Chapter 11
Fault Tree Analysis
11.1 INTRODUCTION
Fault tree analysis (FTA) is a systems analysis technique used to determine the root
causes and probability of occurrence of a specified undesired event. FTA is
employed to evaluate large complex dynamic systems in order to understand and
prevent potential problems. Using a rigorous and structured methodology, FTA
allows the systems analyst to model the unique combinations of fault events that
can cause an undesired event to occur. The undesired event may be a system hazard
of concern or a mishap that is under accident investigation.
A fault tree (FT) is a model that logically and graphically represents the various
combinations of possible events, both faulty and normal, occurring in a system that
lead to an undesired event or state. The analysis is deductive in that it transverses
from the general problem to the specific causes. The FT develops the logical fault
paths from a single undesired event at the top to all of the possible root causes at
the bottom. The strength of FTA is that it is easy to perform, easy to understand,
provides useful system insight, and shows all of the possible causes for a problem
under investigation.
Fault trees are graphical models using logic gates and fault events to model the
cause–effect relationships involved in causing the undesired event. The graphical
model can be translated into a mathematical model to compute failure probabilities
and system importance measures. FT development is an iterative process, where the
initial structure is continually updated to coincide with design development.
In the analysis of systems there are two applications of FTA. The most commonly
used application is the proactive FTA, performed during system development to
influence design by predicting and preventing future problems. The other
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Hazard Analysis Techniques for System Safety, by Clifton A. Ericson, II
Copyright # 2005 John Wiley & Sons, Inc.
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