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 over the risk associated with a specific hazard. The higher the SIL level is, the more efficient that function will be at reducing the risk it mitigates. In other words, SIL can be seen as an indicator of the acceptable failure rate for a security function.
SIL rating is a fundamental parameter to consider when comparing products; it is also an increasingly important requirement in various public and private tender invitations 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 SILs – 1 to 4 – a SIL is determined on the base of both quantitative and qualitative factors based on several methods used in combination, these include risk matrix, risk graphs and Layers of Protection Analysis (LOPA). The higher the SIL, the more serious the potential impact of a failure is, therefore the lower the acceptable failure rate is.SIL certification, within a given system, depends on multiple factors, these include:

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 deemed “suitable for use within a given SIL environment”, in this way the entire system is taken into account.