How do you describe a safety function correctly?

2022-10-17

Technically the description of the safety function is the most important aspect of achieving functional safety and SIL. It is the foundation of all the safety work that follows.

A good safety function description is based on the S.L.A.T.S. concept. This means that you describe the safety function based on what you will Sense, the Logic you will solve, what you will Actuate, how fast, the Time, you need to do it and last but not least how good it needs to be done in terms of SIL. That is a fundamental.

SLATS: Sense, Logic, Actuate, Time, SIL.

Here is an example of a good safety function description:
Measure the level of smoke, and if the level reaches 1000ppm start the ventilation system within 10 seconds. Perform this function according to the SIL 3 requirements.

Pro Tip: Always describe the intention, never the solution. Let the designers come up with a solution. When you describe the intention, you are clear on how you deal with the hazard that you are going to protect with this safety function. That is when the puzzle piece fits together. If you describe the solution, you lose that relationship.