Fault Tree Events

In traditional fault trees, different shapes are used to represent different types of events. These shapes convey additional visual information for the analysis. However, unlike gates, each type of event in a fault tree is treated the same way analytically. Thus, BlockSim does not distinguish between event types during analysis or simulation; the types are purely informational.

In BlockSim, all events can be assigned properties via URDs, just like blocks in reliability block diagrams. This is an enhancement to traditional fault tree events, which generally include only a fixed probability of occurrence and/or a constant failure rate.

BlockSim provides the following event blocks to fit the conventions that are followed in most traditional fault trees. It should be noted that the analysis method is the same for all events; therefore, if you wish to include additional event shapes that are not available, you can use the Block Style window to change the appearance of a block to illustrate a different type of event.

	Basic Event	A basic event (or failure event) is identical to a standard block in a reliability block diagram. Traditionally, a basic event is represented by a circle.
	Undeveloped Event	The diamond representation indicates that the event could have been developed further (i.e., expanded into a separate fault tree), but that it was chosen not to do so for the analysis.
	Trigger Event	A trigger event is an event that can be set to occur or not occur (i.e., it usually has a fixed probability of 0 or 1). It is usually used to turn paths on or off or to make paths of a tree functional or non-functional. Furthermore, the terms failed house and working house have been used to signify probabilities of 0 and 1, respectively. In BlockSim, a trigger event has the same properties as a basic event. To use the block to turn the path off, select the Set block as failed option in the Block Properties window. You can also set this option by selecting the block in the diagram and choosing Fault Tree > Settings > Set Block as Failed. To use the block to turn the path on, make sure no URD is assigned; this means that the block cannot fail.
	Resultant Event	A resultant event is represented by a square and specifies a result in a fault tree diagram.
	Conditional Event	A conditional event is represented by a rounded square and specifies a condition in a fault tree diagram (i.e., an event, possibly external to the configuration represented by the fault tree, that must take place for the gate to succeed). It can be applied to any gate. For example, this symbol would be appropriate to use for the event that is specified in the Conditional Event drop-down list in the Block Properties window for an inhibit gate.
	Subdiagram	A block that is in the shape of a folder indicates that the block represents an existing diagram. Subdiagrams can be used in fault trees in the same way they are used in RBDs.

All events have the same properties available for configuration, with the exception of dependent events. In addition to the common block properties, you will need to describe how the event behaves. These properties are the same as those available for configuring standard blocks in RBDs except that you cannot define throughput properties, as throughput is not calculated in fault trees.

An event can have only one output event, and no input events or gates.