necessary information to ensure correct installation of the equipment provided in a form which is suitable to the personnel responsible for this activity (see IEC 60079-0; Instructions);
  • information necessary for inspection, e.g. list and location of equipment, spares, technical information (see IEC 60079-17);
  • details of any relevant calculation, e.g. for purging rates for instruments or analyser houses;
  • if repairs are to be carried out by the user or a repairer, information necessary for the repair of the electrical equipment (see IEC 60079-19);
  • where applicable, gas or vapour classification in relation to the group or subgroup of the electrical equipment;
  • temperature class or ignition temperature of the gas or vapour involved;
  • external influences and ambient temperature.

From the above points it is realised that all information necessary for the correct and safe installation of equipment must be available for the installer to do his job properly. Thereafter, the information will need to be provided to the user as part of the ‘Verification Dossier’ from which the routine requirements for inspection, maintenance and repair will be decided upon and implemented, as discussed previously. This document will point to the requirements for any special conditions that must be applied and show how the requirements have been met. The manufacturer must provide instruction manuals, certification documents and drawings that communicate sufficient detail of information to allow an assessment of the equipment for the area in which it is to be operated. Other legislation requires that electrical equipment is to be accompanied by adequate documentation including instructions for its intended purpose and its safety whilst in operation.

For intrinsically safe systems, a System Descriptive Document is required which explains how the equipment is connected together with the characteristics of the cable to form a system, is safe and compliant with the Ex i Standards.

This Standard now clearly recognizes the responsibilities of the installer in placing equipment in a hazardous area. The person(s) undertaking the installation must be adequately trained and supervised. Supervisors must also be adequately trained. The declaration of correct installation in accordance with the information provided can only be believably declared by a suitably trained and qualified individual.

12.5.3 Assurance of conformity of equipment

The Part 14 states that electrical equipment with certificate according to the IEC60079 Series meets the requirements for Hazardous Areas when selected and installed according to it.

Where the equipment has no certificate, or has one which is not in accordance with the IEC Standards, the Part 14 states that it should only be used where equipment compliant with IEC60079 cannot be obtained. This is on new installations.

If such equipment is used then the justification for its use, along with the installation and marking requirements, shall be made by the user, manufacturer or a third party and shall be included in the Verification Dossier.

European certified equipment complies with ATEX, which is based on IEC harmonised standards. There is no conflict seen here. Equipment made in North America is ‘listed’ if it is solely for use on that continent. It therefore has no certificate. Many North American manufacturers do now certify to ATEX or IECEx or both, so that users outside are permitted to install and operate the equipment. These terms are explained in the section on Certification in this manual. Where no ATEX/IECEx certification exists for equipment, the Part 14 does permit it to be used under controlled conditions if no other equipment is available.

Equipment already in service is unaffected by this but where equipment has been in service prior to being moved to a new installation, it must be assessed for compliance with the original certificate. It must be verified as being correct and uncompromised. That may require the expertise of the original manufacturer.

12.5.4 Equipment certified to old standards

There is no IEC Series Standard yet for what was conceived under a British document called SFA3009 as ‘Ex s’, yet this type of protection is still quite common in some applications. The Part 14 permits the use of relevant Standards by those who understand them but this is a further example of it throwing the onus onto the user, who must therefore decide whether the use of a solution to a problem can be justified.

Industrial grade equipment not specifically designed for hazardous area use but which can be shown to be safe in normal operation using type of protection n may be certified by the manufacturer according to the ATEX regulations in Europe for use in Zone 2. Many countries and users had requested this approach which has now been formalised. The Dutch, heavily influenced by Shell, BP and the oil industry generally, where the hydrocarbon hazards are predominantly IIA and large areas are Zone 2, had tried to develop a ‘self certification’ scheme before to reduce costs.

It is particularly suitable for solutions to instrument problems where such problems can be confined to zone 2. There is however a cause for concern that large areas will be incorrectly classified as zone 2 to accommodate cheaper and lower grade solutions that are not adequately defined and maintained, thus lowering the integrity of the protection.

12.5.5 Qualifications of personnel

The Part 14 includes requirements for training and competency of personnel performing installation design, selection of equipment and erection. The Part 14 cannot dictate levels of training and competency on the basis that each requirement will be unique. This clearly is down to the plant owner to ensure that the right skill level is reached by personnel undertaking the job functions.

12.6 Selection of electrical equipment

Clause 5 of Part 14, deals with the process of selecting equipment. All Standards and Codes of Practice for installation require that a selection process have been performed (by the plant designer), prior to any erection of equipment, which ensures that the appropriate (type of) equipment is chosen for a given installation. This selection process can only be done correctly when the hazards are known and defined.

Selection criteria depend on the

  • Area Classification               (Zone)
  • Temperature Classification   (T class)
  • Ambient Temperature, and   (written as Ta or Tamb)
  • Equipment Grouping            (Group II, IIA, IIB, IIC, III, IIIa, IIIb or IIIC)

encountered by each piece of equipment.

Figure 12.2
Selection process for equipment

Figure 12.2 offers a simple flowchart for equipment selection. (This is not included in the Standard.) It’s purpose is to instil the discipline of checking the certification of the different types of protection available for a given application to ensure that the chosen equipment has no restriction on its use.

12.6.1 Selection by zoning or basis of risk

Different types of protection have different levels of integrity, as previously discussed and their use must therefore be restricted to suitable zones. The risk must be balanced against the likelihood of the occurrence of a source of ignition. There are two ways in which equipment can be deemed acceptable for the hazardous area.

Firstly, hazardous areas are classified into zones but zoning does not take account of the potential consequences of an explosion. The previous editions of this Standard allocated protection concepts to zones, on the statistical basis that the more frequent the occurrence of an explosive atmosphere, the greater the level of safety required against the possibility of an ignition source. Where only Zones are applied, the following EPLs may be used.

Table 12.3
Equipment protection levels where only Zones are assigned
Zone Equipment protection levels (EPLs)
0 ‘Ga’
1 ‘Ga’ or ‘Gb’
2 ‘Ga’, ‘Gb’or ‘Gc’
20 ‘Da’
21 ‘Da’ or ‘Db’
22 ‘Da’, ‘Db’ or ‘Dc’

Where the EPLs are identified in the area classification documentation, those requirements for selection of the equipment shall be followed. This is perhaps the most popular way of assessing the risk.

Secondly, as an alternative to the relationship given in table 12.3 between EPLs and zones, EPLs may be determined on the basis of risk, i.e. taking into account the consequences of an ignition. This may, under certain circumstances, require a higher EPL or permit a lower EPL than the defined in table 12.3. The recognised types of protection according to IEC Standards have been allocated EPLs according to table 12.4 for gasses and vapours and table 12.5 for dusts.

Table 12.4
Selection of equipment according to EPL (Gasses and Vapours)
EPL Type of protection Code According to
‘Ga’ Intrinsically safe ‘ia’ IEC 60079-11
Encapsulation ‘ma’ IEC 60079-18
Two independent types of protection each meeting EPL ‘Gb’   IEC 60079-26
Protection of equipment and transmission systems using optical radiation   IEC 60079-28
‘Gb’ Flameproof enclosures ‘d’ IEC 60079-1
Increased safety ‘e’ IEC 60079-7
Intrinsically safe ‘ib’ IEC 60079-11
Encapsulation ‘m’ or ‘mb’ IEC 60079-18
Oil immersion ‘o’ IEC 60079-6
Pressurized enclosures ‘p’, ‘px’ or ‘py’ IEC 60079-2
Powder filling ‘q’ IEC 60079-5
Fieldbus intrinsically safe concept (FISCO)   IEC 60079-27
Protection of equipment and transmission   IEC 60079-28
systems using optical radiation    
‘Gc’ Intrinsically safe ‘ic’ IEC 60079-11
Encapsulation ‘mc’ IEC 60079-18
Non-sparking ‘n’ or ‘nA’ IEC 60079-15
Restricted breathing ‘nR’ IEC 60079-15
Energy limitation ‘nL’ IEC 60079-15
Sparking equipment ‘nC’ IEC 60079-15
Pressurized enclosures ‘pz’ IEC 60079-2
Fieldbus non incendive concept (FNICO)   IEC 60079-27
Protection of equipment and transmission   IEC 60079-28
systems using optical radiation    
Table 12.5
Selection of equipment according to EPL (Dusts)
EPL Type of protection Code According to
‘Da’ Intrinsically safe ‘iD’ IEC 60079-11
  Encapsulation ‘mD’ IEC 60079-18
  Protection by enclosure ‘tD’ IEC 60079-31
‘Db’ Intrinsically safe ‘iD’ IEC 60079-11
  Encapsulation ‘mD’ IEC 60079-18
  Protection by enclosure ‘tD’ IEC 60079-31
  Pressurized enclosures ‘pD’ IEC 61241-4
‘Dc’ Intrinsically safe ‘iD’ IEC 60079-11
  Encapsulation ‘mD’ IEC 60079-18
  Protection by enclosure ‘tD’ IEC 60079-31
  Pressurized enclosures ‘pD’ IEC 61241-4

12.6.2 Equipment grouping and temperature classification

Clause 5.5 of the Standard clarifies how equipment must be selected with a suitable Equipment Grouping (see table 12.8) and Temperature Class (see table 12.9):

Table 12.6
Relationship between gas/vapour or dust subdivision and equipment group
Location gas/vapour or dust subdivision Permitted Equipment Group
IIA IIA, IIB or IIC
IIB IIB or IIC
IIC IIC
IIIA IIIA, IIIB or IIIC
IIIB IIIB or IIIC
IIIC IIIC

Clause 5.4 of this Standard states that electrical equipment of types ‘e’, ‘m’, ‘o‘,’p’ and ‘q’ shall be of equipment group II. This is because these types of equipment are not able to emit levels of fault energy by their very design or construction. Hence, sub-classification into energy emission bands of IIA, IIB or IIC is not necessary.

However, types ‘d’ and ‘i’ do need to be sub-classified into the A, B and C sub-groups of Group II. This is because different equipment can emit different energy levels. An enclosure may be certified Ex d IIC or Ex d IIB dependent on its size and construction.

Symbols for the temperature classes marked on the electrical equipment have the meaning indicated in table 12.7 and selection of T rating must be in accordance with Clause 5.6.2 of this Standard. For example, it states that the rating for the T4 class of equipment is 135°C and that this is the maximum surface temperature of that equipment. It then specifies that the ignition temperature of any gas or vapour must be greater than this temperature. This applies to all types of protection. Rules for dust hazards are more complicated and are explained in following paragraphs

The ambient temperature range of unmarked equipment is said by this clause to be -20° to +40°C unless otherwise stated. This concurs with the previously made statement that the Temperature classification applies at the maximum ambient temperature of 40°C.

Table 12.7
Relationship between gas or vapour ignition temperature and temperature class of equipment
Temperature class required by the area classification Ignition temperature of gas or vapour In °C Allowable temperature classes of equipment
T1 >450 T1 - T6
T2 >300 T2 - T6
T3 >200 T3 - T6
T4 >135 T4 - T6
T5 >100 T5 - T6
T6 >85 T6

12.6.3 External Influences

Clause 5.5 recognizes that other external influences that may affect the safe stating. An obvious influence already covered is that of ambient temperature but other considerations may be environmental such as vibration, weather protection, chemical spillage and risk of damage from other activities. The environment in which equipment must operate may influence the decision on the suitability or not of different types of protection.

12.7 Dusts

Dust layers exhibit two properties as layer thickness increases: a reduction in minimum ignition temperature and an increase in thermal insulation.

The maximum permissible surface temperature for equipment is determined by the deduction of a safety margin from the Minimum Ignition Temperature (MIT) of the dust concerned, when tested in accordance with the methods specified in IEC 61241-2-1 (ISO/IEC 80079-20-2, under consideration) for both, dust cloud and layer.

For installations where the layer thickness is greater than 5 mm, the maximum surface temperature shall be determined with particular reference to the layer thickness and all the characteristics of the material(s) being used. Examples of excessively thick dust layers can be found in Annex L in the Installations Standard which now includes areas in which dusts are present.

5.6.3.2 Temperature limitations because of the presence of dust clouds
The maximum surface temperature of equipment when tested in the dust-free test method in accordance with IEC 60079-0 shall not exceed two-thirds of the minimum ignition temperature in degrees Celsius of the dust/air mixture concerned:

      Tmax ≤ 2/3 TCL
where TCL is the minimum ignition temperature of the cloud of dust.

5.6.3.3 Temperature limitation because of presence of dust layers
Where the equipment is not marked with a dust layer thickness as part of the T rating, a safety factor shall be applied taking the dust layer thickness into account as:

– up to 5 mm thickness:

The maximum surface temperature of the equipment when tested in the dust-free test method in accordance with IEC 60079-0 shall not exceed a value of 75 °C below the minimum ignition temperature for the 5 mm layer thickness of the dust concerned:
TmaxT5 mm – 75 °C
where T5 mm is the minimum ignition temperature of the 5 mm layer of dust.

 

– above 5 mm up to 50 mm thickness:

Where there is a possibility that dust layers in excess of 5 mm may be formed on equipment, the maximum permissible surface temperature shall be reduced. For guidance, examples of the reduction in maximum permissible surface temperature of equipment used in the presence of dust having minimum ignition temperatures in excess of 250 °C for a 5 mm layer are shown in the graph below (Figure 1) for increasing depth of layers.

 

– For dust layers above 50 mm, see 5.6.3.4

Before applying the information in Figure 12.3, reference should be made to IEC 61241-2-1.

Figure 12.3
Correlation between the maximum permissible surface temperature and depth of dust layers
Laboratory verification shall be carried out for equipment where the ignition temperature of a 5 mm layer is below 250 °C, or where there is any doubt concerning the application of the graph

Unavoidable dust layers

Where it cannot be avoided that a dust layer forms around the sides and bottom of equipment, or where equipment is totally submerged in dust, because of the insulation effect a much lower surface temperature may be necessary. If equipment protection level “Da” is required in such situations, all specific requirements for EPL “Da” shall be fulfilled.
For installations where the layer depth is greater than 50 mm, the maximum surface temperature of equipment may be marked with the maximum surface temperature TL as reference to the permitted layer depth. Where the equipment is marked TL for a layer depth, the ignition temperature of the dust, at layer depth L, shall be applied in place of T5 mm. The maximum surface temperature of the equipment TL shall be at least 75 °C lower than the ignition temperature of the dust, at layer depth L. Examples of excessively thick dust layers can be found in Annex L.

 

The above rules must be applied with full reference to the Installation Standard to ensure compliance in a given situation.

12.8 Electricity Supply systems

Clauses 7 to 9 of IEC 60079-14 are primarily concerned with the electrical supply and cabling systems integrity. Much of this is not directly relevant to Ex i and is specifically excluded for Ex i Circuits.

The Standard declares that any contact with bare or exposed live parts, other than on Ex i circuits, shall be prevented. The standard does not say how it is to be prevented but clearly requires adequate insulation and shrouding of live parts. It recognizes that there is an adequately low risk associated with Ex i circuits and therefore specifically excludes Ex ‘i’ circuits in order to permit some applications where bare live conductors are accessible. Bursting discs and conductivity probes are examples of this situation that in any case does not occur frequently. This is taken as referring to instrument circuit terminations which are accessible whilst live and do not need shrouding such that it is easy to connect equipment such as test meters and/or hand held controllers into.

Since there are different arrangements for electrical supply systems and the earthing provisions for these, the standard recognizes basic precautions to be taken. For example in type IT where the Earth and Neutral conductors are not connected (or only by a high impedance) it recommends the use of insulation monitoring devices to detect such a fault as a warning to failure.

This does not affect the discussion on earthing in this manual. The use of ‘supplementary equipotential bonding’ to ensure that potentials between conducting parts (i.e. the metal casements of motors and transformers) to prevent dangerous voltages appearing between the frames of electrical equipment is implicit in any requirement.

12.8.1 SELV and PELV systems

The following relevant definitions categorize the level of voltage used in electrical systems associated with Ex i protection. This categorization is referred to in other IEC and national standards and specifies precautions to be taken depending on the circumstances of use.

Extra low voltage: Normally not exceeding 50 Vac or 120 V ripple free dc whether between two conductors or to Earth.
Low Voltage: Normally exceeding Extra Low Voltage but not exceeding 1000 Vac or 1500 Vdc between conductors or 600 Vac or 900 Vdc between conductors and Earth.
SELV: (Separated ELV): an extra low voltage system which is electrically separated from Earth and from other systems in such a way that a single earth fault cannot give rise to the risk of electric shock.
PELV: (Protective ELV): An extra low voltage system which is not electrically separated from Earth but which satisfies all the requirements for SELV.

The IEC 60079-14 standard recognizes the use of Extra Low Voltage systems as applicable to instrumentation by this reference. Circuits may or may not be floating with respect to earth provided that they cannot give rise to the risk of shock. It therefore accepts the use of Galvanic-Isolating interfaces in Ex i circuits. Preferences are expressed towards a floating circuit being earthed at one point, or being monitored with respect to Earth-by-Earth Leakage Detection (ELD) systems.

The Electrical Protection section in the standard specifically excludes Ex i. With other techniques of protection, the intention is to ensure adequate protection or warning devices to operate without ‘inadmissible heating’ occurring. There is a danger that automatic disconnection of equipment could pose a greater safety risk than that of the risk of ignition alone. This emphasis’s the need for safety assessments covering all types of risk.

Ex i circuits, being energy limited by such high levels of integrity arguably fall outside some national regulations for live working on electrical equipment.

12.8.2 Emergency switch-off and electrical isolation

IEC 60079-14: Clause 8 deals with this subject by declaring that:

  • 8.1: For emergency purposes, at a suitable point or points outside the hazardous area, there shall be a single or multiple means of switching off electrical supplies to the hazardous area.
  • Electrical equipment, which must continue to operate to prevent additional danger, shall not be included into the emergency switch off circuit; it shall be on a separate circuit.
  • 8.2: To allow work to be carried out safely, suitable means of isolation shall be provided for each circuit or group of circuits, to include all circuit conductors including the neutral.
  • Labelling shall be provided immediately adjacent to each means of isolation to permit rapid identification of the circuit or group of circuits thereby controlled.

Since this clause does not specifically exclude Ex i it is often misunderstood and instrument installations are sometimes taken to a costly extreme. Users have of each instrument loop. Some think that this is unreasonable. The provision for isolation of the ‘supply’ to equipment is inferred. To reduce the number of connections in an instrument loop must reduce the cost of design, purchasing, installation, maintenance, testing and inspection as well as enhancing the reliability. Sensible grouping of isolation can help in many circumstances.

12.8.3 Portable and Personal Equipment

Transportable, portable and personal equipment may be taken into the hazardous area and it is recommended that it meet the requirements of the location to which it will be exposed and which requires the highest EPL, with appropriate equipment group and temperature classification.

The Standard states that where there is a need to use transportable or portable equipment in a hazardous area for which the normally required EPL is not obtainable, a documented program for risk management shall be implemented. This program shall include appropriate training, procedures and controls. A safe work permit shall be issued appropriate to the potential ignition risk created by the use of the equipment (see Annex D of the Standard).

The Standard now clarifies the situation where items of personal equipment which are battery or solar operated are sometimes carried by personnel and inadvertently taken into a hazardous area.
A basic electronic wrist watch is said to be an example of a low voltage, electronic device which has been independently evaluated and found to be acceptable for use in a hazardous area under both historic and current EPL requirements. All other personal battery or solar operated equipment (including electronic wrist watches incorporating a calculator) shall:

  • conform to a recognised type of protection appropriate to EPL, gas group and temperature class requirements, or
  • be subjected to risk assessment, or
  • be taken into the hazardous area under a safe work procedure.

In situations of the presence of dusts, ordinary industrial portable apparatus should not be used in a hazardous area unless the specific location has been assessed to ensure that potentially combustible dust is absent during the period of use (“dust free” situation).

12.8.4 Rotating machines

Clause 11 of IEC60079-14:2013 gives guidance on the selection of rotating machines. The principle considerations that will impact on Hazardous Areas are listed as:-

  • Duty Cycle
  • Supply Voltage and Frequency range
  • Heat transfer from Driven equipment
  • Bearing and lubrication life
  • Insulation class.

Increased losses that cause excess heating are known to affect the performance and so when selecting a motor for use with a variable speed drive inverter then the assessment of the combination is required as part of the certification.

This section is extremely detailed, covering d, e, t, and nA types of protection for both above and below 1kV.

Bearing failure cannot be included in the protection type and must be predicted by vibration monitoring or adequate bearing replacement intervals as recommended by the motor manufacturer.

12.8.5 Wiring systems

Clause 9 of IEC60079-14 gives the general requirements for wiring systems for all types of Ex Protection. The installation designer and the installer must be familiar with the full requirements in order to specify and connect equipment correctly. The clause is set out under the headings listed below.

Table 12.10
Wiring Systems requirements
Sub-Clause Clause Heading
9: Wiring systems: All Methods
9.1 General
9.2 Aluminium conductors
9.3 Cables
9.3.1 General
9.3.2 Cables for fixed installations
9.3.3 Flexible cables for fixed installations (excluding intrinsically safe circuits)
9.3.4 Flexible cables supplying transportable and portable equipment (excluding intrinsically safe circuits)
9.3.5 Single insulated wires (excluding intrinsically safe circuits)
9.3.6 Overhead lines
9.3.7 Avoidance of damage
9.3.8 Cable surface temperature
9.3.9 Resistance to flame propagation
9.4 Conduit systems
9.5 Additional Requirements
9.6 Installation Requirements
9.6.1 Circuits traversing a hazardous area
9.6.2 Terminations
9.6.3 Unused cores
9.6.4 Openings in walls
9.6.5 Passage and collection of flammables
9.6.6 Accumulation of combustible dust

There is some choice in the way cabling is selected and installed but it is based around the needs of the type of protection primarily and then must take into account the duty of the cable and the environment in which it is sited.

It is worth noting that wiring inside Ex equipment is specified as part of the certification; cabling from the outside world must maintain the integrity of the type of protection to which it is connected. In the case of Ex d, the cable entry method and the cable must withstand the effects of the internal explosion, whereas with Ex e it is only necessary to keep out moisture and dust. Thus the cable entry methods are specified under the construction requirements of the type of protection rather than as a function of installing the equipment. These requirements are stated in Clause 10 of the Standard and listed as clause headings in Table 12.11

Table 12.11
Cable entry systems and blanking elements
Sub-Clause Clause Heading
10: Cable entry systems and blanking elements
10.1 General
10.2 Selction of cable glands
10.3 Connections of cables to equipment
10.4 Additional requirements for equipment other than Ex ‘d’, Ex ‘t’ or Ex ‘nR’
10.5 Unused openings
10.6 Additional requirments for type of protection ‘d’ - Flameproof enclosures
10.6.1 General
10.6.2 Selection of cable glands
10.7 Additional requirments for type of protection ‘t’ - protection by enclosure
10.8 Additional requirments for type of protection ‘nR’ - Restricted breathing enclosure

In general, cable selection must take into account, in no particular order of importance:

  • The duty of the equipment that it is used to supply
  • The conditions in the hazardous area
  • Specific requirements of the type of protection
  • Specific conditions of the certification
  • Possibility of cable damage and means of avoidance
  • Vibration effects
  • Risk of exposure to chemical effects and excess heat
  • Ease of access to terminations

The Part 14 suggests that where integrity may be compromised ‘additional measures shall be taken or appropriate cables selected’. It is not prescriptive here but expects the designer to anticipate what conditions are likely and make choices based on experience and good practice. Whether the choices are usual cables or not, the justification for their use would sensibly be recorded so that others may know why the choice was made.

Practice in Germany prefers the use of the very practical ‘semi-rigid outer sheath’ or toughened plastic sheathed cables whilst other European and non-European countries demand ‘steel-wire-armoured’ (swa.). In the US, conduit has been, up to now, used almost exclusively but some changes are seen owing to the high cost and inconvenience of the use of conduit. Where conduit is used, standard, or non-mechanically protected cables can be used; its only real advantage.

