Fixed the Trip, Now Voltage Imbalance Appears: Two-Fault Decision Tree

Why this matters

You cleared the nuisance trip the customer called about, energized, and now the meter shows a voltage imbalance that was not there before, or that the trip had been masking. This is the two-fault scenario: the first fault hid the second, and fixing one unmasked the other. It is easy to assume your repair caused the new problem and start undoing good work. Usually it did not; the imbalance was always there but the tripped circuit removed the load that made it visible. The discipline is to recognize a pre-existing second fault, isolate it on its own merits, and not chase your own tail. This is most common on three-phase services and on multiwire branch circuits sharing a neutral.

Symptom presentation

After clearing a trip (a tripped breaker, a cleared short, a replaced device, a re-terminated connection), the system now shows unequal voltages: on three-phase, the three line-to-line or line-to-neutral readings differ by more than a couple percent; on single-phase, the two 120 V legs read unequal and shift with load. Motors may run hot or hum, electronics on the high leg may fail, lights on one leg brighten while the other dims. The customer's original complaint is gone, but a new one is forming.

Quick checks

• Re-read all phase/leg voltages at no load and under load. Note the spread. Imbalance that appears only under load points at a high-resistance connection or neutral; imbalance present at no load points at the source or a fixed open.
• Confirm your repair is sound. Re-meter the connection you just fixed; rule out that you introduced the imbalance.
• Check the neutral first on single-phase or wye systems. An open or high-resistance neutral is the most common producer of leg imbalance that a balanced trip previously hid.
• On three-phase, check for single-phasing: one line low or open while the other two read normal, often with a motor as the visible victim.

Isolation tree

Branch 1: Single-phase, two legs now unequal. With the previously-tripped circuit restored, total load shifted across the legs. An open or high-resistance neutral lets the legs share voltage by their load ratio. Meter neutral-to-ground (should be near zero) and watch the legs as you switch loads on one side; if one leg climbs while the other sags, the neutral is the fault. The earlier trip removed the unbalanced load that exposed it. Locate the open neutral at the meter base, main lug, or a splice.

Branch 2: Three-phase, one phase low. Single-phasing: a blown fuse, an open pole, a loose lug on one phase, or a utility primary issue. The cleared trip may have removed a load that was riding the two good phases. Meter each phase line-to-line and line-to-ground; the open phase reads low or shows back-fed voltage through connected motor windings. Find the open pole, fuse, or termination on that phase.

Branch 3: Imbalance only under load. A high-resistance connection on one leg/phase. Under light load it is invisible; under load it drops volts on that conductor. Thermal-image the terminations on the affected phase under load and find the warm joint. This is a separate connection from the one you repaired.

Branch 4: Imbalance present at the service even with the building's main open. The fault is upstream, on the utility side. Confirm by metering at the service entrance with the main off; if the imbalance is in the incoming utility voltage, report to the utility and do not chase it inside.

Branch 5: Your repair is the cause. Rule this in or out honestly. If the imbalance appears the moment your re-terminated conductor carries load and clears when you re-do that exact connection, you under-torqued or crossed a connection. Fix it. But if the imbalance persists independent of your work, it is the second, pre-existing fault.

Confirming diagnosis

• Two-fault confirmation: the imbalance can be reproduced and isolated to a conductor, connection, or source point that is physically separate from your repair. Acting on that second point clears the imbalance; acting on your repair does not.
• Neutral confirmation: leg voltages swing with load distribution and neutral-to-ground reads elevated; repairing the neutral collapses the legs back to balance.
• Single-phasing confirmation: one phase reads low/open at the service; restoring that phase's fuse/pole/termination balances the three.
• Source confirmation: imbalance measured at the service entrance with the building isolated, pointing to the utility.

Remediation

• Open/high-resistance neutral: locate and repair the neutral open; re-verify legs balance under load.
• Single-phasing: replace the blown fuse, re-terminate the open pole, or coordinate the utility primary repair.
• High-resistance connection: re-terminate to listed torque on the affected phase; re-survey thermally.
• Utility-side imbalance: document readings and report; protect motors with phase-monitor relays where appropriate.
• Re-confirm the original complaint stays fixed and the new imbalance is within ANSI tolerance (about 2 to 3 percent for sensitive three-phase loads) before leaving.

References

• NFPA 70 (National Electrical Code), Article 250 grounding and neutral integrity; Article 230 service conductors.
• ANSI C84.1, Voltage Ratings, voltage-unbalance limits for utilization equipment.
• NEMA MG 1, Motors and Generators, voltage-unbalance derating and single-phasing effects on motors.
• IEEE Std 141 (Red Book), Recommended Practice for Electric Power Distribution, on imbalance and neutral conductors.
• NFPA 70E, Standard for Electrical Safety in the Workplace, for energized-work and lockout procedures.