Probability of Failure on Demand in IEC 61508

This paper presents a new and explicit approach for estimation of average Probability of Failure to perform its design function on Demand (PFD), based on the expectations of fault duration / repair time in accordance with IEC 61508. From a practical viewpoint, the formula for estimation of PFD of the negligible Mean Time To Repair (MTTR) configuration Safety-Related System (SRS) operated in low and short duration demand mode is feasible for the non-negligible MTTR configuration SRS with no self-diagnostic function.