Maintenance and selection of digital megohmmeters - Solutions - Huaqiang Electronic Network

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If a multimeter is used to measure the insulation resistance of the device, then the measured value of the insulation resistance at low voltage is not really reflected in the insulation performance under high voltage conditions. The difference between a digital megohmmeter and a multimeter is that it has a high-voltage power supply with a voltage of 500 to 5000V. Therefore, measuring the insulation resistance with a digital megger can obtain the insulation resistance value in accordance with the actual working conditions.
1. Use of digital megohmmeter Before the measurement, the power of the device under test should be cut off, and the conductive part of the device should be connected to the ground to fully discharge to ensure safety. Electrical equipment measured with a digital megohmmeter must also be grounded and discharged in time for re-measurement.
Before measuring, check whether the digital megohmmeter is in good condition. Before the digital megohmmeter is not connected to the measured object, turn on the power switch to detect the condition of the digital megohmmeter battery. If the digital megohmmeter battery is under voltage, replace the battery in time. Otherwise, the measurement data is not desirable. Insert the test leads into the terminal "Line (L) and Ground (E)", select the test voltage, disconnect the test line, press the test button, and observe whether the number shows infinity. Short the terminal "Line (L) and Ground (E)" and press the test button to see if "0" is displayed. If the LCD does not display “0”, it indicates that the digital megohmmeter is faulty and should be used after inspection. Must be wired correctly. There are generally three terminals on a digital megger, labeled L (line), E (ground), and G (shield). Where L is connected to the conductor part of the object to be inspected and grounded, E is connected to the outer casing or earth of the object to be tested, and G is connected to the shield of the object to be tested or the part that does not need to be measured. Terminal G is used to shield the surface current. If the insulation resistance of the cable is measured, the measurement result will be inaccurate due to the leakage current on the surface of the insulation material, especially in the case of high humidity and the cable insulation surface is not clean, the measurement error will be large. To avoid the effects of surface current, a metal shield ring is attached to the surface of the object to be connected to the "shield" terminal of the digital megger. Thus, the surface leakage current IB starts from the positive pole of the generator and flows back to the negative pole of the generator via the terminal G to form a loop. The IB no longer passes through the megohmmeter's measuring mechanism, thus fundamentally eliminating the effects of surface leakage current. The wires connecting the terminal to the device under test cannot be double insulated wires or stranded wires. They should be separated separately by single strands to avoid errors caused by poor insulation of the strands. In order to obtain the correct measurement results, the surface of the device under test should be cleaned with a clean cloth or cotton yarn.
Measure the insulation resistance of a device with a large capacitance. After the reading, the megohmmeter cannot be disconnected immediately. Otherwise, the capacitor that has been charged will discharge the megohmmeter, which may burn the megohmmeter. The test line should be disconnected first after reading, and then the test should be stopped. The conductive part of the device under test cannot be touched by hand until the megohmmeter and the test object are fully discharged. When measuring the insulation resistance of the equipment, you should also record the temperature and humidity during the measurement, the relevant conditions of the test object, etc., in order to analyze the measurement results.
2. The choice of digital megohmmeter The choice of digital megohmmeter is mainly to choose its voltage and measurement range. High-voltage electrical equipment requires high insulation resistance and must be tested with a high-voltage megohmmeter. The internal insulation of low-voltage electrical equipment can not withstand high voltage. To ensure equipment safety, a digital megohmmeter with low voltage should be selected.