I am designing a control system for a very big heavy door which has the capability to crush a person and potentially to death although extremely unlikely. In determining the SIL requirement for the system using BS61508-5 I get the following:
Looking at Table E.1 I can derive a qualitative assessment
Consequence – Serious permanent injury to one or more persons; death to one person – C2
Frequency of exposure – Rare to more often exposure in the hazard zone – F1
Probability of avoiding the hazard – Possible under certain conditions – P1
Probability of the unwanted event – A slight probability – W2
Looking at Figure E.2 this equates to ‘a’ = “No special safety requirements”
However if I use BS62061 looking at table A.1 I find “Irreversible: death, losing an eye or arm” Severity Se = 4, then I go to Table A.6 and irrespective of any other criteria it demands a minimum of SIL2.
The question is why is there such disparity? And which is correct?
Thank you,
Rob
