IEC61511-1:2016, § 3.2.24 defines FSA as: investigation, based on evidence, to judge the functional safety achieved by one or more SIS and/or other protection layers.

FSA should be led by a qualified independent assessor and shall result in a statement that functional safety is achieved for the corresponding SLC-stage. FSA focuses on technical aspects and achieved results in addition to procedures and paperwork (see FS-Audit). Five FSA-stages are defined in IEC 61511-1 Figure 7, of which FSA-3 and FSA-4 are compulsory.