The normally metal cable gland or conduit becomes part of the equipotential bonding system and therefore measures must be taken to ensure good electrical continuity through it. Earthing and bonding integrity must be considered in cabling and connection methods. Provision for earthing and appropriate documentation must be discerned and finalised during the installation phase.

Unused cores of cables must be dealt with in such a way as to prevent induced voltages being built up. Earthing of unused conductors is often preferable to leaving cables terminated but isolated because capacitive coupling can cause unearthed conductors to attain a charge.

Once cabling has been installed and connected, it will be about this time in a project that errors, inconsistencies and missing information about where and how to connect the cables and how to identify cables and termination points, come to light. In the opinion of many experienced personnel, diligence and the ability to record, report and correct such issues will make life easier for the personnel who have to run the plant and maintain it throughout its life. The plant owner’s management should be aware of this precautionary approach at this stage.

12.9 Commissioning

There are no Clauses that deal specifically with the commissioning phase of Ex protected systems. For the purposes of this discussion, operation of any equipment in the presence of hazardous atmospheres must have been proved to be correctly installed and acceptably safe prior to the start-up of the plant. The Standard now requires an initial and detailed inspection to be carried out on completion of the installation. The inspection documentation will form the basis of the verification dossier required by the Standard.

12.10 Installed arrangements

The use of different types of protection in the same part of an installation sometimes causes misunderstandings. The layout shown in figure 12.4 is a typical example of where the connection of equipment that uses different types of protection is quite normal and acceptable.

Figure 12.4
Different types of protection working together

The individual pieces of equipment must be properly selected and installed for the Zone in which it is located, according to the type of protection, Grouping and Temperature Rating. It is not a requirement to use only Ex d if the main equipment is so rated. The JBs should not be thought of as converting Ex n to Ex e to Ex d! This is a potentially confusing concept that the uninitiated use to explain the installation. The choice of the types of protection may be on grounds of availability, cost and convenience of installation amongst other considerations discussed in general terms throughout this course.

12.10.1 Installation of equipment with combined protection

A different approach is required with equipment which comprises more than one type of protection; this is termed ‘combined’ or ‘multiple’ protection. Suppose equipment was marked either “Ex emib” or “Ex em[ib]”; as shown in figure 12.6. The significance is subtle but shows how helpful an understanding of the marking can be.

The Ex certificate will describe which protection type is applied to the various parts of the equipment. It should then be clear what installation requirements must be applied specifically to which part. This would certainly need to be understood during the design phase, as, in this case, the ib version would have Entity Parameters and would require to be treated in a similar way to an Ex i Transmitter. The power to the Ex i circuit would be supplied from the Safe Area. In contrast, the [ib] version would be a self-powered output stating a ‘Safety Description’ and would need a Ex i interface that would be non-energy contributing to the circuit so different equipment and safety characteristics would be needed in the Safe Area for these two options. The Descriptive System Document should show this detail.

The termination box for the power supply circuit would need Ex e component certified plastic glands, or ordinary industrial metal glands which meet IP54 would be quite acceptable. For cable entry to the Ex i termination box, no requirement for cable entry need be specified apart from IP20.

Figure 12.5
Different types of protection working together

The Ex m part of this equipment is internal in this case and would not need any specific consideration for correct installation. It entirely depends how the equipment has been designed and certified. If the flowmeter was small enough then sufficient power from the Ex i circuit may be available but as the diameter increases external power can be supplied via other techniques suitable to the design. This approach is commonplace in instrumentation where it is desirable to retain the live working ability by using Ex i in the signalling circuit.

12.11 Summary

In this chapter, the rules for the installation of equipment comprising the three principle activities of plant design, equipment selection and equipment erection have been examined.

It is vital that personnel involved in selection and installation of equipment are familiar with the contents and requirements of this Standard. The issues of competence for personnel involved in engineering design and supervision as well as actual installation work are stated clearly.

It is important to establish that the equipment has been correctly installed before being brought into service because there is a likelihood that any error made during this phase of plant construction may be carried forward unchecked during the life of the plant. Personnel may accept that the original installation was correct and maintain the plant as it was first installed unaware that the equipment is not being operated as it should be! This represents a potential weakness in the integrity of the protection philosophy owing to human misconception. An initial inspection is therefore required to ensure correct installation and periodic inspection is necessary throughout its life. It is logical to look at these requirements for Inspection and Maintenance in conjunction with Installation aspects and will be studied in the next chapter.


13


Inspection and Maintenance

This chapter exposes the requirements for Inspection and Maintenance of equipment installed in hazardous areas. In the previous chapters describing the principles of operation of the individual types of protection, detail has been given on the requirements to be met. This chapter discusses the approach to the user’s responsibility during the service life of equipment. As a result of findings, remedial work must be undertaken where safety has been compromised.

Learning objectives

  • To study the types and grades of inspection outlined in the Code of Practice
  • To examine routine and breakdown failure
  • To consider inspection duty and implementation

13.1 Introduction

The Law in most countries requires the owners of industrial plant to maintain it in a safe condition.

The condition of the plant and the equipment can only be determined by appropriate inspection in order to ascertain that it is the correct equipment, suitable for the application and any safety features remain according to the design of the plant and equipment. In addition the equipment remains suitable for the operation of the plant.

Where special features are designed into equipment to preserve safety, these must be checked periodically.

It is also necessary to be familiar with the installation requirements of Explosion Protected equipment in order to know that it remains installed correctly.

13.2 Standards

IEC60079-17 is entitled “Recommendations for inspection and maintenance of electrical installations in hazardous areas (other than mines)”. It was first introduced in 1990. Prior to this issue, BS5345 was a well-known document followed by many countries. This was made obsolete in 1999.

IEC60079-17 is introduced as being ‘supplementary’ to IEC60079-14 on the basis that equipment must be installed correctly and knowledge of the state of the installation when first brought into service compared with the state found during inspection is a vital part of safety checking.

It further states:
“Electrical installations in Hazardous Areas poses features specially designed to render them suitable for operation in such atmospheres. It is essential for reasons of safety in those areas, that, throughout the life of such installations, the integrity of those special features is preserved;

They therefore require INITIAL inspection and either:

  • Regular PERIODIC INSPECTIONS thereafter, or
  • Continuous supervision by skilled personnel
    in accordance with this Standard and, when necessary, maintenance”.

The choice provided by the above statement requires decisions by a technical and managerial level of personnel on a site that is fully aware of the conditions under which the plant and the equipment are operated. The Standard cannot prescribe what must be looked at and when because each plant is considered to be unique and operation, affecting plant conditions, will be similarly peculiar to that plant.

There is, therefore, a need to maintain the plant in a safe condition by ensuring that the Ex protected equipment remains in the same safe state as when it was first installed and commissioned. Only by diligent monitoring of these factors can the status be determined. If deterioration is apparent then corrective action must be taken in a timely way to maintain safety (see Figure 13.1).

Figure 13.1
Preventative maintenance

Such factors include:

  • Deterioration in condition of equipment: Wear and tear through a process of ageing may force the deterioration of seals, embrittlement and cracking of enclosures. Metal can ‘relax’ from stress and fixtures become loose if incorrectly designed and specified. Weathering, moisture and ambient temperature cycling can cause or accelerate imperfections in equipment. Constant vibration and mechanical shock are other causes.
  • Environmental attack: Chemicals, vapours and accumulation of condensates may cause primary or additional progressive failure of equipment. A combination of chemicals and other factors mentioned above may have unforeseeable dramatic corrosive results.
  • Modifications: The installation may have undergone modifications during its life or during commissioning in order to improve response to control, quality or efficiency. Such changes may have been done hurriedly and without full control of circumstances. Plant additions and extensions are known to suffer from an increased risk of changing philosophies affecting overall safety. Such changes may demand new area classification.
  • Alteration to hazard: The introduction of new chemicals into a manufacturing process may require the modification of equipment to accommodate a higher risk situation.

13.3 Scope of IEC 60079-17

The scope of this document is stated as being:
“… intended as a guide for users and covers factors directly related to the inspection and maintenance of electrical installations within hazardous areas only. It does not include conventional requirements for electrical installations nor the testing and certification of electrical apparatus. It does not cover Group I (applications for mines susceptible to firedamp) apparatus. The report supplements the requirements laid down in IEC Publication 364-6-61.”

(Note: IEC364: 1986: Electrical installations of buildings. Part 6: Verification. Chapter 61: Initial Verification.)

Mining applications require a somewhat different approach because the construction of equipment to suit mining environments is peculiar to that industry.

The following definitions are taken from IEC 60079-17:

13.4 Definitions

The Standard states the following helpful definitions:-

Maintenance
A combination of any actions carried out to retain an item in, or restore it to, conditions in which it is able to meet the requirements of the relevant specification and perform its required functions.

Inspection
An action comprising careful scrutiny of an item carried out either without dismantling, or with the addition of partial dismantling as required, supplemented by means such as measurement, in order to arrive at a reliable conclusion as to the condition of an item.

Visual Inspection
An inspection which identifies, without the use of access equipment or tools, those defects, e.g. missing bolts, which will be apparent to the eye.

Close Inspection
An inspection which encompasses those aspects covered by a VISUAL INSPECTION and, in addition, identifies those defects, e.g. loose bolts, which will be apparent only by the use of access equipment e.g. steps (where necessary), and tools. CLOSE INSPECTIONS do not normally require the enclosure to be opened, or the equipment to be de-energised.

Detailed inspection
An inspection which encompasses those aspects covered by a CLOSE INSPECTION and, in addition, identifies those defects, e.g. loose terminations, which will only be apparent by opening-up the enclosure, and/or using, where necessary, tools and test equipment.

Initial inspection
inspection of all electrical equipment, systems and installations before they are brought into service

Periodic inspection
inspection of all electrical equipment, systems and installations carried out on routine basis

Sample inspection
inspection of a proportion of the electrical equipment, systems and installations

Continuous supervision
frequent attendance, inspection, service, care and maintenance of the electrical installation by skilled personnel who have experience in the specific installation and its environment in order to maintain the explosion protection features of the installation in satisfactory condition

Skilled personnel
people who meet the requirements for the qualification of personnel in accordance with 4.2

Technical person with executive function
that person providing technical management of the skilled personnel, having adequate knowledge in the field of explosion protection, having familiarity with the local conditions, having familiarity with the installation and who has overall responsibility and control of the inspection systems for the electrical equipment within hazardous areas

13.5 Layout of the standard

The objective of this Standard may be interpreted as providing guidance on the identification of defective apparatus such that it may be maintained to acceptable levels of safety.

13.5.1 General requirements

The sub-clause headings of Clause 4 are stated below:-

4.1 Documentation
4.2 Qualifications of personnel
4.3 Inspections and types
4.4 Regular periodic inspections
4.5 Continuous supervision by skilled persons
4.6 Maintenance requirements
4.7 Environmental conditions
4.8 Isolation of equipment
4.9 Earthing and equipotential bonding
4.10 Conditions of use
4.11 Movable equipment and its connections
4.12 Inspection schedules (Tables 1 to 3)

The recommendations contained in each of the subsequent sections provide guidelines for user companies to organise themselves to ensure that equipment remains safe.

The Standard is not prescriptive in that it dictates how tasks must be done in finite terms. It lays out good terms of reference, which can be built into the management of users’ hazardous areas. The user must read, interpret and act upon the statements contained in the document.

Emphasis must be placed on the two important initial criteria at the beginning of Clause 4 which are the first two items discussed below.

13.5.2 Documentation

Adequate, up-to-date documentation must be available, specifically including previously recorded information. ‘Sufficient’ information should be available in order to enable maintenance to be carried out on hazardous area apparatus ‘in accordance with its type of protection’. Knowledge of classification of the plant is also required in the form of Area Classification Drawings.

Interpreting the intent of this Clause and pre-empting others to be discussed subsequently, the form of presentation of the documentation to ‘an inspector’ must be carefully considered. In a larger organisation, it may be more efficient for an ‘inspection planning authority’ to prepare detailed information (taken from manufacturer’s information, certificates, drawings, Hazardous Area Drawings , etc.) on a ‘check’ sheet and to place it in a format to focus the inspector on what needs to be examined for each piece of equipment or each type of equipment. This would incorporate the tables in Clause 4.12 (to be shown later in this chapter). This is as opposed to providing the inspector with a large bundle of papers and expecting him/her to sort this out for him/herself.

In smaller organisations where manpower is not available to pre-process the required information, additional training (and more time) may need to be given to inspectors to search through documents for the correct information against which to validate the installation.

Inspection cannot be done without documentation, against which to determine that the installation is correct.

13.5.3 Qualifications

Emphasis is placed on the competency of personnel undertaking inspection and maintenance tasks to ascertain that they are suitably qualified and experienced, having undergone training and appropriate refresher training on a regular basis. Annex B of this CoP states requirements for ‘responsible persons and technical persons with executive function’. This inclusion in the competency requirements looks at the role of technical and managerial staff in providing guidance and supervision.

13.5.4 Inspections

There are three ‘grades’ of inspection. The ‘type’ refers to the point-in-time or period of inspection and is laid out in Figure 13.2.

Figure 13.2
Inspection requirements: Grades versus types

The discussion that follows assumes that decisions are made by the plant owner about how safety is to be ensured in a given installation. This may vary from one location on a plant to another. The standard recognizes that each particular installation is unique and it does not attempt to dictate how, when, where and what to inspect and maintain, simply because of the differences encountered in each situation. The responsibility for safety, of which the inspection and maintenance is deemed to play a major part, is always placed upon the owner of the plant. The owner may, perhaps rightly, delegate this detailed decision-making to the plant operating organization that has a detailed knowledge of the process.

After installation and prior to any hazard being introduced onto a plant, there is a specific requirement to perform a ‘Detailed’ inspection (see Figure 13.3). Verification that the installation fully agrees with the documentation must be sought as a starting point in the life of the plant. The results of this as with all inspections shall be recorded.

After this ‘initial’ inspection and when the plant is in service, decisions on periods/types/grades of inspection will be based on the experience of the use of the equipment on the plant. The results of any inspection must be reviewed by a competent authority. The outcome of the review will initiate maintenance procedures and will also consider whether there needs to be any adjustment in the inspection depth or period. Justification of these changes may be necessary by further safety audits. There are obviously cost implications in the performance of inspections and any changes to be made. Appropriate levels of management should be involved in these reviews.

Figure 13.3
Inspection requirements

Where safety issues cannot be verified by a visual or simple physical inspection process, then it may be necessary to measure parameters.

13.5.5 Continuous supervision by skilled persons

Where an installation is visited on a regular basis, in the normal course of work, by skilled personnel then it may be possible to dispense with regular periodic inspection and utilize the frequent presence of the skilled personnel to ensure the on going integrity of the equipment. More precise conditions and terms are stated but not reproduced here.

The objective of continuous supervision is to enable the early detection of arising faults and their subsequent repair. It makes use of existing skilled personnel who are in attendance at the installation in the course of their normal work (e.g. erection work, alterations, inspections, maintenance work, checking for faults, cleaning work, control operations, switching operations, making terminal connections and disconnections, setting and adjustment work, functional tests, measurements) and who use their skill to detect faults and changes at an early stage.

Allied to this discussion, and of interest to inspection planners, may be the statement in Clause 5.3.1 which states:-

“Where the intelligence incorporated in the system permits the frequent monitoring of the status of an instrument loop, some parts of the inspection procedure may be waived. For example, if an installation can confirm the presence of a specific instrument by checking a unique serial number, there is no necessity to read the label periodically.”

From this, it is arguable that Smart instrument systems with communication facilities can be treated as equipment subject to continuous supervision of a form that reduces the inspection burden.

13.5.6 Maintenance requirements

Where faults are found then remedial action must be invoked and safety is to be preserved whilst it is carried out.

13.5.7 Environmental conditions

Aspects of environmental conditions that may affect Ex Protection integrity must be examined but these precautions may be peculiar to the installation and siting of the equipment. Specific information must be given to the inspector in this case.

13.5.8 Isolation of equipment

Except with Ex i systems, electrical isolation must be carried out whilst work on equipment is undertaken.

13.5.9 Earthing and equipotential bonding

Care shall be taken to ensure that the earthing and potential equalization bonding provisions in hazardous areas are maintained in good condition.

Where measurement is deemed necessary then appropriate and safe means of measurement will need to be specified and carried out by competent personnel. The expected maximum values will be declared by the responsible electrical authority based on the supply design capacity, etc.

13.5.10 Conditions of use

Specific conditions of use apply to any type of certified explosion protected equipment where the certificate number has a suffix marking of ‘X’. The certification and instruction documents shall be studied to ascertain the specific conditions of use.

13.5.11 Movable equipment and its connections

Precaution shall be taken to ensure that movable electrical equipment (portable, transportable and hand held) is used only in locations appropriate to its type of protection, equipment group and surface temperature.

13.5.12 Inspection schedule Tables (Tables 1 to 4)

Tables of key inspection activities are included in the Standard. These list the particular requirements that each type of protection must be examined

The table contents are divided into critical areas, A, B and C, as follows;
A General (All Equipment),
A Equipment specific
B Installation - General
B Installation – specific equipment
C Environment
The purpose of this is to differentiate between aspects of an installation, dependent on the type of protection and the consideration of where it is located.

Within each area above there is a set of requirements to check individual aspects considered of importance. Each aspect comprises a statement now given a unique reference, such as A3 or B12 etc., so that it may be referred to easily. It would be declared as necessary or unnecessary to check, depending on the type of protection. The tables are given at the end of this chapter.

The tables in this Standard have been prepared with both Initial Inspection and Periodic inspection requirements in mind. The Initial Inspection requirements are now being included in the construction standards, e.g., IEC60079-1:2015 for Flameproof Equipment. The same reference system explained above is common to all tables. Where necessary the plant owner can implement additional aspects depending on the specific equipment being inspected. The Standard states:-

Items identified in the Tables 1 to 3 and 4.12.2 to 4.12.11 detail only key items related to hazardous area integrity. Other items may also apply along with specific details from the manufacturer’s instructions and application requirements. Inspection schedules should be modified accordingly to suit the specific installation requirements.

13.6 Types of protection

Clause 5 in the Standard refers to additional inspection schedule requirements. This considers each particular type of Ex equipment and what must be examined when inspecting and maintaining it.

5.1 Types of protection ‘d’ Flameproof enclosure (see table 1 and IEC 60079 1)
5.1.1 Flameproof joints (see IEC 60079 1)
5.2 Type of protection ‘e’ Increased safety (see table 1 and IEC 60079 7)
5.2.1 Overloads
5.3 Type of protection ‘i’ Intrinsic safety (see table 2 and IEC 60079 11)
5.3.1 General
5.3.2 Documentation
5.3.3 Labelling
5.3.4 Unauthorized modifications
5.3.5 Associated apparatus (safety interface) between intrinsically safe and non intrinsically safe circuits
5.3.6 Cables
5.3.7 Cable screens
5.3.8 Point to point connections
5.3.9 Earth continuity of non galvanically isolated circuits
5.3.10 Earth connections to maintain the integrity of the intrinsic safety
5.3.11 Intrinsically safe circuit earthing and/or insulation
5.3.12 Separation between intrinsically safe and non intrinsically safe circuits
5.4 Type of protection ‘p’ and ‘pD’ Pressurized enclosure (see table 3 and IEC 60079 2 or IEC 61241 4)
5.5 Type of protection ‘n’ (see table 1 or 2 and IEC 60079 15)
5.5.1 General
5.5.2 Restricted breathing enclosures
5.6 Type of protection ‘tD’ Protection by enclosure (see table 4 and IEC 61241 1)
5.7 Types of protection ‘m’ and ‘mD’ (encapsulation), ‘o‘, (oil-immersion)’op’ (optical radiation) and ‘q’ (powder-filling)
Tables have not been prepared to illustrate the inspection requirements for ‘m’, ‘mD’, ‘o’, ‘op’ and ‘q’ types of protection. Table 1 should be utilised as appropriate for the enclosure and its contents.
  NOTE Inspection details for type of protection ‘o’ are under consideration in IEC 60079-6.
6 Inspection schedules

13.7 Insulation testing

Insulation testing on electrical cables is necessary to ensure that cables are undamaged after installation. The level of testing is based on National Electrical Regulations for the voltage and duty of the circuit. This is normally performed at high voltages and can only be done safely when there is no hazard present on the site.

The use of high voltage tests using Intrinsically Safe to the

The requirement for cables to meet a 500 V ac insulation test for 1 minute expresses the expected grade of cable required to preserve safety integrity. The safety of IS circuits, however, does not depend on a minimum finite insulation resistance level. Faults from core to core of a cable in the same IS circuit are taken into account and cannot cause ignition by design. Faults from cores to “earth” are also designed to be safe by imposing a regime that minimizes risk. It is conceivable that a fault to earth at the same time as other faults occur on a loop could pose a danger but these risks are minimized.

There is no specification for leakage resistance in any of the standards because this is an operational consideration. Poor insulation on RTD circuits will affect loop accuracy whereas Switch-Status loops are incredibly tolerant. The insulation resistance of a cable may degrade over time owing to ageing and environmental attack. It is therefore important to interpret the requirement for inspection as detecting the collapse of cable integrity by the precursor of monitoring the long term trend of insulation even though it is of operational interest.

The rigour with which insulation testing is performed depends on the environmental conditions of the plant. It is usual to apply “sample” inspection testing on cables thought most likely to be affected. It is not recommended that all cables are periodically tested.

The testing of all loops and cables must be performed after a new installation is completed. This is easy to do whilst the plant is not hazardous (during ‘closed’ commissioning); however if the new plant is an extension to an existing installation that is already hazardous, care must be taken with measurement procedures to ensure safety (see Figure 13.4).

Figure 13.4
Inspection of IS earth integrity

13.8 Maintenance

There has been a significant change in the second issue of IEC 60079-17: ‘Maintenance Recommendations’ in the 1990 issue. This has become ‘Maintenance Requirements’ in the second issue in 1996 and in subsequent revisions. It has therefore become more detailed and more stringent in response to earlier criticism with respect to maintenance clauses.

13.8.1 General requirements

The general requirements for all types of protection are summarized as follows:

  • Remedial action is to be carried out where necessary. Caution is expressed in that safety must be preserved prior to and during remedial work.
  • Flexible cables are particularly prone to damage and may require special consideration in that more frequent inspection may be necessary.
  • If any equipment is taken out of service then disconnected cables must be dealt with by correct termination in an appropriate enclosure, being correctly isolated and insulated or being correctly isolated and earthed.
  • Where special bolts and other fastening or special tools are required, these items shall be available and shall be used.
  • Environmental conditions including ambient temperature and chemical attack effects are required to be examined and verified for suitability.
  • All parts of installations shall be kept clean, particularly where accumulations of substances can cause heat build-up.
  • Equipment generally should be undamaged. Weather proofing applied to equipment should be maintained effectively.
  • Care shall be taken in respect of anti-condensation devices operating, vibration not loosening fixings and the generation of static electricity.

This standard expressly permits that live working may be carried out on IS apparatus and systems.

The introduction to this document supports the view that quality and not quantity of detective work in maintaining a plant is necessary.

13.9 Testing

The testing of circuits referred to in the standards, primarily involves insulation testing. This should be undertaken with normal High Voltage test equipment at the pre-commissioning (‘detailed’) inspection stage. Immediately afterwards it is recommended that low voltage tests are carried out using certified equipment and some comparison is made. The results shall be recorded as required.

Thereafter periodic testing may be performed using low voltage (Certified apparatus) test equipment whilst operating to safe procedures even though the plant is hazardous.

The testing of other parameters is discussed in section 10 of this manual.

13.9.1 Trends

The standard correctly infers that the trend of performance of insulation is of the greatest interest. If the sample of measurements show a trend of maintaining adequate insulation properties then there is no cause for concern. Where deterioration is seen, the mechanism of that deterioration should be located and prevented. Alternatively, more suitable apparatus may have to be installed.

13.9.2 Useful data for collection

Other useful phenomena to record are the oscilloscope wave-forms seen between the various earths. Interference patterns can be traced by observing new wave-forms and disconnecting equipment power supplies and earths until a previous situation’s wave-forms are seen. In that case the last earthing system attached will somehow be the cause of the problem and further investigation may start from that point. This technique has been successfully used in the past but it requires a thorough and diligent approach to the investigation with due regard to safety. The use of equipment in the safe area earthing arrangements does not require any assessment. Disconnection of earths for tracing purposes will require careful evaluation.

