The Functional Dependency (FDEP) gate is used to indicate that all dependent basic events are forced to occur whenever the trigger event occurs. The separate occurrence of any of the dependent basic events has no effect on the trigger event. The FDEP gate has one trigger event and can have one or more dependent events. All dependent events are either basic events or spare events. The trigger event can be a terminal event or output of any AND gate, OR gate or dynamic gate, which includes the SPARE gate, PAND gate, Sequence-Enforcing gate (SEQ) and FDEP gate.

Dependent events are repeated events that are present in other parts of the fault tree. The FDEP gate is a dynamic gate, which means the temporal order of the occurrence of events is important to analyse this gate. Generally, the output of the FDEP gate is not that important; however, it is equivalent to the status of its trigger event.

The FDEP gate can also be used to set the priorities for SPARE gates. For example, if multiple spares are connected to a FDEP gate, after the occurrence of the trigger event, all spares that are connected to the FDEP gate will fail. Upon failure of these spares, the next available good spares in those SPARE gates will replace the failed spares. If there exists a conflict in choosing the next available spare between multiple SPARE gates, the priority will be based on the order of the connection of these spares in the FDEP gate from left to right.