Abstract
Each safety-related function must be implemented with a defined safety integrity level (SIL) if the control system implements safety-related functions (SFs) in addition to the standard control functions. The required SIL of the SF depends on the quantity of the risk associated with the failure of this one SF. The SIL against random failure can be expressed through the dangerous failure rate of the SF for an electronic safety-related control system (ESRCS) operating in a continuous mode of operation. The proof must be provided (among other things) that the SIL requirements for the individual SFs are met so the ESRCS can be accepted and implemented. The assessment of the impact of random failures on the SIL of the SF must be performed using the quantitative analysis method. This paper describes the procedure and derives equations for evaluating the impact of random failure on SIL of the SF using Markov chains with two absorption states. The achieved results are presented for SF implemented by ESRCS with dual architecture based on composite fail-safety technique.
Highlights
How “strong” safety measures should be, depends on the results of the risk analysis that is associated with the machinery and equipment used, which are a part of the equipment under control (EUC)
A characteristic feature of electronic safety-related control system (ESRCS) that uses the inherent fail-safety technique is the one-channel architecture. The logic of such systems is realized by special logic elements with asymmetric failure features, whose properties are verified by long-term operational experience, as well as using appropriate methods and procedures in the application of these elements
TLheet DthaenEgSeRroCuSs iFmapilluermeeRnattoenolfytohneeSSFF and all its elements participate in the implementation of this safety-related functions (SFs), the safety integrity (SI) of safety function (SI-SF) can be identified with the SI ESRCSL. et the ESRCS implement only one SF and all its elements participate in the implemBeecnatuasteiornanodf othmisfaSiFlu, rthesenoctchuerSraI nodf osmafleytyanfudnccotniotinnu(SoIu-SsFly) ocavnerbteimide,eCntTifiMeCdswcaitnhbtehe usSeIdEStoRCanSa. lyse the effects of random failures on SI-SF
Summary
A characteristic feature of ESRCS that uses the inherent fail-safety technique is the one-channel architecture. The logic of such systems is realized by special logic elements with asymmetric failure features (most often electromechanical relays, so-called safety relays), whose properties are verified by long-term operational experience, as well as using appropriate methods and procedures in the application of these elements. Increasing the integration of electronic components does not make it possible to compile a list of real failure modes and use only qualitative analysis methods to assess ESRCSs. ESRCS safety assessment requires the application of methods and procedures based on a probabilistic approach. A combination of FMEA and RBD methods in the paper [5] or a combination of FMEA and FTA methods in the paper [14] can be encountered
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