The electrical design industry has historically required electrical system circuit breaker selections and settings be validated with a short circuit and coordination study performed by a licensed engineer.
These studies assure that circuit breakers are capable of interrupting the available current and would operate “selectively.” Traditionally, “selectivity” in a low voltage electrical system meant that the long time and short time portions of time-current curves (TCCs) would be selective, i.e. the circuit breaker closest to the fault would trip first, maximizing the amount of the electrical distribution system left in service. In most cases, the circuit breaker instantaneous overcurrent (IOC) TCCs would not be selective “on paper,” as they typically overlap.
The 2005, 2008 and 20011 NEC extend the selectivity requirement to all possible fault types and magnitudes for certain critical electrical circuits, i.e., those typically fed from automatic transfer switches (ATS). These circuits and requirements are those discussed in the following NEC articles:
• Article 700: Emergency Systems (Legally Required), 700.28 Coordination
• Article 701: Legally Required Standby Systems, 701.17 Coordination (2011 – 701.28)
• Article 708: Critical Operations Power Systems, 708.54 Coordination
These requirements state that overcurrent protective devices (OCPD) must be “fully selective.” In other words, given the range of available interrupting currents, any given pair of overcurrent devices covered by the NEC Articles referenced above must behave in a coordinated fashion as defined in NEC Article 100:
“Coordination (Selective). Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.”
The NEC requirements are desirable design goals, given the adverse consequences of larger than necessary power outages within critical circuits. However, there are other design considerations that these requirements seem to preclude, i.e., arc flash and the specifics of phase and ground fault overcurrent coordination. The ability of fully selective designs to provide sufficient protection to such important, sensitive equipment as generators or automatic transfer switches may be affected.
Thursday, April 19, 2012 – 6:00pm to 8:00pm
IEEE Southeastern Michigan Power Chapter presents: Requirements for Selective Coordination of Low Voltage Circuit Breakers by Edwin Wickman, Specification Engineer, GE Industrial Systems
GE Training Location, 2 Towne Square, Southfield Michigan 48076 http://www.ge-ip.com/traininglocation/NA/directions/SF-Directions
RSVP with Kevin Taylor, taylor@ieee.org
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Source: IEEE Power & Energy Society
