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Functional Safety Related Modelling and Calculations

In: Reliability Assessment of Safety and Production Systems

Author

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  • Jean-Pierre Signoret

    (Total Professeurs Associés)

  • Alain Leroy

Abstract

Safety instrumented systems (SIS) are more and more used in industry to replace conventional protection systems and functional safety standards (e.g. IEC 61508 2010 or ISO/TR 12489 2013) have been developed to deal with them. This chapter presents a critical analysis of the main functional safety concepts (e.g. safety integrity levels) and principles (e.g. fault tolerance), highlights why relying on the safe failure fraction (SFF) is questionable and explains how to perform the related probabilistic calculations (e.g. PFDavg, PFH). The functioning design—loss of power versus power emission, low versus high demand modes of operation—and the impact of fault tolerance are analysed. The mathematics allowing to obtain the ready-made simplified results proposed in IEC 61508-6 are developed and the difficulties and limitations of this approach lead to the conclusion that it should be superseded by the various systemic models described in the book: fault trees, Markov graphs or Petri nets. Models for the various types of failures—dangerous/safe, detected/non-detected by diagnostic or proof tests, non-covered (i.e. never detected)—are proposed. Simple SIS structures as well as multiple SIS are considered. The impact of test staggering is analysed as well as the various ways to take input data uncertainties into account.

Suggested Citation

  • Jean-Pierre Signoret & Alain Leroy, 2021. "Functional Safety Related Modelling and Calculations," Springer Series in Reliability Engineering, in: Reliability Assessment of Safety and Production Systems, chapter 0, pages 749-825, Springer.
    • Handle: RePEc:spr:ssrchp:978-3-030-64708-7_36
    DOI: 10.1007/978-3-030-64708-7_36
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