(1) Before measurement, the power supply of the equipment under test must be cut off and short-circuited to ground. Never allow the equipment to be charged for measurement to ensure the safety of persons and equipment.
(2) For equipment that may induce high voltage. This possibility must be eliminated before the measurement can be performed.
(3) The surface of the test object should be clean. Reduce contact resistance and ensure the accuracy of measurement results.
(4) Check whether the megohmmeter is in normal working condition before measurement. Mainly check its "0" and "∞" two points, namely shaking the handle. Make the motor reach the rated speed; the megohmmeter should be at the "0" position when short-circuited, and at the "∞" position when open.
(5) The megohmmeter should be placed in a stable and firm place when using it. And stay away from large external current conductors and external magnetic fields.
After the above preparations are done, the measurement can be carried out. When measuring, pay attention to the correct wiring of the megohmmeter. Otherwise, unnecessary errors or even errors will be caused.
There are three terminals on the megohmmeter: one is "L", which is the wire end, and the other is "E", which is the ground terminal. Another "G" is the shielding end (also called the protection ring). Generally, the measured insulation resistance is connected between the "L" and "E" ends. But when the surface leakage of the insulator under test is serious, the shield ring of the test object or the part that does not need to be measured must be connected to the "G" terminal, so that the leakage current flows directly back to the negative terminal of the generator through the shield terminal "G" A loop is formed instead of flowing through the measuring mechanism (moving coil) of the megohmmeter, so that the effect of surface leakage current is fundamentally eliminated. It should be especially noted that when measuring the insulation resistance between the cable core and the outer surface, the shield terminal button "G" must be connected, because when the air humidity is high or the cable insulation surface is not clean, the leakage current on the surface will be very high. Large, in order to prevent the leakage of the measured object from affecting its internal insulation measurement. Generally, a metal shielding ring is added to the outside of the cable and connected to the "G" end of the megohmmeter.
When using a megohmmeter to measure the insulation resistance of electrical equipment, you must pay attention to the "L" and "E" terminals cannot be reversed; the correct connection method is: "L" wire terminal button is connected to the conductor of the device under test, "E" Ground terminal button grounded equipment shell, "G" shield terminal is connected to the insulated part of the device under test; if the "L" and "E" are connected reversely, the leakage current flowing through the insulator and the surface will be collected to the ground through the shell. The ground flows into the measuring coil via the "L", which makes the "G" lose its shielding effect and brings great errors to the measurement; in addition, the insulation degree of the internal lead of the "E" terminal and the shell is higher than that of the "L" terminal and the shell. To be low. When the megohmmeter is used on the ground and the correct wiring method is adopted, the insulation resistance of the "E" terminal to the instrument case and the case to the ground is equivalent to a short circuit, which will not cause errors. When "L" and "E" are connected reversely, the insulation resistance of "E" to the ground is connected in parallel with the measured insulation resistance, which makes the measurement result too small, and brings greater error to the measurement.
Therefore, it is necessary to accurately measure the insulation resistance of electrical equipment. The megohmmeter must be used correctly. Otherwise, the accuracy and reliability of the measurement will be lost.