How to use a digital megohmmeter_How to use a digital megohmmeter accurately
Regarding the use of digital megohmmeter, the preparation before use of digital megohmmeter, the correct use method of digital megohmmeter, and the wiring method of the three terminals of digital megohmmeter.
Preparatory tasks before use
1. Before measurement, the power supply of the equipment under test must be cut off and the equipment should be short-circuited and discharged to ground. The equipment must not be charged for measurement to ensure the safety of people and equipment.
2. For equipment that can induce high-voltage electricity, this possibility must be eliminated before measurement can be carried out.
3. The surface of the object to be measured should be clean to reduce contact resistance and ensure the accuracy of the measurement results.
4. Before measurement, check whether the digital megohmmeter is in normal working condition, first check its "0" and "∞" points. That is, shake the handle in to make the motor reach the rated speed. The digital megohmmeter should point to the "0" position when it is short-circuited, and it should point to the "∞" position when it is open-circuited.
5. When using the digital megohmmeter, it should be placed in a stable and solid place, away from large external current conductors and external magnetic fields.
Correct application method
The digital megohmmeter has three binding posts:
1. It is "L" which is the line end
2. "E" is the end of the ground.
3. "G" is the barrier end (also called the maintenance ring),
Generally, the insulation resistance to be measured is connected between the "L" and "E" terminals. However, when the surface leakage of the insulator under test is serious, the barrier 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 shielding terminal "G" to form a loop, instead of flowing through the measuring mechanism (moving coil) of the digital megger.
This basically 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, because when the air humidity is high or the cable insulation When the surface is unclean, the leakage current on the surface will be very large. In order to prevent the measured object from affecting the internal insulation measurement due to leakage, a metal barrier ring is usually added to the surface of the cable, which is in conjunction with the "digital megger" of the digital megohmmeter. G" end is connected.
When using a digital megohmmeter to measure the insulation resistance of electrical equipment, be sure to note that the "L" and "E" terminals cannot be connected reversely. The correct connection method is: the "L" wire terminal is connected to the conductor of the device under test, The "E" ground terminal is connected to the grounded equipment shell, and the "G" barrier is connected to the insulating part of the equipment under test.
If "L" and "E" are connected reversely, the leakage current flowing through the inside and outside of the insulator will collect to the ground through the shell, and then pass through the "L" dry-type high-voltage test transformer, insulating oil dielectric strength tester, and clamp The ground resistance tester, variable frequency series resonance, and wireless high-voltage nuclear phase meter flow into the measurement coil, causing the "G" to lose the barrier effect and bring great errors to the measurement.
In addition, since the insulation level between the internal lead of the "E" end and the casing is lower than the insulation level between the "L" end and the casing, when the digital megohmmeter is placed on the ground and the correct wiring method is used, the "E" end 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 insulation resistance being measured, which will cause the measurement results to change. It is too small, which will bring large errors to the measurement.
