6.6 kV Phase Loss: Coil Damage Is Magnetic, Not Logic

After correctly commissioned three-phase undervoltage protection trips the supply, contactor coils no longer remain in the high-current, partially closed condition that causes thermal failure. The number that matters is not only the reduced voltage; it is the coil current and heating after the magnetic air gap begins to open.

Current, flux, and thermal load

The installation has a 1 MVA, 6.6/0.415 kV, Dyn11 transformer with a solidly earthed low-voltage neutral. One conductor on the incoming 6.6 kV supply opened inside an oil switch, leaving two phases connected. No sustained arcing was reported.

An AC contactor coil is not a fixed-resistance load. With the armature seated, the magnetic path has little air gap, inductance is high, impedance is high, and holding current is relatively low. As voltage falls, magnetic flux and force fall. When magnetic force can no longer oppose the return spring and mechanical load, the poles begin to separate.

Opening even a small air gap reduces inductance. Coil impedance then falls, allowing more current than the sealed coil would draw at the same reduced voltage. The increased current may hold the armature partially closed, producing sustained coil heating instead of a clean release. A fully open contactor also draws its inrush-state current continuously if voltage remains below pickup but high enough to keep the coil energized. This is heat, not logic.

At 50% voltage, open-armature current may be approximately half the rated-voltage inrush current when impedance remains comparable, but that can still be far above continuous sealed current. Coil copper loss follows P = I²R; current therefore decides the thermal outcome. Obtain sealed current, inrush current, pickup voltage, dropout voltage, duty rating, and permissible continuous voltage from the exact contactor datasheet.

Open-phase voltage distribution

With one primary conductor open, one delta winding remains connected directly between the two live lines. The other two delta windings form a series path across those lines. For identical winding and reflected-load impedances, the directly supplied winding receives full line voltage while the two series branches divide the voltage approximately equally. This produces the familiar expectation of one normal secondary phase voltage and two secondary phase voltages near 50%.

That equal division requires balanced impedances. Factory single-phase loads, energized control transformers, contactor coils, lighting, and unequal motor conditions change the impedances reflected into the primary delta. The voltage across the two series branches can consequently divide anywhere between the stated 50/50 case and the limiting 0/100 case. The affected low-voltage phase-to-neutral readings can therefore be very unequal.

The solidly earthed low-voltage neutral fixes the neutral reference; it does not restore missing primary excitation. A grounded neutral prevents the load neutral from freely drifting, but each phase-to-neutral voltage still follows its transformer winding EMF. The 1 MVA nameplate rating describes transformer capacity and does not protect small coils against prolonged undervoltage.

Damage-mechanism comparison

Observed condition	Mechanism	Best discriminator	Relevance
Burned contactor or relay coil	Undervoltage weakens magnetic force; an opening air gap lowers impedance and raises current relative to the sealed state	Record coil voltage, coil current, armature position, and chatter during a controlled test	Primary explanation for coil damage
Hot three-phase motors	Single-phasing produces severe voltage and current imbalance; negative-sequence current adds rotor heating	Capture all three motor terminal voltages and currents	Primary motor-damage mechanism
Heated power contacts	Reduced magnetic force lowers contact pressure; current through a poorly seated contact raises contact resistance and heating	Inspect contact faces and measure pole voltage drop under an approved test condition	Possible secondary contactor damage
Damaged single-phase equipment	Unequal undervoltage, repeated dropout and pickup, or switching transients affect individual loads differently	Oscillography of all phase-to-neutral and phase-to-phase voltages at the event	Investigate load by load
Healthy-phase voltage-to-earth increase	Neutral displacement after an earth fault on an unearthed system changes conductor-to-earth voltage	Measure source neutral earthing and simultaneous phase-to-earth and phase-to-phase voltages	Different condition from the stated solidly earthed low-voltage neutral
Harmonic or skin-effect claim	Harmonics require waveform evidence; skin effect is negligible in ordinary fine magnet wire at power frequency	Voltage and current waveform capture	Not the first explanation for undervoltage coil burnout

Negative-sequence effects explain motor heating but do not directly explain why a single-phase contactor coil burns. Likewise, a lower RMS voltage does not guarantee lower coil heating because the armature position changes the coil impedance. Start with magnetic state, current, and time below the hold threshold.

