Fault Tree Block Types

This section is intended to help you identify the basic types of blocks found in fault trees.

Note: The shapes, colors and locations shown here represent the default settings. For information on modifying appearance settings, see Diagram Skins and Appearance Settings.

	AND Gate	In an AND gate, the output event occurs if all input events occur.
	OR Gate	In an OR gate, the output event occurs if at least one of the input events occurs.
	NOT Gate	In a NOT gate, the output event occurs if the input event does not occur, and vice versa. This gate performs inversion. NOT gates are available only in analytical fault trees.
	NAND Gate	In a NAND gate, the output event occurs if any one of the input events does not occur. NAND gates are available only in analytical fault trees.
	NOR Gate	In a NOR gate, the output event occurs only if none of the input events occur. NOR gates are available only in analytical fault trees.
	Voting Gate	In a Voting OR gate, the output event occurs if some quantity (k) or more of the (n) input events occur. The voting gate is visually distinguished from the OR gate by diagonal lines and the k/n value displayed on it.
	Inhibit Gate	In an Inhibit gate, the output event occurs if all input events occur and an additional conditional event occurs.
	Standby Gate	A standby gate allows you to identify events that operate in a standby configuration. Events that are below a standby gate are dependent events, which describe the characteristics of the items that operate together in the standby configuration.
	Load Sharing Gate	A load sharing gate allows you to identify events that operate in a load sharing configuration. Events that are below a load sharing gate are dependent events, which describe the characteristics of the items that operate together in the load sharing configuration.
	Priority AND Gate	In a priority AND (PAND) gate, the output event occurs if all input events occur in a specific sequence.
	Sequence Enforcing Gate	In a sequence enforcing (SEQ) gate, events are constrained to occur in a specific sequence and the output event occurs if all input events occur in that specified sequence.
	Basic Event	Events are simply representations of some occurrence, typically a failure or error. Different shapes are used to represent different types of events. These shapes convey additional visual information for the analysis but all event types are handled in the same way during analysis or simulation. For information on how these event types are often used, see the topic on fault tree events.
	Undeveloped Event
	Trigger Event
	Resultant Event
	Conditional Event
	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.

For event blocks, there are several status indicators that may appear on the block. Although each status is presented individually for demonstration purposes, a single block can be defined with more than one type of status.

	Inactive Block	An X through the block indicates that the block is inactive for reliability calculations (i.e., set to "off"). This is done by selecting the Set block as failed option in the Block Properties window or by selecting the block in the diagram and choosing [Diagram/Fault Tree] > Settings > Set Block as Failed. This option can be used for "what-if" analyses to investigate the impact of a block on system metrics such as reliability, availability, throughput, etc.
	Block Belongs to a Maintenance Group	A red circle in the upper left corner indicates that the block belongs to a maintenance group. A maintenance group is a set of blocks where some event within the group can trigger either maintenance or state changes for one or more blocks, either within the group or outside of it. Clicking the indicator opens the Maintenance Group Manager. You can change the size of the indicator via the relevant Block Corner Indicators page of the Diagram Style window. You can change the color used for each maintenance group via the Maintenance Group Manager.
	Mirrored Block	A gray box in the lower left corner indicates that the block is a mirrored block. The mirror group to which it belongs is displayed beside the box. All blocks in the mirror group represent the same component in more than one location. Clicking the indicator opens the Mirror Group Manager. You can change the size of the indicator and the appearance of the text via the relevant Block Corner Indicators pages of the Diagram Style window. You can change the color used for each mirror group via the Mirror Group Manager.
	Block Has State Change Triggers Enabled	A blue diamond in the lower right corner indicates that the block has state change triggers enabled. State change triggers allow you to specify events that will activate and/or deactivate the block during simulation. You can change the size and color of the indicator via the relevant Block Corner Indicators page of the Diagram Style window.