Modeling and reliability assessment of a 3-channel safety-instrumented system

Safety-instrumented systems (SISs) are often crucial in the prevention of accidents. In this study, several reliability assessment methods, including the approximation formulas in IEC 61508 and in the PDS handbook, and Markov and Petri net methods, are used to determine the reliability of a redundant SIS configuration with three parallel channels. The average probability of failure on demand (PFD
_avg
) is used as reliability measure, and common cause failures and proof tests are considered in the modeling. The results obtained by the different methods are compared, and characteristics of these methods in SIS reliability analysis are discussed.