Safety Integrity Level (SIL) is a discrete measure of the risk-reduction provided by a safety function, defined by the IEC 61508 family of standards on four levels (SIL 1 to SIL 4). Each level corresponds to a target probability of dangerous failure on demand or per hour. SIL is used to specify, design, and verify that safety-related control systems achieve a tolerable level of risk.
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- SIL classification drives architectural constraints, diagnostic coverage, and proof-test intervals. Determining a target SIL involves risk graphs or layer-of-protection analysis, and verification combines hardware fault-tolerance metrics with systematic-capability evidence across the safety lifecycle.