SINNAMIN, R.M. and ANDREWS, J.D., 1997. Improved accuracy in qualitative fault tree analysis. Quality and Reliability Engineering International, 13 (5), pp 285-292.
The fault tree diagram defines the causes of the system failure mode or ‘top event’ in terms of the
component failures and human errors, represented by basic events. By providing information which
enables the basic event probability to be calculated, the fault tree can then be quantified to yield
reliability parameters for the system.
Fault tree quantification enables the probability of the top event to be calculated and in addition its
failure rate and expected number of occurrences. Importance measures which signify the contribution
each basic event makes to system failure can also be determined. Owing to the large number of failure
combinations (minimal cut sets) which generally result from a fault tree study, it is not possible using
conventional techniques to calculate these parameters exactly and approximations are required. The
approximations usually rely on the basic events having a small likelihood of occurrence. When this
condition is not met, it can result in large inaccuracies. These problems can be overcome by employing
the binary decision diagram (BDD) approach. This method converts the fault tree diagram into a format
which encodes Shannon’s decomposition and allows the exact failure probability to be determined in a
very efficient calculation procedure.
This paper describes how the BDD method can be employed in fault tree quantification.