13.10 Unauthorised modification

The Standard states that inspection should look for signs of unauthorized modification but accepts that it is difficult. This form of inspection would need to be done by a person adequately trained on the specific apparatus type. This may need to be the manufacturer or his representative. The likelihood of this occurrence may be assessed and if adequate systems are in place to ensure that apparatus is repaired by the manufacturer then this level of inspection is not required.

13.11 Earthing integrity verification

Clause 5.3.9 and subsequent of this Standard requires some clarification. Summarising the requirements: where a barrier is used to interface a hazardous area circuit, it requires initial verification that the barrier is in fact connected to ‘earth’. In addition, any parts of the circuit in the hazardous area, or on associated apparatus in the safe area, where parts are earthed, and the connection must be verified. IS approved test devices must, according to the requirements, be used for these purposes.

It goes on to say that if disconnection of the protective earth system is necessary to do this then the hazardous area must be made safe or the power must be removed from the instrument section under test. This shall be done on a sample basis.

As has been discussed, this requirement is obviated by the use of circuits that operate in a self-revealing fault mode. The use of intelligently assigned and set up alarms will assist in providing failure data. This is the current trend in system design.

Note that the standard is unspecific on values, which are acceptable or unacceptable. This is one example of where the document has not succeeded in galvanising the requirements of many different countries. There are divisions even within Europe about the methodology of the earthing requirements. This standard is less specific than existing national standards and therefore has not been adopted.

Inspection procedure

Generally dividing the installations or plant in a geographical manner and then carrying out the appropriate inspections has been found to be more economical than based on a system approach. In checking the installation by systems, it can be time consuming as each system often occupies more than one physical location.

The format of the procedure should be devised such that positive reporting is done. This means compliance of all systems and the full installation with the requirements is ensured. In negative reporting, only non-compliances are reported and a doubt can linger as to whether the full installation has been inspected from every aspect of safety or not.

A suggested method could be:

  • Divide the Plant into proper logical locations.
  • Detail all parts of the electrical installation in sheets of inspection.
  • These sheets should detail type of inspection to be carried out on each apparatus or system or installations in the area identified.
  • The sheets should clearly specify the method of reporting so that no ambiguity remains in the recording.
  • Record compliance and non-compliance separately. This will ensure quick generation of a punch list and its rectification and subsequent re-inspection for compliance.
  • A procedure to integrate all such reports and linking them to each part of installation to allow for verification of the completeness of the inspection process.

13.12 Need for inspection and maintenance

Having stated that ‘Loss prevention’ is one of the main objectives of any ongoing enterprise, it is needless to emphasize that this can be only achieved if all the apparatus, equipment and systems for hazardous area are designed, selected and installed with painstaking care. The equipment must also be maintained as close as possible to the state it was when it was new.

Electrical installations, which comply with standards and regulations, or with other statutory documents dealing with the same subject, possess features specially designed to render them suitable for operation in hazardous areas. It is essential for reasons of safety in those areas that throughout the life of such installations the integrity of those special features is preserved. They therefore require systematic inspection and if necessary, maintenance both initially and at regular periodic intervals thereafter.

It should be noted that the correct functional operation of hazardous area installations does not mean, and should not be interpreted as meaning, that the integrity of the special features referred to above has been preserved.

The features of equipment, which qualify it to be classified as explosion protected, are necessarily in many cases not forming part of its satisfactory operational features. Thus satisfactory operation of apparatus or equipment does not ensure that all the explosion related features are intact. Thus in most cases there is no warning of failure of these features and the apparatus may continue to operate while having faults, which make it ignition capable.

The above makes it imperative that a formal and exhaustive documentation procedure should be put in place. This should also cover the requisite procedures for inspection, testing and certification wherever necessary for the following:

  • Commissioning and test verification
  • Operation,
  • Storage of inflammable material
  • Maintenance schedule
  • Authorized modifications
  • Overhaul and repair

It should be noted that the required certificate / documentation needs to be issued for the maintenance schedule and authorized modifications and these should be included in a Hazardous Area verification dossier (this dossier is mandatory as per the new regulations). Normal overhaul and other activities need also to be suitably documented.

The facility in charge shall nominate a person (or persons) having the appropriate knowledge and experience of hazardous area installations to be responsible for establishing and implementing a system of inspection and maintenance, which shall keep the plant operating safely. Clauses below give guidance on the essential elements of an appropriate system.

13.12.1 Competency and training of personnel

The inspection, maintenance, replacement and repair of apparatus, systems and installations shall be carried out only by personnel whose training has included instruction on the various methods of safeguarding and installation practices and, in appropriate cases, on the general principles of area classification. Appropriate refresher training shall be given to such personnel on a regular basis at intervals, which should not exceed say two years; records being kept of the training given.

Where the inspection of apparatus, systems and installations associated with hazardous areas is carried out by a contractor, the contract shall include a statement to the effect that the inspection shall be carried out only by personnel whose training shall have included instruction on the various methods of safeguarding; evidence of which shall be supplied by the contractor.

13.12.2 Basis and frequency of inspection

A planned scheme of regular routine inspection is the basis of effective maintenance and whilst production requirements need to be considered they shall not result in the postponement of essential inspection and remedial work.

A principal requirement of any scheme is the keeping of records which for this purpose shall as a minimum include for each plant appropriate details of:

  • Up-to-date area classification
  • All apparatus, systems and installations
  • Inspections carried out and faults revealed
  • Corrective action taken to remedy such faults.

13.12.3 Type of inspection with specific reference to hazardous area

Three forms / types of inspection which are given hereunder can be adopted as required by Plant Operation and Maintenance personnel:

1. Initial inspection

All apparatus, systems and installations shall be inspected on initial installation and after modification, in accordance with the ‘Initial’ inspection schedules and the details shall be recorded. These should be carried out as near to commissioning of plant as possible but before flammable or explosive materials are introduced into the plant, so as to avoid any subsequent inadvertent alterations going unnoticed.

We shall now look into some specific guidelines for initial inspection in hazardous area as given hereunder. It should be noted that these are indicative and comprehensive guidelines of manufacturers should be adhered to in addition to the ones given below.

General Initial checks and Inspection checklist

  • External visual inspection
    • Match the protection concept as labelled on the apparatus with the area classification of its location.
    • Match the sub-group (in case the protection concept is subject to a sub-group) as labelled on apparatus with the area classification of its location.
    • Match the surface temperature class as labelled on the apparatus with the appropriate explosive gas, liquid or vapour present in the area classification of its location.
    • The apparatus carries the correct circuit (installation) identification. All installed apparatus / enclosures must be tagged so that they are uniquely identified to trace them at a later date so as to either ensure or cross-check for correct selection and installation information. This is termed as the inspection or conformance of the individual installation with respect to what has been envisaged at the design stage.
    • The initial external visual inspection to ensure that the apparatus is free of corrosion and / or undue dust accumulation or material (corrosion agents) which could lead to corrosion at a later date.
    • To ensure that all cable glands, stoppers and their fastening nuts and bolts are secure. The integrity of the earth connection including its proper tightness to the enclosure / apparatus need to be ensured.
    • To ensure that all cable trays, external special guards around the apparatus, if any, are properly secure and undamaged.
    • The cables and conduits are not damaged. In the case where cables are led to the enclosure through sand filled trenches it is not necessary to remove the sand until physical evidence of any abnormality is noticed.
    • To ensure that the electrical protection is set properly. This is to include checks of fuses, circuit breakers, protection relays etc. Proper labelling of these devices needs to be ensured such that they are clearly identifiable without the need to touch the devices or the need to remove any devices such as fuses etc.
    • To ensure that an external application to prevent corrosion is properly applied, such as –grease applied to gaskets and joints; tape to glands etc.
  • Internal Inspection
    • To inspect for undue accumulation of dust, dirt or corrosion agents inside the enclosure. To check for any damages to gaskets, tightness of electrical connections (to guard against the sparking etc.). Being an internal inspection will require proper electrical isolation of equipment and removal of covers as per requirements. Use of proper tools is a pre-requisite for checking the tightness.
    • Another internal inspection is for checking of the correct installation of lamps / bulbs as per the design. This also necessitates removal of the cover and isolation of electrical apparatus from service and supply.
    • In the case where encapsulating material is used in apparatus / enclosures, cable boxes and stoppers, it should be ensured that the filling is proper. The covers need to be removed for a visual inspection inside the enclosure and requisite isolation done. Generally a visual inspection is sufficient as long as no deterioration or abnormality is observed.
  • Either by internal or external inspections the motor air-gaps and other radial and axial clearances of motors need to be checked against the manufacturer’s design data sheet. This record will help to determine undue wear and tear or distortion, which could be indicative of impending failure in terms of, sparks due to rubbing between rotating parts (such as fan blades, bearing grease guards, rotor body etc.) or any other abnormality.
  • The integrity of the apparatus with respect to design parameters needs to be confirmed with respect to a hazardous dossier and it should be ensured that there are no ‘unauthorised’ modifications. This can be done through internal and external inspections.
  • To ensure by inspection and checking that:
    • Potential equaliser system connections are secure and undamaged
    • Earthing connections are undamaged and secure
    • All enclosure fittings are properly bonded to earth and are secure and undamaged

The initial inspection should also include in addition to the above checks certain other checks, which are specific to the protection concepts in question. Some illustrative checks required to be carried out for important protection concepts are described hereunder:

Flameproof enclosures:

  • To ensure that enclosures, fixing bolts, enclosure glasses, any glass/metal seals are not damaged by external visual inspection.
  • To ensure that the gaps in enclosures are not unduly obstructed by paint or dirt / dust – of which a certain amount is permitted but the gaps are not allowed to be covered with rust.
  • To ensure that the enclosure bolts are of specified tensile strength. Checking the head marking of such bolts can do this.
  • To ensure that there are no unacceptable external obstructions to flamepaths. In order to achieve this, an examination needs to be made for any external hindrances from the obstructions, which are not part of the enclosure but within the specified distance.
  • To ensure that the cable glands and stopper boxes are of the correct type by examining their markings.
  • To ensure that the flange gap dimensions are within the maximum specified, by checking with the help of feeler gauges etc. It is to be noted that there is no specified minimum for this criteria. It may not be possible and necessary to check in the case of cylindrical gaps and spigot joints as they do not get distorted easily.
  • An internal inspection of flamepaths, internal parts of glasses and glass/metal seals needs to be done for any damage and corrosion effects. Being an internal inspection, the covers need to be removed and requisite isolations also need to be done before inspection.

Intrinsically safe installation:

In the case of an internal inspection of an intrinsically safe device or apparatus, isolation is not required, if in the same enclosure:

  • there are no non-intrinsically safe circuits at potentials which constitute risks of shock; or
  • the apparatus is associated apparatus
  • To ensure that there are no unauthorised connections to the potential bonding / equalisation system by carrying out internal inspection. In particular multi-circuit junction boxes need to be checked thoroughly.
  • To ensure requisite segregation is maintained in multi-circuit junction boxes between intrinsically safe and non-intrinsically safe circuits.
  • To ensure that the barrier devices are of correct type and are correctly mounted.
  • To ensure that the cables are connected and segregated correctly. Also, the conduits are likewise carrying properly segregated cables and wires.

Pressurised enclosures:

  • To ensure through external inspection that the inlet and exit ducting is not damaged. Because any gas leakage from the ducting is likely to contaminate either enclosure or exhaust area depending upon the operating system.
  • To ensure that the gas or compressed air supply is given to enclosure free from any contamination and of the appropriate quality. This can be ensured by checking for any exhausts, which can contaminate the air in the area of inlet suction of compressor, and / or the specified gas cylinders are only connected to enclosure.
  • To ensure proper / correct operation of pressurising and flow monitoring devices by checking suitably the operation of interlocking protections.
  • To ensure that adequate and requisite flow and pressure is maintained of air or gas, including purging requirement if any.

Oil-immersed and powder filled apparatus:

  • To ensure that the oil seals and powder retention covers are in a good condition by examining visually and physically.
  • To ensure that there is no leakage of powder or oil from the enclosure.

ii) Inspection after apparatus repair

In the case of repair, adjustment or replacement carried out on any apparatus system or installation, it shall be checked in accordance with the relevant items as per the ‘Initial’ inspection schedule. These checks may be carried out by the person doing the work and need not be recorded.

It has to be ensured that any repair does not change the integrity of the apparatus, vis-à-vis the protection concept as approved and / or certified and documented in the Hazardous Area Dossier of the Plant.

iii) Inspection after change in area classification, sub-group or surface temperature classification

This inspection should ensure that the installation, equipment and apparatus are in conformance to the new classification and are appropriate. This needs to be in particularly checked for sub-group and temperature classification.

iv) Periodic / routine inspection

All apparatus, systems and installations shall be inspected in accordance with the ‘Periodic / Routine’ inspection schedules and the details shall be recorded. These inspections are required to be done to identify the deterioration in installation conditions due to operating conditions or environment or unauthorised modifications.

The nominated person(s) shall determine the frequency of ‘Periodic / routine’ inspections.

Experience has shown that there is a point beyond which increasing the frequency of inspection does not decrease the significant fault rate. The nominated person may extend the interval between ‘Periodic’ inspections if the significant fault rate is not increased consequently.

In extremely adverse conditions, the interval between ‘periodic’ inspections may be as low as three months but should not normally exceed two years. However in extremely good, stable environmental conditions, the interval between inspections may be extended to four years.

Where the interval between ‘Periodic’ inspections exceeds two years, ‘Visual’ inspections shall be carried out as defined below.

It should be noted that a ‘significant fault’ is one in which the certified integrity of the apparatus or system (insofar as it affects safety in flammable atmosphere) is impaired and the significant fault rate is the number of items having significant faults expressed as a percentage of the total number of items inspected.

v) Visual inspection from floor level

The majority of faults on apparatus, systems and installations, which remain undisturbed, are caused by environmental factors and most of these are detectable by a ‘Visual’ inspection from floor level.

Where the ‘Periodic’ inspection interval exceeds two years all apparatus, systems and installations shall be inspected in accordance with the ‘Visual’ inspection schedules and at an interval not exceeding half that determined for the ‘Periodic’ inspection. The details shall be recorded.

Following a major shutdown in a hazardous area all apparatus, systems and installations in that area should be inspected in accordance with the visual inspection schedules. The details should be recorded.

Guideline for routine or visual inspection

The following can act as guidelines for the routine or periodic or visual inspections of installations:

  • To check for deterioration in enclosures, fittings, conduits, cable glands, cable boxes due to corrosion effect.
  • To check for undue accumulation of dust and dirt in cable trays, enclosures, conduits etc. Because these can harbour corrosive liquids and solvents.
  • To check for any physical damages to enclosures, conduits, cables, cable trays etc.
  • To check for any leakage of oil, powder, sand from apparatus having moving parts.
  • To check for looseness in enclosure fittings, mountings, glands, stoppers etc.
  • To check for any deteriorating gaskets thus exposing the components housed in enclosure to harsh environment.
  • To check for any excessive vibrations at the point of mounting which may lead to loosening of cable or conduit connections in enclosures and rotating equipment.
  • To check for the condition of bearings to ensure that no overheating, rubbing or seizing occurs.
  • To check for any abnormal leakage or loss in level of oil or powder or sand indicating that the protection is deteriorating.
  • To check for proper functioning of relays & protection, safety devices used to ensure safety of apparatus, equipment and plant installation.
  • To check for any loose electrical connection in particular with equipotential bonding.
  • To check for any unauthorised changes, such as – in fuse rating or bulbs / lamps.

13.12.4 Periodic inspection

Where apparatus has been operated in service, the above points are mentioned as forming the basis of the periodic inspection without the need for the initial classification audit. In addition, and with the benefit of experience, company’s written procedures often include specific checks for contravention of creepage and clearance distances on termination system during inspections that could occur as a result of mechanical damage.

The recording of results is required by an effective and verifiable means.

Clause 21 begins by saying that, “Where compliance with the appropriate documentation cannot be demonstrated by physical inspection, electrical testing should be carried out in accordance with....” other requirements in Section 21. Little of this advice has been brought into IEC 60079-17. It has however been interpreted in section 10 of this manual.

Figure 13.5
Inspection requirements

Figure 13.5 suggests acceptability bounds for insulation testing that are not in the standards but have evolved as experienced based ‘rules of thumb’.

13.13 IEC inspection tables

In the following tables, reproduced from IEC60079-17:2013, the grade of inspection is as follows:
D = Detailed
C = Close
and
V = Visual.

The purpose of the tables is to prompt the person responsible for organising the inspection function into giving guidance on what to instruct an inspector to look for. It is only a skeleton structure. It will require amplification to suit a particular installation with more specific detail included. Access to supporting documentation will be necessary.

To some extent it depends on the familiarity of the inspector together with the training and experience of that individual as to what level of information will be needed.

The sample tables published were never intended to be copied and made into check lists without further consideration of content but all too often this happens. An inspector can only inspect what is found against supplied documentation to ensure that it is the correct item in the correct location and in the correct state. Experience has determined that a box-ticking approach is insufficient and comments should be solicited from inspecting staff to ensure that diligent examination has taken place. A judgement on the condition of the item is only served by thorough observation.

Table 1
Inspection schedule for Ex “d”, Ex “e” and Ex “n” installations
Check that:
(D = detailed, C = close, V = visual)
Ex “d” Ex “e” Ex “n”
Ex “t/tD”
Grade of inspection
D C V D C V D C V
A GENERAL (ALL EQUIPMENT)                  
1 Equipment is appropriate to the EPL/Zone requirements of the location X X X X X X X X X
2 Equipment group is correct X X   X X   X X  
3 Equipment temperature class is correct (only for gas) X X   X X   n n  
4 Equipment maximum surface temperature is correct (only for “t/tD”)             t t  
5 Degree of protection (IP grade) of equipment is appropriate for the level of protection/group/conductivity X X X X X X t t t
6 Equipment circuit identification is correct X     X     X    
7 Equipment circuit identification is available X X X X X X X X X
8 Enclosure, glass parts and glass-to-metal sealing gaskets and/or compounds are satisfactory X X X X X X X X X
9 There are no unauthorized modifications X     X     X    
10 There is no evidence of unauthorized modifications   X X   X X   X X
11 Bolts, cable entry devices (direct and indirect) and blanking elements are of the correct type and are complete and tight                  
  – physical check X X   X X   X X  
  – visual check     X     X     X
12 Threaded covers on enclosures are of the correct type, are tight and secured                  
  – physical check X X              
  – visual check     X            
13 Joint surfaces are clean and undamaged and gaskets, if any, are satisfactory and positioned correctly X                
14 Condition of enclosure gaskets is satisfactory X     X     X    
15 There is no evidence of ingress of water or dust in the enclosure in accordance with the IP rating X     X     X    
16 Dimensions of flanged joint gaps are:
– within the limits in accordance with manufacturer’s documentation or
– within maximum values permitted by relevant construction standard at time of installation or
– within maximum values permitted by site documentation
X                
17 Electrical connections are tight       X     X    
18 Unused terminals are tightened       X     n    
19 Enclosed-break and hermetically sealed devices are undamaged             n    
20 Encapsulated components are undamaged       X     n    
21 Flameproof components are undamaged       X     n    
22 Restricted breathing enclosure is satisfactory             n    
23 Test port, if fitted, is functional             n    
24 Breathing operation is satisfactory X     X     n    
25 Breathing and draining devices are satisfactory X X   X X   n n  
  EQUIPMENT SPECIFIC (LIGHTING)                  
26 Fluorescent lamps are not indicating EOL effects       X X X X X X
27 HID lamps are not indicating EOL effects X X X       t   t
28 Lamp type, rating, pin configuration and position are correct X     X     X    
  EQUIPMENT SPECIFIC (MOTORS)                  
29 Motor fans have sufficient clearance to the enclosure and/or covers, cooling systems are undamaged, motor foundations have no indentations or cracks. X X X X X X X X X
30 The motor is clean with no signs of vermin or insect activity or other blockages to impede the ventilation airflow X X X X X X X X X
                     
32 Insulation resistance (IR) of the motor windings is satisfactory X     X     X    
B INSTALLATION – GENERAL                  
1 Type of cable is appropriate X     X     X    
2 There is no obvious damage to cables X X X X X X X X X
3 Sealing of trunking, ducts, pipes and/or conduits is satisfactory X X X X X X X X X
4 Stopping boxes and cable boxes are correctly filled X                
5 Integrity of conduit system and interface with mixed system maintained X     X     X    
6 Earthing connections, including any supplementary earthing bonding connections are satisfactory (for example connections are tight and conductors are of sufficient cross-section)                  
  – physical check X     X     X    
  – visual check   X X   X X   X X
7 Fault loop impedance (TN systems) or earthing resistance (IT systems) is satisfactory X     X     X    
8 Automatic electrical protective devices are set correctly (auto-reset not possible) X     X     X    
9 Automatic electrical protective devices operate within permitted limits X     X     X    
10 Specific conditions of use (if applicable) are complied with X     X     X    
11 Cables not in use are correctly terminated X     X     X    
12 Obstructions adjacent to flameproof flanged joints are in accordance with IEC 60079-14 X X X            
13 Variable voltage/frequency installation complies with documentation X X   X X   X X  
  INSTALLATION – HEATING SYSTEMS                  
14 Temperature sensors function according to manufacturer’s documents X     X     t    
15 Safety cut off devices function according to manufacturer’s documents X     X     t    
16 The setting of the safety cut off is sealed X X   X X        
17 Reset of a heating system safety cut off possible with tool only X X   X X        
18 Auto-reset is not possible X X   X X        
19 Reset of a safety cut off under fault conditions is prevented X     X          
20 Safety cut off independent from control system X     X          
21 Level switch is installed and correctly set, if required X     X          
22 Flow switch is installed and correctly set, if required X     X          
  INSTALLATION – MOTORS                  
23 Motor protection devices operate within the permitted tE or tA time limits.       X          
C ENVIRONMENT                  
1 Equipment is adequately protected against corrosion, weather, vibration and other adverse factors X X X X X X X X X
2 No undue accumulation of dust and dirt X X X X X X X X X
3 Electrical insulation is clean and dry       X     X    

NOTE 1 General: the checks used for apparatus using both types of protection “e” and “d” will be a combination of both columns.

NOTE 2 Items B7 and B8: account should be taken of the possibility of an explosive atmosphere in the vicinity of the apparatus when using electrical test equipment.

