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Fault Tree Analysis

Fault tree analysis diagrams are commonly used to illustrate events that might lead to a failure so the failure can be prevented. Fault tree analysis diagrams are commonly used in Six Sigma processes, particularly in the Analyze phase of the Six Sigma business improvements process.



Originally, FTA was used by NASA in the space programs. It is a convenient way of representing the logical connection between the failures modes of a system.

External links: U.S. Nuclear Regulatory Commission Fault Tree Handbook (NUREG-0492)  (Fault Tree Handbook with Aerospace Applications)


Hazard vs Risk

In terms of safety management, hazard and risk are different.


- a set of conditions in the operation of a product or system with potential for initiating an accident sequence.

i.e. a potential source of harm

--> hazard analysis

- the combined effect of the probability of occurrence of an undesirable event, and the magnitude of that event.

i.e. a measure of the probability and severity of harm

--> Risk assessment



Fault Tree Analysis (FTA) is a top-down method for analyzing the potential hazards. An adverse event is the starting point and the analyst works towards the individual components or sub-systems are examined and the consequences of a failure or a series of failures developed.


FTA is defined as the study of the possible sequence of events constituting the failure of a system using the diagrammatic method of algorithms. Some basic Fault Tree symbols are illustrated as following:

Some basic Fault Tree symbols

For example of usage of FTA, please refer to the Case Study - The Amoco Cadiz


Last updated on 28 MAR 2010