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Fault Tree Analysis: How to understand it

The Quality ToolbookFault Tree Analysis > How to understand it

When to use it | How to understand it | Example | How to do it | Practical variations


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How to understand it

The failure of an item in a system is often caused by the failure of other items, for example where a vehicle's braking failure is caused by water in the brake cylinders, which may in turn be caused by failure of the cylinder seals.

Fault Tree Analysis, or FTA, provides a method of breaking down these chains of failures, with a key addition for identifying combinations of faults that cause other faults. Combinations of faults come in two main types: (a) where several items must fail together to cause another item to fail (an 'and' combination), and (b) where only one of a number of possible faults need happen to cause another item to fail (an 'or'' combination).

The FTA diagram shows faults as a hierarchy, with two other symbols to show the 'and' and 'or' combinations, as in Fig. 1. These are called gates, as they prevent the failure event above them occurring unless their specific conditions are met.



Fig. 1. Logical And and Or in Fault Tree Analysis


A third type of gate is called an inhibit gate, as it prevents a failure from happening unless a specific condition is met (it is effectively an 'and' of the failure and some other conditions).

In an FTA diagram, there are two main types of failure event box: combination events, which are the result of other events, and basic events, which are the start points for the chains of events above them. Basic events may be real root events or may simply not be developed further on this diagram. These and other symbols that may be used in FTA diagrams are shown in the table below.


Table 1. FTA symbols


Symbol Name Meaning
And gate Event above happens only if all events below happen.
Or gate Event above happens if one or more of events below are met.
Inhibit gate Event above happens if event below happens and conditions described in oval happen.
Combination gate Event that results from combination of events passing through gate below it.
Basic event Event that does not have any contributory events.
Undeveloped basic event Event that does have contributory events, but which are not shown.
Remote basic event Event that does have contributory events, but which are shown in another diagram.
Transferred event A link to another diagram or to another part of the same diagram.
Switch Used to include or exclude other parts of the diagram which may or may not apply in specific situations.


A common way of reducing the chance of failure of a system is to build redundancy into it, for example by having two sets of critical components running in parallel. It is possible, however, for failures to occur, which results in the fault tolerance of such systems to be negated as one failure causes all redundant parts to effectively not work. This is called common mode failure. For example, a motor system driven by two separate engines may fail when a common fuel line ruptures. FTA is a useful tool for discovering such failures, as it looks back down the chain of events to find possible failures in all areas.



Fig. 2. Selector system



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