Safety Integrity Level Study
(SIL Study)
What Does Safety Integrity Level Study/Assessment (SIL Study) Mean?

Safety integrity level (SIL) studies are a formal method of assessing the probability that a functional safety system will fail when it is needed. The assessed probability of a safety system failing acts as a measurement of the level of risk reduction that the system provides.

A safety integrity level refers to the reliability of an entire safety system, not to the individual risk reduction measures that comprise them.

Safety integrity levels are defined according to an international standard described by the International Electrotechnical Commission (IEC).

Safety Integrity Level Study (SIL Study)

Safety integrity level studies assess the reliability of functional safety systems, that is, those that actively protect workers and the work environment by monitoring-for and responding to potentially dangerous conditions. Active protection includes measures such as the detection of smoke by sensors and subsequent triggering of an automatic sprinkler system. SIL studies do not measure the reliability of passive safety measures, such as fire-resistant doors.

A safety system assessed by a SIL study is classified as either “high demand” (systems that operate continuously or are used more than once a year) or “low demand” (systems used once a year or less). High demand safety systems require a lower probability of system failure than low demand systems do. SIL studies on high demand systems measure the probability of the system having one dangerous failure per-hour. Studies on low demand systems measure the probability of the system failing on demand/use.

SIL studies primarily classify safety systems according to one of four safety integrity levels (1–4). Each level is defined by a maximum allowable probability that a safety system will fail. SIL 1 has the highest allowable probability of failure (0.00001-0.000001 dangerous failures per-hour for high demand systems). SIL 4 has the lowest allowable probability of failure (0.00000001-0.000000001 dangerous failures per-hour for low demand systems). The highest allowable probability of failure allowed in an SIL 4 system equates to a probability of one dangerous failure occurring per 11,400 year timespan. Safety systems that are not required to meet a safety integrity level standard are referred to as SIL 0.

Although individual components of safety systems are not given SILs, these components may be designed by manufactures to be suitable for use at specific safety integrity levels.
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