Table 2
Inspection schedule for Ex “i” installations
Check that: (D = detailed, C = close, V = visual) Grade of inspection
D C V
A EQUIPMENT      
1 Circuit and/or equipment documentation is appropriate to the EPL/Zone X X X
2 Equipment installed is that specified in the documentation X X  
3 Circuit and/or equipment category and group correct X X  
4 IP rating of equipment is appropriate to the Group III material present X X  
5 Equipment temperature class is correct X X  
6 Ambient temperature range of the apparatus is correct for the installation X X  
7 Service temperature range of the apparatus is correct for the installation X X  
8 Installation is clearly labelled X X  
9 Enclosure, glass parts and glass-to-metal sealing gaskets and/or compounds are satisfactory X    
10 Cable glands and blanking elements are the correct type, complete and tight – physical check – visual check X X X
11 There is no evidence of unauthorized modifications X    
12 There are no visible unauthorized modifications   X X
13 Diode safety barriers, galvanic isolators, relays and other energy limiting devices are of the approved type, installed in accordance with the certification requirements and securely earthed where required X X X
14 Condition of enclosure gaskets is satisfactory X    
15 Electrical connections are tight X    
16 Printed circuit boards are clean and undamaged X    
17 The maximum voltage Um of the associated apparatus is not exceeded X X  
B INSTALLATION      
1 Cables are installed in accordance with the documentation X    
2 Cable screens are earthed in accordance with the documentation X    
3 There is no obvious damage to cables X X X
4 Sealing of trunking, ducts, pipes and/or conduits is satisfactory X X X
5 Point-to-point connections are all correct (initial inspection only) X    
6 Earth continuity is satisfactory (e.g. connections are tight, conductors are of sufficient cross-section) for non-galvanically isolated circuits X    
7 Earth connections maintain the integrity of the type of protection X    
8 Intrinsically safe circuit earthing is satisfactory X    
9 Insulation resistance is satisfactory X    
10 Separation is maintained between intrinsically safe and non-intrinsically safe circuits in common distribution boxes or relay cubicles X    
11 Short-circuit protection of the power supply is in accordance with the documentation X    
12 Specific conditions of use (if applicable) are complied with X    
13 Cables not in use are correctly terminated X    
C ENVIRONMENT      
1 Equipment is adequately protected against corrosion, weather, vibration and other adverse factors X X X
2 No undue external accumulation of dust and dirt X X X

 

Table 3
Inspection schedule for Ex “p” installations (pressurization or continuous dilution)
Check that:
(D = detailed, C = close, V = visual)
Grade of inspection
D C V
A EQUIPMENT      
1 Equipment is appropriate to the EPL/zone requirements of the location X X X
2 Equipment group is correct X X  
3 Equipment temperature class or surface temperature is correct X X  
4 Equipment circuit identification is correct X    
5 Equipment circuit identification is available X X X
6 Enclosure, glasses and glass-to-metal sealing gaskets and/or compounds are satisfactory X X X
7 There are no unauthorized modifications X    
8 There are no visible unauthorized modifications   X X
9 Lamp type, rating, and position are correct X    
B INSTALLATION      
1 Type of cable is appropriate X    
2 There is no obvious damage to cables X X X
3 Earthing connections, including any supplementary earthing bonding connections, are satisfactory, for example connections are tight and conductors are of sufficient cross-section
– physical check
– visual check
X X X
4 Fault loop impedance (TN systems) or earthing resistance (IT systems) is satisfactory X    
5 Automatic electrical protective devices operate within permitted limits X    
6 Automatic electrical protective devices are set correctly X    
7 Protective gas inlet temperature is below maximum specified X    
8 Ducts, pipes and enclosures are in good condition X X X
9 Protective gas is substantially free from contaminants X X X
10 Protective gas pressure and/or flow is adequate X X X
11 Pressure and/or flow indicators, alarms and interlocks function correctly X    
12 Conditions of spark and particle barriers of ducts for exhausting the gas in hazardous area are satisfactory X    
13 Specific conditions of use (if applicable) are complied with X    
C ENVIRONMENT      
1 Equipment is adequately protected against corrosion, weather, vibration and other adverse factors X X X
2 No undue accumulation of dust and dirt X X X

Reproduced from IEC 60079-17:2012

Annex A (informative)

Typical inspection procedure for periodic inspections


14


Safe Working Practices

This chapter serves as a reminder of the principles involved in work practice with electrical equipment. It is by no means exhaustive. Safe systems of work and procedures must be written and communicated to personnel involved in managing and performing such activities. Where Hazardous Areas are encountered, the specific risk has to be considered in addition to normal electrical practice.

Learning objectives

  • To understand the need for plant safety rules
  • To note the need of risk assessment
  • To preserve the importance of the principles of observation

14.1 Introduction

The established emphasis placed on Health and Safety in current Regulations, Directives, Standards and Codes of Practice (CoP), demand competent personnel who are well-trained and under proper technical and managerial supervision to work in industry at all levels. The level of risk must be included in the equation required to set and to assess competence levels. Experience is a vital part of the competence issue. Management must take full responsibility for devising strategies to keep the plant and personnel safe whilst ensuring that the skills needed are developed in parallel with the plant operation needs.

The adoption of well tried and tested working methods and practices will help to minimise the risk. There is a danger that calling them ‘safe working practises’ gives a false sense of security; risk is ever-present and the approach must be to ‘minimise’ the risk to acceptable limits.

Industrial accidents caused by human action are known to be caused by one of two dominant situations:-

  • Not understanding the danger or the consequence
  • Ignoring them

There are, perhaps, three stages of information on which to call:-

Plant ‘Safety Rules’
Rules specific to each site devised to guide and inform workers of:

  • Routes of Technical authority
  • Routes of Managerial authority
  • Establishment of authority responsible for safety
  • Fault reporting procedure
  • Hazard reporting procedure
  • Work related communication protocol
  • Method of Signage
  • Location where information is kept
  • Safety procedures
  • Protective equipment
  • Change control procedures
  • Standards to which installation has been done
  • Access to Standards
  • Etc.,
    and the requirement to generate these rules for employees and contractors is embodied in the Law of most countries.

CoPs
Local, National or International Codes of Practices that give recommended guidance

Technical Documentation
A ‘Verification Dossier’ or ‘Explosion Protection Document’ harbouring detailed technical information specific to the installation on a given site.

14.2 Risk assessment

Adult human beings naturally perform ‘risk assessment’. Take, for example, the simple action of walking along. This requires a series of constantly updating assessments; looking for potential obstacles and preparing to take avoiding action, lest one should trip over. This is so automatic that it is not a conscious effort. It is instilled in us as children; we understand that if we are inattentive to our surroundings, then we can hurt ourselves.

When working with any potential threat, the risk posed must be understood and constantly assessed; this is the basis of all Health and Safety Regulations. The ability to assess is allied to skills and competence. It is a process which requires training and constant update with timely reminders.

14.3 General rules

Some obvious rules for ‘Safe Working’ are recommended in the following list:-

  • If something is noticed that is considered unsafe, report it to an authority with responsibility for safety.
  • Never work alone, work as a team
  • Never receive an intentional shock (e.g. by testing)
  • Only work on, operate, or adjust equipment upon which training has been given
  • Don't work on energised equipment unless absolutely necessary
  • Keep loose tools, metal parts, and liquids from the electrical equipment
  • Use correct equipment to make measurements. Multi-meters can be used to measure voltage whilst set to reading current!
  • Use only one hand when operating circuit breakers or switches
  • Use proper tag-out procedures for prescribed circumstances
  • Be cautious when working in voids or un-vented spaces
  • Beware of the dangers of working aloft
  • Keep protective covers, shrouds and panels in place where possible
  • Never bypass an interlock unless authorised under a procedure and in communication with appropriate persons affected by the circuit.
    And many more…..

Informal observation (going round and noticing things) should be encouraged of personnel by plant management. Formal periodic inspection is a requirement but may not occur often enough to pick up damage or failure happening in the running of a plant.

Some safety related practices which need to be more strictly followed when operating machinery in hazardous area are listed below.

14.3.1 Warning signs

These have been placed for personnel protection. To disregard them is to invite personal injury as well as possible damage to equipment. Switches and receptacles with a temporary warning tag, indicating work is being performed, are not to be touched and anyone who interferes with them should be ejected from the site.

14.3.2 Wearing of personal protection equipment, PPE

When work must be performed in areas that demand the wearing of PPE, this shall be checked for suitability and condition before entry into the area.

14.3.3 Working near electrical equipment

When work must be performed in one area, check what other work is being performed in close proximity or close association with it. The authority for issuing permits to works must be consulted.

14.3.4 Authorized personnel only

Only authorised persons shall do maintenance work on electrical equipment because of the danger of fire, damage to equipment, and injury to personnel.

14.3.5 Circuit breakers and fuses

Covers for all fuse boxes, junction boxes, switch boxes, and wiring accessories should be kept closed. Any cover, which is not closed or is missing, should be reported to the authority responsible for maintenance. Failure to do so may result in injury to personnel or damage to equipment in the event accidental contact is made with exposed live circuits.

14.4 Danger signals of electrical malfunctioning

Personnel should constantly be on the alert for any signs, which might indicate a malfunction of electric equipment. Besides the more obvious visual signs, the reaction of other senses, such as:

  • Hearing,
  • Smell,
    And
  • Touch
    should also raise awareness of possible malfunctions.

Examples of signs which one must be alert for are:

  • Fire,
  • smoke,
  • Sparks,
  • Arcing,
  • An unusual/unexpected sound, i.e. from an electric motor
  • Frayed and damaged cords or plugs; receptacles, plugs, and cords which feel warm to the touch
  • Slight shocks felt when handling electrical equipment
  • Unusually hot running electric motors and other electrical equipment
  • An odour of burning or overheated insulation
  • Electrical equipment which either fails to operate or operates irregularly
  • Electrical equipment which produces excessive vibrations, or loud humming

When any of the above signs are noted, they are to be reported immediately. Action to de-energise equipment should be taken if the symptoms are considered serious. Advice should be sought where there is doubt about equipment integrity with which the observer is unfamiliar. Knowledge of how de-energising equipment may affect the safety of other parts of the plant is vital. If unknown seek advice immediately.

Do not operate faulty equipment. Above all, do not attempt to make any repairs yourself if you are not qualified to do so. Stand clear of any suspected hazard and instruct others to do likewise.

14.5 Recording of incidents and observations

The recording of incidents and observations and the action to correct these is considered a vital part of plant management. Procedures for reporting shall be incorporated into the Safety Rules. Review of recorded incidents must be done periodically by competent personnel under whose domain of expertise the incident will fall. If a situation is put right and re-occurs then the remedial action is insufficient or the cause of failure may not have been understood. If making a correction on one part of the plant causes a problem on another then this requires examination.

14.6 Maintenance and safe practices

A practice of safe working must be implemented with special attention to the additional dangers created by the use of electrical equipment in hazardous areas. Electrical Safety Rules developed and published for a specific industrial site must be extended to cover work in Hazardous Areas. A Work Permit and Work-Tag system must be properly implemented and managed to ensure safety. The Management at all levels must be committed to the accurate and timely operation to the rules of such systems in ways that encourage them to be part of the organizations endemic culture.

No work should be done without proper authority and having been subject to thorough risk assessment. The workforce must trust those that operate the system and remain confident in their ability to manage safety from a technical and organisational perspective.

Documented procedures must exist and be worked to consistently. Some aspects of these are given in the following discussion:

14.6.1 Procedures for withdrawing equipment from service

The documentation shall clearly define the “Safety Tag and Lock-out Procedures” for the equipment, when:

  • It is in a dangerous condition
  • It is being worked on
  • It has not been completely installed
  • It is out of operation for repair or alteration
Figure 14.1
Typical tagging shown for safe working

There are two main warning systems used in electrical industry for tagging electrical and non-electrical equipment to indicate isolation before work begins.

These are,

  • Personal Tag
  • Out-of-service Tag

It should be noted that an Out-of-Service Tag is not an indication that the equipment or machinery is safe to work on. Wherever feasible mechanical locking may be done after isolation. No person may work on the equipment until a personal danger tag and where practical, a lock has been placed on equipment isolation. Each person before working on equipment shall be protected by his / her personal tag (see Figure 14.1).

Given hereunder are some general precautions, which must be supplemented with installation specific procedures:

  • To clearly understand and learn local procedure within the installation before starting the work.
  • To employ correct testing procedures so that correct isolation, tagging and lock-out of equipment or that part of installation is done.
  • Never rely on memory. Test before you touch.

14.6.2 Isolation of hazardous area located equipment

Ex protected equipment (other than Ex i) located in a hazardous area must not be opened without isolating all incoming connections including the neutral conductor. Isolation in this context means:

  • the withdrawal of fuses and links
  • the locking-off of an isolator or switch.

It should be noted that if the continuing absence of a flammable atmosphere can be guaranteed by the authority responsible for that area, and a certificate is issued to this effect, essential work for which the exposure of live parts is necessary may be carried out subject to the requirements of Electricity Regulations as applicable.

14.6.3 Modification to certified equipment

Equipment itself must NOT be modified or changed by a user except where specifically permitted in possible variations to certification and under the manufacturer’s instructions. The nature of the intended modification must be examined to determine if the Ex protection characteristics may be invalidated by reference to the manufacturer. For example, adding extra terminals into an Ex e certified junction box will invalidate the certification. By contrast, if, say, an alternative entry point on an enclosure was to be used by switching a cable gland and blanking plug over between two existing entry points, this would not constitute an modification to the equipment, merely a change in the way it has been installed. A clear understanding of what changes can and cannot be made is necessary. Where there I doubt seek advice form a competent authority. Beware that advice from manufacturers may be biased towards encouraging the replacement of equipment which is unnecessary.

14.6.4 Alteration to plant

Alteration to installation must not affect the integrity of the type of protection. For example, power and signal cables may be re-routed for Non-Ex i applications without endangering Ex protection integrity.

Lengthening cables on Ex i circuits will require re-assessment of the System safety for each loop changed. In this case, reference must be made to Descriptive System Documentation to check feasibility. The DSD will require updating but the change may require an entry on a separate modification recording system.

Re-locating equipment in a different position or orientation may alter the area classification. Review and permission before work begins is vital.

14.6.5 Withdrawal of apparatus from service

Permanent withdrawal

When apparatus in a hazardous area is permanently withdrawn from service, the associated wiring should be removed. Where this is not practicable, the exposed conductors shall be correctly terminated in an appropriate enclosure and identified at both ends in a way that indicates the circuit status. This method should be recorded and known to all personnel.

Temporarily withdrawal

When equipment in a hazardous area is temporarily withdrawn from service, the exposed conductors of connecting cables shall be either

  • terminated in isolating terminals inside an appropriate enclosure
    or
  • terminated and solidly bonded together and earthed.

The later method is often preferred. This is in order to prevent a charge via capacitive coupling building up on the cable. If unearthed, controlled discharge is necessary before handling the conductors to reconnect otherwise discharge can cause shock or explosion in the presence of a hazardous atmosphere.

It is preferable that the armouring shall remain earthed at both ends, although this may not always practicable if plastic enclosures are used.

14.6.6 Repairs

Repairs to Ex protected equipment must be carried out in line with the Repair Code of Practice IEC60079-19:2010, ‘Equipment repair overhaul and reclamation’. Repair and overhaul for apparatus used in hazardous atmospheres (other than mines or explosives). More detailed and specific guidance on repairs can be obtained from the manufacturer’s documents.

Third party repair organisations should be assessed for competence to carry out remedial work of the type needed by damaged Ex protected equipment. Some countries now have Ex repairer assessment schemes.

14.6.7 Safety assessment of testing

There is a legal ‘duty of care’ on all industrial personnel that adequate precautions are taken so as not to endanger life or investment during the course of work. The duty is reflected in the Standards discussed, in that they require some assessment of work on electrical apparatus in order to ensure that the integrity of protection is not compromised.

Legislation concerned with electrical testing (at any voltage levels) applies in some countries where work on any apparatus live requires justification.

14.7 Training of personnel

All the Standards and Codes of Practice require that installation, inspection, maintenance, replacement and repair of apparatus, systems and installations shall be carried out only by personnel whose training has included instruction on:

  • the various methods of safeguarding
  • installation practices
  • and, in appropriate cases, the general principles of area classification.

It is now recognised that appropriate refresher training be given to personnel on a regular basis. Records of what training has been provided must also be kept. Modern practice is to provide testing and assessment of the training given which can be used as part of the assessment of competence.

Training can be given by third party training organisations but training on specific equipment and systems can best be provided in house or by manufacturers. The competence to provide training should be assessed by the management of the user company.

14.8 Electrical fire and shock

Appropriate and sufficient training must be given to personnel on how to deal with electrical fire and shock should it occur. A few general points are made here:-

Always

  • De-energise / Isolate electrical equipment which is the subject of a fault as soon as possible. Action to tackle any accident victim may need to begin whilst equipment is live.

Shock

  • If equipment is still live, free the victim from electrical contact: Protection using DRY material, a wooden board or pole a belt, clothing, or other available nonconductive material.
  • Check the victim to see if the person is breathing.
  • If the victim is not breathing, give artificial ventilation. The preferred method is mouth-to-mouth.
  • CPR may be necessary if the heartbeat has stopped, but do not attempt this unless you have been trained in its use.

Obtain Medical Assistance As Soon As Possible

14.8.1 Fires on electrical equipment:

  • Always
  • Only a CO2 gas extinguisher must be used; this will starve the fire of oxygen.
  • Do not use water-based/foam on live electrical equipment as it will conduct and shock the person holding the extinguisher.

14.8.2 Burns from electricity:

The seriousness of a burn depends on two factors:

  • The extent of the burned area
  • The depth of the burn.

The human body will enter a condition of shock from burns involving 15 percent or more of the body. Burns involving 20 percent endanger life. Without immediate and adequate treatment, burns of over 30 percent are usually fatal. The depth of the injury determines whether it is a first, second, or third degree burn.

First-degree burns Symptoms are slight pain, redness, tenderness, and increased temperature of the affected area.
Second-degree burns The inner skin may be damaged, resulting in blistering, severe pain, some dehydration, and possible shock.
Third degree burns In this case skin is destroyed, and possibly the tissue and muscle beneath it. The skin may be charred, or it may be white and lifeless (from scalds). After the initial injury, pain may be less severe because of the destroyed nerve ends. The person may feel very cold and shivery. Some form of shock will result.

The concern is the extent of the burned area. A first-degree burn covering a large area could be more serious than a small third degree burn.

In the absence or delay of trained first-aiders or professional medical help attending (which should be summoned immediately) the most effective immediate treatment of burns and of pain is to immerse the affected area in cold water or to apply cold compresses if immersion is impracticable. Cold water not only minimizes pain, but also reduces the burning effect in the deeper layers of the skin. Continue treatment until no pain is felt when the burn area is exposed to the air. Gently pat dry the area with lint-free cloth or gauze. Burn victims require large amounts of water. Most burns are sterile because of the nature of the injury. The best treatment for uninfected burns, therefore, is to merely protect the area by covering it with the cleanest (preferably sterile) dressing available. Do not apply any creams or ointments as the wound will require assessment by trained personnel.

14.9 Summary

This chapter has reminded personnel that they have a duty of care to themselves and to others to work safely. Some suggestions are made here but companies are often required by Law to write safety rules specific to the hazards on their plants. The IEC60079 Standards, parts 10, 14, 17 and 19 discussed in this manual are necessary documents to set at the heart of such rules where Hazardous Atmospheres are encountered.


15


Fault Finding, Testing and Repairs

In this chapter, a practical view of the procedure for testing and fault-finding Ex equipment is discussed. This leads to issues of competence assessment and which may be different skills to those required to install

Learning objectives

  • To understand the precautions to be taken when fault finding
  • To examine requirements for repair and overhaul in general terms
  • To consider issues of competency of those performing such work

15.1 Fault finding

There is perhaps no right way or wrong way to fault-find. There are however, safe and potentially unsafe ways that are of the greatest importance to consider.

The object of this section is to discuss the approach to fault finding in such a way that guidance can be given on what shall and shall not be done from an explosion protection safety point of view.

15.2 Fault finding routine

It is usual for a fault to be investigated by following a logical routine, an example of which is shown in Figure 15.1.

The approach is described here such that the areas where aspects of safety must be considered can be clearly identified. This is one of a number of possible ways of proceeding; it is not a recommendation of any standard but is distilled from good engineering practice and experience in very general terms.

1) A problem is reported on the plant.

  • Who reported the fault?
  • Will the interpretation of the fault be accurate?
  • What actually happened?
  • Is there really a Fault?
  • Is there a history of failure?
  • Has the fault been recorded
  • Have the safety implications been examined and assessed
  • Have alarms been reset
  • Is there an entry on a fault log?
  • Are there any parallel faults related appearing
Figure 15.1
Possible fault finding routine

2) An understanding of how that part of the plant should perform is necessary.

  • What should happen
  • What should not happen

3) Observation of the current status of the system and how it now behaves.

  • How does this compare with what was originally working?

4) Reason what could cause the observed effect.

  • What has gone wrong?
  • What part of the system is likely to have failed to cause this?
  • Can this be proved?

5) Is the system Explosion Protected?

  • What precautions are necessary?

6) Visually inspect to check the obvious.

  • Confirm explosion protection types. Supply system failure, obvious damage, and visible mechanical problems.

7) Decide on ‘system’ stages.

  • Understand the breakdown of interconnected modules.

8) Decide on a logical fault finding technique or regime and implement it.

  • Options are probably:
  • Measurement
  • Substitution or
  • Simulation or
  • some combination of the above

9) Assess the behaviour of each stage. Eliminate the stages logically. Narrow down the possibilities to home in on the problem and therefore the likely solution.

  • Does this cure the problem?
  • If NOT go back to Stage 3.
  • If YES: Effect a permanent repair. Document the failure.

10) Re-commission the system.

  • Inspection is required in order to verify that safety is not compromised. Return to an operational mode.

The concern with stages 5 to 8 is that the devised test must be safe in that it cannot involve the compromise of any safety system unless under adequately controlled conditions. Such systems are concerned with plant operation safety as well as Explosion Protection.

The model in Figure 15.1 was originally conceived to train personnel working on instrument and control systems but is equally applicable in most circumstances. The modularity of instrument loop system components enable substitution techniques to be used efficiently in order to cure the fault quickly, thereby minimizing ‘down-time’. With some electrical systems this is not usually so easy.

As systems become larger and more complex, however, it is not possible to substitute complete item as this is too costly and takes too long. This aspect of maintenance should be considered during the design of installations.

Most large organizations operate a formal fault reporting system into which the recommendations or requirements of the Codes of Practice can be built. The inspection requirement should become an automatic reaction to any work done on hazardous area apparatus. The recording of the actions taken in the fault reporting system required by the code of practice can be incorporated into the plant documentation. Many think that there must be separate and therefore duplicated systems but there is no requirement in any of the known codes to suggest that these cannot be effectively integrated. Compliance with ISO9000 documentation encourages integration and therefore distribution of useful and safety related information.

15.3 Electrical testing in hazardous area

Electrical equipment designed for the purpose of measurement and testing must comply with the requirements for electrical equipment in Hazardous Areas according to the Installation Code of Practise discussed in Chapter 12. This includes all portable test equipment. There are two aspects to the use of this equipment that must be considered separately:

  • Transport through the hazardous area
  • Operation in the hazardous area

In the first case the Certification of the equipment will determine the safe presence of the equipment for a given Hazardous Area. In the second case, the effect of connecting measurement equipment into any circuit must be the subject of a risk assessment from a plant, personnel and explosion protection point of view before carrying out any such connection.

Of the greatest concern is where measurement of any electrical characteristics in a Hazardous Area is performed by the injection of current or the application of voltage. This could be a potentially dangerous practice and should be prohibited unless adequate risk assessment is undertaken and due precaution is applied.

It is insufficient just to declare that any instrument that is to be used for testing must be ‘Intrinsically Safe’ and, thereby, Intrinsic Safety is applied to a circuit under test so that it cannot cause ignition. This argument is completely the wrong approach but it is quite common amongst the thinking of uninformed technical personnel.

The Ex i equipment certification confirms that the equipment is safe when present in the hazardous area. The equipment may be transported through such a place, but if then connected to any other equipment becomes part of a ‘circuit’, and by definition an Ex i ‘system’ is created.

Stored energy must be proved to be below limits set by the Safety Description of the Ex i equipment. Only then can it be declared as compliant with Ex i system rules. If connected without due consideration, it cannot be proved to be safe.

The values of Inductance and Capacitance present in a circuit under test cannot be predicted. Energy provided from an Intrinsically Safe source of supply can accumulate, which can give rise to incendive discharges in the hazardous area.

Such a condition not understood is an example of personnel believing something to be safe when it is not within the rules. It is this ignorance that the CoPs try to prevent.

The philosophy of testing must be where worst-case conditions are assumed and it be subsequently proved that the conditions are acceptable rather than the approach of assuming that conditions are acceptable and the testing proves this to be untrue!

Figure 15.2
Low voltage insulation testing with a barrier.

Referring to Figure 15.2, five observations can be made:-

  1. An interface on an Ex i instrument loop is a good place to begin checking.
  2. Disconnect the Interface from the Hazardous Area. Testing can be safely performed from the Hazardous Area side of the interface back to the receiving instrument.
  3. Do not bridge the interface while the hazardous area side is connected.
  4. Carry a test-meter into the hazardous Area only if it is certified as safe for the hazard encountered in the area and documentation permits this.
  5. Only connect the test-meter into the circuit if a risk assessment has been performed and a Descriptive System Document to IEC60079-25 covers its safe use.

15.3.1 Insulation testing

Insulation failure can cause fires and explosions. Where the integrity of insulation must be proven, testing may be necessary during normal maintenance or fault diagnosis.

