Risk Assessment: Tools, Techniques, and Their Applications

14.2 Application

Fault trees graphically represent the interaction of failures and other events in a system. Basic events are depicted at the bottom of the fault tree and are linked via logic symbols (known as gates) to one or more of the TOP events. These TOP events represent identified hazards or system failure modes for which predicted reliability or availability data are required. Typical TOP events might be as follows:

• total loss of production;
• explosion;
• toxic emission;
• safety system unavailable.

As indicated, the fault tree begins at the end, so to speak. This top-down approach starts by supposing that an accident takes place. It then considers the possible direct causes that could lead to this accident. Next it looks for the origins of these causes. Finally, it looks for ways to avoid these origins and causes. The resulting diagram resembles a tree, thus the name.

Fault trees can also be used to model success paths as well. In this regard, they are modeled with the success at the top, and the basic events are the entry-level success that put the system on the path to success.