Correct use of electronic megohmmeter
There are three binding posts for a digital megohmmeter:
One is "L" which is the line end, one "E" is the ground end, and the other "G" is the shielding end (also called guard ring). Generally, the insulation resistance to be measured is connected between the "L" and "E" ends. , but when the surface leakage of the insulator under test is serious, the shielding ring of the object under test or the part that does not need to be measured must be connected to the "G" terminal.
In this way, the leakage current flows directly back 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 digital megger. -- Home of Electrical Technology
This fundamentally eliminates the influence of surface leakage current. In particular, it should be noted that when measuring the insulation resistance between the cable core and the surface, the shielding terminal "G" must be connected.
When the air humidity is high or the cable insulation surface 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 generally added to the outside of the cable. Connect to the "G" terminal of the digital megger.
When using a digital megger to measure the insulation resistance of electrical equipment, be sure to note that the "L" and "E" terminals cannot be connected in reverse. The correct connection method is: "L" wire terminal is connected to the conductor of the equipment under test, and "E" The ground terminal is connected to the equipment shell and the "G" shield is terminated to the insulating part of the equipment under test.
If "L" and "E" are connected reversely, the leakage current flowing through the inside and surface of the insulator will be collected to the ground through the shell, and flow from the ground through "L" into the measuring coil, causing "G" to lose its shielding effect and give the measuring band There is a big error.
In addition, because the insulation degree between the internal lead of the "E" terminal and the casing is lower than that between the "L" terminal and the casing, when the digital megger is placed on the ground and used in the correct wiring method, the "E" terminal will The insulation resistance of the shell and the shell to the ground is equivalent to a short circuit and will not cause errors. However, when "L" and "E" are connected reversely, the insulation resistance of "E" to the ground is connected in parallel with the measured insulation resistance, causing the measurement results to change. It is too small, which will bring large errors to the measurement.
