How to use the megohmmeter and what to pay attention to when using it

First of all, let's talk about the selection of megohmmeter. For equipment with a rated voltage of less than 500 volts, use a 500 volt or 1000 volt megohmmeter. For equipment with a rated voltage of more than 500 volts, use a 1000 volt to 2500 volt megohmmeter.

How to use the megohmmeter

The megohmmeter has three measurement terminals, one line terminal (L), another ground terminal (E), and one shield terminal (G). Usually, when measuring the insulation resistance of a photo or power line to the ground, only the L and E terminals are used, and the wiring is shown in the left figure below. The "L" terminal is connected to the "live" or "phase terminal" of the device under test, and the "E" terminal is connected to the "ground" terminal of the device under test. When measuring the insulation resistance of the cable to the ground or when the leakage current of the device under test is severe, use the "G" terminal. The right figure below shows the wiring diagram for measuring the insulation resistance of the cable.

After the circuit is connected, the generator crank of the megohmmeter can be turned clockwise to make the voltage announced by the generator rotor for measurement. The speed of the crank should be from slow to fast, and when the speed reaches a certain value, the speed should be kept uniform and stable. (Usually, the speed of the megohmmeter is about 120 revolutions). When the generator speed of the megohmmeter is stable, the pointer on the dial also stabilizes. At this time, the value indicated by the pointer is the measured insulation resistance value.

Precautions Before measuring, the megohmmeter should be tested for open circuit and short circuit to check whether it is good. If the two connecting wires are open circuited and the handle is gently shaken, the pointer should point to "∞". At this time, if the two connecting wires are short-circuited again, the pointer should be at "0", indicating that the megohmmeter is good, otherwise the megohmmeter is wrong. The power supply of the equipment under test should be disconnected, and the capacitance equipment should be fully discharged to ensure personal safety and measurement accuracy. During the telemetry process, no one can work on the equipment under test. When measuring motors, transformers, cables, and capacitors with large capacitance, there should be a certain charging time, and the larger the capacity, the longer the charging time. The reading after the megohmmeter rolls for one minute is usually used as a standard. Immediately discharge after the measurement is completed to ensure safety. The discharge method is to remove the ground wire used in the measurement from the megohmmeter and short-circuit it on the measured object. Avoid measuring insulation during lightning or near high-voltage conductors. The device can only be measured when it is not energized and cannot be energized by other power sources. Do not touch the measuring part of the device or the terminal of the megohmmeter with your hands before the megohmmeter stops rolling. When removing the wires, do not directly touch the exposed part of the lead wire. The megohmmeter should be calibrated regularly: the calibration method is to directly measure the standard resistance with a determined value to measure whether it is measured incorrectly and whether it is behind the permitted design.