Designing and installing specialized electrical equipment with the intent of reducing the incident energy of an arcing fault was first introduced in 240.87 of the 2011 National Electrical Code (NEC) and was further refined in the 2014 NEC. The requirement’s purpose was to design an electrical system for a facility that would reduce the amount of arcing fault that an electrician or maintenance person would be subjected to while working on the electrical equipment in an energized condition. This requirement initially applied to any circuit breaker that did not have an instantaneous trip setting. Designers, installers, maintainers and inspectors would need documentation.
Subsequent editions of the NEC have provided modifications and refinements that extend into the 2017 NEC requirements. Based on the three editions of the NEC and the revised text each time, many electrical designers, ECs and electricians do not understand what is required and the various methods to achieve incident energy reduction. A brief history and explanation of each special protection technique and a new requirement in 240.67 for similar applications where fuses are used may help clarify the intent of these requirements.
In the 2011 NEC, the original text in 240.87, Noninstantaneous Trip, stated: “Where a circuit breaker is used without an instantaneous trip, documentation shall be available to those authorized to design, install, operate, or inspect the installation as to the location of the circuit breaker(s). Where a circuit breaker is utilized without an instantaneous trip, one of the following or approved equivalent means shall be provided: (1) zone-selective interlocking, (2) differential relaying, or (3) energy-reducing maintenance switching with local status indicator.”
An informational note explained that an energy-reducing maintenance switch permits a worker to set a circuit breaker trip unit to no intentional delay to reduce the clearing time while the person is within an arc flash boundary and then to set it back to its normal setting after the potentially hazardous work is complete.
In 240.87 of the 2014 NEC, the title was changed to “Arc Energy Reduction” and an introductory paragraph was added, stating, “Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted to 1,200 amperes or higher, 240.87(A) and (B) shall apply.”
Section 240.87(A), Documentation, contained the same requirement for documentation as located in 240.87 of the 2011 NEC. Section 240.87(B), “Method to Reduce Clearing Time,” contained the three original methods plus two new ones: “(3) Energy-reducing active arc flash mitigation system, and (4) Approved equivalent means.”
Another informational note stated the active arc flash mitigation system helps reduce the arcing duration in the electrical distribution system by sensing the arc flash very quickly and shutting the system down. Unlike the energy reducing maintenance switch, no change is required for the circuit breaker or the setting of another device. The arc-sensing device starts to shut down within milliseconds of an arc being detected. Something as simple as the light emitted from a camera flash could possibly shut the system down.
In the 2017 NEC, two more energy-reducing systems were added in 240.87(B) as follows: “(5) An instantaneous trip setting that is less than the available arcing current and (6) an instantaneous override that is less than the available arcing current.”
To understand these two new systems, a person must first understand that the utility company provides a bolted fault current value on the secondary side of their transformer, usually based on an infinite bus on the primary side of the transformer. This conservative (high side) fault current value is the bolted fault current value and, using this bolted fault value with a three-phase 480-volt system, the industry normal sustained arcing fault would be approximately 38 percent of the bolted fault. Since the instantaneous trip value of the circuit breaker would be exceeded on the trip current setting or the override setting, the circuit breaker trips much faster than normal and clears the fault much faster, thus reducing the incident energy of the arcing fault.
In addition, the 2017 NEC has new requirements effective Jan. 1, 2020, for arc energy reduction based on fuses rated at 1,200A or higher in 240.67. The same documentation in 240.67(A) is required as stated above for circuit breakers. In 240.67(B), there is a relaxation on the requirement of having a special energy-reducing system. Where a fuse is installed in an electrical service or feeder that has a clearing time of 0.07 seconds or less at the available arcing current, the special arc reduction system is not required.