767 / 1145
The Importance of Functional Safety Assessment and its Application
Page
5
of
12
4. Stages when FSAs are performed
The IEC Functional Safety Standards provide more guidance on the stages when an FSA shall be
undertaken. The IEC 61508 standard requires that FSAs apply to all phases of the safety lifecycle.
For the Process Industries, IEC 61511 provides a staged approach to the Functional Safety
Assessments as shown in
Figure: 2below:
Figure: 2 – SIS safety life-cycle phases and FSA stages
There are five FSA stages defined by the standard indicated from S1 to S5 in the figure above.
At the start of the safety lifecycle and for when process hazard(s) are identified, the associated
operating risks are analysed and the necessary risk reduction is identified for the respective hazard.
Based on this identification, the safety functions are allocated. Typically, each hazard and risk
analysis inclues the description of the assumptions made during the analysis and any human factor
impact associated with specific operational constraints. The safety requirements specification
identifies the necessary requirements for the allocation for a SIS based on these factors. Appropriate
techniques and measures for implementing the above activities are therefore subject to a formal
Functional Safety Assessment as this impacts the subsequent stages of the lifecycle.
As explained in section
2of this paper, one of the major contributors for a potential malfunction of a
SIS is associated with the lack of depth regarding the contents of an appropriate safety requirements
specification. The FSA process if properly applied, should ensure for the development of a robust
SRS prior to design approval. This is identified by the stage S1 in
Figure: 2.Failure to undertake an
FSA at this stage is likely to have greater impact upon the rest of the lifecycle and may even have the
potential to seed errors into the project which may lead to an omisssion or incident at later lifecycle
phases and in doing so, cost the project heavily if not addressed at this earlier opportunity.