Berechnung der Ausfallwahrscheinlichkeit (PFD) von Heterogenen Mehrkanaligen Sicherheitskreisen Mittels Effektiver Fehlerraten

With the introduction of the new standard EN 61508 the probability of failure on demand (PFD) has to be determined for safety-instrumented functions. For single-channel and multichannel systems featuring homogeneous redundancies calculation formulas do exist. However, equivalent formulas for heterogeneous structures have not been published so far. Moreover, their handling would be too tricky for non experts in probability theory due to the higher degree of complexity. In the course of this paper a possibility is presented to refer the PFD of heterogeneous structures to the one of their homogeneous pendants. To achieve this aim a new quantity is introduced called effective failure rate (λeff). Independent from the regarded architecture the calculation of the corresponding effective failure rate follows an easy-understandable pragmatism. Hence, the PFD for a heterogeneous loop structure can be derived similar to the one in the homogeneous case. The only additional effort is given by the calculation of the relevant effective failure rate which is no problem in the practical application.