A megohmmeter, also called an insulation resistance tester or a megger, is a simple and commonly used direct-reading instrument for measuring high resistances. It can be used to measure the insulation resistance of circuits, motor windings, cables, electrical equipment, etc. The megohmmeter has three terminals labeled Ground (N or E), Voltage (U or L), and Guard (G). When using it, not only must the connections be correct and the terminals tightened, but also the following precautions should be observed:
1. Before measurement, perform an open-circuit and short-circuit test on the megohmmeter to check whether it is functioning properly. The specific operation is: with the two connecting leads open, rotate the handle and the pointer should point to infinity; then short the two leads together, and the pointer should point to zero.
2. The device under test must be disconnected from other power sources. After measurement, be sure to fully discharge the device under test (about 2-3 minutes) to protect the equipment and personal safety.
3. The megohmmeter and the device under test should be connected separately with single-strand wires, and the surface of the wires should be kept clean and dry to avoid errors caused by poor insulation between wires.
4. When measuring, place the megohmmeter in a horizontal position. While turning the crank, the terminals must not be short-circuited. When measuring capacitors or cables, the leads must not be disconnected until the crank stops rotating; otherwise, reverse charging may damage the megohmmeter.
5. When rotating the handle, start slowly and gradually accelerate uniformly to 120 r/min, and be careful to prevent electric shock. During rotation, if the pointer indicates zero, do not continue rotating to prevent the internal coil from overheating and being damaged.
6. To prevent surface leakage resistance of the device under test, when using the megohmmeter, connect the middle layer of the device under test (such as the inner insulation between the cable core and sheath) to the guard terminal.
7. Select an appropriate insulation resistance tester according to the voltage level of the device under test. Generally, for devices with a rated voltage below 500V, use a 500V or 1000V megohmmeter; for devices with a rated voltage of 500V and above, use a 1000-2500V megohmmeter. When selecting the range, care should be taken not to let the measurement range exceed the insulation resistance value of the device under test too much, to avoid large errors in readings.
8. It is forbidden to use the megohmmeter during thunderstorm weather or near equipment with high-voltage conductors.
