on December 12th, 2007

Dear Colleagues

On my travels in Southern Africa; this morning I witnessed a horrible car accident with a little kid being tossed into the air after running across a road. Later, a good client working for one of the largest companies in the world, commented that a smelter component had recently exploded and killed an operator. A couple of nights ago, whilst supping in a sea-side café we saw flares being shot off from a small boat in distress and then shortly afterwards a rescue chopper buzzing out. And a few weeks previously, in the self-same city, an engine had fallen off a Boeing 737 whilst taking off. These all have put me to thinking of safety.

Many of these safety issues, are perhaps a little outside our direct engineering province coming down to better road or boat safety practices and a good dose of commonsense, perhaps. But the smelter explosion and engine the falling off the Boeing illustrate the issues of process safety and machinery safety respectively – issues which directly impact on us engineers and techies. I am grateful to my colleague (a real control systems safety guru), Dave Macdonald, for giving me the necessary summary below on current developments in safety control systems. No matter where you are working in engineering, you will increasingly come across these safety standards discussed below. So forgive us for a quick tutorial below. I found it very useful personally. Thanks, Dave.

The first question is of course, why bother about safety standards ?

The great advantage of having international engineering standards for safety is that if we all work to the same principles there will be globally available products that will do the job in the same way in any part of the world. Good standards represent best practice, and who would want to stand in court after an accident and not be able to claim that they followed best practice? So we are moving away from locally generated prescriptive standards, which are difficult to maintain and use on a global basis. After all, we are rapidly becoming a global engineering community and we all want to use the best solutions.
As the best reference source for designing and managing automatic safety systems for process and machinery hazards the standards IEC 61508 and IEC 61511 have been a resounding success. They are not just European standards; but truly global standards covering the subject of functional safety, which means an active function or response to protect against a hazardous condition. Think of safety interlocks, trips and critical alarms.

But what the hell are these IEC 61508 and 61511 standards ?

IEC 61508 has been in place for nearly 10 years. It provides a broad-based set of principles and advice on how to specify and build safety control systems using electrical or programmable electronic systems such as PLCs. It has a strong emphasis on safety management through the entire safety life cycle of a product or application ranging from a pressure sensor to full sized ESD for a large oil and gas installation. It’s emphasis on management methods is because human factors remain the dominant cause of failures in functional safety systems. Whilst IEC 61508 is a generic standard for any form of electronic safety controls, it has in turn led to the publication of the more specialised standards: IEC 61511 for functional safety in the process industries and, more recently, IEC 62061 for machinery safety controls. IEC 61511 has been adopted in many industrialised countries for safety controls in their process and energy sectors and is published in the EU as EN 61511, in the USA as ANSI/ISA S84.01(IEC 6151 Mod) and in Australia as AS 61511. 
These standards ask you to define each safety function accurately and to set target safety integrity levels (SILs) in the range 1 to 4 according to the scale of risk reduction you need to make things safe. The equipment you provide must be capable of meeting the SIL-rated performance features.  They will not allow you to use just any old instrument for sensing and they have stringent requirements for control equipment such that most standard industrial PLCs will not be acceptable for safety duties. If this sounds like something you have already inherited, now would be a good time to consult IEC 61511. (Try. www.iec.org  or www.saiglobal.com )
Where do we start with safety standards ?

The first step is to decide which standard applies to your situation. IEC 61508 is predominately used for the design and development of safety equipment and hence is used mainly by manufacturers to ensure that the hardware and software they supply for safety applications are right for the job.  But if you are engineering a process control project with safety involved you will need to use IEC 61511. It is laid out in the form of a safety life cycle with a series of easy to follow steps matching the stages of a typical project.  Most significantly all the standards we have mentioned call for the competency of engineers and technicians working on safety systems to be appropriate to the job in hand. This means your employer should recognize your skills if you have them and get you trained if you are falling short.
We have spoken about Process safety – but what about machinery safety ?

IEC 61508 has influenced a substantial revision of existing machinery safety codes resulting in two new standards: ISO 13849-1 which is now replacing the widely used EN 954-1 in the field of machinery safety interlocks, IEC 62061 which is aimed at typical manufacturing automation applications and robotics. To get started in machinery safety practices requires a grounding in the principles of risk assessment and the application of all forms of safety features. These in turn lead to the understanding of how safety devices are to be integrated into the overall control scheme for a machine.

Without wanting to sound like a Persian rug salesman, we obviously provide training and books on the above topics. As do many other very reputable organisations throughout the world. But there is a wealth of free material on the web to assist you. Despite all this above, we should never forget, as Jeff Cooper, wryly observes:

Safety is something that happens between your ears, not something you hold in your hands.

Yours in engineering learning


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