Safety Handbook - JOKAB SAFETY - #15

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www.jokabsafety.com 1:13 1 2 3 4 5 6 7 8 9 10 11 13 12 How do you achieve the performance level (PLr) that is determined by the risk graph? There are a number of parameters to take into account when designing a certain protection function. These are as follows: • MTTFd for each individual component – Mean Time To Failure, dangerous • DC, Diagnostic Coverage – to what extent dangerous faults are detected • the structure – which categories are to be used for the parts in their protective functions • CCF, Common Cause Failure – measures that will be relevant when working with some form of redundancy (for categories 2,3 and 4) • how the function acts in a fault condition – also given by the selected category • systematic fault – measures to avoid these in the design The Performance Level (PL) is expressed as the Probability of Dangerous Failure Per Hour within certain ranges. Performance Level, PL PFHD a 10-5 till <10-4 b 3x10-6 to <10-5 c 10-6 to <3x10-6 d 10-7 to <10-6 e 10-8 to <10-7 The standard gives the designer several choices for achieving a certain PL. The following illustration shows how this is possible, depending on which levels the parameters achieve. The values of MTTFd, DC and the categories of our products will be given to you when you contact us. In some cases we can state a PL straight away for a protective function or part of a protective function. In the case of a risk assessment of a machine or type of machine, one determines which protective measures are necessary in order to ensure that the safety of the machinery will agree with the Machinery Directive and the standards that are to be applied. In respect of the safety measures that involve the control systems of machines, one must decide "how safe" the safety must be - the higher the risk the more reliability is demanded of the safety function. In EN 954-1 categories are selected – B, 1, 2, 3 or 4 for these parts. Here it is not always possible (nor desirable) to be able to assign a category to the entire safety function. With the method in EN ISO 13849-1 one selects instead a required Perfomance Level (PLr) and this applies to the entire safety function. The choice of required Perfomance Level (PLr) takes place in a similar way to the selection of a category, see the risk graph. Förklaring PL prestandanivå 1 MTTFd för varje kanal = låg (kort tid) 2 MTTFd för varje kanal = medel (medellång tid) 3 MTTFd för varje kanal = hög (lång tid)