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Codes Working Together: NEC fault-current marking related to NFPA 70E


By Mark C. Ode | Apr 15, 2016
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As Section 110.2 States, The National Electrical Code (NEC) provides installation requirements for electrical conductors and equipment that are either required or permitted to be installed. However, these conductors and equipment are only acceptable if the installation is approved. NEC Article 100 defines “approved” as being acceptable to the authority having jurisdiction. The NEC’s purpose is the “practical safeguarding of persons and property from hazards arising from the use of electricity,” and part of that safeguarding is that listed or labeled equipment shall be installed and used in accordance with any instructions in the listing or on the labeling [110.3(B)].


Related to the NEC but somewhat separate, NFPA 70E, Standard for Electrical Safety in the Workplace, covers electrical safety. Its purpose is to provide employees with a practical safe working area relative to the hazards arising from electricity use, whereas the NEC covers electrical installations. NFPA 70E addresses the “safety of workers whose job responsibilities entail interaction with electrical equipment and systems with potential exposure to energized electrical equipment and circuit parts.”


Within the past few NEC cycles, additions related to electrical workplace safety have supplemented or strengthened NFPA 70E requirements. For example, Section 110.16 requires arc-flash hazard warnings for electrical equipment, such as switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures and motor control centers, where this equipment is likely to require examination, adjustment, servicing or maintenance while energized. Meanwhile, in Section 130.5(D), NFPA 70E requires similar but more extensive marking. Section 110.21(B) requires field-applied hazard markings— such as caution, warning and danger signs and labels—to help electrical service personnel deal with hazardous electrical installations. This NEC marking is similar to the marking in NFPA 70E Section 130.6, and it is intended to increase awareness of electrical hazards.


NEC Section 110.24 requires service equipment field-marking in other than dwelling units with maximum available fault current. The field marking must include the fault current calculation’s date and must be of sufficient durability to withstand its installed environment. When modifications in the electrical installation affect the maximum available fault current at the service—e.g., the utility company transformer is replaced with a lower impedance transformer or the transformer size is increased—the required field markings must be adjusted to reflect the higher values.


A new informational note in the 2014 NEC indicates the field marking in 110.24 relates to the equipment’s required short-circuit current ratings. Section 110.9 applies to interrupting ratings, and Section 110.10 requires circuit protective devices to be selected and coordinated to clear the fault without extensive equipment damage.


The informational note in 110.24 also states that NFPA 70E provides assistance in determining the severity of potential exposure to arc flash, arc blast, planning safe work practices and selecting personal protective equipment (PPE). The maximum available fault current marking at the electrical service equipment provides a basis within NFPA 70E for incident energy calculations based on IEEE 1584, the Guide for Performing Arc Flash Calculations, or determining if the equipment maximum fault current values are within the parameters of the arc flash PPE categories method of 130.7(C)(15) and 130.7(C)(16).


Both the incident-energy calculation method and the arc flash PPE category method rely on calculating the maximum available fault current, the reaction times of the overcurrent protective devices in series with the fault, the minimum fault level at which the arc will self-sustain (sustaining arc fault level), and the distance the person should be located from the prospective arc. If the equipment fits within the parameters given in the arc flash PPE category tables, appropriate PPE equipment, safety accessories and testing equipment can be determined. If the equipment doesn’t fit within the tables’ parameters, the incident energy calculation method must be used.


Related to the overcurrent protective device reaction time, new Section 240.87 helps provide arc energy reduction as part of the electrical equipment design. The section only applies where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker (e.g., a circuit breaker rating plug) is rated or can be adjusted to 1,200 amperes or higher. Installing zone-selective interlocking, differential relaying, energy-reducing maintenance switching and energy-reducing active arc flash mitigation systems can result in faster acting devices and can cut arcing time down considerably.


Future NEC editions will likely contain enhancements for electrical safety based on NFPA 70E.

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]

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