Available fault current, short-circuit current rating, arc energy, arc flash hazards, and incident energy are closely related in both the National Electrical Code (NEC) and NFPA 70E, Standard for Electrical Safety in the Workplace. However, enforcement of the NEC and NFPA 70E are handled very differently.
The NEC is basically an electrical installation code, primarily dealing with de-energized installations, and enforced by the local authorities having jurisdiction (AHJ) or municipal or state inspectors. Once a system is energized, NFPA 70E requires employers and employees to comply with certain safety standards, and it is enforced by either local or federal Occupational Safety and Health Administration (OSHA) regulators. While the NEC historically recognizes and deals with the issue of available fault current and short-circuit current ratings, NFPA 70E also deals with available fault current as it applies to arc energy, arc flash, and incident energy. Care must be taken at the installation stage of electrical circuits and during the operational stage with the amount of fault current that is available where an arcing fault may occur.
A quick review of the pertinent definitions in the NEC and NFPA 70E provides a basis for the study of arc flash and arc energy reduction. For example, short-circuit current rating is defined as the prospective symmetrical (rms) fault current at a certain voltage to which an electrical apparatus or system can be connected without sustaining damage that exceeds a certain defined acceptance criteria.
Arc flash hazard is defined as a dangerous condition associated with the possible release of energy caused by an electrical arc (may also be called arcing fault) with temperatures that may reach 35,000°F and arc blast pressures in excess of 2,160 pounds per square foot at a person’s chest level.
Neither the NEC nor NFPA 70E define bolted fault, but, in effect, a bolted fault does not involve arcing and may occur where a conductor has been bolted or welded to a conductor of opposite polarity or to a grounded conductor or surface.
Incident energy is defined as the amount of energy impressed on a surface a certain distance from the source and generated during an electrical arc event. The unit used to measure incident energy is calories per centimeter squared and is an important value for determining the arc rating of personal protective equipment for protection against the high amount of heat involved in an electrical arcing event. Annex D of NFPA 70E provides three factors that affect incident energy: (1) the total protective device clearing time (the faster the device clears, the less time a worker would be exposed to the arc flash and the less energy that will be permitted within the arc); (2) the distance the worker is from the prospective arcing fault; and (3) the maximum fault current available at the point of the arc. The maximum bolted fault for a three-phase, short-circuit current available at the equipment and the minimum fault level at which the arc will self-sustain is critical in determining the incident energy level of the fault. The industry accepted minimum level for sustaining an arcing fault, based on a 480-volt system, is 38 percent of the available bolted fault.
Code-Making Panel (CMP) 10 has recognized the importance of arc energy and arc energy reduction by inserting text dealing with arc-energy reduction in 240.87 of the 2014 NEC. This section states “where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1,200 amps or higher, documentation must be available to those authorized to design, install, operate, or inspect the installation as to the location of the circuit breaker(s).”
In addition, there must be a method to reduce these breakers’ clearing times. CMP 10 requires one of the five following methods: (1) zone-selective interlocking (solid-state circuit breakers that electronically talk to each other during a faulted condition to shut the system down faster); (2) differential relaying (usually using current transformers for current sensing with connection to a circuit breaker or relay initiate faster shut down); (3) energy-reducing maintenance switching with local status indicator (a switch that effectively cuts the time delay on an instantaneous-trip breaker down to 100 percent during service work); (4) energy-reducing active arc-flash mitigation system (senses an arc flash and shuts the system down very quickly); (5) or approved equivalent means.
Any of these methods will effectively cut the arcing time down and lower the incident energy. However, remember that the arcing fault is generally 38 percent of the bolted fault level. Using the bolted fault value will erroneously increase the incident energy value and require a higher level of personal protective equipment.