Protective Device Coordination

An electrical power system is designed so that if a short-circuit occurs, a protective device (such as a fuse or circuit breaker) will operate to open the circuit and prevent the continued flow of electrical energy to the faulted area. To minimize interruption of electrical service to other areas of the power system, the system is also designed so that the protective device closest to the short-circuit operates first to clear the fault.

See the figures below for the Time Current Curves (TCC): in figure 1 since the curves overlap they are not coordinated. In figure 2 there is no overlap and the devices are coordinated.