PFDavg CALCULATION METHODS - CONTROVESY OR CONFUSION?

摘要

IEC61508 and the new IEC 61511 allow probabilistic evaluation of each set of equipment used to reduce risk in a safety related system. Different order of magnitude risk reduction levels are achieved depending on the average probability of failure on demand (often called average probability of dangerous failure). In practice, a number of different methods have been used to calculate this probability. Among the most popular are fault tree analysis, reliability block diagrams, simplified equations (derived using a number of different ways) and Markov models. For those who use Markov models, different solution techniques are used. A debate has existed in various circles about the appropriateness of various methods. The fundamental problem is that these different methods give results that vary by 2X+ for same set of input parameters.