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Anomaly between BS62061 and 61508

Rob Eagle

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


Parents
  • 0
    Thank you for the 2 responses however the question still remains.

    The question is why is there such disparity between the two? 



     A hazard is a hazard whether it is from a machine (62061) or a process (61508) and the end result is the same, why should two standards evaluate the risk reduction requirement so markedly differently?  It is effectively placing a significantly different importance on the outcome.

    Why is BS62061 less tolerant than BS61508, what is different in the 62061 standard that demands SIL2 for a single death however improbable, ISO13849 is more aligned with 61508 in that aspect.

    We have done a risk assessment (BS12100) and we think the risk is tolerable especially when considering other mitigating measures including human factors (HEART and TESEO), however we can't comply with BS62061 unless we design and substantiate to SIL2.

     


Reply
  • 0
    Thank you for the 2 responses however the question still remains.

    The question is why is there such disparity between the two? 



     A hazard is a hazard whether it is from a machine (62061) or a process (61508) and the end result is the same, why should two standards evaluate the risk reduction requirement so markedly differently?  It is effectively placing a significantly different importance on the outcome.

    Why is BS62061 less tolerant than BS61508, what is different in the 62061 standard that demands SIL2 for a single death however improbable, ISO13849 is more aligned with 61508 in that aspect.

    We have done a risk assessment (BS12100) and we think the risk is tolerable especially when considering other mitigating measures including human factors (HEART and TESEO), however we can't comply with BS62061 unless we design and substantiate to SIL2.

     


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