AS IEC 62502:2011 Analysis techniques for dependability— Event tree analysis (ETA)
7.2.3 Step 2: Identification of the initiating events of interest
This step usually involves the use of a broad hazard identification technique, such as what-if, preliminary evaluation, or preliminary hazard analysis, to evaluate systematically all activities within the scope of the study, e.g. the consideration of the operational experience in the field of the specific industry. This step helps to identify the hazards and the possible initiating events that arise from these hazards. These identification methods broadly consider all operations within the scope of the study and seek to identify the full range of potential initiating events and the range of outcomes associated with such events. For an extensive list and description of various methods, see [1 2]. The outcome of these identification processes is usually an extensive list of potential events and their expected consequences.
It should then be the general aim to identify the entire spectrum of events that can occur within the scope of the analysis. After this has been done, the analysts apply screening criteria to identify the initiating events of most interest that will be considered in the event trees. Basically, there are two options for screening out initiating events, namely exclusion due to unlikely physical properties (e.g. specific values for pressure, temperature or fire loads are not exceeded) or due to low initiating event frequencies usually estimated in a conservative manner. This step helps identify those events that have to be analysed further to understand the complex interactions of systems. During this analysis one has to check the possibility of any interaction among initiating events and mitigating factors, e.g. whether the environment as caused by the initiating event, such as loss of all energy supplies after an earthquake, can adversely affect the performance of the mitigating factors.
After the initial list of events is identified and screened, the remaining list of initiating events includes those that will be considered in event trees. These are the events that are identified by experienced experts as complex enough to require additional analysis of the various system and personnel interactions that cause different outcomes from the initiating event.