Basic reliability analysis applied to safety systems

This chapter focuses on the process of reliability analysis of safety instrumented system (SIS) design for its overall safety integrity. Reliability analysis provides an early indication of a system's potential for meeting the design requirements, enables life-cycle cost comparisons, determines the weak link in the system and shows the way to improve performance, and allows comparisons to be made among other designs. Reliability analysis supports IEC 61508 requirements for estimating the probability of failure of safety functions due to random hardware failures. A description of formulae based on simplified versions of well-established reliability analysis methods is presented, which is applicable to the analysis of the whole SIS or to any subsection. With the help of the basic formulae, one can model the SIS into component parts and work out the probability of failure on demand (PFD) and spurious trip rates for each section and for the overall SIS function.