Because the output voltage of a multimeter's resistance mode is low, generally 9 volts, so it can't be used to measure the insulation resistance of electrical equipment (general low-voltage electrical equipment should be more than 1,000 volts).
How to wire a megohmmeter?
The correct wiring of the megohmmeter
Generally, there are three terminals on the megohmmeter, one for the line terminal labeled "L", one for the ground terminal labeled "E", the other for the protection or shielding terminal labeled "G"; the other for the protection or shielding terminal labeled "G"; the other for the protection or shielding terminal labeled "G"; the other for the protection or shielding terminal labeled "G"; the other for the protection or shielding terminal labeled "G". "G"; in measurement, "L" is connected to the insulated conductor part of the measured equipment and earth, and "E" is connected to the shell or other conductor part of the measured equipment. Generally in the measurement of only "L" and "E" two terminals, but when the measured equipment surface leakage is serious, on the measurement of the results of the larger impact and is not easy to eliminate, such as air is too humid, the surface of the insulating material is eroded and can not be wiped clean when it must be Connect the "G" end button. At the same time in the wiring must also pay attention to can not use double stranded wire, should be used with good insulation and different colors of the single wire, especially for the connection of the "L" terminal wire must have good insulation.
Measurement and readings
Measurement should be the first megohmmeter placed smoothly, turn the handle of the megohmmeter at a constant speed, so that its speed to maintain in the rated range of speed (generally 120 (-2 ~ +5) rpm), avoid fast and slow. As the insulation resistance of the measured equipment is different with the length of the measurement time, usually using a constant speed in a minute before starting to read, when encountered in the measured object of particularly large capacity, must wait until the pointer is stable and unchanged before reading and recording. If the work requires, should also record the measurement of the ambient temperature and climatic conditions and the megohmmeter used in the voltage level, the measurement range, the condition of the equipment to be measured, in order to analyze the results.
Safety precautions when using megohmmeter
In practice, when using megohmmeter insulation resistance measurement, often the workplace is more complex, there are many high and low voltage equipment around the operation of the charged as well as the performance characteristics of the equipment to be measured to recognize the lack of hazardous sources, if you do not take it seriously, in our measurement of the insulation resistance will be a serious danger to the safety of persons and equipment If we do not take it seriously, it will bring serious harm to the safety of people and equipment when we measure insulation resistance. Should pay attention to the following aspects:
In the vicinity of the energized equipment using megohmmeter insulation resistance measurement must be at least two people to participate in the instrument, wire, human body on the energized equipment should maintain a sufficient safety distance, and can not be vigorously pulling the test lead and expand the scope of the work;
Measurement of the equipment must be connected to cut off and the measurement of all the power supply, and will be measured by the equipment to be connected to the ground short-circuit discharged.
Before measurement, all power supplies connected to the equipment to be measured must be cut off and the equipment to be measured must be grounded and discharged in a short circuit. Never allow the megohmmeter to measure the insulation resistance of the charged equipment or the equipment after the power failure without discharging, so that not only do not get the correct measurement results, but also on the person and the equipment constitutes a great danger or damage;
For the possibility of inducing a high voltage equipment, in the possibility of eliminating the previous, never blindly use the megohmmeter to measure it;
In the megohmmeter Before the megohmmeter stops rotating or the equipment under test is not discharged, do not touch the terminals of the megohmmeter or the equipment under test with your hands. Especially after the test with large capacitance equipment, you must first short-circuit the DUT to ground to discharge, and then stop turning the megohmmeter, carry out the work of removing wires.