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The present paper examines the effect of safe failure fraction (SFF) constraints on hazardous-event rates, and discusses the validity of the SFF constraints in IEC 61508. First, the safe states are categorized into three types of states, and overall systems involving safety-related systems are classified into six types of systems based on the safe-state categorization, and the completeness of trips. Next, state-transition models for the systems where the effect of SFF is the greatest are presented, and the hazardous-event rates are analysed for the systems. Then, it is found that, when the effect of the SFF constraints is positive, it is negligible; and when it is negative, it is not negligible for safety. Thus, we recommend that the application of the SFF constraints to the standard should be put on hold.
Date of Publication: Dec. 2008