A SIS is a set of devices and software that perform one or more Safety Instrumented Functions (SIFs). Each SIF has a stated Safety Integrity Level (SIL) that is related to the probability that the SIF will NOT work when challenged (when needed).
In order to begin this assessment, one must understand the rules of a SIS and how Independent Protection Layers (IPLs), SIFs and SILs are all related. For example, in order to be considered IPLs (including SIFs), there are several rules that must be satisfied, some of which include: each protection layer must be truly independent of the others (i.e., no failure can deactivate two or more protection layers); the IPL must be specifically designed to prevent or mitigate the consequences of a potentially hazardous event; the IPL must be dependable (must have the stated reliability); the IPL must be validated periodically and the validation system must be audited. A probability of failure (PFD) of each IPL (including SIFs) must also be identified. One of the biggest challenges a PHA or design team faces is the determination of when a SIF is the appropriate choice for reducing the risk and then determining the appropriate target SIL for an identified SIF.