MAP-01-011 HFI Technical Guide - Human Factors Integration ...

Annex 3 – HF Techniques, Methods and Tools• If used correctly, the technique could potentially depict anything that couldpossibly go wrong in a system.• Event tree analysis is a relatively easy technique that requires little training.• Event tree analysis has been used extensively in PSA/HRA.• Disadvantages• For large, complex tasks, the event tree can become very large andcomplex.• Can be time consuming in its application.• Task steps are often not explained in the output.A3.10.3Fault Tree AnalysisFault trees are used to depict system failures and their causes. A fault tree issimilar to an event tree diagram and defines the failure event and displays thepossible causes in terms of hardware failure or human error. Fault tree analysiswas originally developed for the analysis of complex systems in the aerospaceand defence industries and they are now used extensively in probabilistic safetyassessment (PSA). Although typically used to evaluate events retrospectively,fault trees can be used at any stage in the design process to predict failureevents and their causes. The fault tree can be used to show the type of failureevent and its various causes. Typically, the failure event or top event is placed atthe top of the fault tree, and the contributing events are placed below. The faulttree is held together by AND and OR gates, which link contributing eventstogether. An AND gate is used when more than one event causes a failure i.e.contributing factors are involved. The events placed directly underneath an ANDgate must occur together for the failure event above to occur. An OR gate isused when the failure event could be caused by more than one contributoryevent in isolation, but not together. The event above the OR gate may occur ifany one of the events below the OR gate occurs. A fault tree analysis could beused in the design of a system in order to contribute to the eradication ofpotential failure causes.• Advantages• Fault trees are useful in that they define possible failure events and theircauses. This is especially useful when looking at failure events withmultiple causes.• Fault tree type analysis has been used extensively in PSA.• Although most commonly used in the analysis of nuclear power plantevents, the technique is generic and can be applied in any domain.• Fault trees can be used to highlight potential weak points in a systemdesign concept (DTC115 - Kirwan & Ainsworth 1992).May 2006 Page A3-91 Issue 4