Safety Integrity Level – what is it and why does the industry need it?

Safety Integrity Levels (SILs) are a measure of the impact that a Safety Instrumented Function (SIF) has on the risk associated with a specific hazard. The higher the SIL level, the more effective that function will be in reducing the risk it is intended to mitigate. In other words, SIL can be seen as an indicator of the acceptable failure rate for a safety function. SIL classification is a fundamental parameter to consider when comparing products; it is also an increasingly important requirement in various public and private tenders for the supply of mechanical systems and electrical and electronic products. SIL standards, developed by the International Electrotechnical Commission (IEC), focus on functional safety. IEC standards define four SIL levels – 1 to 4 – a SIL is determined based on both quantitative and qualitative factors using different methods in combination, including risk matrices, risk graphs, and Layers of Protection Analysis (LOPA). The higher the SIL, the more severe the potential impact of a failure, and therefore the lower the acceptable failure rate. SIL certification within a given system depends on multiple factors, including:

• type of technologies
• system architecture
• number of system components
• probability of failure on demand (PFD) of each component
• diagnostic test intervals

A product with SIL certification is considered “suitable for use within a specific SIL environment”; in this way, the entire system is taken into account.