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RR29 - Proposal for requirements for low complexity safety related systems.

Proposes a framework for the application of the standard IEC 61508 to "low complexity" systems such as simple relay based interlock arrangements commonly found in machinery safeguarding applications. Presents a methodology for the design, integration and validation of low complexity electrical/electronic programmable electronic safety-related systems. A scheme for architectural constraints is proposed which limits the Safety Integrity Levels (SILS) that can be claimed for low complexity systems of various degrees of hardware fault tolerance. The scheme is consistent with the principles of IEC 61508 while simplifying the requirements. Comparisons of numerically and qualitatively assessed SILS on the basis of annual proof testing; annual functional testing only, and taking into account Common Cause Failure (CFF) are included for 18 example circuits. The proposed scheme has been shown to be consistent with the achievement of the target failure rate and Probability of Failure on Demand (PFD) of the relevant SIL for "low complexity systems". In order to simplify the process of reliability analysis to satisfy the requirements for hardware reliability, conservative values based on generic reliability data are proposed based on IEC 61508 but tailored for low complexity systems. The examples in this report are taken from the machinery sector but the principles described will also be applicable in other sectors.

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Updated 2010-03-19