02 March 2011
It has been accepted for many years that EN 954-1, ’Safety of Machinery, Safety related parts of control systems‘, which was introduced as long ago as 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, and 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 many objections from machine builders and other interested parties, it was also agreed that, although the new standard had been published, EN 954-1 could continue to be used until the end of 2011. In broad terms, this is still the current situation. So what’s new?In a sense not a lot, except that the end of 2011 isn’t now very far away, especially as 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 to the new standard ’represents a drastic evolution in the safety philosophy for control systems.’ Machine builders need to start taking action now or, in just a few short months, they will find that they may have a serious problem.But what’s all the fuss about? 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. 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. Fortunately, 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. This package, which clearly 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, of course, as the standard beds in and the novel concepts it embodies become more familiar. At the present time, however, a very good case can be made for calling on the help of expert consultants who should be able to provide dependable advice and assistance. While it may undoubtedly appear that money can be saved by ’going it alone‘ it is worth remembering that, as always when it comes to matters involving standards, getting it wrong can be 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 is happening for good reasons and it is not going to go away. It would be ill advised indeed to wait until the last minute to carry out the necessary preparatory work, so the message has to be invest now in making the appropriate preparations.
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