Risk Assessment: Tools, Techniques, and Their Applications

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 ...