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Fault Tree Analysis Diagram

Event Tree Analysis

An event tree is a graphical representation of the logic model that identifies and quantifies the possible outcomes following an initiating event. Event tree analysis provides an inductive approach to reliability assessment as they are constructed using forward logic. Fault trees use a deductive approach as they are constructed by defining TOP events and then use backward logic to define causes. Event tree analysis and fault tree analysis are, however, closely linked. Fault trees are often used to quantify system events that are part of event tree sequences. The logical processes employed to evaluate event tree sequences and quantify the consequences are the same as those used in fault tree analyses.

FaultTree+ in Reliability Workbench

Fault tree analysis techniques were first developed in the early 1960's. Since this time they have been readily adopted by a wide range of engineering disciplines as one of the primary methods of performing reliability and safety analysis.

Fault trees graphically represent the interaction of failures and other events within a system. Basic events at the bottom of the fault tree are linked via logic symbols (known as gates) to one or more TOP events. These TOP events represent identified hazards or system failure modes for which predicted reliability or availability data is required. Typical TOP events might be:

  • Total loss of production
  • Safety system unavailable
  • Explosion
  • Loss of mission

Basic events at the bottom of the fault tree generally represent component and human faults for which statistical failure and repair data is available. Typical basic events are

  • Pump failure
  • Temperature controller failure
  • Switch fails closed
  • Operator does not respond

Fault trees may be used to analyse large and complex systems. They are particularly adept at representing and analysing redundancy arrangements. In addition common cause events are easily handled.

Isograph's FaultTree+ computer program allows you to perform event tree analysis in an integrated environment. The program also provides fault tree and Markov analysis capabilities. For more information on FaultTree+ visit our web site at


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