Safety Instrumented Systems

The ANSI/ISA-84.00.01-2004 (IEC 61511) standard (Ref. ^[1]) defines a safety instrumented system (SIS) as an "instrumented system used to implement one or more safety instrumented functions. A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s)." IEC 61508 (Ref. ^[2]) does not use the term SIS but instead uses the term "safety-related system." That term defines the same concept but uses language that can be broadly applied to many industries.

Practitioners often prefer a more functional definition of SIS such as: A SIS is defined as a system composed of sensors, logic solvers and final elements (Figure 2-1) designed for the purpose of:

• automatically taking an industrial process to a safe state when specified conditions are violated;

• permit a process to move forward in a safe manner when specified conditions allow (permissive functions); or

• taking action to mitigate the consequences of an industrial hazard.

Figure 2-1: Basic Process Control System versus Safety Instrumented System

The definition tells us that a SIS may be responsible for shutdown functions, permissive functions, and even consequence reduction (mitigation) functions. All these functions have a common attribute they all reduce risk. One common interpretation of a SIS definition is, therefore, "automatic risk reduction systems." In some cases the function is designed to reduce risk by decreasing the likelihood of a potential hazard. In some cases the function will decrease risk by reducing the magnitude of the consequence.