A spare event can be an input to only a spare gate or a functional dependency gate. Because both of these gates are dynamic gates, you can insert them and spare events only in time dependent fault trees.

While spare events are similar to basic events in functionality, they allow only failure rates as inputs to accurately model temporal behavior. The dormancy factor of the spare event indicates the ratio of the failure rate in the standby mode and the failure rate in the operational mode.

A spare event can have a spares pool, which represents the number of identical instances of that spare component (event). For example, if the spares pool of an event is two, there are two identical spare components of that spare event.

For an example of how spare events are used with spare gates, see Spare Gate.