This is a new section for the 2023 edition of NFPA 70 that recognizes the hazards of equipment when supplied by transformers and tapped conductors.
“Barriers shall be placed such that no uninsulated, ungrounded busbar or terminal is exposed to inadvertent contact by persons or maintenance equipment while servicing load terminations in panelboards, switchboards, switchgear, or motor control centers supplied by feeder taps in 240.21(B) or transformer secondary conductors in 240.21(C) with the disconnecting device, to which the tap conductors are terminated, in the open position.”
The hazards associated with the first panel on the secondary of a transformer include the following:
- Shock: When the main OCPD in the panel is opened, the line side terminals are still energized and present a shock hazard. This is technically the case for any panelboard that has a main breaker but in this case the severity of this hazard also includes the fact that this unintended consequence could be an arc flash event covered as part of the second hazard associated with this location in the circuit. The shock hazard is not specific to the type of transformer connections. This means that a delta-delta, delta-wye, wye-wye, and other configurations of the transformer have no impact on whether or not the secondary equipment introduces a shock hazard when, for example, the main OCPD in the secondary equipment is in the open position.
- Arc Flash: Incident energy for the first equipment on the secondary of any transformer is high for the following reasons:
- The protecting OCPD: The incident energy for the panel on the secondary of the transformer is determined by the clearing time of the circuit breaker on the primary of the transformer.
- The protecting OCPD: The incident energy for the panel on the secondary of the transformer is determined by the clearing time of the circuit breaker on the primary of the transformer.
Intentional delays: The primary circuit breaker selected for a transformer must be sized such that the primary circuit breaker has an intentional delay to permit for inrush current of the transformer. Inrush is not specific to the type of connection of the transformer. This means that a delta-delta, delta-wye, wye-wye, and other configurations of the transformer will have to address inrush currents. This will require the primary OCPD to be selected with an intentional delay that would permit these inrush currents to flow without tripping. This causes a larger than necessary OCPD for the application which will drive incident energy to high values for the first equipment on the secondary of the transformer.
Public Input noted the following:
“Barriers shall be placed such that no energized, uninsulated, ungrounded busbar or terminal is exposed to inadvertent contact by persons or maintenance equipment while servicing load terminations in panelboards, switchboards, switchgear, or motor control centers supplied by feeder taps in 240.21(B) or transformer secondary conductors in 240.21(C) whenwith
Eaton’s Breaker Integrated Transformer offers a solution to address the incident energy at the secondary of the transformer by integrated the secondary overcurrent protection with the transformer itself. In this case, the secondary conductors of this transformer are feeders and are not transformer secondary conductors. This gives the first equipment on the secondary of the transformer the ability to leverage the fast acting nature of the upstream circuit breaker which now is not the primary feeder circuit breaker supplying power to the transformer.
Clicking on the image to the left will gain you access to a video that helps you understand this solution.