The concept of safety integrity levels (SILs) was introduced during the development of BS EN 61508 (BSI 2002) as a measure of the quality or dependability of a system which has a safety function – a measure of the confidence with which the system can be expected to perform that function. It is also used in BS IEC 61511(BSI 2003), the process sector specific application of BS EN 61508.
The standards recognize that safety functions can be required to operate in quite different ways. In particular they recognize that many such functions are only called upon at a low frequency / have a low demand rate.
