The safety function description is the single most important element in achieving functional safety and SIL compliance. If this foundation is weak, everything built on top of it will be flawed.
Every safety lifecycle activity—design, verification, validation, operation—depends on how clearly and correctly the safety function is defined.
A poor description leads to:
A robust safety function description follows the SLATS concept:
What parameter are you measuring? This defines how the hazard is detected.
What condition triggers the safety action? This defines the decision-making criteria.
What action is taken to bring the system to a safe state?
How quickly must the action occur? Timing is directly linked to risk reduction and verification.
How reliable must the function be? This defines the required Safety Integrity Level.
Risknowlogy Insight: If one SLATS element is missing or unclear, the safety function is incomplete.
Measure the level of smoke, and if the level reaches 1000 ppm, start the ventilation system within 10 seconds. Perform this function according to SIL 3 requirements.
A critical mistake in many projects is specifying how the safety function should be implemented instead of what it must achieve.
The safety function description is not just documentation—it is the blueprint of your safety system.
When done correctly, it ensures alignment between hazard analysis, design, implementation, and verification. When done poorly, it guarantees problems later in the lifecycle.
