AT THE MOMENT, NFPA codes aren’t quite in agreement on the idea of "selective coordination" of overcurrent devices. But at least the interested parties have agreed to disagree.
In the NEC ® , selective coordination is used to increase the reliability of the overall electrical distribution system. It is a design and application technique that coordinates protective devices, such as fuses or circuit breakers, so that an overcurrent situation, such as an electrical overload or short-circuit, is limited to the smallest possible section of the circuit. Ideally, this localization of the problem allows upstream overcurrent devices to remain online while power is maintained to all other loads supplied by that system. Requirements covering selective coordination are limited to specific types of equipment (including elevators, critical operations data systems, and fire pumps) and systems (including emergency, legally required standby, and critical operations power). The only occupancy type where selective coordination is expressly required is health care facilities.
This is where the disagreement comes in. Hospitals and other health care facilities are subject to the provisions of NFPA 99, Health Care Facilities, which means the responsibility for developing requirements for the performance of electrical systems is assigned to the NFPA 99 Technical Committee on Electrical Systems. The committee addressed the issue as part of its discussion of the 2012 edition of NFPA 99 and took a comparatively limited approach to electrical system coordination. The committee said that selective coordination is not the only factor to be considered in the selection of overcurrent protective devices, and it limited the level of system coordination to only electrical circuit or equipment problems lasting longer than 0.1 seconds — six electrical cycles in a 60-hertz system. This requirement, first specified in the 2012 edition of NFPA 99, has been extracted into Section 517.30(G) of the 2014 NEC — a change from the 2011 NEC, which required a fully selective system for health care facilities.
The change has produced sharp disagreement among some in the NEC community, who maintain that health care facilities should also include full selective coordination for emergency power systems, which has been part of the NEC since 2005. They argue that, in the electrical world, a tenth of a second can be an eternity — fast-acting, current-limiting fuses and circuit breakers can react to an overcurrent condition in a fraction of just one electrical cycle — and that designing systems to handle problems lasting six cycles or longer will not provide sufficient system protection. Under certain conditions, they say, localization won’t occur because the overcurrent protection devices won’t respond quickly enough to work in a coordinated manner, increasing the risk of a more widespread power outage.
Some of the strongest objections to the limited level of coordination came from members of the NEC community. However, NFPA’s Standards Council made it clear that, while NEC Code-Making Panel 15 is responsible for installation requirements in health care facilities, electrical system performance requirements are the responsibility of the NFPA 99 Technical Committee on Electrical Systems.
So the new reality is that the NEC contains different requirements for electrical system coordination depending on occupancy type; the emergency power system in a high-rise office tower is required to have full selective coordination, while the hospital next door is only required to have limited coordination. The good news is that, with the advent of mandatory requirements on selective coordination, the industry is responding with new and better ways to accomplish the end result of a more reliable electrical system.
Jeffrey Sargent is a regional electrical code specialist for NFPA.