what is sil

SIL stands for Safety Integrity Level, which is a measure of safety system performance in terms of the probability of failure on demand (PFD) . In functional safety, SIL is defined as the relative level of risk-reduction provided by a safety instrumented function (SIF), which is the measurement of the performance required of the SIF. The applicable SIL is determined based on a number of quantitative factors in combination with qualitative factors, such as risk assessments and safety lifecycle management. In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with SIL4 being the most dependable and SIL1 the least. The SIL allocation is an exercise in risk analysis where the risk associated with a specific hazard, which is intended to be protected against by a SIF, is calculated without the beneficial risk reduction effect of the SIF. That unmitigated risk is then compared against a tolerable risk target. The difference between the unmitigated risk and the tolerable risk target determines the required risk reduction, which is then assigned a SIL. SIL is used in most functional safety standards, including several industry-specific standards.