Understanding the Use, Misuse and Abuse of Safety Integrity Levels1

Modern standards on system safety employ the concept of safety integrity levels (SILs). Increasing numbers of system purchasers are expecting their suppliers to demonstrate that they use the concept, so system developers are seeking to apply it. But the standards differ in their derivation of SILs and none explains the concept satisfactorily, with the result that it is often misunderstood and used inconsistently, incorrectly, and inappropriately. This paper explains the concept and its application, giving examples of how SILs are derived in three current safety standards. It then shows a number of ways in which the SIL concept is misinterpreted and used misleadingly. Further, it considers the relationship between SILs and risk-tolerability decisions.