There are three binding terminals of the megger: one is "L" which is the line terminal, one "E" is the ground terminal and the other "G" is the shield terminal (also called guard). ring).
Generally, the insulation resistance to be measured is connected between terminals "L" and "E". However, when the surface leak of the insulation to be measured is serious, the shielding ring or the part which constitutes it. does not need to be measured must be connected to terminal "G".
In this way, the leakage current returns directly to the negative terminal of the generator through the shielded terminal "G" to form a loop, instead of flowing through the measuring mechanism (moving coil) of the megger. . This basically eliminates the influence of surface leakage current. Particular attention should be paid to the fact that when measuring the insulation resistance between the cable core and the surface, the shield terminal "G" must be connected.
Because When the air humidity is high or the surface of the cable insulation is not clean, the leakage current on its surface will be very large. In order to prevent the measured object from affecting the internal insulation measurement due to leakage, a metal shielding ring is usually added. outside the cable. Connect to the “G” terminal of the megger.
Detailed information
Precautions for use:
(1) Depending on the withstand voltage level of the equipment tested, select different output voltages. Generally, for low voltage electrical appliances with insulation voltage below 500V, megohmmeters below 500V are used to test between motor windings, transformer windings and ground. High-voltage meggers cannot be used to test low-voltage equipment to avoid insulation breakdown; structure of theequipment.
(2) During the shaking test, if the megger needle has reached zero and shaking the generator is "hard", it means the circuit under test is short circuited. Don't keep starting. The handle is more difficult to avoid damaging the instrument.
(3) After the test is completed, the filter capacitor of the instrument still contains a charge, and the test terminals should be short-circuited and discharged to avoid electric shock.
(4) When testing the line, make sure the line is disconnected from the power supply.