QEHSRisk
Safety Integrity LevelSIL
A discrete level (SIL 1–4) defining the reliability required of a safety function.
Defined in IEC 61508 and IEC 61511. SIL 1 requires a PFD of 0.1–0.01; SIL 4 requires 0.0001–0.00001. SIL is determined through LOPA or risk graph methods. Safety Instrumented Systems (SIS) must be designed, verified, and validated to achieve the required SIL. SIL is not a product rating — it is a system-level achievement.