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

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.