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The FMEDA report states that radar transmitter is a Type B component and with a SFF of only 81%
(below the 90% threshold) it must have a HFT of 1 for a SIL 2 SIF. However, the report then goes to
claim that these radar transmitters meet the requirements of BS EN 61511 section 11.4.4, ‘Prior
Use’, and a HFT of 0 is sufficient for SIL 2 applications.
The FMEDA report justifies its prior use claim by detailing data from a field tracking database and
operating experience of over 100,000,000 hours.
The HSE Principal Inspector was quite specific in his reasoning for not accepting this justification, the
inspector pointed out that: -
·
The (FMEDA) report was over 10 years old and doesn’t suitably consider any subsequent
issues that may have occurred on later build versions
·
The software operating hours were detailed for v3.22 of the radar level transmitter, which
although substantial, the installed product is v3.8 and it is unclear how long this software
has been in operation and what issues have been identified.
·
A generic prior use argument by a 3
rd
party is unlikely to satisfy the standard in regard to the
operating experience/ reliability claimed and the robustness of the examination of the
manufacture’s quality management systems. These areas within the report are qualitative in
nature, and are not site / operating environment specific and are significantly out of date.
Based on these points the Specialist Inspector concluded that the information would not form the
basis of a suitable demonstration that Relevant Good Practice has been met and the company were
advised to consider alternate measures – the HSE Inspector said ‘No’ to the SIL 2 certificate.
It can be seen that the certification report is using HFT concepts from both BS EN 61511 and BS EN
61508. The relationship between the ‘parent’ BS EN 61508 and the process sector variant, BS EN
61511 is clearly shown in clause 1 of BS EN 61511 Ed 1.
Figure 6 – Relationship between BS EN 61508 and BS EN 61511
Manufacturers
and suppliers of
devices
BS EN 61508
Process Sector Safety
Instrumented System
Standards
Safety
instrumented
systems
designers,
integrators and
users
BS EN 61511