Cables, Switches, and Motor Circuit Testing
Cables, Switches, and Motor Circuit Testing covers insulation, continuity, contact condition, and motor winding balance. Cable tests include insulation resistance and, where specified, time-domain reflectometry or high-voltage proof testing. Switch tests include contact resistance and insulation. Motor tests include winding resistance, insulation resistance, rotation, and no-load current.
Core rules
- Isolate both ends of a cable before insulation testing and discharge it afterward; capacitance can hold a charge.
- Disconnect VFDs, surge suppressors, and sensitive electronics before megging a motor circuit.
- Measure motor winding resistance phase-to-phase and compare balance. A common field guide is to investigate deviations beyond about 5-10%, but manufacturer criteria control.
- Check switch contact resistance with a micro-ohmmeter and inspect contacts for pitting or misalignment.
- Verify motor rotation uncoupled where possible and record no-load current against nameplate.
Worked example
A three-phase motor reads 1.20 Ω, 1.20 Ω, and 1.80 Ω. Average is 1.40 Ω; the high phase deviates by 0.40 / 1.40 = 28.6%, far beyond a typical balance limit. The technician checks lead placement, connection tightness, and temperature, then investigates the winding or internal connections. The reasoning is that a large phase imbalance is a red flag even when the motor still runs.
Traps
Megging a circuit with a VFD connected; not discharging a cable after test; testing motor windings at unequal temperatures; and accepting a running motor as proof of winding balance.