14.4 Event Symbols

The symbols shown in Table 14.1 are the most common fault tree symbols. These symbols represent specific types of fault and normal events in FTA. In many simple trees only the basic event, undeveloped event, and output event are used.

Table 14.1 Common Fault Tree Symbols

Symbol name	Symbol	Description
Basic event		A basic initiating fault (or failure event)
Undeveloped event		An event that is no further developed. It is a basic event that does not need further resolution
Output event		An event that is dependent on the logic of the input events
External event (house event)		An event that is normally expected to occur. In general, these events can be set to occur or not occur, that is, they have a fixed probability of 0 or 1
Conditioning event		A specific condition or restriction that can apply to any gate
Transfer		Indicates a transfer to a subtree or continuation to another location

Events representing failures of equipment or humans (components) can be divided into failures and faults. A component failure is a malfunction that requires the component to be repaired before it can successfully function again. For example, when a pump shaft breaks, it is classified as a component failure. A component fault is a malfunction that will “heal” itself once the condition causing the malfunction is corrected. An example of a component fault is a switch whose contacts fail to operate because they are wet. Once they are dried, they will operate properly.

Output events include the top event, or ultimate outcome, and intermediate events, usually groupings of events. Basic events are used at the ends of branches since they are events that cannot be further analyzed. A basic event cannot be broken down without losing its identity. The undeveloped event is also used only at the ends of event branches. The undeveloped event represents an event that is not further analyzed either because there is insufficient data to analyze or because it has no importance to the analysis.