Quick diagnostic table
| Symptom | Likely causes | First checks |
|---|---|---|
| Motor hums, won't start or overheats | Phase loss, blown fuse, failed contactor pole | Measure all three line voltages and currents |
| Motors running hot | Voltage unbalance, overload, low voltage | Calculate % unbalance; compare to nameplate |
| Neutral overheating | Triplen harmonics, unbalanced single-phase loads | True-RMS neutral current; harmonic analysis |
| Breakers trip randomly | Loose connections, harmonics, ground faults, undersized device | Thermal scan; trend currents; insulation test |
| Lights flicker when equipment starts | Voltage drop from motor inrush, undersized feeders | Record voltage during starting |
| Capacitor bank failures | Harmonic resonance | Harmonic survey; consider detuned bank |
Phase loss (single-phasing)
If one phase opens upstream, a running three-phase motor may keep turning on two phases while drawing much higher current in the remaining lines, and it quickly overheats. Phase-loss relays and modern electronic overloads protect against this.
Voltage unbalance
% unbalance = (max deviation from average ÷ average) × 100. Example: 460, 467, 450 V → average 459 V; max deviation 9 V; unbalance ≈ 1.96%. NEMA MG 1 recommends derating motors above 1% and avoiding operation above 5%. Rebalance single-phase loads and check for utility issues or bad connections.
Harmonics
VFDs, UPS systems, LED drivers and IT equipment draw non-sinusoidal current, which heats transformers and neutrals and can resonate with capacitors. Mitigations include line reactors, K-rated transformers, active or passive filters, and multi-pulse drives. See Power Factor Correction and Choosing a VFD.
Loose and overheating connections
Thermal cycling and vibration loosen terminations over time. Infrared thermography under load is the standard way to find them. Compare similar connections across phases, because a large temperature difference between phases is a red flag.
When to call in help
Recurring problems often point to a design or capacity issue. Our Services page explains how we can help you find troubleshooting and engineering support.