I summarized the working principle and usage instructions of megohmmeter

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Megohmmeter, commonly known as megohmmeter, is a special instrument commonly used to measure the insulation resistance and high-value resistance of electrical equipment and cables. 1. Structure and working principle of megohmmeter The megohmmeter mainly consists of a hand-operated DC generator, a magnetoelectric current ratio meter, and three terminals (i.e., L: Line end, E: Ground terminal, G: shielded end). The movable part of the flow meter has two coils winding in opposite directions and at a certain angle to each other, and the magnetic induction intensity in the air gap is uneven. Coil 1 is used to generate rotational torque, and coil 2 is used to generate reaction force torque. The resistance to be measured is connected to the two terminals L (line) and E (ground), forming two loops, one is the current loop and the other is the voltage loop. The current loop returns from the positive terminal of the power supply to the negative terminal of the power supply through the measured resistor Rx, the current limiting resistor RA, and the movable coil 1. The voltage loop returns from the positive terminal of the power supply to the negative terminal of the power supply through the current limiting resistor RV and the movable coil 2. Since the magnetic induction intensity in the air gap is uneven, the torques T1 and T2 generated by the two coils are not only related to the currents I1 and I2 flowing through the coils, but also related to the deflection angle α of the movable part. The current-limiting resistors RA and RV are fixed values. When the generator voltage remains unchanged, the current I2 of the voltage loop is constant. The size of the current loop current I1 is inversely proportional to the size of the resistance Rx being measured. Therefore, the deflection angle α of the current ratio meter pointer can directly reflect the size of the resistance Rx being measured. The deflection angle of the flow ratio meter pointer has nothing to do with changes in the power supply voltage. The fluctuation of the power supply voltage U has the same interference with the rotational torque and reaction torque. Therefore, the accuracy of the flow ratio meter has nothing to do with the voltage. 2. Use of megohmmeter 1. Choose a megohmmeter that meets the voltage level. Under normal circumstances, equipment with a rated voltage below 500V should use a 500V or 1000V megger. ； For equipment with a rated voltage above 500V, choose a megger with a range of 1000V~2500V. 2. It can only be measured when the device is not powered and there is no induced electricity. 3. Before measurement, the megohmmeter should be subjected to an open circuit and short circuit test to check whether the megohmmeter is in good condition. Open the two connecting wires, shake the handle, the pointer should point at "∞", then short-circuit the two connecting wires, the pointer should point at "0", if the above conditions are met, it is good, otherwise it cannot be used. 4. When measuring insulation resistance, generally only the "L" and "E" terminals are used. However, when measuring the insulation resistance of the cable to the ground or the leakage current of the device under test is serious, the "G" terminal must be used and the "G" terminal must be connected to the shielding layer or shell. In this way, the current flowing through the insulation surface no longer passes through the measuring coil of the current ratio meter, but directly flows through the G terminal to form a loop. Therefore, the measured insulation resistance is only the volume resistance of the cable insulation. 5. After the circuit is connected, you can turn the crank clockwise. The shaking speed should be from slow to fast. When the rotation speed reaches about 120 rpm (ZC-25 type), keep rotating at a constant speed, and take readings while shaking. Do not stop to take readings. 6. It is strictly forbidden to touch the megohmmeter with your hands before it stops rotating or before the device under test is discharged. At the end of the measurement, large capacitance devices need to be discharged. The discharge method is to remove the ground wire used during measurement from the megohmmeter and short-circuit it with the device under test. 7. Generally, the minimum scale is 1M ohm, and the measuring resistance should be greater than 100K. 8. It is forbidden to measure insulation resistance during lightning or near high-voltage equipment. During the shaking measurement, no one can work on the equipment under test. In addition, its accuracy should be checked regularly.