Event trees are used to analyze continuously operating and standby systems. An event tree is a visual representation of all the events that can occur in the system. It displays the sequences of events that result in the success and/or failure of system components. You use event trees to determine the probability of a particular event based on the events leading up to it.

The starting point of an event tree, referred to as the initiating event. When an event tree is used to analyze a continuously operating system, events (component failures) can be analyzed in an order. The initiating event for a continuously operating system is typically some normal operation. The events or columns inserted in the event tree then indicate the success or failure of particular system components.

When an event tree is used to analyze a standby system, particularly safety-oriented or mission-oriented systems, the sequence of events becomes critical as operational logic and the switching of components is involved. Because the operation of one component is dependent on the success or failure of other components, the events must be analyzed in chronological order. The initiating event for a standby system is typically some undesirable event or situation.

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