The megohmmeter, also known as a megger or insulation resistance tester, is a simple and commonly used direct-reading instrument for measuring high resistance. 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 if 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 briefly 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, it is essential to fully discharge the device (about 2-3 minutes) to protect the equipment and personal safety.
3. Use single-strand wires to connect the megohmmeter and the device under test separately, and keep the surface of the wires clean and dry to avoid errors caused by poor insulation between wires.
4. When measuring, place the megohmmeter on a level surface. While turning the crank, the terminals must not be short-circuited. When measuring capacitors or cables, the leads must only be disconnected while the crank is still turning; otherwise, reverse charging may damage the megohmmeter.
5. When turning the handle, start slowly and gradually accelerate uniformly to 120 r/min, and be careful to prevent electric shock. During the turning process, if the pointer indicates zero, do not continue turning 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 (such as the inner insulation between the cable core and sheath) to the guard terminal.
7. Choose an appropriate insulation resistance tester based on the voltage level of the device under test. Generally, for devices with rated voltage below 500V, use a 500V or 1000V megohmmeter; for devices with rated voltage of 500V and above, use a 1000-2500V megohmmeter. When selecting the range, care should be taken not to have the measurement range excessively exceed the insulation resistance value of the device under test, to avoid large errors in readings.
8. It is forbidden to use the megohmmeter during thunderstorm weather or near equipment with high-voltage conductors.
