Fault Tree Analysis (FTA)

Fault tree analysis is a widely used method in analysing the behaviour of failures in complex systems and is a method used in estimating the probability of occurrence for a particular failure mode. The main purpose of fault tree analysis is to evaluate the probability of a top event using analytical or statistical methods. This approach is most useful when several factors play a role in the occurrence of the failure mode.

Fault tree analysis is a systematic method of analysing contributing events that may lead to accidents or incidents in the workplace. Fault trees can be used to determine the probability of failure of a system (or top event), to compare design alternatives as a process of identifying critical events that will significantly contribute to the occurrence of a top event and to determine the sensitivity of the probability of a top event to the contributions of various fundamental events.

Fault trees describe a sequence of events that lead to system failure. The leaves of the tree can be used to represent the initial causes of the accident or incident according to Leplat, 1987. In this article however we prefer to rather refer to the network of roots of a tree as this more accurately describes the network of events leading to a top event, accident or other undesired result.

Nodes and Logic Gates

The logical dependencies at each level of the tree is specified by using the so-called AND and OR gates. The AND gate indicates that the output occurs if and only if all of the input events occur. The OR gate indicates that the output occurs only if at least one of the input events occur. Basic tree nodes can make reference to any dependability model element such as LRU class, hardware configuration, reliability graph, embedded fault tree, or Markov model. Reliability and availability values are calculated for each node and logic gate and are expressed as probability numbers.

Conclusion

The fault tree technique or derivatives of it is widely used in industry to investigate accidents and incidents. The success of the method can be found in the logical and sequential approach that separates different cause consequence trails.

This author is of the opinion that the major disadvantage of fault treeanalyses as an accident or incident investigation technique is that it does not present an objective mechanism to discover fundamental contributing factors as independent contributors to accidents or incidents. The fault tree approach is a quantitative system only aimed at establishing the probability of an accident re-occurring rather than establishing failure modes.

© 2009 Carl Marx