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Fault Tree

Fault tree shows the structure of the system failures in a graphical format. It contains events that are describing failures. An unwanted event is set as the top node in the tree structure and this is called the TOP event. Factors that affect the top event are typically component failures and human errors, but the versatile method itself allows taking into account also other types of events. The event realization is defined with the gate condition and the probability condition of the event. Gate condition includes the inputs, i.e. the other events in the fault tree, that affect the realization of the event. In the picture below the gate condition is defined with the values a and b. The gate condition will be realized if at least a and at the most b inputs will realize.

Fault tree gate conditions

The probability condition of the event is the probability value for the event to realize if the gate condition is already realized. The event will realize if the gate condition and the probability condition both realize. A fault tree event can be represented as a vector:

ID is the identification tag of the event
a is the at least -value of the gate condition
b is the at most -value of the gate condition
P is the probability for the event to realize when the gate condition is valid
in is the tag of the event input n

Theoretical example of a fault tree:

Fault tree is a versatile and effective method for examining and documenting system failures. Without a proper tool the drawing process of a fault tree is very time consuming. The documentation of the failure logic is an iterative process that tries to achieve as exact end result as possible without losing any of the overall clarity. With the help of a fault tree tool the model can be built quickly and it is easily editable. A good fault tree tool has to offer multiple supporting methods for documentation and analysis. The picture below shows an example of a fault tree drawed with the ELMAS fault tree tool. ELMAS is a versatile fault tree tool with powerful analysis qualities.

ELMAS: Fault tree example