A Functional Safety Assessment (FSA) is an investigation and judgement as to whether a Safety Instrumented System will properly perform its intended function to mitigate the hazard to the target risk level. The IEC61511 standard has proposed five levels off assessment as follows:

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Stage 1 FSA – After the risk assessment, SIL target and Safety Requirements Specification defined

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Stage 2 FSA – After the Design and Engineering of the SIS and includes the Factory Acceptance Testing

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Stage 3 FSA – Completed after the installation and commissioning and before the SIS enters service

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Stage 4 FSA – During Operation and Maintenance an assessment of the SIS/SIF on a periodic basis

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Stage 5 FSA – Any modification of a SIS/SIF will require an FSA 5 and an Impact Assessment

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The FSA assessment shall be carried out by an independent competent person not involved in the lifecycle activities being assessed.