Fault Tree Analysis
Build a top-down fault tree from an undesired top event, combine contributing events with AND/OR logic, identify qualitative minimal cut sets, and explore how prevention, redundancy and common-cause exposure affect the risk picture.
Reason from an undesired system event down to combinations of contributing events.
Fault Tree Analysis is a deductive, top-down technique. The analyst defines a top event and uses logical gates to represent how intermediate and basic events can combine to produce it.
This builder uses a fictional remote mission power system. It focuses on event definition, AND/OR logic, qualitative minimal cut sets and intervention logic. It does not claim quantitative probability without valid event probabilities and appropriate dependence assumptions.
Fault Tree Analysis — Step by Step
Eight stages establish the top event, add intermediate events, assign gate logic, add basic events, reveal qualitative cut sets, assess prevention and redundancy, apply a separation challenge and conclude with a management systems view.
Top Event, Logic Gates, Basic Events and Qualitative Cut Sets
Reveal the top event context to begin.
Review event level, gate logic, event definition and qualitative contribution to the top event.
Explore Fault-Tree Logic Without Changing the Guided Analysis
Adjust illustrative event exposure, redundancy effectiveness and common-cause exposure. This experiment is qualitative and isolated from the guided fault tree.
Test your fault-tree judgement.
Fault-tree analysis cues
Top Event
The clearly defined undesired system-level event being analysed.
OR Gate
The output event occurs if any one of the input events occurs.
AND Gate
The output event occurs only when all required input events occur.
Basic Event
A lowest-level event not developed further in the current analysis.
Intermediate Event
An event produced by lower-level causes and feeding higher-level logic.
Minimal Cut Set
A smallest combination of basic events sufficient to cause the top event under the tree logic.
A fault tree is useful when event definitions and gate logic are explicit enough to challenge and verify.
Use the tree to expose vulnerable combinations and guide design actions, but do not assign numerical top-event probability without appropriate data and modelling assumptions.