Abstract
A trend of increasing application of computer systems to safety-related systems (SRS) forced IEC to compile IEC 61508, Functional safety of electrical/electronic/programmable electronic SRS. In the standard, fail-to dangerous failures of SRS are classified into fail-to dangerous undetected (DU) and detected (DD) failures. In the present paper, the causation of hazardous events in a dynamic demand state is modeled by use of sequential failure logics taking account of such factors as DD failures and maintenance of SRS with self-diagnostic functions. Then, formulas to estimate hazardous evnet rates are derived based on the model. As a result, safety integrity levels (SIL) of SRS can be determined easily and reasonably by applying the formulas.
