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The fraction of faults detected when the proof test is performed are
generally termed the Proof Test Coverage (C
PT
) and the fraction of faults
not detected during Proof test are termed (1-C
PT
). These latter faults will
only be detected when a demand is made and the SIF fails to function or
will remain undetected due to not having an impact on the safety path.
Why are we interested in the C
PT
, because it impacts on the achieved
Average probability of failure on demand (PFD
AVG
) of a SIF and therefore
impacts on the ability of the SIF to meet the target SIL.
It is critical therefore that the manufacturer’s requirements for testing are
complied with, so the SIF will continue to provide the “as designed” risk
reduction for the required equipment lifetime or until it is no longer
required as a protection layer for the plant.
So how do we go about including the impact of C
PT
in our calculations?
Introduction