31st European Safety and Reliability Conference

31st European Safety and Reliability Conference | 19-23 September 2021, Angers, France

EUR ING Ben Easton MSc CEng MIET

Managing Director & Principal Safety Consultant

Technis, Reliability and Functional Safety Engineer (Diploma [008])

FS Expert (TÜV Rheinland, #299/19, SIS, PH & RA), FS Eng (TÜV Rheinland, #21705/ 21, Machinery)

Ben will be presenting his paper on Impact of Imperfect Proof Testing on the Performance of Safety Instrumented Functions at the European Safety and Reliability Conference on 19-23 September in Angers, France shortly.  Further details can be accessed via the following link:  http://esrel2021.org/en/index.html An abstract of the content is shown below. 

"Periodic proof testing (PT) is critical in providing 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 (λDU) that cannot be detected by diagnostics.  However, it is recognized 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 detected by the proof test is referred to as the Proof Test Coverage Factor (PTC).  This paper shall define proof test coverage, identify areas of consideration as to what can impact the PTC, propose methods for determining the PTC for greenfield and legacy equipment, how the PTC can impact the average probability of failure on demand (PFDAVG).

This paper shall conclude that the impact of imperfect proof testing can have a significant impact on the designed risk reduction requirements and the suitability of the defined proof testing method when the PTC is not considered.  Therefore, a theoretical and pragmatic approach should be adopted considering the prescribed proof testing methods in the safety manual and its predefined PTC for the selected operation mode.  Consideration should also be given to the persons’ responsible for writing and conducting the proof testing and their ongoing relevant competency requirements.

Keywords: Functional Safety, IEC 61511:2016, Proof Test Coverage"

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