Quantities and decision limits

Quantity	Why it matters	Where to read it
Three primary line-to-line voltages	Confirms the open conductor and distinguishes it from a ground-reference shift	Protection relay event record or approved primary voltage instrumentation
Three secondary phase-to-neutral voltages	Shows which single-phase loads received normal, reduced, or unequal voltage	Power-quality recorder at the low-voltage bus
Three secondary phase-to-phase voltages	Defines the voltage imposed on three-phase equipment	Power-quality recorder at the same timestamp
Coil voltage and current	Separates harmless release from sustained open-gap heating	Oscilloscope or recorder with suitable isolated probes and current measurement
Pickup and dropout voltage	Defines when the armature can close and when it begins to release	Exact contactor or relay datasheet
Inrush and sealed current	Sets the thermal difference between open-gap and closed-gap operation	Exact coil datasheet or controlled bench measurement
Event duration	Heating depends on how long abnormal current persists	Relay sequence-of-events record and waveform capture
Motor phase currents	Quantifies single-phasing and current imbalance	Motor protection relay or current recorder

A referenced dropout range of 53% to 68% explains why a coil exposed to approximately 50% voltage may release even though it was already pulled in. Pickup and dropout hysteresis delays release; it does not guarantee indefinite operation below the continuous-voltage rating. The field information also identifies -15% as a typical continuous design allowance and reports failures at -20%. Treat those figures as screening values only; the exact coil datasheet controls the protection decision.

Three-phase undervoltage protection recommendation

Use protection that evaluates all three phases and trips before contactors or motors remain in a damaging undervoltage or imbalance state. Monitoring a single phase is inadequate because the surviving primary lines can leave one secondary phase near normal while the other two are depressed unequally.

The protection function should distinguish a sustained open-phase condition from acceptable short disturbances. Select its voltage threshold from the most restrictive connected device: coil continuous-voltage range, contactor dropout behavior, motor undervoltage and imbalance limits, and process ride-through requirements. Select the delay from equipment thermal tolerance and coordination studies rather than an assumed universal setting. No exact trip threshold or delay follows from the transformer rating alone.

Place the sensing point where it sees the voltage actually supplied to the vulnerable loads. For this installation, three low-voltage phase-to-neutral and phase-to-phase measurements expose the distorted transformer output directly. Where primary protection already records all three 6.6 kV voltages, coordinate primary phase-loss detection with low-voltage undervoltage and motor protection. Configure a latched trip or controlled restart if automatic re-energization would cause widespread contactor cycling.

Installation and commissioning procedure

1. List every energized motor, contactor coil, relay coil, control transformer, drive, and sensitive single-phase load supplied by the transformer. Record rated voltage, permissible continuous range, dropout value, pickup value, and ride-through requirement from each manufacturer document.
2. Identify the device with the narrowest permitted voltage range or shortest thermal tolerance. Use that device to establish the maximum acceptable detection and clearing time.
3. Install or configure sensing for all three low-voltage phase-to-neutral and phase-to-phase voltages. Include phase-loss or voltage-imbalance logic where the selected relay provides it.
4. Coordinate the undervoltage threshold above the region where coils can hover partially closed, while allowing only the disturbances that connected equipment is rated to ride through. Use the actual dropout data rather than assuming every contactor remains safely seated at 50%.
5. Set the trip delay from documented equipment limits and the protection-coordination study. A delay long enough to ride through a harmless dip may be long enough to overheat an open-gap coil, so verify both requirements.
6. Define the trip target: affected motor feeders, the low-voltage incomer, or the transformer supply. The selected boundary must remove voltage from every load exposed to the abnormal transformer output.
7. Configure event recording for three-phase voltage, trip initiation, breaker position, and motor current where available. Preserve a common time reference so primary and secondary records can be compared.
8. Test the scheme by secondary injection or an approved relay test method. Never create an intentional open phase on an energized 6.6 kV system.
9. Verify reset and restart behavior. A returning phase must not cause uncontrolled simultaneous pickup, repeated chatter, or restart of machinery whose process state requires operator intervention.

Verification and post-fault inspection

A successful injection test must drive each simulated abnormal phase condition through detection, timing, trip output, breaker opening, alarm indication, and reset. Test unequal cases as well as the balanced 50/50 division because real single-phase loading changes the voltage split. Confirm that loss of any one monitored phase produces the intended response.

Compare measured operating time with the maximum time accepted in the coordination study. Review the recorded voltage at trip and verify that sensing did not depend on the one secondary phase that remained near normal. Check that the relay records identify which quantity crossed its threshold.

After an actual event, replace visibly burned coils and inspect surviving contactors for discolored coils, mechanical drag, pole-face damage, chatter marks, welded contacts, reduced contact pressure, and overheated terminals. Insulation testing alone will not reveal weakened contact pressure or a distorted shading structure. Check motor winding condition and protection records separately because motor single-phasing follows a different heating mechanism.

Capture waveforms during future incidents. RMS snapshots can miss rapid dropout, repickup, and switching transients that damage electronic or single-phase loads. Simultaneous voltage and current records establish whether each failure came from sustained undervoltage, abnormal current, transient overvoltage, or a separate earth-reference problem.

Frequently asked questions