In the following series of blogs, we'll go back to basics and run down everything you need to know to get started in functional safety.  We'll start with some more general terms and descriptions and make our way to more advanced material.

4. SIS

A SIS – Safety Instrumented System – is an implementation of one or more Safety Instrumented Functions (SIF) . A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s). It usually has a number of safety functions with different safety integrity levels (SIL), so it is best avoid describing it by a single SIL. 

A SIS is different from a SIF, which can encompass only a single function and acts in a single way to prevent a single harmful outcome. One SIS may have multiple SIFs with different individual SIL, so it is incorrect—and ambiguous— to define a SIL for an entire safety instrumented system. 

An example of a SIS is shown below. Note: The Basic Process Control System (BPCS) is also shown, where the SIS is highlighted in yellow.