(1) Before measurement, the power supply of the equipment to be measured must be cut off, and the equipment must be short circuited to the ground for discharge. The equipment must not be charged for measurement to ensure the safety of personnel and equipment.

(2) For equipment that may induce high voltage, the possibility must be eliminated before measurement.

(3) The surface of the measured object shall be clean to reduce the contact resistance and ensure the correctness of the measurement results.

(4) Before measurement, it is necessary to check whether the megger is in normal working condition, mainly to check its “0” and “∞” points. That is, shake the handle to make the motor reach the rated speed. The megger shall point to the “0” position in case of short circuit and the “∞” position in case of open circuit.

(5) The megger shall be placed in a stable and firm place and away from large external current conductors and external magnetic fields.

After the above preparations are made, the measurement can be carried out. During the measurement, pay attention to the correct wiring of the megger, otherwise unnecessary errors or even errors will be caused.

There are three terminals of the megger: one is “L”, which is the line end, one is “e”, which is the ground end, and the other is “g”, which is the shielding end (also known as the protective ring). Generally, the measured insulation resistance is connected between the “L” and “e” ends. However, when the surface leakage of the measured insulator is serious, the shielding ring or the part that does not need to be measured must be connected with the “g” end. In this way, the leakage current directly flows back to the negative end of the generator through the shielding end “g” to form a loop, instead of flowing through the measuring mechanism (moving coil) of the megger. In this way, the influence of surface leakage current is fundamentally eliminated. In particular, it should be noted that when measuring the insulation resistance between the cable core and the outer surface, the shielding terminal button “g” must be connected properly, because when the air humidity is high or the cable insulation surface is not clean, the leakage current on the surface will be very large. In order to prevent the influence of the measured object on its internal insulation measurement due to leakage, a metal shielding ring is generally added to the outer surface of the cable, It is connected to the “g” end of the megger.

When megger is used to measure the insulation resistance of electrical equipment, it must be noted that “L” and “e” terminals cannot be connected reversely. The correct connection method is: “L” line terminal is connected to the conductor of the equipment under test, “e” ground terminal is connected to the equipment shell, “g” shielding terminal is connected to the insulation part of the equipment under test. If “L” and “e” are connected reversely, the leakage current flowing through the insulation body and surface will flow through the shell to the ground, and then flow into the measuring coil through “L”, so that “g” loses its shielding effect and brings great errors to the measurement. In addition, because the insulation between the “e” end internal lead and the housing is lower than that between the “L” end and the housing, when the megger is used on the ground and the correct wiring method is adopted, the insulation resistance of the “e” end to the instrument housing and the housing to the ground is equivalent to a short circuit and will not cause error. When the “L” and “e” are reversed, the insulation resistance of the “e” to the ground is parallel with the measured insulation resistance, resulting in a small measurement result, It will bring great error to the measurement.

It can be seen that in order to accurately measure the insulation resistance of electrical equipment, the megger must be used correctly, otherwise, the accuracy and reliability of the measurement will be lost.

Responsible editor: LQ

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