Start at the Beginning

By Jan 15, 2009
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In the last few editions of the National Electrical Code (NEC), more electrical equipment has been required to be rated for short-circuit current. How many people in the electrical industry really understand the short-circuit current definition added to Article 100, the requirement for short-circuit current ratings for motor controllers in 430.8 and 440.4(B), and the short-circuit current rating for industrial control panels in 409.110?

Although many of these requirements were added in either the 2005 or the 2008 NEC, short-circuit rating requirements have been in the NEC for years without many in the electrical industry truly understanding them. With the 2011 NEC process just starting, it may be time to analyze and remove the mystique of short-circuit current rating of electrical equipment.

The best place to start understanding this difficult issue is to analyze the definition provided in Article 100 for “short-circuit current rating” as “the prospective symmetrical fault current, at a nominal voltage, that an electrical apparatus or system can be connected without sustaining damage exceeding a defined acceptance criteria.”

This definition applies to both electrical apparatus or equipment and electrical circuits operating within the rated voltage of the circuit and being able to withstand short-circuit current without sustaining extensive damage to the circuit or the equipment. In other words, the electrical overcurrent--protective devices must react fast enough to not allow damage to the conductors, the circuit controllers, electrical utilization equipment, and other components in the circuit. In addition, the overcurrent protective device also must be able to safely interrupt the fault current available on the line terminals of the device without sustaining substantial external damage to the circuit breaker or fuse.

Determining the available short-circuit current for the system usually starts by contacting the electrical utility company to provide a site or phone assessment based on the voltage of the primary side of the utility company transformer, the impedance of the transformer, and the secondary fault current available at the line terminals of the service equipment, if possible. A fault current study of the entire electrical system of the facility, based on the impedance of the conductors for each feeder, would permit determination of the available fault current at each feeder panelboard. Once the level of fault current is calculated at each panelboard, compliance with Section 110.9 and 110.10 can be determined. Overcurrent protection can then be selected for the electrical components of the system.

Section 110.9 requires equipment, such as circuit breakers and fuses, intended to open under fault conditions, to be rated for fault currents at the line side of the devices. For example, an available fault current of 50,000 amps requires the overcurrent protective devices as well as the panelboards and switchboards containing the overcurrent protective devices to be rated at least 50,000 amps. Any electrical equipment, such as switchboards, panelboards and similar equipment, must be braced for that fault current at the rated voltage.

Any electrical components in the circuit subjected to fault current must be able to withstand the available fault current or be provided with current-limiting overcurrent protection. Based on 110.10, the overcurrent protective devices protecting the circuits, the total impedance of the circuit, the short-circuit current ratings of the components in the circuit, and other electrical characteristics of the circuit must be carefully selected and coordinated, so the protective devices can clear the fault without extensive damage to these components. The protective devices must open fast enough to limit the amount of fault current that may adversely affect any conductors, controllers, switches, receptacles and similar components within the circuit.

Many listed components and equipment are tested for an applied fault current and, where used appropriately within the listing requirements, will be able to withstand the fault current of the system. Use of current-limiting devices will provide fast acting protection for the circuit and are designed to operate within the first half cycle of the fault, thereby limiting the let-thru current and protecting the downstream components of the circuits.

Current-limiting overcurrent protective devices, defined in 240.2, are devices that reduce the current flowing in a faulted circuit to a magnitude substantially less than would be if the overcurrent device was replaced a solid conductor of the same impedance as the overcurrent device. Current-limiting devices must be applied within the current-limiting range of the device. Installed in a circuit where the available fault current is at a much lower value than the current-limiting range of the device, the current-limiting device will not operate as quickly, permitting more arcing with more damage to the components in the circuit.

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 protected].

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