Protective Device Coordination

Coordination of circuit breakers, fuses and protective relays is critical to a modern commercial, industrial or medical facility. In the event of an overload or fault, improper device settings can result in major system disruption by allowing a main circuit breaker to open instead of a downstream feeder device which is closer to the problem. In a case such as this, a whole switchboard can be de-energized when the problem should have been isolated to just the affected area.

The National Electrical Code(NFPA70) specifies coordination and short circuit protection in Articles 110-10 and 240-12:

110-10. Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit withstand ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway.

240-12. Electrical System Coordination.
Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted:
(1) Coordinated short-circuit protection.
(2) Overload indication based on monitoring systems or devices.
(FPN): Coordination is defined as properly localizing a fault condition to restrict outages to the equipment affected, accomplished by choice of selective fault-protective devices. The monitoring system may cause the condition to go to alarm, allowing corrective action or an orderly shutdown, thereby minimizing personnel hazard and equipment damage.