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FTA: Logic, Symbols, Rules and Other Considerations

A description of the logic and event symbols used is given in the Figure, and the rules for constructing a fault tree is given in the Table.

Rules for fault tree constructionFTA Symbols

The AND logic function is very important for describing processes that interact in parallel. This means that the output state of the AND logic function is active only when both of the input states are active. The INHIBIT function is useful for events that lead to a failure only part of the time. For instance, driving over debris in the road does not always lead to a flat tire. The INHIBITgate could be used in the fault tree to represent this situation.

Some of the factors to consider when using a FTA:

Other considerations for fault tree construction are:

(1) Assume that faults propagate through normally operating equipment. Never assume that a fault is stopped by the miraculous failure of another piece of equipment
(2) Gates are connected through labeled fault events. The output from one gate is never connected directly into another

It is important in fault tree analysis to consider only the nearest contributing event. There is always a tendency to jump immediately to the details, skipping all of the intermediate events. Remember that the purpose of the fault tree is to determine the individual event steps that must occur to produce the top event. Some practice is required to gain experience in this technique.


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