Glossary · Functional safety
Safety instrumented function (SIF)
Also known as: SIF
German: Sicherheitsgerichtete Funktion (SIF)
In process safety under IEC 61511, a safety instrumented function (SIF) is a safety function with a specified safety integrity level, implemented by a safety instrumented system to achieve or maintain a safe state of the process with respect to a specific hazardous event.
- Functional safety
In one sentence
A safety instrumented function (SIF) is a safety function with a specified SIL, carried out by a safety instrumented system for one hazardous event.
Example
SIF-101 closes the feed valve of a reactor when two of three temperature transmitters exceed the trip limit; it is specified as SIL 2 with a response time of 5 seconds.
How it applies
- Engineering: Each SIF protects against a defined hazardous event and comprises sensors, logic and final elements. Several SIFs usually share one Safety instrumented system (SIS).
- Specification: The Safety requirements specification (SRS) states for each SIF the trip condition, safe state, required SIL, response time, proof-test interval and bypass rules.
- Operation: Bypasses, overrides and demands on a SIF are recorded and reviewed, because frequent demands may indicate that the process or the assumptions of the analysis have changed.
- Documentation: Give every SIF a stable identifier and use it across the cause-and-effect matrix, test procedures and operating instructions. Operating manuals should describe what the operator sees when a SIF trips and what must happen before a restart.
SIF vs. SIS
The SIF is the function: what must happen, with which integrity. The SIS is the equipment that implements one or more SIFs. SIL is assigned to a SIF, not to the SIS as a whole.