Risk Assessment: Tools, Techniques, and Their Applications

14.6 Analysis Procedure

There are four steps to perform an FTA:

1. Defining the problem.

2. Constructing the fault tree.

3. Analyzing the fault tree qualitatively.

4. Documenting the results.

14.6.1 Defining the Problem

A top event and boundary conditions must be determined when defining the problem. Boundary conditions include:

• system physical boundaries;
• level of resolution;
• initial conditions;
• not allowed events;
• existing conditions;
• other assumptions.

Top events should be precisely defined for the system being evaluated. A poorly defined top event can lead to an inefficient analysis.

14.6.2 Constructing the Fault Tree

Construction begins at the top event and continues, level by level, until all fault events have been broken into their basic events. Several basic rules have been developed to promote consistency and completeness in the fault tree construction process. These rules, as listed in Table 14.4, are used to ensure systematic fault tree construction (3).

Table 14.4 Rules for Constructing Fault Tree

Fault tree statements

Write the statements that are entered in the event boxes and circles as malfunctions. State precisely a description of the component and the failure mode of the component. The “where” and “what” portions specify the equipment and its relevant failed state. The “why” condition describes the state of the system with respect to the equipment, thus explaining why the equipment state is considered a fault. Resist the temptation to abbreviate during ...