Estimation of Average Hazardous-Event Rate for Steady-State Demands and Determination of SIL

Abstract

The international standard on functional safety, IEC 61508, requires to specify safety integrity levels (SILs), i. e., target failure measures for safety functions of electrical/electronic/programmable electronic safety-related systems (SRSs), taking account of the necessary risk reduction achieved by SRSs. In order to reflect SIL to the risk reduction through designing and operation of the SRS, it is essential that the relationships between a number of systems characteristics involving the SRS and the achieved-risk reduction should be explored. In the present paper, generalized modes of operation are recommended for the proposition that risk reduction ratios and average hazardous event rates should be made use of as target failure measures instead of the conventional ones, i.e., the average probability of failure on demand and dangerous failure rate. Thus, SILs are assigned to SRSs more easily and reasonably.