Energized Secondary Repair Under Time Pressure
A 25 kVA overhead transformer feeds a 120/240 V single-phase service to a small commercial customer. The customer reports partial power: half the building's circuits are dead while the other half work. You arrive with a two-person crew. The service drop neutral shows signs of arcing at the weatherhead, and the utility's transformer secondary neutral connection looks discolored. The customer is pressing hard to restore power immediately because they have refrigeration loads. Your crew has rubber gloves rated for the voltage class, insulating blankets, and a live-line clamp tool. The feeder is on a fused cutout that can be opened, but the customer's meter is on the load side and the meter socket has no bypass. Weather is clear and dry.
1. Identify the hazard mechanism causing the partial outage and explain why de-energizing is the correct first action rather than working it hot.
Read the worked answer
The symptom - some 120 V circuits live, others dead - points to an open or high-resistance neutral on the service or transformer secondary, not a lost phase (this is single-phase, so there is no phase to lose). With the neutral open, the two 120 V legs become a series circuit across 240 V. Load imbalance drives one leg high and the other low: the leg with lighter load rises toward or above 240 V and can destroy equipment or shock persons, while the loaded leg sags. This is a floating neutral condition.
Working it hot is inappropriate because:
- The neutral is the very conductor that is compromised; you cannot rely on it for equipotential or for the glove rating to protect you from a voltage that appears on enclosures and grounding.
- The customer's bonded metal and the meter socket may be energized at line-to-neutral potentials that shift unpredictably.
- The hazard is not a simple live conductor you can cover; it is a system-level fault that changes as loads switch.
The correct sequence is to open the cutout, verify absence of voltage, then repair the neutral connection with the circuit dead. Speed does not justify exposing the crew to a shifting potential they cannot control.
2. Outline the isolation and verification steps you would take before touching the neutral, including what you must do about the customer's loads.
Read the worked answer
- Notify and secure: tell the customer power will be interrupted; treat the service as de-energized only after isolation.
- Open the cutout feeding the transformer (or the service disconnect if that is the isolation point). Lock/tag per the utility's clearance procedure.
- Test for absence of voltage at the service entrance and at the transformer secondary using a meter rated for the voltage and proven on a known source before and after the test.
- Address backfeed sources: the customer may have PV, a generator, or UPS. Confirm none are backfeeding the service; if present, isolate at the customer's transfer equipment.
- Disconnect or warn about loads: because the neutral is open, any 240 V or 120 V load left connected can hold potential or be damaged on re-energization. Have the customer shut off sensitive loads, or open the main.
- Repair the neutral at the weatherhead and the transformer secondary lug; replace discolored conductors and retorque to spec.
- Re-energize and verify balanced voltage leg-to-neutral and leg-to-leg, then check the customer's equipment.
3. After repair, what measurements confirm the neutral is sound before you release the service?
Read the worked answer
With the service energized and representative load connected:
- Line-to-line should read approximately 240 V.
- Each leg-to-neutral should read approximately 120 V and be roughly equal; a large split (e.g., 135 V / 105 V) indicates a still-resistive or open neutral.
- Neutral-to-ground at the service should be near zero volts under load; a significant reading indicates a poor neutral or grounding electrode issue.
- Voltage drop under load: apply a load and confirm the neutral does not cause excessive voltage rise on the lightly loaded leg.
Document the readings. Do not release the service on a visual repair alone; the floating-neutral condition is only proven cleared by balanced loaded voltages.