Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A master electrician is performing a continuity test on a newly installed 3-phase motor winding. The multimeter reads a very low resistance (e.g., 0.5 ohms) between each winding and between the windings and the motor frame. What does this reading indicate?
- AThe motor winding is properly insulated.
- BThe motor winding has a short circuit.
- CThe motor winding is grounded.
- DThe motor winding is open.
Show answer & explanationAnswer & explanation
Correct answer: C. The motor winding is grounded.
A very low resistance reading between the windings and the motor frame indicates a ground fault, meaning the insulation has failed and the winding is making contact with the frame.
Why the other options are wrong
- A. Proper insulation would show very high (ideally infinite) resistance to the frame.
- B. A short circuit between turns would show lower resistance than normal, but a short to the frame is a ground fault.
- D. An open winding would show infinite resistance.
Continuity Test (Motor Windings)
A continuity test on motor windings checks for open circuits, short circuits between windings, and ground faults (short to frame) by measuring resistance.
- Tests for open: infinite resistance (bad).
- Tests for shorts between windings: abnormal low resistance (bad).
- Tests for ground faults: low resistance to frame (bad, indicates insulation failure).
Memory trick: Motor Tests: Ohms Tell All – Open, Short, Grounded, Good!