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Standards for safety related parts of the control system

Standards for safety related parts of the control system Paul Laidler of Laidler Associates discusses how EN 13849-1 requires a new approach to safety for complex machinery, with the need to focus on the concept of functional safety.

Within the field of machinery safety, standards relating to the safety related parts of the control system have been a topic of concern for some time. Since it was first published in 1996, EN 954-1 has been used as the standard for safety related parts of the control system both for machine builders and end users. However, state-of-the-art has moved on dramatically and components and systems are now available which are too technologically advanced to use it. The transition period for its withdrawal ran until December 2011 which meant that machine builders could choose whether to work to EN 954-1, EN ISO 13849-1 or EN 62061. As this date has now passed EN ISO 13849-1 (or EN 62061) must now be followed

EN ISO 13849-1 and EN 62061 are more advanced standards which can deal with the newer technologies, they also require increased levels of knowledge to apply them correctly. Having two different standards for safety related controls that are both harmonised to the Machinery Directive has left many people confused about which standard should be applied in a particular application. However, EN 62061 applies to electrical, electronic and programmable electronic control systems, so could not directly replace EN 954-1, whereas EN ISO 13849-1 covers electrical, pneumatic, hydraulic and mechanical safety systems.

While EN ISO 13849-1 follows a similar method to EN 954-1 for defining a Performance Level required (PLr), the user then has to verify the Performance Level achieved by taking account of a number of other factors and calculations (for example diagnostic coverage, mean time to dangerous failure, architecture and common-cause failures) to validate that the safety functions in question have achieved the performance requirements.

This approach is based on probabilities. Performance Levels (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 of greater than or equal to 10-5 to less than 10-4, while for PLe it must be in the range greater than or equal to 10-8 to less than 10-7. While this new quantitative approach is more appropriate for complex machinery, it does mean that designers have to consider many more aspects than before. 'Component performance' and the impact of 'diagnostic coverage' are the two main new requirements. With EN 954-1 it was a case of designing the system and relying on the design being right, but EN ISO 13849-1 forces you to validate that the control system really does do what is required of it.

To make EN ISO 13849-1 work for them, machine builders need to pay more attention to the concept of functional safety and identify the individual safety functions of a machine, assigning performance requirements against each of these to ensure that they comply. Breaking each function into further sub systems, can help with the calculations whilst also ensuring that no functions are missed.

Performance data is available from most of the safety product manufacturers which can be used for the calculations. However, even when the relevant data is available, 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 software packages have been produced that guide users through the process.

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, but in the meantime, remember that a little money spent on consultancy may well save a lot of time, trouble and expenditure later.
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