On Uniformly Staggered Testing Strategy for Redundant Safety Instrumented Systems

In redundant safety instrumented systems (SIS), staggered proof testing is effective in reducing the system unavailability or PFD (Average Probability of Failure on Demand) of SIS than simultaneous proof testing. A method of computing the PFD for SIS with k-out-of-n structure is proposed and PFD Formulas in the uniformly staggered and simultaneous schemes in periodic proof testing are analysed. The optimality of uniformly staggered testing scheme is reviewed for SIS with k-out-of-n structures in which redundant units have identical and different failure rates, respectively. And in this paper, the staggered testing schemes for k-out-of-n structures with common cause factor (CCF) models used in IEC 61508 are also examined and compared with a numerical example.