A safety function is anything that brings the process to a safe state, or holds it there, for one specific hazardous event. It doesn’t have to be instrumented. A relief valve lifting on overpressure and an operator responding to an alarm are both safety functions. When the function is carried out by a safety instrumented system (SIS), it becomes a safety instrumented function (SIF).
Key Points
- IEC 61508 is written around the generic safety function; process-sector work talks almost entirely about SIFs because the SIS is the protection layer the process standard governs.
- One safety function can be spread across more than one protection layer, but keeping one function per layer makes the risk math far cleaner.
- Each one is tied to a specific hazardous event and a defined safe state, and for a SIF both get written into the safety requirements specification.
Example
For a separator high-pressure scenario, the safety function is “stop feed to the vessel before pressure exceeds its design limit.” The team implements it as a SIF with two pressure transmitters, the logic solver, and an inlet shutdown valve. The safety function is the what. The SIF is the how.
See Also: SIF, safe state, IPL
Cited Sources
- IEC 61511-1:2016, Clause 3.2.65
- IEC 61508-4:2010, Clause 3.5.1