Process industry Functional Safety Assessment (FSA) has been a requirement in IEC 61511 - Safety Instrumented Systems since the first edition published back in 2003. An FSA 4 is one of the clear activities required to claim compliance with the IEC 61511 standard. The stated objective is to ensure that functional safety and safety integrity are achieved.
In practice, many organisations have viewed process industry Functional Safety Assessment (FSA) as an activity to be completed when a new safety instrumented system (SIS) gets installed, and of course, that is absolutely correct. But what about existing, or "legacy" systems? The question is, would it even be beneficial to carry out an FSA on an SIS that has been installed for many years and possibly even pre-dates the IEC61511 standard?
Well, the latest edition of IEC 61511 (edition 2, published in 2016) seems to make it clearer that FSA is not just a design phase activity, but also an ongoing operational requirement
Notes in part 1 clause 220.127.116.11.4 require the FSA scope to include:
" giving consideration to relevant factors such as time in operation, number and scope of changes in operation, proof test frequency"
(i.e. ACTUAL frequency of proof tests). Furthermore, a new clause in part 1: 18.104.22.168.10 states
"A FSA shall also be carried out periodically during the operations and maintenance phase..."
This at least partially answers the earlier question in this article about legacy systems and those SIS's in place that pre-date IEC 61511.
There are many management and verification activities mandated in IEC 61511 that could help to reduce systematic errors and improve safety, and the FSA 4 should be just the mechanism for highlighting the gaps.