New NEC Could Change Emergency Power Distribution System Design

08/01/2005


The new 2005 National Electrical Code (NEC) has been out and in circulation and is close to being adopted in the Northwest. Based on conversations with local code officials in the Seattle area, it is my understanding that the 2002 NEC will no longer be utilized and the 2005 NEC will take effect around September. There are some changes brewing in the 2005 NEC that could significantly transform the way emergency distribution systems are designed and built in the near future, and I think it is important to look at these changes, as well as some new definitions.

NEC 100 Definitions: 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 and settings.

NEC 700.27: Coordination - Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

NEC 701.18: Coordination - Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

NEC 517.26: Application of Other Articles - The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.

For clarity it is important to include the NEC definition (Section 100) of overcurrent and the fine print notes defining "Emergency Systems and Legally Required Standby Loads" in Sections 700 and 701 below:

Overcurrent: Any current in excess of the rated current of the equipment or ampacity of the conductor. It may result from overload, short circuit or ground fault.

It is significant that a "short circuit" is noted as one of the items that can cause an overcurrent. The typical molded-case circuit breaker combination, with the upstream breaker somewhat larger than the downstream breaker, does not have a problem coordinating in the overload area of the time-current curve, but a high level of current in the short circuit area of the time-current curve can represent significant problems to selective coordination.

NEC Section 700.1 FPN no. 3: Emergency systems are generally installed in places of assembly where artificial illumination is required for safe exit and panic control... Emergency systems may also provide power for such functions as ventilation..., fire detection and alarm systems, elevators, fire pumps, public safety communication and industrial processes...

NEC Section 701.2 FPN: Legally required standby systems are typically installed to serve loads, such as heating and refrigeration systems, communication systems, sewage disposal, lighting systems and industrial processes, that, when stopped during any interruption of normal electrical supply, could create hazard or hamper rescue or firefighter operations.

Elevators are noted in Section 700.1 as an emergency system load. Some jurisdictions in our area also consider elevators to be a legally required standby load. In either case, the NEC, for some time, has required elevators in certain situations to be selectively coordinated. This selective coordination has been required per NEC 620%%MDASSML%%62.

NEC 620.62: Selective Coordination - Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective device in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.

The coordination of the overcurrent protective device protecting a feeder serving a multi-position elevator control panel can be achieved with an upstream breaker feeding downstream fuses. Fig. 1 (p. 24) is an example of an overcurrent protection coordination study illustrating the feeder breaker overcurrent protection, the elevator fuse overcurrent protection and the elevator motor start-up curves. The graph indicates a 200-amp breaker in the main distribution gear feeding an elevator control panel with a 100-amp dual-element RK5 fuse and a 70-amp dual-element RK5 fuse. The protective coordination study must ensure that the two fuses will trip in a fault condition in any one of the separate elevator feeders and will not trip the 200-amp main breaker. A fault in one of the elevator feeders that took out the main breaker would essentially take out both elevators from the electrical distribution system.

As you can see from Fig. 1, there is no overlap between the fuses and the breaker. This selective coordination can be achieved because the breaker is upstream from the fuses; the "blob" on the lower-right corner of the breaker curve represents a mechanical device opening to clear the fault. The bottom part of the blob is the time at which the contacts on the breaker unlatch, arcing occurs and the current continues to flow in the breaker until there is physical separation of the contacts and the arc is extinguished. This coordination is not so simple if the breaker is on the left (downstream side) of the fuses or if the system utilizes all thermal magnetic breakers for the overcurrent protective devices.

To illustrate the difficulty in selectively coordinating thermal magnetic breakers, Fig. 2 (p. 24) provides an example of two breakers—a 100-amp and a 400-amp molded-case circuit breaker—that are coordinated in the overload region but would not be considered to be selectively coordinated in the short-circuit region as per the definition of overload in the 2005 NEC. The typical emergency distribution system utilizes breakers downstream from breakers. In Fig. 2, if the available fault current was above 3,000%%MDASSML%%4,000 amps downstream from the 100-amp breaker, the electrical distribution system would not satisfy the wording of the revisions to the 2005 NEC. A 3,000%%MDASSML%%4,000-amp fault current would probably unlatch the 100-amp breaker before the 400-amp breaker, but before the 100-amp breaker extinguishes the arc and completely opens the circuit, the 400-amp breaker would start to unlatch, thus—in this situation—taking out both breakers in the emergency electrical distribution system.

The role of fuses

In addition, for complete selective coordination, series rating would no longer be allowed. Series rating requires that both the downstream and upstream breakers trip to reduce the available fault current rating at the downstream device to lower levels. The combination of overcurrent protective devices must be tested together to verify performance based on UL 489. The rating of the combination cannot exceed the rating of the upstream overcurrent protective device.

On the other hand, fuses can be completely coordinated. The melting time of a fuse is significantly faster than the mechanical operation of a breaker and will allow the downstream device to clear the fault without causing the upstream device to open, thereby exposing the smallest amount of the electrical distribution system as possible to an outage. Fig. 3 shows a 400-amp dual-element fuse and a 100-amp dual-element fuse.

Furthermore, larger, more expensive electronic power breakers can be programmed to remove the instantaneous portion of the breaker curve. This can allow the two breakers to be fully selectively coordinated. Fig. 4 illustrates a 2,000-amp main breaker and an 800-amp sub-breaker. The instantaneous portion of the main breaker has been turned off to allow for complete selective coordination between the two breakers. The sub-breaker should trip any fault downstream without tripping the main breaker. In this case, the 2,000-amp breaker will not trip for approximately 0.2 to 0.3 seconds or about 12 to 18 cycles, and switchgear construction would have to be utilized. Switchboard construction is only rated to handle its rated fault current for three cycles.

More on switchgear and switchboards in next month's installment of Codes and Standards.

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