AS IEC 62502:2011 pdf download.Analysis techniques for dependability - Event tree analysis (ETA)
From the qualitative point of view, ETA helps to identify all potential accident scenarios (fanning out like a tree with success- or failure-branches) and potential design or procedural weaknesses. The success branch models the condition that the mitigating factor is operating as intended. As with other analysis techniques, particular care has to be taken with the modelling of dependencies, bearing in mind that the probabilities used for quantifying the event tree are conditioned on the event sequence that occurred prior to the occurrence of the event concerned. Clause 8 deals with these qualitative aspects of the analysis as well as the basic quantitative rules for the calculations required to estimate the (dimensionless) probabilities or frequencies (1/h) of each of the possible outcomes. Though one could, in theory, model the effect of failures of the operator or software by an event tree, this standard does not deal with their quantification since these issues are covered by other IEC
publications, e.g. IEC 62508 [23] and IEC 62429 [22].
The advantages of ETA as a dependability and risk-related technique, as well as the limitations, are discussed in Clause 5. As an example of the limitations of ETA, the time-dependent evolution has to be considered cautiously because it can be handled properly only in particular cases. This limitation has led to the development of strongly related methods such as the dynamic event tree analysis method, which facilitate the modelling of time-dependent evolutions. This dynamic event tree analysis method will not be detailed in this standard; however, references are included in the bibliography for further information.
ETA bears a close relationship with FTA whereby the top events of the FTA yield the conditional probability for a particular node of the ETA. This is explained more fully in Clause 6 which also covers the relationships between ETA and other analysis techniques such as cause-consequence analysis (CCA) and layer of protection analysis (LOPA). CCA combines cause analysis and consequence analysis hence using deductive and inductive approaches. LOPA has been developed by the process industry as a special adaptation of the ETA.
Since the first steps and a well constructed approach are crucial for success, Clause 7 describes the development of the event tree, starting with a precise system definition.Furthermore, Clause 7 deals with the different aspects of the system (technical, operational,human and functional) as well as the depth of the analysis. Another important issue is the question of how to establish the list of relevant initiating events.AS IEC 62502 pdf download.