Question 155Electricity & Electronics - Assistant Engineer

The correct answer is D) disconnect each shunt field coil before testing. To accurately determine which shunt field coil is grounded in a DC machine using a megohmmeter, you must disconnect each shunt field coil before testing. This is because if the coils are still connected, the megohmmeter will measure the resistance across all the coils in parallel, which could mask a ground fault in one of the individual coils. By disconnecting each coil and testing them individually, you can isolate the specific coil that is grounded. The other answer choices are incorrect because: A) a high capacity megohmmeter is not necessary, B) insulating the field frame is not required for this test, and C) removing main line lead connections is not the proper procedure for this test.