Functional Safety 2016
November, 2016 - London
Page 2 of 17
Abstract
Periodic proof testing (PT) is critical to provide adequate assurances that the Safety Instrument
Functions (SIFs) provide the required risk reduction throughout its lifecycle. The purpose of the
PT is to detect dangerous undetected failures
!"
that cannot be detected by diagnostics.
However, it is recognised that not all failures can be detected by diagnostics or PT and will only
be identified either at equipment overhaul or when a demand is placed on the SIF.
The fraction of failures undetected by the proof test is refered to as the proof test coverage
factor
!"
. This paper aims to identify how the C
PT
can be determined, the impact it has on
the average probability of failure on demand (PFD
AVG
), where this information can be located
and how it can be applied to industry.