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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


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  • 0
    Thanks Robert,



    All the overlapping standards in the realm of machine safety, system safety etc do not seem to obey a basic stipulation of BS0, the standard for standards, (section 5.2) that when a person or organisation proposes a new standard it should not conflict with a previously existing standard. Since the standards bodies don't even try to obey this rule in this area and then compound this error by having no published mechanism for resolving conflicts (because by definition they don't exist) what hope is there for the rest of us.



      http://www.iso.org/sites/PEG/docs/PEG%20Documents/04_bs02011.pdf



    As far as I can gather (in terms of what is written down only) if there is a conflict you must refer back to BS0 and attempt to argue that the earlier standard of the two holds precedence when two standards conflict. This is not satisfactory in my view.



    BSI has confirmed to me that they have no published mechanism for handling conflicts between standards. If they did the procedure would depend to some extent on whether one or both of the standards originated in Europe.



    All I can suggest is that you contact the knowledgecentre@bsigroup.com and detail the conflict between the two standards that exists and see what happens.



    James 
Reply
  • 0
    Thanks Robert,



    All the overlapping standards in the realm of machine safety, system safety etc do not seem to obey a basic stipulation of BS0, the standard for standards, (section 5.2) that when a person or organisation proposes a new standard it should not conflict with a previously existing standard. Since the standards bodies don't even try to obey this rule in this area and then compound this error by having no published mechanism for resolving conflicts (because by definition they don't exist) what hope is there for the rest of us.



      http://www.iso.org/sites/PEG/docs/PEG%20Documents/04_bs02011.pdf



    As far as I can gather (in terms of what is written down only) if there is a conflict you must refer back to BS0 and attempt to argue that the earlier standard of the two holds precedence when two standards conflict. This is not satisfactory in my view.



    BSI has confirmed to me that they have no published mechanism for handling conflicts between standards. If they did the procedure would depend to some extent on whether one or both of the standards originated in Europe.



    All I can suggest is that you contact the knowledgecentre@bsigroup.com and detail the conflict between the two standards that exists and see what happens.



    James 
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