Functional Safety 2016
November, 2016 - London
Page 15 of 17
Conclusion
The impact of imperfect proof testing can have significant impact on the designed risk reduction
requirements and the suitability of the defined proof testing method when the C
PT
is not
considered.
It has been shown the impact imperfect proof testing can impact the PFD
AVG
in relation
hardware calculations, and has been illustrated in the practical example of where the proof test
methodolgy has been based on manufacturers requirements. By following the requirements of
the manufactures testing, inspection and competency requirements within the safety manuals
obtaining a 100% C
PT
is a near impossible target to achieve.
The application of C
PT
should be applied to a project involving a SIS as early as is practicable,
where C
PT
is not available either due to equipment not yet selected or equipment not certified,
using a systematic approach such as the approach identified in (Abdelrhafour, Bajaj, & Boily,
2012) would be benficial.
As shown in Table 4 there is potential for a final system to not provide defined risk reduction
requirements, which may lead to an end-user rejecting or involvement from the HSE.
Therefore, a theoretical and pragmatic approach should be adopted taking into account the
prescribed proof testing methods in the safety manual and its prefined C
PT
for the selected
operation mode. Consideration should also be given to the persons responsible to writing and
conducting the proof testing for their ongoing relevant competency requirements.