The SPARE gate is used to model the behaviour of spares in the system. The SPARE gate is used to indicate that the output occurs if and only if all input spare events occur. All inputs of a SPARE gate are spare events. A SPARE gate can have multiple inputs. The first event (left-most event) is known as the primary input, and all other inputs are known as alternative inputs. The primary event is the one that is initially powered on, and the alternative inputs and are initially in standby mode.

After a failure, the active/powered unit that is the first available spare from left to right will be chosen to be active. If all units are failed, then the spare will be considered as failed (output occurred). Depending on the dormancy factor of spares, spares can fail even in standby mode.

If the dormancy factor of all spares connected to a SPARE gate are 0, then the spare acts like a cold spare. If the dormancy factor of all spares connected to a SPARE gate is 1, then the spare acts like a hot spare. If the dormancy factor of all spares connected to a SPARE gate are the same (and are between 0 and 1), then the spare acts like a warm spare. If the dormancy factors of its inputs are different, then it handles generalised situations. The SPARE gate is a dynamic gate, which means the temporal order of the occurrence of events is important to analyse this gate.