Which statement best describes the relationship between Safety Instrumented Functions (SIFs) and Safety Instrumented Systems (SIS)?

• A. A SIF is a signal controlled by a human operator, while SIS is automatic.
• B. SIS is a type of SIF.
• C. SIFs are a separate system from SIS.
• D. A SIF is a function performed by a Safety Instrumented System.

Answer: (D)

Think of SIFs as the automated safety actions designed to bring the process to a safe state when a hazard is detected, and the SIS as the hardware and software stack that actually carries those actions out. A Safety Instrumented System includes sensors to detect conditions, a logic solver to decide what to do, and final elements (like actuators) to execute the action. A SIF is not a separate, manually operated signal; it’s a function that the SIS performs automatically when triggered. An SIS can implement multiple SIFs, each achieving a different safety outcome. So the best description is that a SIF is a function performed by a Safety Instrumented System.