What is Safety Integrity Level (SIL)?

Safety Integrity Level (SIL) is a measure of safety system reliability defined in IEC 61508 and IEC 61511. There are four SIL levels: SIL 1 (lowest), SIL 2, SIL 3, and SIL 4 (highest). Each level represents a range of risk reduction capability measured by PFDavg (Probability of Failure on Demand average). SIL 1: 10^-1 to 10^-2, SIL 2: 10^-2 to 10^-3, SIL 3: 10^-3 to 10^-4, SIL 4: 10^-4 to 10^-5. Higher SIL requires more reliable components, redundancy, and rigorous testing.