The megohmmeter (commonly called a megger in the field) is used to measure the resistance of insulation in megohms (thousands of ohms).
Test results indicate the presence of dirt, moisture, and insulation deterioration.
Megohmmeter instruction manuals provide detailed information about connecting to and testing various types of equipment. The following sections provide general guidance for common types of troubleshooting tests.
Testing DC Motors and Generators on ship
Disconnect a DC motor and a DC generator from its load. Then attach the negative test lead of the megohmmeter to the machine ground and the positive lead to the brush rigging. Measuring the insulation resistance in this manner indicates the overall resistance of all components of the unit.
To measure the insulation resistance of the field or armature alone, either remove the brushes or lift them free of the commutator ring and support the brushes using a suitable insulator. Connect one test lead to the frame ground and the other to one of the brushes. Insulation resistance of the field alone will then be indicated. With the brushes still removed from the commutator ring, connect one of the megger test leads to one of the segments of the commutator and the other to the frame ground. The insulating resistance of the armature alone will then be indicated. This test may be repeated for all segments of the commutator.
Testing AC Motors on ship
To test an ship AC motor, first disconnect the motor from its power source, either by using the switch or by disconnecting the wiring at the motor terminals. If the switch is used, remember that the insulation resistances of the connecting wire, switch panel, and contacts will all be measured at the same time. Connect the positive megger lead to one of the motor lines and the negative test lead to the frame of the motor. Compare meter readings to the established insulation resistance minimums.
Testing ships Circuit Breakers
Disconnect the circuit breaker from the line and connect the megger black lead to the frame or ground.
Check the insulation resistance of each terminal to ground by connecting the red (positive) lead to each terminal in turn and making the measurements.
Next, open the breaker and measure the insulation resistance between terminals by putting one lead on one terminal and the other on the second for a two terminal breaker; for a three-pole breaker, check among poles 1-2, 2-3, and 1-3.
Testing Safety of ships Switches and Switchgear
Completely disconnect from line and relay wiring before testing. When testing manually operated switches, measure the insulation resistance from ground to terminals
and between terminals. When testing electrically operated switches check the insulation resistance of the coil or coils and contacts. For coils, connect one megger lead to one of the coil leads and the other to ground. Next, test between the coil lead and core iron or solenoid element.