IEC 61508 Safety Instrumented Systems

This standard involves a systematic approach to Life Cycle Safety of Safety Instrumented Systems (SIS). Systems such as these need to be maintained to be sure of a certain safety level during operation. It is concerned specifically with Electrical/Electronic/Programmable Electronic Safety-Related Systems (E/E/PESs).IEC 61508 provides guidelines to classify these systems by Safety Integrity Levels (SIL levels). Four SILs can be defined according to the risks associated with the system requirements with SIL4 being assigned to the highest risks. The standard adopts a risk based approach to calculate the required SIL, which represents the Probability of Failure on Demand of the target system.IEC 61508-1 show the four SILs in two tables. The first of these (shown below) is for systems operating in the ‘low demand mode of operation’, displaying the associated average probability of failure to perform its design function on demand.

Safety Integrity Level Low Demand Mode of Operation
4 >=10-5 to <10-4
3 >=10-4 to <10-3
2 >=10-3 to <10-2
1 >=10-2 to <10-1

As part of this approach, the calculation of system reliability is obviously vital. The use of various system modeling approaches are applicable (fault tree, reliability block diagram, markov) to return the necessary PFD.

Isograph software is ideal for this purpose. Reliability Workbench has many custom features for IEC 61508 compliant safety analyses, such as

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