In the following series of blogs, we'll go back to basics and run down everything you need to know to get started in functional safety.  We'll start with some more general terms and descriptions and make our way to more advanced material.

2. Safety Integrity Level (SIL)

SIL – Safety Integrity Level – is a quantitative target for measuring the level of performance needed for a safety function to achieve a tolerable risk for a process hazard. It is defined in both . Defining a target SIL level for the process should be based on the assessment of the likelihood that an incident will occur and the consequences of the incident. 

It is a discrete level (one out of four) for specifying the safety integrity requirements of the safety instrumented functions to be allocated to the safety instrumented systems. SIL 4 has the highest safety integrity and SIL 1 the lowest.

IEC 61511 only defines requirements for SIL 1 to SIL 3, as it is expected that SIL 3 will be a maximum level in the process sector (excepting Nuclear). For SIL 4, IEC 61511 refers the user back to the detail in IEC 61508.

IEC 61508 and IEC 61511 both have tables with the Safety Integrity Levels associated with Probability of Failure, Risk Reduction Factor, Hardware Fault Tolerance and Architecture.