Safety integrity level determination techniques

Refer to lEC 61508-1, table 2 (for low demand mode operation) or table 3 (for continuous or high demand mode operation) to determine the safety integrity level (SIL). The SIL then guides the selection of the techniques used for the avoidance of systematic faults in both hardware and software, so that as the risk reduction increases, or the hazard rate decreases, there is a reduction in the likelihood that systematic failures (including those resulting from incorrect specification) will result in a hazard. 

ISA-TR84 00.02-2002-Part 2 Safety Instrumented Functions (SIF)-Safety Integrity Level (SIL) Evaluation Techniques Part 2 Determining the SIL of a SIP via Simplified Equations Introduction, ISA, June 2002. 

Technical Report ISA-TR84.00.02-2002-Part 3 Safety instrumented functions (SIF)-safety integrity level (SIL) evaluation techniques. Part 3 determining the SIL of a SIF via fault tree analysis, June 2002. 

This places much emphasis on the engineering of sufficient safety integrity into the safety-related systems to ensure that risks affected by those systems are reduced to a level that is as low as reasonably practicable (ALARP). There are many methods and techniques for achieving this. The lEC standard 61508 Functional safety of electrical/electronic/programmable electronic safety-related systems, for example, advocates a risk-based approach to determining the required safety performance of safety-related systems, and does so in the context of an overall safety life cycle. The success in the implementation of such an approach demands a suitable level of competence in the engineers involved in the determination of the safety requirements and in the realisation of such systems. 

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