Tests are to be performed at suitable voltage levels according to Electrical Regulations. This depends on the type of cable used and the supply voltage expected in normal operation. Test voltages are then appropriately increased. In normal industrial applications no hazardous atmosphere need be considered and ordinary equipment using high voltages for testing is required. If testing is required in a place normally designated as a hazardous area but the flammable atmosphere is guaranteed absent (by plant operator action and adequately controlled under ‘hot-work’ permits or ‘gas-clearance’ schemes) then there is also no restriction on testing. This is provided that the test can be guaranteed not to affect any adjacent place which may contain a flammable atmosphere. This is a difficult situation to be sure about because, if there is a fault in wiring then it is possible that the energy from the test may be injected into a path unexpectedly entering that hazardous area. Realistically this is more likely to occur due to mis-wiring on plants during commissioning but nevertheless the risk must be considered and evaluated.

Standard insulation testers are designed to release stored energy in a controlled way usually by placing a low resistance across the load circuit when being switched off. This is referred to as a ‘discharge circuit’. This facility is in-built into a ‘press-to-test’ push-button. An insulation tester should not be disconnected without such a button being released otherwise a cable under test could maintain a considerable charge. This provision, however, gives no protection against a possible discharge within the circuit during the application of a test.

15.3.2 Insulation testing of circuits in hazardous areas

Insulation testing at elevated voltages in hazardous areas is an unsafe practice. It can only be done if the area is declared ‘gas-free’. The industry standard approach is that necessary tests should be completed and the results should be recorded during the plant commissioning phase so that reference conditions are known. At the same time alternative low-energy test are completed and recorded for comparison purposes.

Figure 15.3
Low voltage insulation testing with a barrier.

These low energy tests (using certified equipment) can be repeated safely whilst the plant is hazardous for trend analysis purposes to show up worsening or failure conditions. A low voltage test is as shown in Figure 15.3. Determining the current drawn without the cable and/or instrument connected and then comparing the difference when connected will provide figures for calculation. A 28v Barrier will permit 24V DC to be applied with negligible leakage.

Although intrinsically safe ohm-meters are available, the circuit excitation to measure resistance is only about 1 volt and so is not considered an effective means of measurement for insulation testing.

Ex i certified insulation testers (to BS1259) that can apply 500V dc (limited to about 10µA) are available. Their ability to charge a cable that has only a small amount of capacitance to a level of energy above the limit for Ex i purposes means that they are not useable within the limits of an Ex i system. It is unsafe to believe that just because an Intrinsically Safe Certified piece of test-equipment is being used then it is unequivocally safe and no further consideration need be given to safety. It is agreed that the likelihood of the use of a test instrument can cause ignition but assessment of safety is still necessary when using any test equipment and especially for where testing is associated with Hazardous Areas. In some cases, gas-free assessment is the only practical and safe way to proceed.

15.4 Earth testing in a hazardous area

For the same arguments as for insulation testing, where earth path testing is required, the plant should be made and declared gas-free by a permit system. If the hazard cannot be removed it must be realised that there is always a danger that a test in one part of the plant may result in energy release in another part of the plant which could give rise to incendive conditions. There is no guarantee that it is unconditionally safe: the test is being done to prove conditions and if those conditions are incorrect the test itself may cause potential or actual danger. Assumptions on return paths for current should not be made.

No Standard can dictate how to do the testing, only what must be applied and must be achieved. The method of test must be assessed for safety bearing in mind the risk that sneak current paths may be present if insulation has failed or equipment earths are connected incorrectly in the first place.

15.4.1 Assessment of earth-path resistance

Tests may be required for the following conditions by some Local, National and International Regulations and Standards:

  • Earth loop impedance
  • Earth loop resistance
  • Earth return resistance

Plant safety documentation should identify the object of any connection made and its specification along with requirements and methods for testing. Verification of the requirements will occur during the inspection requirements outlined in the next chapter.

Obvious checks of a visual and close nature can be carried out but where measurement is required, safety must be preserved.

Continuous monitoring of critical plant bonding paths such as that required where static bonding must be established, may be performed with Ex i equipment. In Figure 15.4, a galvanically isolating Solenoid/Alarm Driver interface is permanently energised and used as an ‘Ex i power supply’ to provide a continuous current through an earth loop.

The voltage drop across a resistance in series with the supply and the monitored earth bond path is connected to a trip-amplifier set to trip if the voltage rises above a pre-determined value representing the limit resistance of the monitored bond. The low trip point represents failure of circulating current. Such a system is used for earth proving units on tanker loading systems and similar applications.

Figure 15.4
Monitoring an Earth bond between two structures.

These approaches require engineering into the design of plant and operational requirements.

15.5 Repairs

Ex protected equipment can suffer damage by a variety of means. From the chapters explaining the principles of Ex protection and considering the requirements for inspection discussed in the previous chapter it will be understood that where the damage is detrimental to its Ex protection ability then remedial action is required and in a timely fashion. Informed knowledge is needed to effect repair and is often beyond the scope of most users. The manufacturers may offer repair facilities. Some types of repair involve different skills to that which is required to manufacture.

The Standard IEC60079-19:2010, ‘Equipment repair, overhaul and reclamation’, for apparatus used in hazardous atmospheres (other than mines or explosives) has been issued. The original versions of this standard issued in 1993 and updated in 2006 were largely based on the original BEAMA/AEMT Repair Code of Practice issued in the UK in the mid 1980s.

A repair CoP is necessary because it is recognised that the skills of a manufacturer who designs and fabricates equipment will be different from an organisation that is set up to repair, overhaul or recondition equipment. Metal spraying or machining to accommodate new bearings but maintaining adequate flamepaths in a flameproof motor are examples of skills required to repair damaged equipment that a manufacturer might not be able to provide.

A repairer will receive equipment and perform an assessment to ensure that the defect is identified and that there are no other defects in the equipment. This can only be done by comparison with manufacturer’s drawings and data. The findings of the assessment are recorded in a report that will eventually be provided to the equipment owner.

15.5.1 Identification of repairs

Repaired equipment must be identified. The CoPs state that a label must be affixed to the equipment stating the Repairers name, the job reference and a symbol which states in what manner the repair has been carried out.

The triangle with the apex upwards and an uppercase ‘R’ as in Figure 15.2, depicts that it was repaired to the old BEAMA/AEMT CoP. A triangle with the apex downwards means that it was repaired in line with the IEC60079-19 CoP and a Square means that it has been carried out in line with the appropriate Ex protection Construction Standard and has been recertified by a Notified Body.

Figure 15.5
Repair marking

15.5.2 Repair guidelines

Flameproof equipment having damaged flamepath joints of the flange type can be repaired depending on the nature and degree of damage. Spigot Joints with excessive gaps can have metal built up and machined to within limits. Where damage has occurred to threads, inserts can be applied but machining to remove metal must not weaken the structure. Careful assessment and measurement must be made.

Ex e gaskets can be replaced if supplied by the manufacturer but the method of adhesion must be correct. Ex e motors cannot use “copy-windings” whereas this is acceptable with Ex d and Ex n motors.

In general Ex i equipment is not repairable by any other than the manufacturer.

The cost of repair may exceed the cost of replacement and this aspect must be taken into the decision making process between the two possible approaches. Small items of equipment will be cheaper to replace whereas larger and more specialised equipment may be repairable more cost effectively.

15.5.3 Repairer registration schemes

There are no formal certification systems known at present that can declare organisations offering repair services as being duly capable. The users are required to assess the competence of such organisations prior to them undertaking work. A scheme of registration is in place in the UK, albeit voluntarily organised at the present time.

15.6 Competency assessment

The user of Ex protected equipment must assess the competence of all who design plant, and install or inspect, repair and overhaul equipment. There are no laid down requirements for experience, qualification or supervision. Users must develop appropriate monitoring systems to ensure that these requirements, topped up with training, are in place in a service providing organisation whether it is internal or external to the user company.

15.7 Summary

The continued safe operation of Ex protected equipment depends on inspection and, when necessary, maintenance. To find out what is wrong and that maintenance is necessary skills in fault finding and diagnosis are specialist and wide-ranging across various engineering disciplines. The owner of the equipment is responsible for ensuring that safety is preserved during all the stages of repair and putting back into service.


16


Standards, Certification and Marking of Equipment for use in Hazardous Areas

In this chapter, the issue of safety compliance is discussed. The theory, technology and practice of Ex protection has been examined and explained in the previous Chapters but in order to manage the safety of an installation, verification of compliance with acceptable safety Laws, Regulations and Directives must be demonstrated by the plant owner, otherwise known as the ‘duty-holder’. The policy of safety, its management and communication to a workforce is at the heart of this requirement. This chapter will bring together issues at a Management level.

Learning objectives

  • To summarise the subject of Certification
  • To explain the background to Approval
  • To summarise the Equipment Marking requirements
  • To explain the IEC approach to Certification

16.1 Introduction

The intention of this Chapter is to bring together the aspects of Standards, certification, approval and marking of equipment which are now applicable (almost) internationally and which have arisen in the previous chapters of this manual. This information may be of more help if it is in one place, particularly for certain readers that are involved in the documentation process for demonstrating the safety of equipment and installations.

Some historical background may be helpful to explain how much of the terminology and equipment ‘marking’ has evolved. Some of this will be recognised in older publications still in circulation.

The term ‘Marking’ refers to the requirement for coding comprising letters, numbers and symbols on equipment to depict safety related information.

This Chapter sometimes refers to ‘ATEX’, for which a more detailed explanation is given in Appendix F of this manual.

16.2 Progression of standards

The harmonisation and adoption of Standards, written and circulated by an internationally accepted organisation such as the IEC, has benefits in both economic and safety terms.

The European Community (EC), being one of the earliest industrialised zones, has contributed a lot towards Standardisation in the field of Explosion Protection. A time-line view of the Standards development situation in the UK and Europe is shown in Figure 16.1.

Figure 16.1
Timeline of Standards

The common thread amongst these complementary approaches is that they limit public intervention to what is essential and leave business and industry the greatest possible choice on how to meet their public obligations.

Progressive globalization obviously also includes industries such as large-scale chemicals, pharmaceuticals, petroleum and gas extraction and processing and many other industries directly or indirectly involved with hazardous areas. Hence, in the long term, it will be unacceptable for globally active companies to have to conform to different safety standards in different areas of the world. The development of internationally valid Explosion Protection Directives and Standards will therefore continue to accelerate.

16.3 A brief history of ‘certification’ and ‘approval’

When Ex protected equipment first emerged, probably in the early 1920s in UK and Germany, it was ‘approved’ for use by a government department. The Ministry of Power in the UK did issue certificates confirming that testing had been performed on some equipment. A separate organisation run by the UK government, called the British Approvals Service for Electrical Equipment in Flammable Atmospheres, BASEEFA, emerged to perform the approval. It was not until formal Construction Standards were written that Certificates (of Conformity) began to appear. Equipment certified to BS229 was marked with the letters ‘FLP’ to depict Flameproof Protection as shown in Figure 16.2(a). The shape used is that of a ‘crown’ as in His or Her Majesty’s Government appointment.

Figure 16.2
Early Certification Marking

When formal committees were set up to write the standards for adoption and recognition by the British Standards Institute, certificates were considered necessary to convey safety critical information to the user of the equipment. This was formalised in the UK in the 1960s when the letters ‘Ex’ were followed by the letter depicting the type of protection employed and to which the certification related. BS889 and BS4683 are common examples. It is believed that Germany had a similar system around that time. The early German ‘PTB’ certification mark is shown in Figure 16.2(b)

Certification does not affirm the safety of certified equipment. It is a statement to inform that the equipment has been assessed and conforms to the requirements laid out in a Construction Standard, hence the correct title of a certificate of conformity.

16.3.1 Ex and EEx

Equipment certified to old (British) Standards bore the symbol ‘Ex’ to depict Explosion Protected ‘Apparatus’.

The ‘Old approach’ Directive in Europe saw the generation of Standards written by CENELEC, published as the EN50 Series. This used much of the UK format for certificates but the letters ‘EEx’ were now used to depict Ex protection as certification was issued in conformity with a European Standard.

Under the harmonised International Standards, IEC60079 Series, the prefix ‘Ex’ is used again and the term ‘apparatus’ is being replaced by the term ‘equipment’. The term ‘equipment’ has been used in this manual.

16.3.2 Previous Standards and their marks

Where Standards were obsolete or in the process of being written, BASEEFA, then the UKs government department for equipment testing, wrote their own ‘Standards for Assessment’ known as SFAs.

The old-approach Directive saw the formalisation of the marking format e.g., “EEx d IIB T6” prefixed by the use of the symbol (c) in Figure 16.2. The ‘e x in the hex’, as it became known, was not a certification mark but a product mark to indicate that the equipment must be allowed into any European country with no trade embargoes placed upon it and without the need to be locally recertified. This was a major breakthrough for the unification of safety principles.

Each piece of equipment must meet the general requirements of the Explosion Protection Standards and the particular Standard for the type of protection used. Thus a certificate will state for an Ex d equipment that it meets IEC 60079-0 and IEC60079-1. The date of the Standard which depicts its issue or revision level must also be stated when it is quoted.

16.3.3 Certification process

The certification process or procedure has really never changed. The testing authority will receive an application, check that the product meets the requirements of the applicable standards and will agree the marking that will appear on the product. A confidential report, outlining the testing and the performance of the product, is written by the testing authority. It is held as confidential so as to prohibit the authority from disclosing sensitive commercially-advantageous technical information to the applicant’s competitors.

Variations and amendments may be made as the product matures. This may be for many reasons, not least the availability of components used in the products construction. Throughout the life cycle, variants to satisfy customer and marketing demands so as to provide additional facilities or methods of use might be sought.

16.4 Types of Certificates

There are several types of certificates which are explained in this section. The format for each certificate comprises clauses which are numbered in a standardised fashion.

16.4.1 Component certificates

Certificates are issued for components parts that are considered to provide ignition-capability unless protection is applied to the design. If it was deemed ‘a component upon which safety depended’ then the certificate number would be of the form, for example, ‘Ex89B1234U’.

  • The U depicted it was a component certified part.
  • 1234 was the serial number of the equipment
  • B was the second amendment to the standard to which it was certified, stated on the certificate
  • 89 was the year of certification
  • Ex meant that it was an explosion protected device.

This same basic format is still used and the component will carry a safety code, say, ‘Ex d IIC’. There is no T rating assigned or stated by the mark at this stage. This cannot be assigned until testing is performed on the complete apparatus with all the designed components inside and energised to the specification for the equipment as a whole. A component may not be used on its own in hazardous area. It is required to be part of a certified piece of equipment.

It should be noted that there is a trend for certifying authorities to certify Ex d cable Glands as Equipment and use a T6 rating, whereas these were formerly considered to be Components. Some Ex e terminals which are deemed components according to the latest Standards also carry T ratings. This practise is frowned on by many Certifying. Authorities.

16.4.2 Equipment certificates

An Equipment Certificate is issued for a complete piece of equipment that can be located in a hazardous area. It has the same number form without the ‘U’.

For example a product label may state: “EEx d IIB T4 Ta= +55°C Ex95D1289X/3”

It is therefore known (in order of the symbols above) that the equipment is:

  • Certified to a European Standard (EN50014 and 018)
  • Flameproof
  • Unsafe in Hydrogen and other gasses requiring IIC Equipment
  • Can be used up to a +55°C ambient Temperature (-20°C is the default lower limit)
  • The certification was issued in 1995
  • ‘D’ is the Fourth amendment to the Standard
  • 1289 is the certificate serial number
  • The X denotes special conditions of use, and;
  • ‘/3’ indicates there are the original certificate and three supplements
Figure 16.3 The Equipment Safety Code Marking

16.4.3 System certificates

Where equipment is intrinsically safe and marked as such using square brackets, as in ‘[Ex ia]’, then it is termed ‘Associated Equipment’. Such equipment can only be mounted in a ‘Safe Area’ and can only be connected to an Intrinsically Safe circuit in the hazardous area. Where an I.S. circuit comprises associated equipment and equipment in the hazardous area, a certificate for the system may be obtained for the combination of equipment. This is called a systems certificate.

This type of certificate is not mandatory but was often obtained from Certification Authorities by field equipment manufacturers for the convenience of users. It states the electrical safety characteristic assessment (matching of the Safety Description with Entity Parameters) and cabling characteristic limitations for the combination of interconnected equipment.

16.4.4 Conditions and ‘special’ and ‘specific’ conditions of certification

As a result of the certification process, there may be conditions of usage stated on the certificate to be complied with by the installer or the user in order to preserve the safety of the equipment and the installation in which it is used. If conditions are deemed ‘Special’ then the certificate number is appended with an ‘X’. Previously, i.e., before about 1976, the letter ‘B’ was used to denote this for equipment certified to older British Standards.

Some certifying authorities recognise that there are conditions but do not deem these to be special, in which case the X does not appear in the certificate number. They are therefore considered to be ordinary conditions. This does promote the idea that the certificate of any equipment being chosen for use in a hazardous area must be examined to determine if such condition pose an advantage or disadvantage in the selection of the particular equipment.

Standards writing authorities are now updating new editions of the Standards to use the term ‘specific conditions’ as this is felt to be more appropriate. This change has been placed into IEC60079-0:2011 ‘Explosive atmospheres. General requirements’, IEC60079-14:2012 ‘Selection and installation’ and in IEC60079-17:2011, ‘Inspection and maintenance’.

16.4.5 Control Drawings

Standard IEC60079-0: requires that the manufacturer provides instructions for installation of Ex equipment. The term ‘control drawing’ is defined in the Construction Standard specifically for Ex i equipment, IEC6009-11:2011, as:-

a drawing or other document that is prepared by the manufacturer for the intrinsically safe or associated apparatus, detailing the electrical parameters to allow for interconnections to other circuits or apparatus.

Since the connections of some devices can be quite complex, with a range number of optional configurations, it is desirable to be able to show these in diagrammatic format for the sake of clarity in each application. Clause 13 of this Standard, recommends this as a form for ‘consolidating connection information and special requirements for installation and use’.

At present, the organisations that provide ‘approvals’ in North America seem to encourage the use of this drawing more readily than elsewhere in the world but this is changing as this terminology comes into more use. The use of these as formal drawings and the benefits they provide seem to be bringing the certification and approval procedures closer.

16.4.6 Licence

In the UK, BASEEFA issued a licence to use their certifying authority mark (as in figure 16.1c) as the testing authority on a product. The mark was the Crown, as used for ‘FLP’ equipment, but it was replaced with the letters ‘Ex’ (see Figure 16.1d). This mark was used to signify that BASEEFA had performed the testing and then, to ensure that the consistency and quality of the equipment was maintained for the product life-cycle, they would visit the ‘licence-holder’ on a regular basis to maintain surveillance. The Licence does not form part of the user’s safety documentation. This surveillance concept has now been adopted by Product directive and the UK licence was dispensed with when ATEX came into operation in Europe.

16.5 Combined Protection

Where equipment is protected by more than one type of protection, a certificate will reflect this by quoting the standards to which the equipment has been tested for compliance. Thus an Ex d small volume switch unit in an Ex d housing will be marked ‘Ex ed’ (plus the Group and T rating, etc.). In the past, the order of the letters ‘d’ and ‘e’ were based on agreement between the manufacturer applying for the certification and the certifying authority. This was done on the basis of which was considered to be the major method of protection being stated first. This had lead to a number of disputes and so the Standards now state that they shall be in alphabetical order. This explains why two pieces of similarly protected equipment have their marking with the types of protection letters interchanged.

The use of the square-brackets marking (explained earlier) is available for any situation where, for example, an Ex d enclosure contains an Ex ia supply for powering an Intrinsically Safe circuit in the hazardous area. It would be written as, say, ‘Ex d[ia]’.

Where there is a greater number of types of protection used in different ways, appropriate marking will be used. For example, consider the marking of an Ex p enclosure mounted in Zone 1 which is monitored by an Ex ib protected pressure-switch. The switch is connected into an Ex d enclosure housing a time switch and the circuit breaker interlocking circuits. The Ex p enclosure then contains an [Ex ia] interface. It would be marked, typically, ‘Ex dpx[ia]ib IIB T5’.

16.6 Approval versus Certification

Before certificates were issued in the UK and in Europe, equipment was deemed to be ‘approved’ after testing by a government-appointed authority. The equipment comprised a label stating the document to which it had been approved known as the ‘approval number’. After the issue of formal national Standards, conformity to those standards became attested by the issue of a formal certificate of conformity.

In the North American continent, USA and Canada, the insurance-based organisations (rather than government departments) evolved ways to examine safety precautions being taken to mitigate their risk. Such organisations eventually formed independent testing facilities that have become known as Underwriters Laboratories (UL) and Factory Mutual (FM). They have maintained the ‘approval’ system by ‘listing’ acceptable products in a yearbook that is updated regularly.

The main differences are firstly, that the onus is placed on the Manufacturer to pass on all information related to safety. (Note that in the rest of the world, critical information for the safe use of the equipment in a hazardous area is stated on the certificate.) Secondly, the equipment is not marked with the type of protection applied; it is only marked with the flammable atmospheres for which it is deemed safe. (Note that this is not acceptable under the International Standards because identification of the type of protection employed by the equipment is considered to be part of the safety-maintenance requirements.)

16.7 The IEC-Ex scheme

The IEC-Ex scheme was proposed in the early 1990s. It introduces itself as a single global certification ‘Framework’ based on the International Electro-technical Commission’s international standards. It caters for countries whose national standards are either identical to those of the IEC or else very close to IEC standards. The IECEx is truly global in concept and practice, reduces trade barriers caused by different conformity assessment criteria in various countries, and helps industry to open up new markets. The goal is to help manufacturers reduce costs and time while developing and maintaining uniform product evaluation to protect users against products that are not in line with the required level of safety.

The aim of the IECEx Scheme and its Programs is to ease international trade of Explosion Protected Equipment (termed Ex equipment) by eliminating the need for duplication of testing and certification, while preserving safety. IECEx operates as an International Certification System covering products and services associated with the Ex industries.

The IECEx Equipment Certification Program provides both:

1. A single Global Certificate of Conformity, that requires manufactures to successfully complete:

  • Testing and assessment of product samples for compliance with IEC Standards;
  • Assessment and auditing of manufacturing premises;
  • On-going surveillance audits of manufacturing premises

2. A “fast track” process for countries where regulations still require the issuing of national Ex certification or approval by way of global acceptance of International IECEx product test and assessment reports (ExTR).

In essence, the IECEx scheme will issue a Certificate that should be acceptable internationally. Alternatively, an IECEx Test Laboratory, known as ExTL , can perform the testing and make the results available for assessment by a Country’s equivalent of a government appointed testing authority. They can then issue the Certificate for local use. The scheme is proving popular with many products now IECEx certified.

The reproduction of any Ex protection certificate must be in full, i.e., the original and other variations produced up to the time of the request for the certificate. The advantage of the IECEx scheme is that the certificates are held on a website and need not be printed. Thus they are always up-to-date and complete.

In the North American continent, the IEC rules are accepted but they implement testing to their version of the IEC Standards and designate tested equipment with the letters ‘AEx’.

The IECEx scheme also introduces the Certified of Personnel Competence Scheme. (CoPC). This scheme is designed to allow training organisations to prepare engineers and technicians for examinations that will test true competence in a theoretical and practical framework.

The units are listed below to show how the scheme is modularised so that personnel with differing job functions may be appropriately qualified with particular units.

Unit Ex001 – Basic principles of protection in explosive atmospheres
Unit Ex002 – Perform classification of hazardous areas
Unit Ex003 – Install explosion-protected equipment and wiring systems
Unit Ex004 – Maintain equipment in explosive atmospheres
Unit Ex005 – Overhaul and repair of explosion protected equipment
Unit Ex006 – Test electrical installations
Unit Ex007 – Perform visual & close inspection
Unit Ex008 – Perform detailed inspection
Unit Ex009 – Design electrical installations
Unit Ex010 – Perform audit inspection of installations

The scheme is voluntary at present but employers are increasingly aware of the helpfulness of the training and assessment.

