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Safety Integrity Level (SIL) - IEC 61508/61511 Overview
Safety Integrity Level (SIL) is defi ned as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In (somewhat) simple terms, SIL is a measurement of performance or probability of failure on demand (PFD) required for a Safety Instrumented Function (SIF) within a Safety Instrumented System (SIS) based on the ANSI/ISA 84, IEC 61508, and IEC 61511 standards.
All organisational and technical risk reduction measures act as a counterweight to the risk potential. The values SIL 1 to SIL 4 (SIL = Safety Integrity Level) are derived from the risk analysis. The greater the risk, the more reliable risk reduction measures must be implemented and, consequently, the greater the reliability the components used must exhibit. Typically, as the SIL level increases, the cost and complexity of the hardware/system also increase. The four SIL levels are defi ned, with SIL4 being the most dependable and SIL1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. The requirements for a given SIL are not consistent among all of the functional safety standards.
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