A spare event is used to specify spares in dynamic fault trees. Spare events are similar to basic events in functionality; however, they allow only rates as inputs. The dormancy factor of the spare indicates the ratio of failure rate in the spare mode and the failure rate in the operational mode. Spare events can have a spares pool, which represents the number of identical instances of that event. For example, if a spares pool of an event is two, there are two identical spare components of that spare event. Spare events are restricted to use as either spares to SPARE gates or as dependent events to Functional Dependency gates.