By hvhipot 
Insulation resistance testers are usually called megohmmeters or megohmmeters. It can be widely used to measure the insulation resistance of generators, motors, power transformers, circuits, electrical appliances and other electrical equipment (such as control, signal, communication, power cables). They are usually used in routine maintenance procedures, high-voltage insulation resistance testers to indicate changes in motor insulation resistance over months or years.
Large changes in insulation resistance may indicate a latent failure. Therefore, it is necessary to calibrate the megohm meter regularly to ensure that the meter itself does not change over time. Then use Ohm’s law to measure a current produced by the device.
Before using the insulation resistance tester, you should understand these points:
One: When measuring, pay attention to the correct wiring of the megohmmeter. Otherwise, the high-voltage insulation resistance tester will cause unnecessary errors or even errors. The megohmmeter has three terminals, three “L” for the wire end, one “e” for the ground terminal, and the other “g” for the shielding end, also known as the guard ring. Usually, the measured insulation resistance is connected between the “L” and “e” terminals. But when the measured insulation surface leaks severely, the shielding ring or the untested part must be connected to the “g” terminal. In this way, the leakage current will flow directly back to the negative r of the generator through the shielded terminal “g”, forming a loop instead of flowing through the measuring mechanism of the megohmmeter.
This eliminates the effect of surface leakage current. It should be noted that when the high-voltage insulation resistance tester measures the insulation resistance between the cable core and the surface, the shield button “g” must be connected. Because when the air humidity is high or the surface of the cable insulation is not clean, the surface leakage current will be very large. In order to prevent the leakage of the measured object from affecting the internal insulation measurement, a metal shielding ring is usually set on the outer surface of the cable and connected to the “g” terminal of the megohmmeter.
Two: When measuring the insulation resistance of electrical equipment with a megger, pay attention to the “L” and “e” terminals not to be reversed. The correct connection method is to connect the “L” wire end to the conductor of the device under test, the “e” ground terminal or to the grounded device enclosure, and the “g” shield terminal to the insulated part of the device under test. If “L” and “e” are reversed, the leakage current flowing through the insulator and the insulating surface will be collected to the ground through the housing, and then flow into the measurement coil through the “L” from the ground, so that the “g” terminal will lose the shielding effect, which will bring a lot of trouble to the measurement. Big error.
In addition, since the insulation between the “e” terminal and the inner lead with a shell is lower than the insulation between the “L” terminal and the shell, when the insulation resistance tester is used on the ground and the correct wiring method is used, “e” The insulation resistance between the end-to-instrument case and the case-to-ground is equivalent to a short circuit