Introduction #

Single phasing (loss of one supply phase) overheats motors, trips protection, and can destroy windings within minutes. This guide covers detection, root causes, and corrective actions.

What Is Single Phasing #

  • One of the three phases is lost or severely unbalanced.
  • Current in the remaining phases can rise to 1.7–2.0× normal.
  • Torque drops; motors may stall and overheat.

Common Causes #

  • Blown fuse on one phase (especially with fuse links on outgoing feeders).
  • Loose terminations or broken conductor.
  • Contact wear in starters/contractors.
  • Utility phase loss or severe imbalance.
  • Cable damage or water ingress.

Symptoms and Field Checks #

  • Motor hums, won’t accelerate, or trips overloads quickly.
  • Elevated current on two phases; near-zero on the lost phase.
  • Voltage: one phase low/zero at motor terminals.
  • Thermal imaging: one leg hotter; cables or terminations show hotspots.

Protection and Controls #

  • Phase-loss/phase-imbalance relays: trip contactor on loss or imbalance.
  • Overload relays (IEC/NEMA): ensure properly set; many miss fast phase loss.
  • Undervoltage relays: catch deep sags that mimic phase loss.
  • Soft starters/VFDs: some have phase-loss detection—enable it.

Immediate Response #

  1. De-energize the affected feeder.
  2. Check fuses/MCBs; replace blown links only after finding root cause.
  3. Inspect terminations and contactor tips; tighten/replace as needed.
  4. Megger the motor if any doubt about insulation before restart.

Root-Cause Checklist #

  • Was there a utility event or upstream breaker operation?
  • Are fuses mismatched or underrated?
  • Any recurring loose lugs or overheated terminals?
  • Water/oil ingress in cable terminations?
  • Excessive motor starts causing contact wear?

Monitoring and Prevention #

  • Add phase-loss relays to critical motors and MCC buckets.
  • Use thermal scans quarterly on MCCs and feeders.
  • Trend phase currents with submeters where downtime is costly.
  • Maintain torque on lugs; document and re-check after 24 hours of operation.

Integration With Calculators #

Conclusion #

Single phasing is highly destructive but easy to prevent with phase-loss protection, good terminations, and routine thermal/visual inspections. Detect quickly, fix the mechanical/electrical cause, and verify current balance before returning to service.