Frequently Asked Questions

During our SIL verification assessments we came across a case where the initiator of a SIF is a push button (pressed by an experienced operator) which through a SIL 3 PES closes two SIL 2 MOVs in 1oo2 architecture. My question is whether such a SIF could
Last Updated a year ago

We have had many interesting discussions on this topic. Technically speaking the function's initiator/measurement is a human being and therefore it is not an instrumented function. As such it is not a SIF. Assigning a SIL to it would therefore also not be applicable.
Having said that, I do see operator initiated functions (as you can see I am avoiding the word SIF) that have a particular reliability requirement. So one could determine the PFD of such a function, keeping in mind that the human operator will still need to be added to the final result to be able to determine an overall risk reduction.
I would therefore model the function, including the push button and conclude that it meets a particular PFD value, instead of claiming a SIL level. I would also indicate that the SIL claim does not apply because of the human operator and therefore for this function, uncheck the "consider SIL capability" checkbox.

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