Event Tree Analysis (ETA)

An event tree is constructed by defining an initial event and the possible consequences that flow from it. An event tree graphically shows all of the possible outcomes following the success or failure of protective systems, given the occurrence of a specific initiating cause (equipment failure or human error). Event trees are also used to study other events, such as starting at a loss event and evaluating mitigation systems.
Event trees are used to identify the various incidents that can occur in a complex process. After these individual event sequences are identified, the specific combinations of failures that can lead to the incidents can then be determined using Fault Tree analysis.
The results of the Event Tree Analysis are event sequences; that is, sets of failures or errors that lead to an incident. An Event Tree Analysis is well suited for analyzing complex processes that have several layers of safety systems or emergency procedures in place to respond to specific initiating events.
Reference: CCPS: guidelines for hazard evaluation procedures, third edition
A graphical logical model that identifies possible outcomes following an initiating event. With suitable data it can be used to quantify the occurrence of an event.
Reference: HarsNet working group, 2002, HarsBook, A technical guide for the assessment of highly reactive chemical systems, Frankfurt.
ETA is a technique, either qualitative or quantitative, which is used to identify the possible outcomes and if required, their probabilities, given the occurrence of an initiating event. ETA is widely used for facilities provided with engineered accident mitigating features, to identify the sequence of events which lead to the occurrence of specified consequences, following the occurrence of the initiating event. It is generally assumed that each event in the sequence is either a success or a failure.
ETA is an inductive type of analysis in which the basic question addressed is “what happens if…?”. It provides the relationship between the functioning or failure of various mitigating systems and ultimately the hazardous event following the occurrence of the single initiating event, in a clear way. ETA is very useful in identifying events which require further analysis using FTA (i.e. the top events of the fault trees). In order to be able to do a comprehensive risk assessment, all potential initiating events need to be identified. There is always a potential, however, for missing some important initiating events with this technique. Furthermore, with event trees, only success and fault states of a system are dealt with, and it is difficult to incorporate delayed success or recovery events.
ETA can be used both for hazard identification and for probability estimation of a sequence of events leading to hazardous situations.
Reference: IEC 60300-3-9
Technical Tools/Risk Assessment/Quantitative Risk Assessment/Frequency Techniques