What Does Software Safety Integrity Mean In The Context Of Safety Integrity Being Defined As Probability Of Failure?

A safety integrity level (SIL) applies to an end-to-end safety function of the safety-related system. Like any other system component, software has no safety integrity level in isolation from the safety-related system. When integrated into a system, software may be capable of supporting a particular safety function at some safety integrity level, depending on how the software was specified, designed, implemented, verified, etc. SILn software is a short way of saying “software developed using appropriate techniques and measures to ensure that the software meets the systematic failure requirements of a specific safety function X at SILn”.

Hardware suffers physical degradation and the resulting random failure rates can be described numerically using well established methods of statistical reliability. In contrast, software does not degrade physically, and all failures result from systematic factors in its construction and use. It is not currently widely accepted that conventional reliability analysis can be applied to systematic behavior. Therefore, the standard recognizes that a quantitative demonstration that the target failure measures for safety integrity levels in tables 2 and 3 of IEC 61508-1 have been met is in general possible only for random hardware failures (see note 8 of 7.6.2.9 of IEC 61508-1). The effectiveness of the measures and precautions used to meet the target failure measures for systematic safety integrity (and specifically software) generally needs to be assessed qualitatively.

However, despite the above difficulties, tables 2 and 3 of IEC 61508-1 provide a valuable framework for comparing different levels of achievement of systematic safety integrity.

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