EN954-1: It's time to let go

The death throes of the EN954-1 safety standard have been long drawn out and it has unexpectedly rallied on more than one occasion.Now,however, the date of its final demise has been fixed and system integrators and machine builders across Europe must start to deal with this change without delay, says Paul Laidler of Laidler Associates

It has been accepted for many years that EN 954-1, 'Safety of Machinery, Safety related parts of control systems', which was introduced in 1992, needed to be replaced. It has many shortcomings, just one example of which is its failure to deal with the programmable safety systems that are now becoming so popular. Accordingly, a new standard, EN ISO 13849-1 was produced and was published in October 2006. It was planned that it would replace EN 954-1 in December 2009.

In the event, the new standard was itself revised - the current version is EN ISO 13849-1:2008 - and, in response to objections from machine builders and other interested parties, it was agreed that, although the new standard had been published, EN 954-1 could continue to be used until the end of 2011.

So what's new? In a sense not a lot, except that the end of 2011 isn't very far away, and the changes introduced by the new standard are numerous. Even the Machinery Directive Working Group, which is not a body noted for exaggeration, has said that the transition 'represents a drastic evolution in the safety philosophy for control systems.' In other words, those machine builders who have been burying their heads in the sand hoping that EN 954-1 would live forever need to start taking action now or, in just a few months, they will have a serious problem.

Why the fuss? The truth is that EN ISO 13849-1 adopts a totally different approach from EN 954-1.

This means that achieving compliance with the new standard isn't merely a matter of tweaking the existing documentation; instead it means starting over from scratch, and it's by no means a simple task.

The approach to safety used by EN ISO 13849-1 is based on probabilities. Gone are the familiar safety categories of EN 954-1, replaced by Performance Levels (PLs). These PLs relate directly to the probability of a system failing to danger. To achieve PLa, for example, the average probability of a failure to danger per hour must be in the range >10-5 to <10-4, while for PLe it must be in the range >10-8 to < 10-7.

This is all well and good, but how are those probabilities determined? The most usual answer is that they are calculated on the basis of MTTF (mean time to failure) data for the components used in the safety system. Until recently, this was a large and sometimes insurmountable problem, as the necessary data was often not available from the component suppliers. This issue has now largely been addressed, and all major suppliers are now offering libraries of data specifically for use with the new standard.

Even when the relevant data is available, however, it would be misleading to pretend that carrying out the calculations required by EN ISO 13849-1 is a straightforward task. To make things a little easier, several organisations have produced software packages that guide users through the process. Some of the packages are produced by commercial organisations and understandably they may have a commercial or proprietary bias.

For those who may wish to avoid such bias, a useful package has been produced by a non-commercial body - SISTEMA from IFA, the institute for research and testing that is associated with the German insurance industry. The package, which describes all aspects of the analysis procedure contained in the standard for determining the probability of failure of control systems, can be downloaded free of charge from the IFA English website: www.dguv.de/ifa/en/pra/en13849/index.jsp.

In spite of the availability of software support, ensuring compliance with EN ISO 13849-1 is still going to be a task which few system integrators or machine builders will want to undertake for themselves, or indeed have the resources available to do so. This may change as the standard beds in and the concepts it embodies become more familiar.

At the moment, however, a good case can be made for calling on the help of consultants, such as Laidler Associates, who have made a detailed study of the new standard and who can provide dependable advice and assistance.While it may undoubtedly appear that money can be saved by going it alone, it's worth remembering that, as always when it comes to matters involving standards, getting it wrong can prove very expensive! The transition from the familiarity of EN 954-1 to the relatively uncharted territories of ISO EN 13849-1 is not something anyone is anticipating with relish, but it's happening for good reasons and it's not going to go away. The message has to be to invest now in making the appropriate preparations; a little money spent on consultancy may well save a lot of time, trouble and expenditure later.