The course for which this manual is designed addresses much of the theoretic requirements of these units. More details can be read from the Operational Document IECEx OD 504 Edition 3 2014-09 which is published and updated on the IECEx Website. This, like all IECEx documents, is written in the same uniform method as a standard for ease of navigation and understanding.

16.8 Conclusion

Standards and certification are constantly evolving as technology and experience changes. The legacy of old standards will be present for the life of the equipment installed on a plant: it is therefore important that users understand how the equipment they have installed is marked and the meanings of those markings.

The installation and maintenance of this certified equipment must also comply with evolving Standards for its use. Every time a Standard is updated it does not mean that equipment has to be reinstalled, but during inspection and maintenance the quality of the installation and procedures may also need to be reviewed.

16.9 Course Conclusion

In conclusion to this manual, and the course for which it is prepared, perhaps the main point of learning is that the safety aspects of Ex protection depend on the correct approach to the fine detail.

Competent technical personnel with the correct level of engineering training and experience who work closely with the equipment can appreciate the subtlety. Management must lead an integrated approach by ensuring the flow of information and education that this subject demands for safety.


Appendix A


IEC Series Standard Titles For Explosive Atmospheres

Here follows a list of the IEC Standards relevant to Hazardous Areas and Explosion Protection.

The date of the latest publication has been included in this listing. Standards are reviewed at least every 5 years. It is the responsibility of user to ensure that the latest issue is consulted for new applications. If dealing with existing equipment or installations then the date of the Standard shall be recorded for reference purposes.

IEC Standards for electrical equipment

The IEC Standard 60079 was previously entitled “Electrical equipment in hazardous areas”. The process of harmonization has led to the Series looking at all aspects of the subject including dusts and in future non-electrical sources of ignition. The series has become retitled “Explosive Atmospheres”.

In countries who subscribe to the harmonised IEC Standards, they will use the IEC number but prefixed with their Standards reference, for example BS EN 60079 would be used as the British Standard where the EN letters denote that it is also a ‘Euro-Norm’.

The status of Standards can be ascertained on the website: https://webstore.iec.ch/ under which will advise if the standard has been updated or withdrawn. Many previously well-known Standards relating to explosive atmospheres have been included into or superseded by IEC so many BS and EN issues are now withdrawn.

Standard Number Date Explosive Atmospheres
IEC60079-0: 2011 Part 0: Equipment - General Requirements
IEC60079-1: 2014 Part 1: Equipment protection by flameproof enclosures “d”.
IEC60079-2: 2014 Part 2 Equipment protection by pressurized enclosure “p”.
IEC60079-3:   Part 3: Spark test apparatus for intrinsically-safe circuits.
IEC60079-4:   Part 4: Testing of gases and vapours
First Supplement.
Note: This supplement applies also to the second edition of 1975.
IEC60079-5: 2015 Part 5: Equipment protection by powder filling “q”
IEC60079-6: 2015 Part 6: Equipment protection by liquid immersion “o”
IEC60079-7: 2015 Part 7: Equipment protection by increased safety “e”
IEC60079-10-1: 2015 Part 10: Classification of areas - Explosive gas atmospheres
IEC60079-10-2: 2015 Part 10: Classification of areas - Explosive dusts atmospheres
IEC60079-11: 2011 Part 11: Equipment protection by intrinsic safety “i”
IEC60079-12:   Part 12: Assessment of gas characteristics
     
     
IEC60079-13:   Part 13: Equipment protection by pressurised room “p”
IEC60079-14: 2013 Part 14: Selection and installation of electrical equipment in hazardous areas
IEC60079-15: 2010 Part 15: Type of protection “n”
IEC6 TR 60079-16: 1990 Electrical apparatus for explosive gas atmospheres: Part 16: Artificial ventilation for the protection of analyser(s) houses.
IEC60079-17: 2013 Part 17: Electrical installation and maintenance
IEC60079-18: 2014 Part 18: Equipment protection by encapsulation “m”.
IEC60079-19: 2010+amd1:2015 CSV Part 19: Equipment repair, overhaul and reclamation
IEC60079-20-1: 2010 Part 20-1: Material characteristics for gas and vapour classification – Test methods and data
IEC60079-25:   Part 25: Intrinsically safe electrical systems
IEC60079-26 2006 Part 26: Equipment with Equipment Protection Level (EPL) Ga
Now withdrawn
IEC60079-27: 2002+2005+2008 Part 27: Fieldbus intrinsically safe concept (FISCO) and Fieldbus Non-incendive concept (FNICO)
Now withdrawn and replaced by IEC 60079-11:2011
IEC60079-28 2015 Part 28: Protection of equipment and transmission systems using optical radiation
IEC60079-29-1 2016 Part 29-1: Gas detectors – Performance requirements of detectors for flammable gasses
IEC60079-29-2 2015 Part 29-2: Gas detectors – Selection, installation use and maintenance of detectors for flammable atmospheres and oxygen
IEC60079-29-3 2014 Part 29-3: Gas detectors –. Guidance on functional safety of fixed gas detection systems
IEC60079-29-4 2009 Part 29-4: Gas detectors –. Performance requirements of open path detectors for flammable gases
IEC60079-30-1: 2015 Electrical resistance trace heating. General and testing requirements
IEC60079-30-2: 2015 Electrical resistance trace heating. Application guide for design, installation and maintenance
IEC60079-31: 2013 Part 31: Equipment dust ignition protection by enclosure “t”.
IEC60079-32: 2013 Part 32-1: Electrostatic hazards, guidance
IEC60079-32: 2015 Part 32-2: Electrostatics hazards - Tests
IEC60079-33: 2015 Part 33: Equipment protection by special protection “s”.
IEC 60079-35-1: 2011 Part 35-1: Caplights for use in mines susceptible to firedamp - General requirements - Construction and testing in relation to the risk of explosion
IEC 60079-35-2: 2011 Part 35-2: Caplights for use in mines susceptible to firedamp - Performance and other safety-related matters
IEC60079-39: 2015 Part 39: Intrinsically safe systems with electronically controlled spark duration limitation
IEC60079-40: 2015 Part 40: Requirements for process sealing between flammable process fluids and electrical systems
IEC61241   Explosive dusts: WITHDRAW replaced by IEC 60079.
IEC 61779   Electrical apparatus for the detection and measurement of flammable gases: WITHDRAWN.

CEN Standards for explosion protection (General)

CEN Standard Number Date Part
EN 1127   Explosive atmospheres - Explosion prevention and protection
EN 1127-1: 2011 Part 1: Basic concepts and methodology
EN 1127-2: 2014 Part 2: Basic concepts and methodology for mining
     
EN 1755: 2000+A2:2013 Safety of industrial trucks - Operation in potentially explosive atmospheres - Use in flammable gas, vapour, mist and dust

CEN standards for explosion protection of non-electrical apparatus

CEN Standard Number Date Part
EN 13237: 2012 Potentially explosive atmospheres - Terms and definitions for equipment and protective systems intended for use in potentially explosive atmospheres
EN 13463   Non-electrical equipment for potentially explosive atmospheres
EN 13463-1: 2009 Part 1: Basic method and requirements
EN 13463-2: 2004 Part 2: Protection by flow restriction “fr”
EN 13463-3: 2005 Part 3: Protection by flameproof “d”
EN 13463-4: 2003 Part 4: Protection by inherent safety “g”
EN 13463-5: 2003 Part 5: Protection by constructional safety “c”
EN 13463-6: 2005 Part 6: Protection by controlled ignition sources “b”
EN 13463-7: 2003 Part 7: Protection by pressurisation “p”
EN 13463-8: 2003 Part 8: Protection by liquid immersion ‘k’
EN 13673-1: 2003 Determination of the maximum explosion pressure and the maximum rate of pressure rise of gases and vapours Part 1: Determination of the maximum explosion pressure
EN 13760: 2003 Automotive LPG filling system for light and heavy duty vehicles - Nozzle, test requirements and dimensions
EN 13821: 2002 Potentially explosive atmospheres - Explosion prevention and protection - Determination of minimum ignition energy of dust/air mixtures
EN 13980: 2002 Potentially explosive atmospheres - Application of quality systems

Appendix B


Ingress Protection Code for Enclosures of Electrical Equipment

The Ingress Protection Code IEC60529:2013 specifies the degree of protection provided by enclosures to levels of solids and liquids as depicted in the code list.

Note that ‘water’ is used as the test for liquid ingress. Enclosures may suffer a greater degree of penetration from other liquids such as solvents where corrosion or reactions may also result. Assessment of ingress by other liquids shall therefore be determined by testing.

The text is reproduced to assist in the understanding of the application against each definition.

Table I of IEC 60529: 2013
Degree of protection against access to hazardous parts indicated by the first characteristic numeral.
FIRST CHARACTER NUMERAL Degree of protection
Brief description Definition
0 Non-protected -
1 Protected against access to hazardous parts with a finger The access probe, sphere of 50 mm diameter, shall have adequate clearance from hazardous parts.
2 Protected against access to hazardous parts with a tool The access probe, sphere of 12 mm diameter, 80 mm length, shall have adequate clearance from hazardous parts
3 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts
4 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts
5 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts
6 Protected against access to hazardous parts with a wire The access probe 2,5 mm diameter shall have adequate clearance from hazardous parts

 

Table II of IEC 60529:2013
Degree of protection against solid foreign objects indicated by the first characteristic numeral.
FIRST CHARACTER NUMERAL Degree of protection
Brief description Definition
0 Non-protected -
1 Protected against solid foreign objects of 50 mm diameter and greater The object probe, sphere of 50 mm diameter, shall not fully penetrate a
2 Protected against solid foreign objects of 12,5 mm diameter and greater The object probe, sphere of 12,5 mm diameter, shall not fully penetrate a
3 Protected against solid foreign objects of 2,5 mm diameter and greater The object probe, sphere of 2,5 mm diameter, shall not penetrate at all a
4 Protected against solid foreign objects of 1 mm diameter and greater The object probe, sphere of 1,0 mm diameter, shall not penetrate at all a
5 Dust-protected Ingress of dust is not totally prevented, but dust shall not penetrate in a quantity to interfere with the satisfactory operation of the apparatus or impair safety
6 Dust-tight No ingress of dust
a The full diameter of the object probe shall not pass through an opening of the enclosure

 

Table III of IEC60529:2013
Degree of protection against water indicated by the second characteristic numeral.
FIRST CHARACTER NUMERAL Degree of protection
Brief description Definition
0 No protection -
1 Protection against vertically falling water drops Vertically falling drops shall have no harmful effects
2 Protection against vertically falling water drops when the enclosure tilted 15° from vertical Vertically falling drops shall have no harmful effects when the enclosure is tilted at any angle up to 15° on either side from the vertical
3 Protection against spraying water Water sprayed at an angle up to 60° on either side of the vertical shall have no harmful effects
4 Protection against splashing water Water splashed against the enclosure from any direction shall have no harmful effects
5 Protection against water jets Water projected in jets against the enclosure from any direction shall have no harmful effects
6 Protection against powerful water jets Water projected in powerful jets against the enclosure from any direction shall have no harmful effects
7 Protection against the effects of temporary immersion in water Ingress of water in quantities causing harmful effects shall not be possible when the enclosure is temporarily immersed in water under standardised conditions of pressure and time
8 Protection against the effects of continuous immersion in water Ingress of water in quantities causing harmful effects shall not be possible when the enclosure is temporarily immersed in water under conditions which shall be agreed between the manufacturer and the user but which are more severe than for numeral 7
9 Protection against high pressure and temperature water jets Water projected at high pressure and high temperature against an enclosure from any direction shall not have any harmful effects

 

Code
First Digit
Physical Protection and foreign Solid body ingress Protection Code
Second Digit
Protection against ingress of Liquid
0 No protection against ingress of solid foreign bodies.
No protection of persons against contact with live or moving parts inside the enclosure.
0 No protection
1 Protection against inadvertent or accidental contact with live or moving parts inside the enclosure by a large surface of the human body, for example a hand, but not protections against deliberate access to such parts.
Protection against ingress of large solid foreign bodies (50 mm diameter).
1 Protection against drops of condensed water.
Drops of condensed water falling vertically on the enclosure shall have no harmful effect.
2 Protection against contact with live or moving parts inside the enclosure by fingers.
Protection against ingress of large solid foreign bodies (12.5 mm diameter).
2 Protection against drops of liquid.
Drops of falling liquid shall have no harmful effect when the enclosure is tilted at any angle up to 15 deg. from the vertical.
3 Protection against contact by objects of thickness greater than 2.5 mm.
Protection against contact from tools.
3 Protection against rain.
Protection against water-spray at an angle up to 60°.
4 Protection against contact by objects of thickness greater than 1 mm.
Protection against contact with a wire.
4 Protection against splashing liquid.
Protection against water spray from all directions.
5 Complete protection against contact with live or moving parts plus harmful deposits of dust. The deposit of dust may not be fully prevented but sufficient amount of dust will not be allowed to enter so as to harmful effect on operation of equipment.
Protection against contact with a wire
5 Protection against water jets.
6 Complete protection against contact and ingress of dust, i.e., it shall be dust-tight. 6 Protection against conditions on ships decks.
Protection against strong water jets.
    7 Protection against immersion in water.
    8 Protection against indefinite immersion in water.

Appendix C


Case Studies

In September 1998, the following made the headlines in Australian papers:

“ESSO blamed for Australian gas explosion”

Former High Court Justice Dawson had passed the following remarks concerning the above September 1998 gas explosion in Victoria:

“The major cause of the accident was the failure of ESSO to equip its employees with appropriate knowledge to deal with the events which occurred”.
The lack of adequate procedures for the identification of hazards in Gas Plant 1 contributed to the occurrence of the explosion and fire”.

The idea of bringing this up early in the chapter is to emphasize the importance of the identification of hazards and hazardous areas”. If this is not done in a systematic manner with due care and diligence then the results can be catastrophic as it happened in ESSO’s Gas Plant 1.

  • Urban Structure Fires. Perhaps the most common human-caused hazard (often a disaster) is fire in large occupied buildings. Causes can be accidental or deliberate, but unless structures have been built to safe fire standards, and sound emergency procedures are used, heavy loss of life can result. Disastrous fires have affected most countries. Notable overseas cases include a high-rise building fire in Sao Paulo, Brazil; the Kings Cross Station inferno in London and Bradford Soccer Stadium, both in England; and hotel fires all around the world, such as at Pattaya, Thailand, in July 1997, when 100 died. These, and many like them, have cost thousands of lives, injuries and untold property. In August 1981, 19 people died in the Rembrandt Hotel fire, Sydney, NSW, Australia.
  • BLEVE. An entire community was involved at Mississauga, Ontario, Canada when 250,000 had to be evacuated to avert disaster following a train accident which triggered a series of BLEVEs (Boiling Liquid Expanding Vapour Explosions). Liquefied gas BLEVEs have occurred in Cairns (1987 - one dead, 24 injured) and in Sydney where fortunately, there were no casualties.
  • Other Explosions.Great loss of life occurred in Halifax, Nova Scotia, Canada in 1917 when a ship carrying explosives collided with another. The resulting explosion destroyed large sections of the town and killed 1,963 people! Australia’s most disastrous explosion was in the Mt Kembla mine, Wollongong, in 1902, when 95 miners died. One of the worst non-mining explosions occurred in 1974 at the Mt St Candice Convent in Hobart, when seven died in a boiler explosion.
  • Toxic Emission.Not all hazardous materials accidents involve transport, and some can result in worse disasters. During 1984 cyanide gas escaped from a fertilizer factory in Bhopal, India. The resulting deadly cloud caused the deaths of approximately 2,000 people living close-by. In Australia in August 1991, the Coode Island fire burnt 8.6 million litres of chemicals in the heart of Melbourne and loomed as a potential disaster. Good luck rather than good management resulted in winds dispersing toxic fumes away from residential areas. Over 250 workers were evacuated from nearby ships and factories but only two injuries occurred (to fire fighters).

Appendix D


Hazardous Area Classification Tabulation Format taken from IEC60079

The following two tables are recommended formats for the recording of data concerned with Hazardous Area definition and are taken from IEC60079-10:2009.

The latter two tables are those published in the current Area Classification Standard IEC60079-10-1:2015 which permits the recording of more details. The details on the tables are part of an example published in the Standard in Appendix E so as to show how it has been completed.

These are recommended formats and can be modified and extended to cover more specific conditions

IEC60079-10-1 Example Number 10 Sheet 1

IEC60079-10-1 Example Number 10 Sheet 2

IEC60079-10-1:2015 Table E1: Hazardous Area Classification data sheet - Part I: Flammable substance list and characteristics

IEC60079-10-1:2015 Table E2: Hazardous area classification data sheet - Part II: List of sources of release


Appendix E


Practical Exercises for Hazardous Areas Course

E.1 Introduction

The following is a list of exercises that may be worked through in the class setting or may be used for further study as time allows and depending on the class profile. The exercises are designed to be carried out in conjunction with the information presented in the manual. The exercises are not in a specific order.

Clearly some exercises would take considerable time to complete. It may be that additional information is needed. The more important point of learning here may be in deciding the approach, realising the limitations and discerning the level of confidence required with which the problem is tackled.

Please read the information fully before proceeding.

Exercise titles

  1. Area Classification of your work place – discussion in class of a few case studies of participants
  2. Area Classification of a Fuel Station
  3. Brain Teasers on Hazardous Areas
  4. A Case Study for Additions or Modification in Hazardous area of a Plant
  5. A Case Study in Hazardous Area Classification of Equipment
  6. Area Classification of a process plant – a Schematic
  7. Hazardous Area Classification for ‘A fixed roof tank’
  8. Exercise on Protection concepts and Inspection
  9. Hazardous Area Classification for Compressor Housing
  10. Exercise on Protection concepts Ex ‘d’
  11. Switch Status Input System Calculations
  12. Solenoid Valve Operational Characteristics

E.2 Exercises

Exercise 1

Area classification of your work-place

1.1 Think of your work area. Make a list of the “fuels” that are present that could be a hazard.

1.2 When, where and how are these substances present?

1.3 What could cause these substances to ignite? Be specific.

1.4 Describe the ventilation of the area.

1.5 Given all the above information, what do you think the area classification of the work area would be?

Exercise 2

Area classification of a fuel station

Figure L.1
From SABS 0108: 1995

From Figure L.1 answer the following:
2.1 Make a list of the allowable protection methods for Zone 1.

2.2 The area classification is for a fuel station.

  • a. What types of equipment will typically be used?
  • b. What would the typical Ex rating be for each of these pieces of equipment?

2.3 Fuel and diesel fuel are mixtures of several chemical compounds. How do you think complex chemicals such as these are classified?

2.4 If you consider the situation inside the storage tanks, what do you think would the area classification be? Why?

2.5 What type of electronic sensing device/equipment would you recommend to determine the amount of fuel left in the tank?

Exercise 3

Brain teasers on hazardous areas

3.1 Name several factors that could determine the frequency of maintenance and inspection of equipment in hazardous areas.

3.2 Why are light metal alloys, such as magnesium not allowed in hazardous areas?

3.3 How would you determine whether an Ex d enclosure is damaged?

3.4 How would you repair Ex i equipment?

Exercise 4

A case study for additions or modifications in hazardous areas of a plant

Eric Patterson just left the office of the Plant Manager of the Petrochemical Company that he works for. He had been given a task that he felt uneasy about. The company did a study and found that it could be economically viable to purify and then sell the cyclohexane that is produced as a by-product from one of the other processes in the plant.

Being a chemical engineer, he knew that cyclohexane is highly flammable and that he had to be careful in considering the safety of the personnel and the plant in the design of the process. He did feel, however, that his knowledge of dealing with this type of hazard was limited and decided to give a friend from university a call.

Linda Watson was one of the recognised experts in the field of hazardous areas and arranged to visit Eric at the plant to discuss the problem.

The meeting was convened in Eric’s office a week later with the following people present: Linda, Eric, two other process engineers, an electrical engineer and the chief control system engineer.

4.1 If you were Linda what would the first things be that you would advise Eric to do?

4.2 Make a schematic/block diagram of such a plant. The following are the basics of the process:

  • Unpurified cyclohexane is pumped into the plant from the other process and stored into large tanks.
  • The solution is heated to 100°C and another chemical is added that binds to the metals that are present in the solution.
  • The impurities are heavy and will drop to the bottom of the tank.
  • The impurities are now extracted and drained away from the tank.
  • The purified cyclohexane is now pumped into storage tanks from where it will be pumped to tanker vehicles or other containers to be transported to their customers.

4.3 Do an area classification for your plant. What shortages do you notice? What else may be needed?

4.4 Select equipment for use in your plant.

4.5 How would you do the electrical installations in your plant when considering the plant is fully automated and controlled from a central location?

4.6 How would you address earthing in your plant when taking into account that a fair number of lightning strikes occur each year?

4.7 Draw up a maintenance and inspection schedule.

Additional information

Cyclohexane has an ignition temperature of 285°C and requires Group IIA marked equipment.

Exercise 5

A case study of hazardous area classification of equipment

Since many delegates will be familiar with the basic design and construction of motor vehicles, this exercise has been specifically chosen to prompt discussion on the concepts of area classification. For the purposes of instruction, the delegates are asked to examine how the ‘industrial practice’ would be applied.

It should be noted that extensive vehicle testing is performed to meet exacting international standards for fire safety. Motor vehicle manufactures have agreed standards, which are suitable for these specialised conditions of risk.

Objectives

Petrol-driven motor vehicles such as the typical domestic car comprise mechanical and electrical systems that generate heat and energy in close proximity to the fuel, petrol. Since the layout of most cars is somewhat similar and known to many delegates, in this hypothetical exercise, you are required to area classify the vehicle shown in figure L.2 and L.3.

It is suggested that you work in small teams and brainstorm some ideas. The task is complete when the sketch of the car has been clearly marked with adequate information on the “hazard” and the area classification assigned to the vehicle in its various conditions of use. The team should agree on the conclusions. Indicate any areas you wish to designate Zone 0, 1 or 2.

Supplementary instructions

It is appreciated that you may have insufficient information to complete the task. Where you require more information, note this need adjacent to a relevant part or area.

The vehicle detail shown is diagrammatic and is specifically for the purposes of this exercise. The ‘area classification team’ may augment the drawing with additional information.

Any assumptions made should be recorded by appropriate annotations.

Figure E.2
Side elevation of car
Figure E.3
Plan view of car

Method of approach

Use the principles and terminology outlined in Chapter 3 of this course. Assess the risk by taking account of the nature of the hazard. Consider likely scenarios and postulate possible situations to be considered.

How would the modern car be area classified according to industrial practice? Assuming the layout to be as shown in Figure E.2 & E.3 (Plan and Elevation), what should be considered? What other information do you need to know?

Consider the four likely operational situations.

  1. Parked in a garage.
  2. Parked on the road.
  3. Low speed.
  4. High speed.

Your ideas should list the methodology used including:

  1. The identification of all sources of ignition (electrical and mechanical).
  2. The identification of all locations and routes (storage and piping) of fuel.

Petrol flammability characteristics

Flashpoint: -40°C
Ignition Temperature 310°C
Vapour density 1.24
Ignition energy: IIA
Vapour dispersion in still air: 4 metres in diameter from source of release. 0.5 metres high from the level ground

You are urged to consider the following situations:

  1. The vehicle is first started (from cold).
  2. The engine running but the vehicle is stationary.
  3. The car moves slowly, say, below 10mph/16kph
  4. Higher speeds above this value.
  5. Accidents of varying degrees of severity.
  6. Long term storage.
  7. Fuel leaks and spillage.

What other influences are present on the behaviour of the fuel and/or vapour?
How do you think a car made ‘safe’ from risk of explosion protection?

Exercise 6

Area classification of a process plant – a schematic

This exercise has been specifically chosen to prompt discussion on the protection concepts once area classification has been done. For the purposes of instruction, the delegates are asked to examine how the ‘industrial practice’ would be applied.

6.1 Objectives
The schematic of a process area is given below in Figure L.4. Indicate type of protection concepts you will choose for each of equipment in various Zones to make it compliant to Standards. The economics of selection and ease of maintenance should not be forgotten.

