The question “What is SIL?” is very easy to answer: Safety Integrity Level, or "SIL" for short, is a unit of measurement for quantifying risk reduction. Just as wind strength is quantified using units on the “Beaufort” scale, risk reduction can be quantified using the concept of the Safety Integrity Level SIL.
Technical equipment can pose a safety risk so dangerous that people should not be exposed to the equipment. In such cases, the relevant risks must be reduced to meet the need for safe operation. It must be possible to quantify (and measure) risk reduction in order to satisfy this requirement. This is achieved using the SIL "unit", whereby only whole values are defined within a range from 1 to 4.
When risk reduction is complete, it must be proven that the extent of the risk reduction corresponds to the minimum required parameters (i.e. the SIL requirement). This approach results in the following procedure: