How to Use a Megohmmeter
The megohmmeter, also known as the insulation resistance tester, is mainly used to measure the insulation resistance of electrical equipment.
The following are the usage methods of the megohmmeter:
1. **Preparation Work**
- **Select the appropriate megohmmeter**: Select a megohmmeter with the corresponding voltage level according to the voltage level of
the device under test. Generally speaking, for electrical equipment with a voltage below 500V, a 500V or 1000V megohmmeter should be
selected; for electrical equipment with a voltage between 500V and 1000V, a 1000V or 2500V megohmmeter should be selected; for electricalequipment with a voltage above 1000V, a 2500V or 5000V megohmmeter should be selected.
- **Check the megohmmeter**: Before use, conduct an open-circuit and short-circuit test on the megohmmeter to check whether it is in good
condition. Separate the two leads of the megohmmeter, shake the handle, and the pointer should point to "∞"; then short-circuit the two leads,
gently shake the handle, and the pointer should point to "0". If the pointer cannot point to the corresponding position, it indicates that the
megohmmeter has a fault and needs to be repaired.
- **Cut off the power supply**: Before measuring the insulation resistance of electrical equipment, the power supply of the device under test
must be cut off first, and discharge should be carried out to ensure the safety of personnel and equipment.
- **Clean the device under test**: Wipe the surface of the device under test clean with a clean cloth or brush to remove impurities such as dust
and oil stains, so as not to affect the measurement results.
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2. **Wiring**
- **Connect the "L" terminal**: Connect the "L" terminal (line terminal) of the megohmmeter to the conductor of the device under test with a
wire. For example, when measuring the insulation resistance of an electric motor, connect the "L" terminal to the winding lead of the electric motor.
- **Connect the "E" terminal**: Connect the "E" terminal (ground terminal) of the megohmmeter to the casing or grounding body of the device
under test with a wire. If the device under test is an electric motor, connect the "E" terminal to the casing of the electric motor.
- **Connect the "G" terminal (if necessary)**: For devices under test with a dirty surface or high humidity, in order to eliminate the influence of
surface leakage current, the "G" terminal (shield terminal) of the megohmmeter can be connected to the insulation layer of the device under test.
For example, when measuring the insulation resistance of a cable, connect the "G" terminal to the insulation layer of the cable.
3. **Measurement**
- **Shake the handle**: Shake the handle at a uniform speed so that the speed of the megohmmeter gradually reaches the rated speed (generally
120r/min). During the shaking process, keep the speed of the handle stable and do not shake it too fast or too slow, so as not to affect the accuracy
of the measurement results.
- **Read the data**: After the pointer of the megohmmeter stabilizes, read the scale value indicated by the pointer, which is the insulation resistance
value of the device under test. When reading the data, pay attention to the range and scale of the megohmmeter to ensure the accuracy of the reading.
4. **Disconnect the wires and discharge**
- **Disconnect the wires**: After the measurement is completed, first remove the lead wire connected to the device under test from the megohmmeter,
and then stop shaking the handle.
- **Discharge**: Discharge the device under test to eliminate the residual charge on the device. The discharge method is to connect the conductor
of the device under test to the grounding body with a wire, allowing the charge to be released into the ground through the wire.
When using the megohmmeter, it is necessary to strictly follow the operation steps to ensure the accuracy of the measurement results and the safety
of personnel and equipment.
