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My previous post, “Selective Coordination and the NEC: Defining Terms and Understanding Updates,” gave a general overview of selective coordination requirement changes in the 2014 edition of the National Electrical Code® (NEC®). In this post, I’m going to delve more deeply into specifics important to designers working on health care projects. Updates in the 2011 and 2014 editions of the NEC address some challenges raised by language in prior editions, and these changes could enhance the ability to achieve coordination and make engineers’ work a little easier.
The 2011 NEC introduced two major changes to Section 517.17 regarding ground-fault protection of equipment (GFPE). The first of these was in subsection (B), in which the previous prohibition against a second level of GFPE on the generator feeder was deleted. This change should help designers meet the basic goal of coordination, which is to have the overcurrent device closest to a fault be the one to clear the fault. Should a ground-fault in a feeder occur when power is being derived from an alternate source, it would be far better for the feeder circuit breaker to clear the fault than for the alternate source main circuit breaker to do so. The 2011 update enables such designs, as follows:
(B) Feeders. Where ground-fault protection is provided for operation of the service disconnecting means or feeder disconnecting means as specified by 230.95 or 215.10, an additional step of ground-fault protection shall be provided in all next level feeder disconnecting means downstream toward the load. Such protection shall consist of overcurrent devices and current transformers or other equivalent protective equipment that shall cause the feeder disconnecting means to open.
The additional levels of ground-fault protection shall not be installed on the load side of an essential electrical system transfer switch.
The second change, also a deletion, is in subsection (C), in which the old requirement for 6 cycles of separation between the two levels of GFPE was removed. Today, GFPE is available in the trip units of low-voltage circuit breakers, and their time-current curves (TCCs) reflect the total clearing time for ground-faults. As a result, it’s no longer necessary to add 6 cycles of “white space” between the curves to account for the clearing time of the downstream circuit breaker. That added “white space” often caused the ground-fault TCC of the feeder circuit breaker to overlap the TCCs of downstream overcurrent devices, resulting in a loss of coordination, so this change really helps design better coordinated systems. This subsection now reads:
(C) Selectivity. Ground-fault protection for operation of the service and feeder disconnecting means shall be fully selective such that the feeder device, but not the service device, shall open on ground faults on the load side of the feeder device. Separation of ground-fault protection time-current characteristics shall conform to manufacturer’s recommendations and shall consider all required tolerances and disconnect operating time to achieve 100 percent selectivity.
The 2005 NEC added a new Section 517.26, mandating that the essential electrical system (EES) in a health care facility meet the requirements outlined in Article 700, including the requirement for selective coordination in Section 700.27. This change had several costly impacts, given that the life safety, critical and equipment branches of the EES might account for two thirds to three quarters of the load in a health care facility. Some engineers estimated a 10 percent hike in the cost of hospital electrical systems, along with a need for larger electrical rooms to accommodate related selectively coordinated equipment, was the result. Additionally, large facilities – especially those who used their emergency generation capacity for peak shaving – faced particular challenges in applying the new 700.27 requirement because the prospective fault current from those generators sometimes exceeded the fault current from the utility. Selective coordination becomes harder to achieve with such high levels of prospective fault current.
The 2014 NEC addressed this issue, and now only the life safety branch of the EES must meet the requirements of Article 700. The section now reads as follows:
517.26 Application of Other Articles. The life safety branch of the essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.
Better correlation with NFPA 99
The final change in the 2014 edition of the NEC correlates the installation requirements in the NEC with the performance requirements described in the National Fire Protection Association’s NFPA 99, the Health Care Facilities Code. This change involved adding a new subsection 517.30(G).
(G) Coordination. Overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.
Exception No. 1: Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary.
Exception No. 2: Between overcurrent protective devices of the same size (ampere rating) in series.
Informational Note: The terms coordination and coordinated as used in this section do not cover the full range of overcurrent conditions.
Finally, I’d like to offer two broad tips to keep in mind when designing electrical systems that will meet the NEC requirements for health care facilities:
- Overcurrent protective devices need to be coordinated for all types of overcurrents; overloads, short circuits and ground-faults. Regarding ground-faults, it is not just the two levels of GFPE that need to coordinate with one another. The feeder GFPE needs to coordinate with the downstream phase overcurrent protective devices as well so that it is those devices that open should a ground-fault occur on their load side.
- The NEC and NFPA 99 establish minimum requirements. They do not prohibit the design of systems that coordinate to a higher level than 0.1 sec.
My next post will address challenges in meeting the NEC selective coordination requirements. In the meantime, for more information on selective coordination and the NEC, check out Schneider Electric’s Codes and Standards Technical Library on the topic. And, as always, should you have questions regarding selective coordination in any current or future project, please visit Schneider Electric’s dedicated consulting engineer portal for a wide range of resources, including access to one-on-one support from expert staff engineers.