Transformers and Arc Flash

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From the archives—March 14, 2022

Do transformers require arc flash labels, and what voltages are arc flash labels required for in low-voltage panels?

Section 110.16 addresses your question and states electrical equipment, such as switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures and motor control centers, that is in other than dwelling units, and is likely to require examination, adjustment, servicing or maintenance while energized, shall be field- or factory-marked to warn qualified persons of potential electric arc flash hazards. The marking shall meet the requirements in 110.21(B) and shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing or maintenance of the equipment. The key information to focus on in this rule is if the transformer is equipment likely to be worked on while energized. Transformers are not mentioned, at least directly. The other bit of useful information in this rule worth noting is the phrase “shall be field or factory marked to warn qualified persons of potential electric arc flash hazards.” Often manufacturers include danger markings on this type of equipment and arc flash hazard warnings from the factory. Applicable voltages are not mentioned in Section 110.16.

From the archives—July 15, 2024

Good morning CQD, does an increase in distance from the source breaker to the equipment/load increase incident energy related to the arc flash hazard? Does an increase in distance from the secondary side of a transformer decrease the incident energy?

There are several factors that impact the available incident energy, including the available fault-current and the clearing time of the OCPD. A longer distance from the source to the equipment/load may result in a lower available fault-current; however, a lower available fault-current may result in the OCPD taking more time to operate/open. Therefore, a larger distance does not always equate to lower incident energy. 2024 NFPA 70E, Section 130.5(C), Informational Note No. 3 provides information on this topic. Additionally, Informational Annex D of NFPA 70E provides guidance on the methods for calculating incident energy.

Post sponsored by Eaton Corp.

The first panelboard on the secondary of a standard transformer can have very high incident energy because the protecting OCPD is on the primary and sized to permit inrush currents to flow. Eaton’s Breaker Integrated Transformer (BIT) provides a safer solution integrating the protecting OCPD for the secondary panelboard on the secondary of the transformer.