Editing Fault tree analysis (section)

===Event symbols===
Event symbols are used for ''primary events'' and ''intermediate events''. Primary events are not further developed on the fault tree. Intermediate events are found at the output of a gate. The event symbols are shown below:
<gallery>
File:FTA_basic_event.jpg|Basic event
File:FTA_initiating_event.jpg|External event
File:FTA_undeveloped_event.jpg|Undeveloped event
File:FTA_conditioning_event.jpg|Conditioning event
File:FTA_intermediate_event.jpg|Intermediate event
</gallery>
The primary event symbols are typically used as follows:
* '''Basic event'''{{snd}}failure or error in a system component or element (example: switch stuck in open position)
* '''External event'''{{snd}}normally expected to occur (not of itself a fault)
* '''Undeveloped event'''{{snd}}an event about which insufficient information is available, or which is of no consequence
* '''Conditioning event'''{{snd}}conditions that restrict or affect logic gates (example: mode of operation in effect)
An intermediate event gate can be used immediately above a primary event to provide more room to type the event description.

FTA is a top-to-bottom approach.