Because of the different technical criteria used in IEC 61508 and EN 954-1:1996 (also published as ISO 13849-1:1999), these two standards do not provide a sufficient technical basis for directly linking a measure of safety performance based on the requirements for categories in EN 954-1:1996 with a similar measure of performance based on the requirements for safety integrity levels in IEC 61508.

However, in the context of safety performance, a pragmatic view is that an E/E/PE safety-related system capable of supporting safety integrity level 1 (SIL1) safety functions (according to IEC 61508) broadly satisfies the requirements for a category 1 or a category 2 system (according to EN 954-1:1996). Likewise, SIL2 can be said to satisfy category 3 and SIL3 can be said to satisfy category 4.

It is essential to note that there is no equivalent correlation in the reverse direction. For example, it cannot be said that any category 3 E/E/PE safety-related system is capable of supporting SIL2 safety functions. This is because many of the requirements of IEC 61508 do not have an equivalent in EN 954-1:1996.

Further detail is available in IEC 62061.

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