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Risk Analysis Techniques 69

12.1.2 Theory
An FT is a graphical representation of parallel and sequential events that are intercon-
nected by logic gates, leading up to a top event. The top event usually represents an
undesired outcome, such as a Hazardous Situation, and the lower events include faults,
user errors, and normal conditions. An example of an FT can be seen in Fig. 12.1.
The logic gates show the required relationships among the lower level events that are
needed to cause the output of the gate in question. The event at the top of a gate is
called the “higher” event and is the output of the gate. The events below a gate are
called the “lower” events and are the inputs to the gate.
Due to its logical construct, an FT can always be translated into a set of Boolean
equations. As such, rules of Boolean algebra can be applied to FTs to simplify and
reduce them. This simplification is beneficial both in understanding of the System
under analysis, and also for the derivation of minimal cut sets of the tree.
Analysis of FTs can provide us with:

• minimal cut sets of the tree
• qualitative component importance
• knowledge of cut sets that are susceptible to CCFs

If probabilities of basic events are known, the quantitative analysis can provide
probability of occurrence of the top event; quantitative importance of the minimal cut

Figure 12.1 Example of Fault Tree Analysis diagram.