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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