When using a megohmmeter to determine which shunt field coil is grounded in a DC machine, what must you do?

Correct Answer: Option D — disconnect each shunt field coil before testing

Isolating individual shunt field coils is mandatory during insulation testing with a megohmmeter per IEEE 45 standards. Leaving parallel paths connected will cause misleading insulation resistance readings and fail to pinpoint the specific grounded coil winding within the complex direct current machine frame.