6.2 Supplementary instructions
It is appreciated that you may have insufficient information to complete the task. Where you require more information, note this need adjacent to a relevant part or area.

You may augment the drawing with additional information.

Any assumptions made should be recorded by appropriate annotations.

The method of protection for following equipment need to be marked for the following equipment and apparatus:-

  • Lighting
  • Enclosure – suggest the best method for protection of various apparatus housed in it.
  • i/p valve positioner
  • Motor
  • Motor Starter
  • Speed Monitor
  • Remote control Pendant with Indicator lights
  • Magnetic Flowmeter
  • Temperature Tx
  • Pressure Tx
  • Digital Indicator
Figure L.4
The hazardous area …application of protection techniques

Notes –

Exercise 7

Hazardous area classification for ‘A fixed roof tank’

Since many delegates will be familiar with the basic design and construction of a Process Plant, this exercise has been specifically chosen to prompt discussion on the concepts of area classification. For the purposes of instruction, the delegates are asked to examine how the ‘industrial practice’ would be applied.

7.1 Objectives
A product storage tank is of fixed roof type construction as shown in Figure L.5. It contains liquid above its flashpoint. Indicate any areas you wish to designate Zone 0, 1 or 2 in the areas already defined in the drawing.

7.2 Supplementary instructions
It is appreciated that you may have insufficient information to complete the task. Where you require more information, state this requirement adjacent to a relevant part or area.

You may augment the drawing with additional information. Any assumptions made should be recorded by appropriate annotations. You should also try to estimate the extent of area falling under the influence of the zone classified.

Figure L.5
The elevation of the tank with bund wall all around

7.3 Method of approach
Use the principles and terminology outlined in Chapter 3 of this course. Assess the risk by taking account of the nature of the hazard. Consider likely scenarios and postulate possible situations to be considered.

Exercise 8

Exercise on protection concepts and inspection

Instructions: -

You are to draw up some general and practical Hazardous Area Equipment Inspection Tables for technicians to follow when examining installations.

Under the headings of ‘Initial’, ‘Periodic’ and ‘Visual Inspection’ fill in the following tables by indicating with a tick which activities apply to the different types of protection. If there are missing requirements or unclear statements please indicate this in an appropriate way for later clarification.

A. Inspection Schedule for Ex p Equipment

Check that Types of inspection
Initial Periodic Visual
Apparatus is appropriate to area classification.      
Apparatus surface temperature class is correct.      
Apparatus carries the correct circuit identification,      
There are no unauthorized modifications.      
Earthing connections, including any supplementary earthing connections are clean and tight.      
Earth loop impedance or resistance is satisfactory.      
Lamp rating and type is correct.      
Source of pressure/purge medium is free from contaminants      
Pressure/flow is as specified      
Pressure/flow indicators, alarms and Interlocks function correctly.      
Pre-energizing purge period is adequate.      
Ducting. Piping and enclosures are in good condition.      
No undue external accumulation of dust and dirt,      

B. Inspection Schedule for Ex e equipment

Check that Types of inspection
Initial Periodic Visual
Apparatus is appropriate to area classification.      
Apparatus group (if any) is correct.      
Apparatus surface temperature class is correct.      
Apparatus carries the correct circuit identification.      
Enclosures, glasses and glass / metal parts are satisfactory.      
There are no unauthorised modifications.      
Earthing connections, including any supplementary earthing connections are clean and tight.      
Earth loop impedance or resistance is satisfactory.      
Lamp rating and type is correct.      
Bolts, glands and stoppers are of the correct type and are complete and tight.      
Enclosed-Break and Hermetically sealed devices are undamaged.      
Condition of enclosure gaskets is satisfactory.      
Electrical connections are tight.      
Apparatus is adequately protected against corrosion, the weather, vibrations, and other adverse factors.      
There is no obvious damage to cables.      
Automatic electrical protection devices are set correctly.      
Automatic electrical protection devices operate within permitted limits.      
No undue external accumulation of dust and dirt.      

C. Inspection Schedule for Ex i System

Check that Type of inspection
Initial Periodic Visual
System and/or apparatus are appropriate to area classification.      
System group or class is correct      
Apparatus surface temperature class is correct      
Installation is correctly labelled.      
There are no unauthorised modifications (including readily accessible lamp and fuse ratings).      
Apparatus is adequately protected against corrosion, the weather, vibration and other adverse factors.      
Earthing connections are permanent and not made via plugs and sockets.      
The INTRINSICALLY-SAFE CIRCUIT is isolated from earth or earthed at one point only      
Cable screens are earthed in accordance with the approved drawing.      
BARRIER UNITS are of the approved type, installed in accordance with the certification requirements and securely earthed.      
Electrical connections are tight.      
Point to point check of all connections      
Segregation is maintained between intrinsically- safe and non-intrinsically safe circuits in common marshalling boxes or relay cubicles      
There is no obvious damage to apparatus and cables.      

Exercise 9

Hazardous Area Classification for a compressor house

9.1 Objectives
The compressor type could be either Reciprocating or Centrifugal. It is supposed to be handling gas, which is heavier than air. The sides of the building are open and roof- ventilation is envisaged. Indicate any areas you wish to designate Zone 0, 1 or 2.

9.2 Supplementary instructions
It is appreciated that you may have insufficient information to complete the task. Where you require more information, note this need adjacent to a relevant part or area.

You may augment the drawing with additional information.

Any assumptions made should be recorded by appropriate annotations.

You should also try to estimate the extent of area falling under the influence of the zone classified (see figure L.6).

Figure L.6
The elevation of the building housing the compressor

9.3 Method of approach
Use the principles and terminology outlined in Chapter 3 of this course. Assess the risk by taking account of the nature of the hazard. Consider likely scenarios and postulate possible situations to be considered.

Solutions to Exercises

Exercise 1

For discussion in class

Exercise 2

The answer to Exercise 2 is as follows:
2.1 Ex ‘ia’, Ex ‘ib’, Ex ‘d’, Ex‘e’, Ex ‘p’, Ex ‘m’, Ex‘s’.
2.2 Pumps, Meters, Controllers, Lighting.
2.3 Ex ‘d’ /‘e’, Ex‘i’/‘d’/‘e’, Ex‘i’/‘e’, Ex ‘d’/‘e’
2.4 Identify the individual chemicals. The worst compound determines the classification. Fuel and diesel fuel = IIA.
2.5 Inside the liquid (Oxygen level too low for ignition) = safe area. Above the liquid there would be vapour that could be at LEL or above = Zone 0. The vapour concentration would depend on the ventilation of the tanks.
2.6 Simple apparatus like a capacitive sensor inside the liquid (same as motor vehicle), ultra sonic, laser, flow sensors. All these devices will have to be IS and suitable for use in this type of environment.

Exercise 3

The answers to exercise 3 are:
3.1 Corrosive environments (chemicals, sea, etc), High humidity, High lightning strikes, Cable damage, Accumulation of dust and dirt, Mechanical damage, Vibration, High temperatures, Training and experience of people, Likelihood of unauthorized modifications, Inappropriate maintenance.
3.2 The light metal alloys are prone to sparking when struck with another metal object.
3.3 Inspect it after an explosion occurred. Visual inspection of damage to the enclosure, glands, flanges, bolts, thread, seals, windows.
3.4 Repairs to IS devices must be done by people trained in IS requirements. The circuits must be repaired to the original type approval and all protection must be restored. The protection could include, conformal coating, encapsulation, seals for IP ratings.

Exercise 4

Group discussion only.

Exercise 5

To be discussed in class.

Exercise 6

Possible types of protection to be chosen are:-

Lighting:

Zone 1; Ex e
Zone 2; Ex n.
These would be the Lowest Cost options.
Ex e could be fitted in all locations to keep down the inventory and for ease of spare part holding. Ex d would be possible but too expensive and inconvenient.

Enclosure.
Analyser House: Ex px (Zone 1 to safe area!) Little choice!

i/p Valve positioner
Ex ib/ia or Ex d but the latter would be more expensive.

Motor
Depends on duty: Ex e lowest cost of ownership but only for non-arduous duty otherwise Ex d.

Motor Starter
Place in safe area where possible! Otherwise Ex d if it includes contacts

Speed sensor
Push button remote pendant
Transmitters
Digital Indicator
all Ex ib or Ex ia

Exercise 7

Answer: (Drawings not to scale)

Blanket Zoning:- If the ‘traditional’ approach is taken, the Zoning might end up as similar to Diagram A. Distances would require calculation but more likely compared with Institute of Petroleum, Code of Practice, IP15 at probably 1.5M from the tank wall for Zone 1.

Diagram A

The more expected extent of Zone 1 with no Zone 2 would be as shown in Diagram B. With ventilation taken into account, the zone would be immediately around the vent points and would be unlikely to extend down the side of the tank.

Diagram B

Exercise 8

Suggested answers (non-definitive)

A. Inspection Schedule for Ex p System

Check that Types of inspection
Initial Periodic Visual
Apparatus is appropriate to area classification  
Apparatus surface temperature class is correct.  
Apparatus carries the correct circuit identification,  
There are no unauthorized modifications.    
Earthing connections, including any supplementary earthing connections are clean and tight.    
Earth loop impedance, or resistance is satisfactory.    
Lamp rating and type is correct    
Source of pressure/purge medium is free from contaminants    
Pressure/flow is as specified  
Pressure/flow indicators, alarms and Interlocks function correctly.    
Pre-energizing purge period is adequate    
Ducting, piping and enclosures are in good condition.  
No undue external accumulation of dust and dirt    

General comments:
All the tables are incomplete!
Periodic inspection purpose must be specified as either: Detailed, Close or Visual
Other factors require addition, specific to what is being inspected.

B. Inspection Schedule for Ex e Equipment

Check that Types of inspection
Initial Periodic Visual
Apparatus is appropriate to area classification.  
Apparatus group (if any) is correct.  
Apparatus surface temperature class is correct.  
Apparatus carries the correct circuit identification.  
Enclosures, glasses and glass / metal parts are satisfactory.    
There are no unauthorised modifications.    
Earthing connections, including any supplementary earthing connections are clean and tight.    
Earth loop impedance, or resistance is satisfactory.    
Lamp rating and type is correct.  
Bolts, glands and stoppers are of the correct type and are complete and tight.  
Enclosed-Break devices are undamaged.    
Condition of enclosure gaskets is satisfactory.    
Electrical connections are tight.    
Apparatus is adequately protected against corrosion, the weather, vibrations, and other adverse factors.  
There is no obvious damage to cables.  
Automatic electrical protection devices are set correctly.    
Automatic electrical protection devices operate within permitted limits.    
No undue external accumulation of dust and dirt.  

C. Inspection Schedule for Ex i Systems

Check that Type of inspection
Initial Periodic Visual
System and/or apparatus is appropriate to area classification.  
System group or class is correct  
Apparatus surface temperature class is correct  
Installation is correctly labelled.  
There are no unauthorised modifications (including readily accessible lamp and fuse ratings).    
Apparatus is adequately protected against corrosion, the weather, vibration and other adverse factors.  
Earthing connections are permanent and not made via plugs and sockets.  
The INTRINSICALLY-SAFE CIRCUIT is isolated from earth or earthed at one point only    
Cable screens are earthed in accordance with the approved drawing.  
BARRIER UNITS are of the approved type, installed in accordance with the certification requirements and securely earthed.  
Electrical connections are tight.    
Point to point check of all connections    
Segregation is maintained between intrinsically- safe and non-intrinsically safe circuits in common marshalling boxes or relay cubicles  
There is no obvious damage to apparatus and cables.  
No undue accumulation of dust and dirt    

Exercise 9

Area Classification : Zone 2 most likely, not Zone 1.

Based on a gas at high pressure which is heavier than air, in a building space with considerable heat source and a chimney effect then the expected shape might be similar to that shown. Convection currents could cause the rise of gas, depending on the extent of ventilation.


Appendix F


ATEX: European Directives

Introduction

The following is a list of exercises that may be worked through in the class setting or may be used for further study as time allows and depending on the class profile. The exercises are designed to be carried out in conjunction with the information presented in the manual. The exercises are not in a specific order.

Clearly some exercises would take considerable time to complete. It may be that additional information is needed. The more important point of learning here may be in deciding the approach, realizing the limitations and discerning the level of confidence required with which the problem is tackled.

Please read the information fully before proceeding.

Exercise titles

  1. Area Classification of your work place – discussion in class of a few case studies of participants
  2. Area Classification of a Fuel Station
  3. Brain Teasers on Hazardous Areas
  4. A Case Study for Additions or Modification in Hazardous area of a Plant
  5. A Case Study in Hazardous Area Classification of Equipment
  6. Area Classification of a process plant – a Schematic
  7. Hazardous Area Classification for ‘A fixed roof tank’
  8. Exercise on Protection concepts and Inspection
  9. Hazardous Area Classification for Compressor Housing
  10. Exercise on Protection concepts Ex ‘d’
  11. Switch Status Input System Calculations
  12. Solenoid Valve Operational Characteristics

Learning objectives

  • To study ATEX and its effect on the IEC Standards
  • To examine the UK DSEAR

F.1 Introduction

There are two parts to this discussion.

F.2 Exercises

Exercise 1

F.3 Introduction to ATEX

Two European Community Directives came into force in the EC as from July 2003, known as the ATEX Directives.

  • Directive 94/9/EC

    (derived from Article 100 of the Engineering Working Group agreement)
    Equipment and protective systems for use in potentially flammable atmospheres and

  • Directive 1999/92/EC

    (derived from Article 137 of the EWG agreement)
    Minimum requirements for the protection of workers in potentially explosive atmospheres.

Whereas the ‘Equipment’ Directive is the concern of manufacturers of Ex protected electrical equipment and of the ‘Notified Bodies’, appointed to perform the testing, compliance with the ‘Workers Directive’ is mandatory for Users of the equipment.

In essence, the so called European ‘Old Approach’ referred to the period after CENELEC began to write European Standards. Equipment was, thereafter, marked ‘EEx’ to signify this. The use of CENELEC Standards was construed in Law in such a way that if the Standards were updated then the Laws of EC member countries had to be changed each time to accommodate them!

The New Approach allowed the development of a series of Directives where “Essential Safety and Health Requirements” (ESHR) were stated. The Law was clarified to state the need to ensure safety, so that the ESHRs must be met. It was therefore not necessary to make changes in Legislation every time a Standard was changed or a new one was developed. As a result new Standards emerged addressing other issues that had been identified as necessary. Non-electrical equipment that could be ignition-capable became subject to assessment to meet the newly stated ESHRs.

ATEX now requires the assessment of ‘equipment and protective systems’ for its ignition-avoidance capability and integrity to meet the ESHRs whence it may carry the European Conformance Assessment mark ‘CE’.

On March 1, 1996, a transitional period began for the implementation of the ATEX Directive (94/9/EC). This Directive applies to electrical and non-electrical equipment/components and protective systems intended for use in potentially explosive atmospheres. The ATEX Directive became mandatory on July 1, 2003.

ATEX was proposed in 1994 and the New Approach and ATEX compliance were run in parallel up until the time ATEX was fully in place. Manufacturers had the choice of certification to the European Standards or by demonstrating that the ESHRs under ATEX had been met.

The ‘Transitional Period’ which lasted up to 30 June 2003, when the ATEX-Directive 94/9/EC cancelled all the old approaches. At the end of the transition period, manufacturers in the European Union (EU) had to comply with the relevant National Regulations, which will be in line with the ATEX directive. All equipment and protective systems intended for use in potentially explosive atmospheres which are made or sold in the EU, including imports, had to:

  • satisfy the wide-ranging essential health and safety requirements;
  • in some cases, be subject to type-examination by a notified body;
  • in many cases be subject to conformity assessment procedures by a notified body;
  • carry CE marking and information (generally about the manufacturer).

F.4 Product directive

The ATEX Directive 94/9/EC has been adopted by the European Union (EU) to facilitate free trade in the EU by aligning the technical and legal requirements in the Member States for products intended for use in potentially explosive atmospheres. The full text of the Directive was published in the Official Journal of the European Communities No L 100, dated 19 April 1994.

The fundamental underlying principles enshrined in ATEX directive are,

  • Prevent the formation of explosive atmospheres
  • Prevent the ignition of unavoidable explosive atmospheres
  • Control the effects of unavoidable explosions

The ATEX Directive sets out the essential requirements that products must meet and defines procedures,

  • For the evaluation of a product’s design
  • For manufacture (production) based on Equipment Groups and Categories.
  • For the conformity assessment that manufacturers must undertake before affixing the CE marking to them.

Equipment located outside potentially explosive atmospheres are also covered by the ATEX Directive if the equipment includes:

  • A safety device,
  • Controlling device or
  • Regulatory device
  • Required for the safe function of equipment
  • Required for protective systems with respect to the risk of explosion.

F..5.1 Change in philosophy

The change to the Directive is not merely an update, but a major change in philosophy, extending to cover under normal operating conditions,

  • All equipment and protective systems, which may be used in areas endangered by potentially explosive atmospheres created by the presence of flammable gases, vapours, mists
  • Dusts
  • Both electrical and non-electrical equipment
  • Constructional features
  • Methods of testing
  • Certification requirements and procedures
  • Requirements for safety related devices (flame arrestors, suppression systems etc) and safe area equipment
  • Marking of equipment
  • Additional quality system requirements
  • The need to produce a ‘Technical File’

‘USE’ or ‘Worker’s’ Directive

Along with the ATEX Directive EU has also issued a new Directive for ‘The Protection of Workers at Risk from Potentially Explosive Atmospheres’ (1999/92/EC), on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres (15th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC) - commonly known as the ‘Use’ directive. This Directive also became mandatory under EU law from 1st July, 2003.

The essential requirements under this directive are that Sites shall document evidence where potentially explosive atmospheres (gas or dust) may develop by carrying out;

  • Risk analysis;
  • Area classification
  • Site inspections.

This ‘Use’ Directive ensures that only ATEX certified electrical, mechanical and safety related systems are installed in potentially explosive atmospheres.

Some other salient features of this Directive are,

  • The requirement to create documentation on explosion protection with a comprehensive risk assessment,
  • The duty to protect workers,
  • The safety strategy in force at the operator’s premises,
  • The determination of the risk posed by the explosion danger,
  • The classification of areas (Zone 0, 1, 2, 20, 21, 22)
  • The requirements relating to qualifications of employees working in the hazardous areas,
  • The procedure for giving approvals to work,
  • Regular testing,
  • The criteria governing the selection of working substances and installation materials for the different zones,
  • The identification of the areas.
  • The criteria for the authorization of work in the various zones.

The establishment of a coherent strategy for the prevention of explosions requires that organizational measures complement the technical measures taken at the workplace.

Documentation

As mentioned above, Directive 89/391/EEC requires the employer to be in possession of an assessment of the risks to workers’ health and safety at work. This Directive specifies that the employer is to draw up an explosion protection document, or set of documents, which satisfies the minimum requirements laid down in this Directive. In addition to this, the employer is to keep it up to date.

The explosion protection document may be part of the assessment of the risks to health and safety at work and should include,

  • The identification of the hazards
  • The evaluation of risks
  • The definition of the specific measures to be taken to safeguard the health and safety of workers at risk from explosive atmospheres

An assessment of explosion risks may be required under other Community acts; whereas, in order to avoid unnecessary duplication of work, the employer is allowed, in accordance with national practice, to combine documents, parts of documents or other equivalent reports produced under other Community acts to form a single ‘safety report’.

F.5.2 Definitions

ATEX

“ATmospheres EXplosibles” is French for “potentially explosive atmospheres”. The Directive 94/9/EC is frequently called the “ATEX-Directive”. In other parts of the world, areas with potentially explosive atmospheres are often called hazardous locations.

CE

The CE Marking is a distinctive Community Mark of the European Union. It is a marking that signifies Declaration by the Responsible Party (usually the manufacturer) that a product is compliant with all appropriate European Union New Approach Directives, such as the ATEX, Low Voltage, EMC, Machinery Directives and quality assurance, third party assessment, depending on the directives.

Product

It covers equipment, protective systems, devices, components and their combinations.

Notified body

A Notified Body is a Testing Laboratory and Certification Organization. Every National government notifies (or, designates) one or more testing/certification lab as an agency able to issue certificates pertaining to the Directives.

Categories of equipment

Equipment when assessed for compliance is assigned a Level of Protection. This can now be awarded by the Notified Body. Compliance with certain Standards suggests specific levels are awarded to equipment but this can be modified by an authority and allows some flexibility not previously given. For example, ‘Ex ia’ systems are useable in a Zone 0 and are for applications requiring a ‘Category 1’ approach, equivalent to the IEC’s ‘EPL Ga’ designation. Alternatively a combination of types of protection submitted by a manufacturer as a solution to an application and the Notified Body can assign a Category (or EPL) after adequate testing and evaluation.

F.5.3 Scope product directive

The objective of directive 94/9/EC is to ensure free movement for the products to which it applies in the EU territory. Therefore the directive, based on Article 95 of the EC Treaty, provides for harmonized requirements and procedures to establish compliance.

The directive notes that to remove barriers to trade via the New Approach, provided for in the Council Resolution of 7 May 1985(17), essential requirements regarding safety and other relevant attributes need to be defined by which a high level of protection will be ensured. These Essential Health and Safety Requirements (EHSRs) are listed in directive 94/9/EC at Annex II. The essential requirements must be met by equipment and protective systems intended to be used in potentially explosive atmospheres.

The essential requirements fall into three groups:

  • Common requirements
  • Requirements for equipment
  • Requirements for protective systems

After 30th June 2003 products could be placed on the market in the EU territory, freely moved and operated as designed and intended in the expected environment only if they comply with directive 94/9/EC (and other relevant legislation).

Atmospheres covered

An explosive atmosphere for the purposes of directive 94/9/EC is defined as:

  • a mixture, of flammable substances in the form of gases, vapours, mists or dusts with air; under atmospheric conditions in which, after ignition, the combustion spreads to the entire unburned mixture.

It has to be noted that not always the whole quantity of dust is consumed by the combustion. A second combustion could take place at a later time.

An atmosphere, which could become explosive due to local and/or operational conditions, is called a ‘potentially explosive atmosphere’ or PEA. It is only this kind of potentially explosive atmosphere which products falling under the directive 94/9/EC are designed for.

It is important to note, that products are not covered by directive 94/9/EC (27) where they are intended for use in or in relation to atmospheres which might potentially be explosive, but one or more of the defining elements as above are not present.

Product and equipment covered

The word ‘equipment’ is used instead of ‘apparatus’. It should be noted that directive 94/9/EC provides for the first time Essential Safety and Health Requirements for non-electrical equipment intended for use in potentially explosive atmospheres and is to include both electrical and non-electrical equipment.
Equipment, as defined in directive 94/9/EC, means

  • Machines,
  • Apparatus,
  • Fixed or mobile devices,
  • Control components and instrumentation thereof
  • Detection or prevention systems, which, separately or ‘jointly’, are intended for
    • The generation,
    • Transfer,
    • Storage,
    • Measurement,
    • Control
    • Conversion of energy
    • The processing of material

and which are capable of causing an explosion through their own potential sources of ignition. It should be noted that intrinsically safe equipment is included in the scope of the directive.

The following is a partial list of equipment, which as an example is subject to Directive 94/9/EC, Article 1.2, due to being a potential source of ignition:

  • Electrical equipment and apparatus (motors, lamps, switches, measuring devices, gas detection devices, etc.)
  • Ventilators, fans, compressors
  • Pumps
  • Forklift trucks
  • Fast-running mechanical machinery
  • Centrifuges
  • Drive belts
  • Mixers
  • Hoisting gear

Electrical equipment

Directive 94/9/EC does not define “Electrical Equipment”. However, because such equipment is subject to its own conformity assessment procedure it may be useful to provide a definition, which has been generally accepted by the majority of Member States, as follows:

Electrical Equipment: Equipment containing electrical elements is used for the generation, storage, measurement, distribution and conversion of electrical energy, for controlling the function of other equipment by electrical means or for processing materials by the direct application of electrical energy. It should be noted that a final product assembled using both electrical and mechanical elements may not require assessment as electrical equipment provided the combination poses no additional risks.

