Availability of systems with self-diagnostic components—applying Markov model to IEC 61508-6

Of all the techniques applicable to safety-related analyses, each one may be adaptable to some aspects of the system safety behavior. On the other hand, some of them can fit to analysis on one aspect of the system behavior concerning risk, but they do not always lead to the same results. Rouvroye and Brombacher made a comparison of these techniques and indicated that Markov and Enhanced Markov analysis techniques can cover most aspects of systemʹs safety-related behavior. According to their conclusion, the Markov method is introduced to Part 6 of the standard IEC 61508 for quantitative analysis in this paper. The purpose is to present explanation in details for solutions given in the standard because there are not clear descriptions for many results and it is not easy for a safety engineer to find the clue. In addition, the down time tc1 shown in the standard is newly defined because it is the basis to get the results of average probability of failure on demand of system architectures and its meaning is not clearly explained. Through derivation, however, a discrepancy is found in the standard. From this point of view, new suggestions are proposed based on the results obtained.