Fault Tree Block Types

Visual Indicator

Block Type

Description

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 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 blocks 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 blocks 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.