Event tree analysis (ETA), also known as system event treeanalysis (SETA), is a systematic method for analysing the possible consequencesof a triggering event in a system. It is one of the inductive methods of systemanalysis and is described as a quality method in DIN EN 62502.
Basic principle
The EBA starts with a defined initial event and analyses itspotential effects on the overall system. The event tree is built from left toright, with each branch representing two possible outcomes: success or failureof a system component or safety function.
Structure and implementation
1. Identification of the initiating event:
o An undesired event is definedas the starting point.
2. Development of the event tree:
o Branches represent possiblesystem reactions.
o Upper branches representsuccessful functions, lower ones failures.
3. Calculation of probabilities:
o Each branch is assigned aprobability.
o Path probabilities arecalculated by multiplying the individual probabilities.
4. Analysis of the end scenarios:
o Each path leads to a specificend state or accident sequence.
o The overall probability of anundesirable outcome is the sum of the relevant path probabilities.
Areas of application
EBA is used in various areas:
• Hazard analyses to identifypotential system weaknesses
• Probabilistic safety analyses(PSA), often in combination with fault tree analyses
• Risk analyses in productdevelopment and plant operation
Advantages and limitations
Advantages:
• Visualisation of complex eventsequences
• Quantitative risk assessmentpossible
• Support for decision-making insystem design
Limitations:
• Very large trees can arise incomplex systems
• Computer support may berequired for extensive analyses
• The quality of the resultsdepends heavily on the completeness of the factors considered
Compared to other methods
In contrast to fault tree analysis (FTA), which isbackward-looking and investigates the causes of a top event, EBA isforward-looking and considers the consequences of an initial event. Inpractice, the two methods often complement each other and can be used togetherwith scenario techniques.
Event tree analysis is a valuable risk management tool thatmakes it possible to systematically capture and evaluate potential riskscenarios. It helps to improve the safety and reliability of systems byidentifying weak points and creating the basis for targeted preventivemeasures.
