Recommended Reading: What Does Root Cause Analysis Really Mean?

This is an excerpt from a recent educational article by exida:

The IEC61511 standard requires that any failures that occur within the Safety InstrumentedSystem (SIS) are investigated.  The reason for this is that we need to understand whether the failure was dangerous or safe, and whether the failure was random or systematic.  This is important for several reasons, not least of which is to be able to properly count random failures that would be used in future PFDavg or PFH calculations, as opposed to viewing them as systematic and not including them in the random failure count.  This would then lead to dangerously low and optimistic random failure numbers.  The 2nd edition of IEC61511 now includes Clause 11.9.3 that states that failure rate data used in the reliability calculations (PFDavg/PFH) must be: Credible, Traceable, Documented and Justified, for the purposes of preventing unrealistically and dangerously low random failures being used in SIL verification calculations.  The outcome of which would be to have lower PFDavg or PFH results, that would result in a higher risk reduction than the SIF is actually providing, which could lead to a false sense of safety.

To read the complete article, click on this link:


Julius Frago,
The Automation School

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