Selective coordination has been a requirement in the National Electrical Code (NEC) for many years but has been limited to a few required applications. With the adoption of the 2005 NEC, selective coordination will apply to a much broader application with an emphasis on coordinating overcurrent protective devices for emergency systems, as well as the original primary issue of minimizing electrical hazards to equipment.
In the 2002 and previous editions of the NEC, the primary requirements for selective coordination and the definition were found in Article 240, covering overcurrent protection of conductors. Selective coordination was normally required where an orderly shutdown was necessary to minimize the hazards to personnel and equipment.
The definition in 240.2 read as follows: “Coordination: The proper localization of a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault-protective devices.”
This definition has been modified and moved to Article 100 in the 2005 NEC.
The main goal of selective coordination is to isolate an electrical fault to the closest overcurrent protective device upstream while maintaining power to the remainder of the electrical system.
For example, if a ground fault or a short circuit were to occur in a 20-ampere circuit, a properly coordinated system would ensure that only the 20-ampere overcurrent protective device protecting the branch circuit would open. The branch circuit would be protected while maintaining power to the other branch circuits, feeders and service.
If a fault occurred at the feeder level, only the feeder overcurrent protective device would open, leaving other feeders and the service energized. This ensures power loss to only the circuit directly affected by the fault.
A short circuit or a ground fault will cause an overcurrent protective device, such as a circuit breaker or a fuse, to react in a certain amount of time, based upon the amount of fault current flowing in the faulted circuit. These are called the “time/current characteristics” of the circuit protective device.
When a fault occurs, impedance in the circuit decreases, causing the amperage in the circuit to increase. The amount of fault current in the circuit will often depend on the amount of arcing that occurs, since this arcing results in higher impedance in the faulted circuit. The higher the fault current and the longer fault current can flow in the circuit before the overcurrent protective device opens, the more damage can occur to the equipment.
The reaction time of the fuse or circuit breaker is critical in determining which overcurrent protective device reacts to the fault first, the one closest upstream to the fault or additional devices upstream. Based upon the amount of available fault current for the circuit, fuses and circuit breakers can be placed in the circuit and coordinated to ensure selective coordination.
To determine selective coordination of circuit breakers, a series of overlays can be used to compare each circuit breaker with its particular time/current characteristics to the upstream circuit breaker, based upon the available fault current in the circuit. The overlays will show the point at which each circuit breaker will start to open and permit coordination of the response of the breakers to fault currents. Similar studies can be done for fuses.
In the 2005 NEC, selective coordination is now required for emergency systems in accordance with 700.27 and for legally required standby systems in accordance with 701.18.
Section 700.27 and 701.18 both state the requirements as follows: “Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.”
This addition in the 2005 NEC involves selectively coordinating the overcurrent protective devices in an emergency system or a legally required standby system to ensure that power is not lost for the entire system. This is intended to ensure that a fault in these critical circuits will only affect that circuit.
Selectively coordinating circuit breakers, or circuit breakers and fuses, can be a very complex and difficult task. Eliminating the loss of power to life safety equipment is the ultimate goal of this particular addition to Article 700 for emergency systems and Article 701 for legally required standby systems.
Providing this coordination study would be the responsibility of the electrical engineer of record for new construction but new branch circuits and feeders installed in existing facilities often do not have an engineer of record making a coordination study.
Where selective coordination is mandated, the only recourse is to apply either fuse coordination studies, as supplied by the fuse manufacturers, or the circuit breaker overlays as supplied by the breaker manufacturer. At the very least, both electrical engineers and electrical contractors must become familiar with selective coordination since more of this will be seen in the future. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at email@example.com.