What is a Safety Instrumented Function (SIF) and how does it relate to SIL?

• A. A type of hardware fault that requires monitoring.
• B. A software module that has nothing to do with safety levels.
• C. A safety function designed to achieve a specified safety level, with its target SIL determined by the required reliability.
• D. A maintenance procedure for resetting faults.

Answer: (C)

A Safety Instrumented Function is a safety action built into a Safety Instrumented System that, when triggered by an abnormal condition, automatically moves the process to a safe state to reduce risk. The level of protection this function provides is quantified by a Safety Integrity Level. The target SIL is set by the risk assessment and represents how reliable the function must be to achieve the required risk reduction. Designing a SIF to meet its target SIL involves choosing appropriate sensors, a reliable logic solver, and final elements, plus redundancy, diagnostics, and testing to keep the probability of a dangerous failure within the required range. So the correct choice describes a safety function designed to achieve a specified safety level, with its target SIL determined by the required reliability.