Editing Fault tree analysis (section)

==Graphic symbols==
The basic symbols used in FTA are grouped as events, gates, and transfer symbols. Minor variations may be used in FTA software.

===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.

===Gate symbols===
Gate symbols describe the relationship between input and output events. The symbols are derived from Boolean logic symbols:
<gallery>
File:FTA_OR_gate.jpg|OR gate
File:FTA_AND_gate.jpg|AND gate
File:FTA_XOR_gate.jpg|Exclusive OR gate
File:FTA_priority_AND_gate.jpg|Priority AND gate
File:FTA_inhibit_gate.jpg|Inhibit gate
</gallery>

The gates work as follows:
* '''OR gate'''{{snd}}the output occurs if any input occurs.
* '''AND gate'''{{snd}}the output occurs only if all inputs occur (inputs are independent from the source).
* '''Exclusive OR gate'''{{snd}}the output occurs if exactly one input occurs.
* '''Priority AND gate'''{{snd}}the output occurs if the inputs occur in a specific sequence specified by a conditioning event.
* '''Inhibit gate'''{{snd}}the output occurs if the input occurs under an enabling condition specified by a conditioning event.

===Transfer symbols===
Transfer symbols are used to connect the inputs and outputs of related fault trees, such as the fault tree of a subsystem to its system. NASA prepared a complete document about FTA through practical incidents.<ref name=fthbaa />
<gallery>
File:FTA_transfer_in.jpg|Transfer in
File:FTA_transfer_out.jpg|Transfer out
</gallery>