Crossover Arcs: NEC And 70E

NFPA 70, the National Electrical Code (NEC), is an installation code, while NFPA 70E is the Standard for Electrical Safety in the Workplace. There is an unofficial line of demarcation between the two documents. The NEC is used for design, installation and inspection of the electrical installation before the system is energized. NFPA 70E provides practical and safe working guidelines for employees relating to the hazards from the use of electricity—in other words, after the system is energized. 

With this demarcation in mind, design applications have been added to the last few NEC editions, and the additions deal with safety issues where the direct results of these changes benefit the electrical safety for people working on these systems after being energized. Discussion and understanding of these design applications can help apply safety to future building installations.

Text crossover between the NEC and NFPA 70E first appeared in 110.16 of the 2002 NEC with mandatory language specifically requiring switchboards, panelboards, industrial control panels and motor control centers—in other than dwelling occupancies—to be field-marked to warn qualified people of potential arc flash hazards. The fine print note located directly below that Code requirement stated that NFPA 70E provides assistance in determining the severity of exposure to an arc flash event.

The 2011 NEC added 110.24, which requires service equipment in other than dwelling units to be legibly marked in the field with the maximum available fault current. This marking must include the date when the calculation was performed. Where modifications to the installation might affect the available fault current, the fault current must be verified or recalculated and the equipment marked again as necessary. 

There is an exception to the field-marking requirement for industrial installations where conditions of maintenance and supervision ensure that only qualified people service the equipment. This marking provides the maximum bolted fault current at the service. It is the basis for the incident-energy calculation for Table 130.7(C)(15) of the 2012 NFPA 70E. The calculation determines the hazard-risk category of the personal protective equipment service that personnel must wear.

The 2011 NEC also added 240.87, which covers noninstantaneous trip circuit breakers. Where a circuit breaker is used without an instantaneous trip, the NEC requires documentation that indicates the location of the circuit breaker for personnel authorized to design, install, operate or inspect the installation. This documentation permits the user to locate the breaker and determine if it has zone-selective interlocking (internal electronic communication between breakers to quickly shut the system down in an arcing event), differential relaying (used with current transformers to closely monitor and shut circuits down in an arcing event), energy-reducing maintenance-switching with local status indication (a switch permitting workers to set the breaker trip unit to “no intentional delay” to reduce clearing time in an arc event), or other approved means.

New in 240.87 of the 2014 NEC is a change in title from “Non-Instantaneous Trip” to “Arc Energy Reduction” and new subsection titles of (A) for Documentation and (B) for Method to Reduce Clearing Time. There is also new introductory text 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 is 1,200 amp or higher, then arc-energy reduction can be used.” This text places a limit of at least 1,200A or higher on the circuits that qualify for this arc reduction. Energy-reducing active arc flash mitigation system was added as an arc-energy-reduction method. An informational note was added to explain that the energy-reducing active arc-flash-mitigation system helps in reducing arcing time duration in the electrical distribution system. Unlike other arc-reducing methods provided in 240.87, energy-reducing arc-flash-­mitigation systems do not require any change in the circuit breaker or settings of other devices as is required when a maintenance worker is within an arc flash boundary as defined in NFPA 70E.

Text similar to 240.87 of the 2011 NEC was added to Section O.2.3 of Annex O in the 2012 edition of NFPA 70E for arc-energy reduction. Changes in O.2.3 for the 2015 edition of NFPA 70E should reflect the same wording found in 240.87 of the 2014 NEC. Further development of arc-reduction techniques in the near future will help reduce the number of serious electrical arcing events and help protect personnel in the field. Stay tuned for further developments in the 2017 NEC and major changes in NFPA 70E.

About the Author

Mark C. Ode

Fire/Life Safety Columnist 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 .

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