Defining safety instrumented functions

One of the first steps in implementing a safety instrumented system (SIS) is defining the individual safety instrumented functions (SIF)that comprise that system. This step is critical in ensuring that safety goals have been achieved, but is the source of many of the errors in SIS design. Often, inputs and outputs that are not involved in detecting the hazardous condition or moving the process to a safe state are included in the function, causing increased equipment and testing requirements to meet SIL targets. This paper provides a discussion of defining SIF, including example cause-and effect diagrams that are dissected to identify several safety and non-safety functions. This paper also defines new terms for effectively subdividing large instrumented functions into their safety and non-safety components. These terms include primary safety function, secondary safety function, auxiliary inputs,, and auxiliary outputs.