Examples: A pump (non-electrical) is assessed under the appropriate conformity assessment procedures and is then connected to an electric motor (electrical equipment), which has already been assessed. As long as the combined equipment poses no additional hazards, then no further assessment for the electrical part is necessary.

If the same pump and electric motor have not been through the appropriate conformity assessment procedures and are connected, then the resulting product is to be regarded as electrical equipment and the conformity assessment should treat it as such.

Protective systems

“Protective Systems” means design units, other than components, which are intended to halt incipient explosions immediately and/or limit the effective range of explosion flames, and include items that prevent an explosion that has been initiated from spreading or causing damage. They include,

  • Flame arrestors,
  • Quenching systems - water trough barriers
  • Pressure relief panels or explosion relief systems (using e.g. bursting discs, vent panels, explosion doors, etc.);
  • Fast-acting shut-off valves
  • Extinguishing barriers
  • to name but a few.

    Products are to be categorized by the level of protection that they offer against the risk of them becoming a potential source of ignition of an explosive atmosphere.

Some examples of devices falling under protection systems are:

  • A power supply feeding an intrinsically safe (Ex ‘i’) measurement system used for monitoring process parameters;
  • A pump, pressure-regulating device, backup storage device, etc. ensuring sufficient pressure and flow for feeding a hydraulically actuated safety system (with respect to the explosion risk);
  • Overload protective devices for electric motors of type of protection EEx ‘e’ ‘Increased Safety’;
  • Controller units in a safe area, for an environmental monitoring system consisting of gas detectors distributed in a potentially explosive area, to provide executive actions if dangerous levels of gas are detected;
  • Controller units for sensors temperature, pressure, flow, etc, located in a safe area, for providing information used in the control of electrical apparatus, used in production or servicing operations in a potentially explosive area..

Components

A component means any item essential to the safe functioning of equipment and protective systems but with no autonomous function.

Components intended for incorporation into equipment or protective systems which are accompanied by an attestation of conformity including a statement of their characteristics and how they must be incorporated into products, are considered to conform to the applicable provisions of directive 94/9/EC. Ex – components as defined in the European standard EN 50014 are components in the sense of the ATEX directive 94/9/EC as well. Components must not have the CE marking affixed unless otherwise required by other directives (e.g. the EMC directive 89/336/EEC). Examples:

  • Terminals;
  • Push button assemblies;
  • Relays;
  • Empty flameproof enclosures;
  • Ballasts for fluorescent lamps;
  • Meters (e.g. moving coil);
  • Encapsulated relays and contactors, with terminals and/or flying leads.

Assemblies

From the term ‘jointly’ in the definition above it follows that an assembly, formed by combining two or more pieces of equipment, together with components if necessary, has to be considered as a product falling under the scope of directive 94/9/EC, as a single functional unit.

Such assemblies may not be ready for use but require proper installation. The instructions (Annex II, 1.0.6.) will have to take this into account in such a way, that compliance with directive 94/9/EC is ensured without any further conformity assessment, provided the installer has correctly followed the instructions.

In the case of an assembly consisting of different pieces of equipment as defined by directive 94/9/EC which were previously placed on the market by different manufacturers these items of equipment have to conform with the directive, including being subject to proper conformity assessment, CE-marking, etc. The manufacturer of the assembly may presume conformity of these pieces of equipment and may restrict his own risk assessment of the assembly to those additional ignition and other relevant hazards (as defined in Annex II), which become relevant because of the final combination. If additional hazards are identified a further conformity assessment of the assembly regarding these additional risks is necessary. Likewise, the assembler may presume the conformity of components, which are accompanied by a certificate, issued by their manufacturer, declaring their conformity.

However, if the manufacturer of the assembly integrates parts without a CE-marking into the assembly (because they are parts manufactured by himself or parts he has received from his supplier in view of further processing by himself) or components not accompanied by the above mentioned certificate, he shall not presume conformity of those parts and his conformity assessment of the assembly has to cover those parts as required.

Annexure H may be referred to for further details.

Installations

A common situation is that pieces of already certified equipment are placed on the market independently by one or more manufacturer(s), and are not placed on the market by a single legal person as a single functional unit. Combining such equipment and installing at the user’s premises is not considered as manufacturing and thus does not result in equipment; the result of such an operation is an installation and is outside the scope of directive 94/9/EC. The installer has to ensure that the initially conforming pieces of equipment are still conforming when they are taken into service. For that reason he has to carefully follow all installation instructions of the manufacturers. The directive does not regulate the process of installation. Installing such equipment will generally be subject to legal requirements of the Member States. An example could be instrumentation consisting of a sensor, a transmitter, a Zener barrier and a power supply if provided by several different manufacturers installed under the responsibility of the user.

Product type

A product type is a unique design that is sufficiently well defined that items manufactured to that definition would be in compliance with the requirements applicable to the product type. The concept of product type is of particular importance where products are to be manufactured in volume and where the cost of a notified body verifying the conformity of each item produced would be prohibitive.

In compiling the technical file, the manufacturer must prepare a set of documents, normally in the form of drawings that specify every feature of the product that could affect conformity with the essential requirements. Different features will be addressed by different requirements. For example the materials of which the product is made must have,

  • Adequate strength
  • Stability
  • Be resistant to the operating conditions (including corrosion, heat and ultraviolet light for example) defined by the manufacturer.
  • The dimensions of component parts must have suitable tolerances that the required degree of fit will be achieved.

The sample(s) presented for inspection and testing must be in conformity with the specified design features. Where a particular test, for example mechanical impact, is to be applied, the sample(s) must represent the worst-case condition such as the minimum wall thickness of an enclosure. The sample(s) must also be sufficiently representative of the production methods to be used not to adversely affect the outcome of the tests. If a plastic case has been fabricated from sheet materials for testing purposes and the final product is to be moulded, it may be necessary to repeat some tests on the moulded article if there is a possibility that the moulding process may give rise to weaknesses such as flow lines.

Exclusions of products and equipment

The following product types are excluded from the Directive only because other requirements apply:

  • Medical devices intended for use in a medical environment
  • Equipment and protective systems where the explosion hazard results exclusively from the presence of explosive substances or unstable chemical substances
  • Products for use in the presence of explosives
  • Equipment intended for use in domestic and non-commercial environments where potentially explosive atmospheres may only rarely be created, solely as a result of the accidental leakage of fuel gas
  • Sea-going vessels and mobile offshore units together with equipment on board such vessels or units
  • Military equipment
  • Personal protective equipment covered by directive 89/686/EEC
  • Means of transport, i.e. vehicles and their trailers intended solely for transporting passengers by air or by road, rail or water networks, as well as means of transport in so far as such means are designed for transporting goods by air, by public road or rail networks or by water. Vehicles intended for use in a potentially explosive atmosphere shall not be excluded, the equipment covered by Article 223 (1) (b) of the Treaty.

Groups and categories

All the equipment covered under this directive is divided into traditional groups of:

  • Group I – mining
  • Group II – non-mining

Group I - Mining equipment

The Categories of equipment for gassy mines are defined as hereunder,

Equipment Category Protection Comparison To Current IEC Classification
M1 2 levels of protection; or 2 independent faults Group I
M2 1 level of protection based on normal operation Group I

The M1 category of equipment is designed to operate continuously in explosive atmosphere and must be,

  • Equipped with additional special means of protection. It must remain functional with an explosive atmosphere present.
  • So constructed that no dust can penetrate it.
  • So constructed that the surface temperatures of equipment parts are kept clearly below the ignition temperature of the foreseeable air/dust mixtures in order to prevent the ignition of suspended dust.
  • So designed that the opening of equipment parts, which may be sources of ignition, is possible only under non-active or intrinsically safe conditions. Where it is not possible to render equipment non-active, the manufacturer must affix a warning label to the opening part of the equipment. If necessary, equipment must be fitted with appropriate additional interlocking systems.

Some features of equipment coming under category M2 which are required to operate in a potentially explosive atmosphere are,

  • The equipment is intended to be de-energized in the event of an explosive atmosphere.
  • Equipment must be so designed that the opening of equipment parts, which may be sources of ignition, is possible only under non-active conditions or via appropriate interlocking systems. Where it is not possible to render equipment non-active, the manufacturer must affix a warning label to the opening part of the equipment.

`Mines cannot be zoned’ like a petrochemical plant. There is no distinct source of release and gas is emitted by,

  • Most of the walls,
  • Floors
  • Ceilings
  • Main product (coal) during transport.

Consequently mines have two different situations, neither depending on the place, nor depending on the time.

Sometimes the gas (methane) content in the air (at those places where measuring instruments are installed or where somebody uses a portable instrument) is below the permissible value, normally fixed by the inspecting bodies at levels between 1% and 2%, well below the lower explosive limit (LEL). Of course at other places at the same time there may be other, higher concentrations, even higher than LEL.

This increase of the gas from source (or decrease of the ventilation) normally is recognized very soon in a properly operated mine. Then the power is switched off and the machinery is stopped. During the time before switching off, the electrical equipment [category M2] - which is designed using identical principles as Zone-1-equipment - of course may operate in explosive atmosphere; this is the intended use. After switching off the M2-equipment, there is a need for instrumentation for safety purposes and communication. This equipment needs to meet criteria similar to Zone-0-equipment. Consequently it is named “Category M1”.

Category M1 / M2 equipment and coal dust
The M1 and M2 equipment are not only safe in explosive methane atmosphere, they are also safe with respect to coal dust layers and coal dust clouds.

Coal dust explosions in underground mines normally are triggered by an ignition of a methane explosion. But also coal dust alone is able to form an explosive atmosphere, which can be ignited by ignition sources with sufficient energy (e.g. high power arcing, wrong use of explosives, hot spots, smouldering nests in coal dust layers). Consequently since 1977 the CENELEC standards (which since that time contain basic requirements for 2D and M2 equipment) required what we call now M2 equipment to meet IP 54 requirements and to limit the maximum external surface temperature to 150° C. For flameproof (American English: explosion proof) electrical equipment - the gaps, which do not transmit a methane explosion, are also safe for a coal dust explosion.

A 200 m gallery for coal dust explosions was installed in Dortmund 1911. With this set-up for the first time it could be demonstrated, that with coal dust alone (without any methane) a self-propagating explosion is possible. The gallery is still in use.

The CENELEC standard EN 50014 is slightly amended and re-introduces special requirements concerning electrostatic ignition risks and EN 50020 - 2 is supplemented by a separate standard for intrinsically safe systems for Group I. This is in line with the original approach in Directive 82/130/EEC, “electrical equipment for use in underground parts of mines susceptible to firedamp which may be endangered by firedamp’.

Group II - Equipment
Equipment intended for use in other than Equipment Group I, i.e. non-mining places that are liable to be endangered by explosive atmospheres is defined as hereunder,

Equipment Category Definition Protection Comparison To Current IEC Classification
1 Very High level of protection 2 levels of protection; or 2 independent faults Group II, Zone 0 (gas)
Zone 20 (dust)
2 High level of Protection 1 level of protection based on frequent disturbances; or equipment faults Group II, Zone 1 (gas)
Zone 21 (dust)
3 Normal level of protection 1 level of protection based on normal operation Group II, Zone 2 (gas)
Zone 22 (dust)

Verifying conformity with the ESHRs

The manufacture of products for use in potentially explosive atmospheres has to be monitored as part of the surveillance activity undertaken by the Notified Bodies in the Member States.

Associated with each equipment category, whether for electrical or non-electrical equipment, is a complex conformity assessment procedure defined by the Ex Product Directive 94/9/EC. The following outlines the main features of the conformity assessment procedures for electrical and mechanical equipment, apparatus, machines and internal-combustion engines and the documents that are generated from the procedures.

The following table lays out the alternatives to formal product certification:-

Equipment category 1 2 2 3 Annex of 100a
Equipment type All Electrical Non-Electrical All  

 

Certification Phase  
Certification by Notified Body Yes Yes Yes   III
Certification by manufacturer       Yes VIII
Unit verification by Notified Body U N I V E R S A L O P T I O N IX

 

Surveillance  
QA of production by Notified Body Yes       IV
QA of product by Notified Body   Yes     VII
QA by manufacturer     Yes Yes VIII

Notes to the above table:-

The annexes referred to give a more detailed explanation of the requirements under each option.

The Internal Combustion Engine is treated as electrical equipment for the purpose of this part of the directive.

Unit verification is normally used for special/small quantity manufacture equipment.

The ATEX Directive now means that far more equipment will require certification. This relates mainly to the need for mechanical equipment and protective systems to comply with the Directive. Presently there are over 70 new standards being prepared by CEN committees specifically for these types of equipment.

All the equipment to be used in Hazardous atmospheres under this directive will have to bear distinctive marking of category. The marking of the equipment with the category will help the end-user with their selection of the equipment in that it identifies which Zone it can safely be installed in.

Harmonized standards
CENELEC and CEN technical committees are recognized as the bodies competent to adopt harmonized standards, which follow the general guidelines for cooperation between the Commission of the European Communities (CEC) and those two bodies, signed on 13 November 1984.

CENELEC is the European Committee for Electro-technical Standardization.
CEN is the European Committee for Standardization.

For the purposes of this Directive, a harmonized standard is a technical specification (European Standard or harmonization document) adopted by one or other of those bodies, or by both, at the prompting of the Commission pursuant to Council Directive 83/189/EEC of the 28 March 1983 providing for a procedure governing the provision of information on technical standards and regulations and pursuant to the general guidelines referred in the Directive.

CENELEC and CEN are European standardization bodies recognized as competent in the area of voluntary technical standardization and listed in Annex I of the Union Directive 98/34/EC (replacing 83/189/EEC) concerning ‘the information procedure’ for standards and technical regulations. Together they prepare European Standards in specific sectors of activity. When these standards are prepared in the framework of the ‘new approach’ directives, they are known as ‘harmonized standards’ and will be cited in the Official Journal. Products manufactured in accordance with these standards benefit from a ‘presumption of conformity’ to the essential requirements of a given directive.

Existing standards may be suitable in their existing form or may require amendment in order to address the essential requirements. Harmonized standards may originate as existing standards or be created at the specific request of the CEC (see figure F.4).

CEN or CENELEC carries out the process of creation or amendment as appropriate. The work is assigned to the relevant Committee or, if none exists, a new committee is set up. The member bodies of CEN or CENELEC will appoint the members of the committee. The member bodies are the national standards bodies of each member country. In practice the appointment of members is delegated to the corresponding committee at national level. The Chairman and Secretary of the committee are appointed following consultation with the members.

The process of creating a standard starts with the adoption of the item on the work program of the committee. A draft is prepared, usually by a small working group or one of the members and is circulated to national committees for their comments. The comments are discussed at a meeting of the committee and a decision taken on either amending the draft for further discussion, or putting the draft forward.

The national committee considers comments before sending the official national comments to the European Committee. The European committee considers the comments and decides either to put the document out for voting or to refer the draft back for further consideration.

Once the document is sent out for voting the national committees must decide whether to accept or reject the document. The accepted document is translated into English, French and German by the European committee and sent to the CEC for them to accept as a harmonized standard according to the mandate. Provided that the document is acceptable, the CEC will publish the number and title in the OJEC together with a reference to the directive to which it relates.

The published standards are produced by each national standards body in their own language and as a direct transposition of the text into a national standard.

There is an agreement between CENELEC and the International Electro technical Commission (IEC) to co-ordinate their standards work. Thus, if an IEC standard already exists or is in preparation, it will be used as the starting point for a CENELEC standard. In this way products produced in Europe should be acceptable in other countries where IEC standards are used. Equally products from these countries should comply with.

The preparations of the standards (EN50 series) covering electrical products have been completed by CENELEC and are now included in the Official Journal. The standards for mechanical products and protective systems are still either under development or under approval.

Category 1 and M1 equipment
Category 1 and M1 require the highest levels of explosion protection, because they are the types of equipment intended to remain operational in a continuously present explosive atmosphere (see figure F.5). Even though no constructional details are given in the directive but the basic requirements as explained earlier are,

  • With even one fault the equipment is safe against ignition
  • To have two independent protection types applied in such a way that if one fails the other will protect against ignition.

CEN and CENELEC have produced the following harmonized standard to enable Notified bodies to issue certificate of conformity,

  • For category 1G equipment it is EN50284: 1999
  • For category M1 equipment it is EN50303:2000

The majority of equipment in this category is likely to be of the type as indicated above. Here, as shown in the example either Increased Safety ‘e’ or Intrinsically Safe ‘ib’ apparatus is located inside a flameproof ‘d’ enclosure or pressurized ‘p’ enclosure. This provides two independent means of protection and to ensure the integrity of the protection concept the power supply to such equipment is through cables carrying intrinsically safe ‘ia’ circuits.

Category 2 and M2 equipment
The protection concepts for Category 2 and M2 electrical equipment are already well known and are being used.

As mentioned before Internal Combustion engines are the new type of equipment, which will be covered under this new Directive. These diesel engines when used in powering locomotives and free steered vehicles have been traditionally of flameproof type. But now new standards have been produced to allow ATEX conformity. These are:-

  • EN 1834-1:2000 – Reciprocating internal combustion engines – safety requirements for design and construction of engines for use in potentially explosive atmospheres – Part 1 – Group II engines for use in gas and vapour atmospheres.
  • EN 1834-2:2000 – Reciprocating internal combustion engines – safety requirements for design and construction of engines for use in potentially explosive atmospheres – Part 2 – Group I engines for use in underground workings susceptible to firedamp and / or combustible dust.
  • EN 1834-3:2000 – Reciprocating internal combustion engines – safety requirements for design and construction of engines for use in potentially explosive atmospheres – Part 3 – Group II engines for use in flammable dust atmosphere.

F.5.4 New series of standards for Non-electrical equipment

Traditionally so many other equipment are being used in potentially explosive atmosphere for centuries. These could include the mechanical de-watering pumps used in mines to the modern pneumatically or hydraulically driven equipment, like – pumps etc.

CEN is now producing a new series of standards, which are similar to the ones in existence and has allotted a ‘symbol letter’ to each type of protection so as to allow them to be identified with a ignition protected electrical equipment as given hereunder,

Standards for Non-Electrical Protection Concepts
Equipment Protection method Description and notes
EN 13463-1 Basic methodology and requirements
EN 13463-2
Flow restricting enclosure ‘fr’
The explosive atmosphere is prevented from reaching the ignition source by the tight seals of the enclosure. The seals restrict the breathing of the enclosure as internal air heats and cools through operation of the equipment.
EN 13463-3
Flameproof enclosure ‘d’
EN 13463-3:2005 Protection by flameproof enclosure An ignition inside the equipment does not propagate the external atmosphere. This type of protection relies on closely machined joints and a tough enclosure.
EN 13463-4
Inherent safety ‘g’
Protection by inherent safety (symbol: g) - low potential energy.
EN 13463-5
Constructional safety ‘c’
Protection by constructional safety (symbol: c) - Ignition hazards are eliminated by the specification of the equipment.
EN 13463-6
Control of ignition sources ‘b’
Sources of ignition are only present in the event of a malfunction. The equipment is fitted with control equipment to detect malfunctions and prevent ignition sources arising.
EN 13463-7
Pressurisation ‘p’
The enclosure is purged with a protective gas (air) and pressurised to ensure that an external atmosphere cannot re-enter the enclosure.
EN 13463-8
Liquid immersion ‘k’
The enclosure has a suitable liquid to prevent the explosive atmosphere reaching the ignition source or to cool a hot surface (for example a gearbox).

Note: Part 1 of the above Standard is similar to EN50014 in that it and includes general requirements for all the types of protection of non-electrical equipment.

This places responsibility on manufacturers to,

  • Assign a ‘Maximum surface temperature’ to equipment
  • Take in to account the restrictions on use of exposed light metals, like – alloys containing aluminium, magnesium, titanium and zirconium
  • Avoid the risk of electrostatic discharge
  • Perform impact tests on the parts that could have their ignition protection hampered in case of impact damage
  • Mark the equipment with ATEX Group, category and ignition protection symbol
  • Submit technical file (as described below)

Technical file
The technical documentation to be submitted by manufacturer to Notified Body is termed as ‘Technical File’. A technical file is a dossier of information specifying a product in sufficient detail for it to be manufactured in accordance with the requirements of the ATEX Directive and containing the evidence that the product conforms to those requirements. The ATEX Directive does not use the term “technical file” but refers to “technical documentation”. It has become common in the field of EU directives to refer to such an item as a technical file.

Annexes III, VIII and IX of the ATEX Directive specify that the technical file shall contain:

  • A general description of the product
  • Design and manufacturing drawings and layouts of components, sub-assemblies, circuits, etc.
  • Descriptions and explanations necessary for the understanding of the drawings and layouts and the operation of the product
  • A list of harmonized or other standards that have been applied in full or in part
  • For aspects where standards have not been applied, descriptions of the solutions that have been adopted to meet the essential requirements of the Directive
  • Results of design calculation examinations carried out, etc

Certification & CE marking
The CE Marking is intended to facilitate the free movement of products within the EU by signifying that essential health and safety requirements have been met.

The CE Marking comprises the symbols CE together with such other information as may be required by the European Union directives, which apply to a particular product. For the ATEX Directive, the symbols CE must be accompanied by the following:

  • Name and address of manufacturer
  • Designation of series or type
  • Serial number, if any
  • Year of construction

The specific marking of explosion protection followed by the symbol of the equipment group and category,

  • For equipment-group II, the letter G’ (concerning explosive atmospheres caused by gases, vapours or mists), and/or
  • The letter ‘D’ (concerning explosive atmospheres caused by dust).

Furthermore, where necessary, they must also be marked with all information essential to their safe use.

The manufacturers normally affix the CE Marking to the products. The manufacturers legally appointed representative in the EU might affix where products are manufactured outside the EU the CE Marking. However, the representative would then be taking legal responsibility for verifying the conformity of the products with the requirements of the relevant directives. In that case the representative would have to comply with the conformity assessment procedures, including, where required, Type Examination and Quality Modules.

For the ATEX Directive, the CE Marking must be affixed to each item of equipment or to each protective system. The CE Marking must not be affixed to components which do not of themselves comply with all relevant requirements but which must be combined with other parts in order to comply.

Conversions of existing certification
The vast majority of equipment certified before 1994 will not comply with the latest Harmonized Standards for the ATEX Directive and, manufacturers who do not consider the design implications now are liable to be caught out in the very near future. It is imperative that before putting your equipment up for certification you ensure that you are now compliant with all of the ‘latest’ relevant Harmonized European Standards and the Essential Health and Safety Requirements (EHSR) of the Directive.

Conformity assessment procedure

Quality requirements

The annexe to directive specifies quality assurance requirements, known as ATEX Quality Modules. These modules are issued to the product manufacturer by the Notified Bodies. The issuing of the modules is dependent upon the manufacturer achieving a satisfactory level of quality control, which is determined by an external audit performed by the Notified Body. Maintaining the quality module will be dependent upon a periodic audit program, i.e., Surveillance under the responsibility of the notified body, which again is carried out by the Notified Body.

There are two quality modules specified in the directive as hereunder,

  • Production Quality Assurance: which applies to equipment in categories 1 and M1 and to protective systems
  • Product Quality Assurance: which applies to electrical equipment and internal combustion engines only, in categories 2 and M2

The Directive requires the quality assurance system to address the following points:

  • Quality objectives, organizational structure, responsibilities and powers of management with regard to equipment quality
  • Manufacturing, quality control and quality assurance techniques, processes and systematic actions that will be used
  • Examinations and tests which will be carried out before, during and after manufacture and frequency with which they will be carried out
  • Verification and testing of each piece of equipment shall be carried out as set out in the relevant standard(s) in order to ensure their conformity with the type as described in the EC-type-examination certificate and the relevant requirements of the Directive.
  • Quality records (inspection reports, test data, calibration data, qualifications of personnel, etc)

These points will be covered by a quality system complying with ISO 9002:1994.

A Manufacturer’s perspective
The very fundamental reason for formulation of ATEX Directives is that there should be common certification in EU. Manufacturers can have much wider market. Users can therefore have wider choices for safety products.

Any equipment manufactured for use in a Hazardous Area for whatever purpose must be assessed for ignition capabilit