SIL

Safety Integrity Level (SIL) is a relative level of risk reduction provided by a safety function, or a target for that level of risk reduction. Defined in IEC 61508 and IEC 62061 as SIL 1 through SIL 4, where SIL 4 represents the highest degree of risk reduction.

SIL is determined by the risk reduction required to achieve tolerable risk. The quantitative target for each SIL is expressed as a Probability of Failure per Hour (PFH) for continuous / high demand mode safety functions, or as a Probability of Failure on Demand (PFD) for low demand mode.