Symbology
===================

In a Fault Tree, two types of nodes are usually used: primary events and logic gates. 

The primary events are failure events that are expected to occur with a given probability. The logic gates descibe the Boolean relationship beetween the primary events or other logic gates.

The primary events symbols can be the following:

+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Primary Event                | Symbol                      | Description                                                                                                                                                                                          |
+==============================+=============================+======================================================================================================================================================================================================+
| Basic                        | .. image:: img/BE.png       | A basic event represents the lowest level in a fault tree. A basic event has no inputs.                                                                                                              |
|                              |                             |                                                                                                                                                                                                      |
|                              |                             | It simply represents the occurrence of an event in the system being analyzed.                                                                                                                        |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Basic Repeated               | .. image:: img/BE_r.png     | A repeated event is a single event that affects multiple portions of a fault tree. When a repeated event occurs, it occurs simultaneously everywhere it is inserted in the tree.                     |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Basic Dormant                |                             | A dormant event is a basic event with no identifiable means of detecting the specific failure mode during normal operation, until it causes an evident failure or there is a maintenance action.     |
|                              | .. image:: img/BE_d.png     |                                                                                                                                                                                                      |
|                              |                             | The calculated probability is based on the applicable maintenance interval.                                                                                                                          |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Basic Repeated Dormant       | .. image:: img/BE_d_r.png   | A dormant event repeated. See repeated event.                                                                                                                                                        |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Basic Hidden                 |                             | A hidden event is a basic event that is not detectable neither as it occurs nor later, in any normal operation, neither during maintenance.                                                          |
|                              |                             |                                                                                                                                                                                                      |
|                              | .. image:: img/BE_h.png     | No identifiable means of detecting until it will cause an evident failure.                                                                                                                           |
|                              |                             |                                                                                                                                                                                                      |
|                              |                             | The calculated probability is based on the system life hours.                                                                                                                                        |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Basic Repeated Hidden        | .. image:: img/BE_h_r.png   | A hidden event repeated. See repeated event.                                                                                                                                                         |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Undeveloped                  |                             | An undeveloped event is used when further resolution of the event is not necessary or when the information for developing the event is not currently available.                                      |
|                              |  .. image:: img/UE.png      |                                                                                                                                                                                                      |
|                              |                             | While an undeveloped event is sismilar to a basic event, it is represented by a different symbol to indicate that is possible to break this event down into associated gates and events.             |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Undeveloped Repeated         |   .. image:: img/UE_r.png   | An undeveloped event repeated. See repeated event.                                                                                                                                                   |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Undeveloped Dormant          | .. image:: img/UE_d.png     | An undeveloped event with dormant failure. See Basic Dorment.                                                                                                                                        |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Undeveloped Repeated Dormant | .. image:: img/UE_d_r.png   | An undeveloped event with dormant failures repeated.                                                                                                                                                 |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Undeveloped Hidden           | .. image:: img/UE_h.png     | An undeveloped event with hidden failures. See Basic Hidden.                                                                                                                                         |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Undeveloped Repeated Hidden  | .. image:: img/UE_h_r.png   | An undeveloped event with hidden failures repeated.                                                                                                                                                  |
+------------------------------+-----------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+


The logic gate symbols are the followings:

+----------+-------------------------------+------------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------+
| Gate     | Symbol                        | Description                                                                                          | Gate logic                                                                                       |
+==========+===============================+======================================================================================================+==================================================================================================+
| AND      | .. image:: img/AND.svg        | The AND gate is used to indicate that the output occurs if and only if all the inputs events occurs. | All events must be TRUE for the output to be TRUE                                                |
+----------+-------------------------------+------------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------+
| OR       | .. image:: img/OR.svg         | The OR gate is used to indicate that the output occurs if any of the input events occurs.            | If at least one event is TRUE, the output is TRUE. If all events are FALSE, the output is FALSE. |
+----------+-------------------------------+------------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------+
| Transfer | .. image:: img/TF.png         | A transfer gate is used to indicate continuation of the fault tree in a separate area.               | N/A                                                                                              |
+----------+-------------------------------+------------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------+
| Remark   |.. image:: img/R.png           | A remark gate is used to add relevant information without being involved in the FTAs calculation.    | N/A                                                                                              |
+----------+-------------------------------+------------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------+