USCG Exam PracticeElectrical measuring devices and instrumentation
When using an ohmmeter to measure the resistance of a motor winding, the technician must first ensure the motor is:
- A. Running at no-load to stabilize the winding temperature
- ✓ De-energized and isolated from the circuitCorrect
- C. Connected to a reduced voltage source to limit current
- D. Warmed up so insulation resistance is at its lowest
Why B is correct
An ohmmeter uses its own internal battery as the voltage source. Applying it to an energized circuit introduces external voltage that produces false readings and can damage the meter. The circuit must be de-energized and isolated before measurement.
Cited:NEETS Mod. 3 §1-3