Both the National Electrical Code (NEC) and NFPA 70E, Standard for Electrical Safety in the Workplace, use the phrase “arcing current.” It is located in 240.87(B)(5) and (B)(6) in the NEC and in many places within NFPA 70E. The phrase “bolted fault current” is used in various locations within NFPA 70E, such as in Annex D.3, explaining the calculation of incident energy, but a search for the phrase in the NEC does not show any results. Why would both phrases be used in NFPA 70E but only “arcing current” show up in the NEC? Is there a major difference between the two phrases? The differences surrounding these important phrases must be studied to understand how to address these issues for both NFPA 70E and the NEC.
Definitions of fault current and related terms are extremely important in understanding the overall usage of these two phrases. According to Wikipedia, fault current is any abnormal electric current, such as a short circuit where the current bypasses the normal load and can involve one or more phases and ground or may be phase to phase. A prospective short-circuit current of a predictable fault can be calculated for most situations. A bolted fault is an extreme fault where the fault has zero impedance, thus giving the maximum prospective short-circuit current in the faulted circuit. A symmetrical fault is a balanced fault that affects all three phases equally, as opposed to an asymmetrical fault that does not.
Both the NEC and NFPA 70E define short-circuit current rating as “the prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.” The NEC and NFPA 70E define interrupting rating as “the highest current at rated voltage that a device is identified to interrupt under standard test conditions.”
So, assume the utility company provides power to a service for a building using a pad- or pole-mounted transformer. Depending upon the impedance of the transformer, the primary voltage and the secondary voltage, the maximum available fault current can be determined at the secondary side of the utility transformer. Usually, this information is available from the utility company or can be calculated. This maximum available fault current is the bolted fault value that could be delivered to a fault that occurs at the point of connection at the secondary of the transformer.
Depending upon the size, type (copper or aluminum) and length (from the transformer secondary to the point of connection at the service to the building) of the conductors, the original maximum available (bolted) fault current will be somewhat less than the original value at the transformer. The service equipment manufacturer builds the equipment, as the electrical contractor orders, to withstand the amount of available fault current at the point of connection of the utility company conductors. Overcurrent protective devices provided in the service equipment are designed to withstand the amount of symmetrical and asymmetrical current in the bolted fault and will be able to interrupt the current at that value. This is commonly called the interrupting current of the device and is marked on the overcurrent device [unless the circuit breaker is a 5,000 AIC breaker based on 240.83(C)] as the ampere-interrupting-current (AIC) rating of the overcurrent device. Section 110.9 states equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the equipment line terminals. This current is normally the maximum (bolted) short-circuit current available, as used in the NEC.
In Annex D.3.1 covering the Doughty Neal Paper in NFPA 70E, the parameters for calculating incident energy on electrical systems, rated at 600 volts (V) and below, require (1) the maximum bolted fault, three-phase short-circuit current available at the equipment and the minimum fault level at which an arcing current will self-sustain, (2) the total protective device clearing time upstream of the prospective arc location at both the bolted fault and the arcing fault, and (3) the distance the worker will be from the prospective arc.
The Doughty Neal Paper states, for a 480V system, the electrical-industry-accepted value for a self-sustaining arcing fault is about 38 percent of the bolted fault current of the system. The use of arcing fault and maximum bolted short-circuit current should be clearer now.
About The Author
ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected].