About Components of Fault Trees

Fault Tree Components

Events


Event	Definition
Top Event	The top event corresponds to the failure of the system, so it points to the root failure mode. It is developed until basic events are encountered. Note: You can have only one top event per fault tree.
Basic Event	A basic event is the lowest level event. It corresponds to a leaf of a fault tree. A basic event points to a failure mode that ends the failure scenario. To successfully compute the fault tree, all basic events must be assigned a reliability formula.
Intermediate Event	A top event is split into intermediate events. An intermediate event depends on several basic events or other intermediate events. The top event is a particular case of intermediate event. These intermediate events can be: Fault tree elements pointing to a failure mode. For more information about failure modes, see Architecture Tree Elements. Gates: structure elements that are used to define a logical expression. A gate is not aggregated in an architecture reference, does not point to a failure mode, and cannot be assigned a reliability formula. Intermediate events are computed.

Operators


Operator Name	Description	Cardinality
-	Parent event occurs if the child event occurs	For events that have only one child
NOT	Parent event occurs if the child event does not occur	For events that have only one child
OR	Parent event occurs if any one of the child events occur	For events that have two or more children
AND	Parent event occurs if all child events occur	For events that have two or more children
Exclusive OR (XOR)	Parent event occurs if the number of child events is odd	For events that have two or more children
NOR	Parent event occurs if none of the child events occur	For events that have two or more children
NAND	Parent event does not occur if all the child events occur	For events that have two or more children
VOTE	Parent event occurs if a number of child events occur. This value is comprised between the minimal number of events `k` and the maximal number of children `n`. In the example, the `i` value must be between 1 and 3 included. Note: The VOTE operator corresponds to an OR symbol with two values: The right value `k` corresponds to the minimal value involving the occurrence of the parent. `n` is included between 0 and the maximal value. The left value `n` corresponds to the maximal value involving the occurrence of the parent. `n` is included between `k` and the number of children. From the work area, you can click the display number to change its value.	For events that have three or more children

Important: During the installation of the app, the Administrator can define logical operators available in the app. Consequently, you may not have the access to all the operators of this table.

Symbol on Leaves

The leaves of fault tree elements correspond to the basic events or intermediate events. The following symbols can appear on a leaf of the fault tree:


Symbol	Name	Description
	Undeveloped event	It corresponds either to an event with: No associated failure mode, that is a gate. An associated failure mode without a formula. For the purpose of computation, a probability value of 1, that is no credit taken, is assigned to this event. A formula can be associated with the event to define the event as basic. You can hover over this symbol, if the Navigate option appears, you can click this symbol to navigate to the corresponding fault tree element.
	Basic event	It corresponds to an event, which points a failure mode with a reliability formula.
	Transfer	It corresponds to an intermediate event, and more precisely, to an instance of a fault tree element, that is a portion of a tree not yet visible. tree composed of several instances from other fault tree elements Hover over this symbol, if the Navigate option appears, you can click this symbol to navigate to the corresponding fault tree element.