Reduction Revisted: Arc energy reduction for electrical maintenance safety

Electrical Box with wires going in one side and coming out the other
Shutterstock / Medvedikov
Published On
Dec 15, 2020

Arc energy reduction is critical for working on high-energy electrical equipment. However, many electrical designers, contractors and inspectors don’t understand the issues surrounding energy reduction. Arc energy reduction starts on the design table and continues when the electrical equipment is energized. If any step of the process is not adequately followed, from design and installation based on the National Electrical Code through written safety procedures provided by NFPA 70E, electrical maintenance personnel may be subjected to high fault currents and high incident energy. The higher the available fault current and the longer the overcurrent protective device remains closed in the arc faulted circuit, the more likely the electrician may end up with third-degree burns.

Arc energy reduction of circuit breakers was first inserted into the 2011 NEC in 240.87, however, the text only applied to “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.”

The lead-in text to the section 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.”

An informational note below actually provided more valuable information than the mandatory text. It states: “An energy reducing maintenance switch allows a worker to set a circuit breaker trip unit to ‘no intentional delay’ to reduce the clearing time while the worker is working within an arc-flash boundary as defined in NFPA 70E, and then to set the trip unit back to a normal setting after the potentially hazardous work is complete.” Neither differential relaying nor zone-selective interlocking was explained by the informational note.

The 2014 NEC provided a better description for the title of 240.87, new text dealing with arc energy reduction, and the informational notes explaining the purpose of arc energy reduction. The new text stated, “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 1200 amperes or higher, 240.87(A) and (B) shall apply.” The term “Documentation” was inserted as the title for new 240.87(A) without any additional change to the documentation requirements from the 2011 NEC. “Method to Reduce Clearing Time,” 240.87(B), added “energy reducing active arc flash mitigation system” and “approved equivalent means” to the three other methods, zone-selective interlocking, differential relaying and energy-reducing maintenance switching with local status indicator, as provided in the 2011 NEC.

A new informational note explained arc flash mitigation as: “An energy-reducing active arc flash mitigation system helps in reducing arcing duration in the electrical distribution system. No change in the circuit breaker or the settings of other devices is required during maintenance when a person is working within an arc flash boundary as defined in NFPA 70E.”

A new 240.67 was inserted in the 2017 NEC dealing with fuses rated at 1,200A and higher for arc energy reduction. The new section read as follows: “240.67 Arc Energy Reduction. Where fuses rated 1,200A or higher are installed, 240.67(A) and (B) shall apply. This requirement shall become effective Jan. 1, 2020. (A) Documentation. Documentation shall be available to those authorized to design, install, operate, or inspect the installation as to the location of the fuses. (B) Method to Reduce Clearing Time. A fuse shall have a clearing time of 0.07 seconds or less at the available arcing current, or one of the following shall be provided: (1) Differential relaying, (2) Energy-reducing maintenance switching with local status indicator, (3) Energy-reducing active arc flash mitigation system, and (4) An approved equivalent means.”

We also added two additional methods of arc energy reduction for circuit breakers, new “(5) An instantaneous trip setting that is less than the available arcing current” and new “(6) An instantaneous override that is less than the available arcing current.” An informational note was added that explained “an instantaneous trip is a function that causes a circuit breaker to trip with no intentional delay when currents exceed the instantaneous trip setting or current level. If arcing currents are above the instantaneous trip level, the circuit breaker will trip in the minimum possible time.”

There was nothing added to restrict using the instantaneous trip setting of the circuit breaker to temporarily reduce the setting time of the instantaneous trip circuit breaker with a screw driver while working on the 1,200A and higher electrical equipment, rather than installing the maintenance switch. In the 2020 NEC, text added to 240.87(B)(5) states, “where dealing with an instantaneous trip setting, temporary adjustment of the instantaneous trip setting to achieve energy reduction shall not be permitted.” Too many people would forget to return the instantaneous setting back to the original setting after finishing their work on the equipment.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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