How to correctly use a megger to measure insulation resistance

Electricity safety issues exist in the process of using electricity, in electrical equipment, such as motors, cables, household appliances, etc. Modern life is changing with each passing day, and people cannot live without electricity for a moment. One of their normal operations is the degree of insulation of their insulating material, that is, the value of the insulation resistance. When exposed to heat and moisture, insulation deteriorates. Its insulation resistance is reduced. This may cause leakage or short circuit accidents in electrical equipment. In order to avoid accidents, it is required to frequently measure the insulation resistance of various electrical equipment. Determine whether the insulation level meets the needs of the equipment. There are usually two ways to measure ordinary resistance: low voltage measurement and high voltage measurement. The insulation resistance generally has a high value (generally megohm level). Measured values ​​at low voltages do not reflect the true insulation resistance values ​​operating under high voltage conditions. Also called insulation resistance meter. It is the most commonly used instrument for measuring insulation resistance. It has high voltage itself when measuring insulation resistance, which is what makes it different from resistance measuring instruments. Megohmmeters are convenient and reliable for measuring insulation resistance. But if used improperly, it will bring unnecessary errors to the measurement. We must correctly use the megger to measure the insulation resistance.

When the megohmmeter is working, it generates high voltage itself, and the measurement object is equipment, so it must be used correctly, otherwise it may cause personal or equipment accidents. Before use, you must first make the following preparations:

(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 allowed to be powered for measurement to ensure the safety of people and equipment.

(2) For equipment that may induce high-voltage electricity, this possibility must be eliminated before measurement can be performed.

(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 megohmmeter is in normal working condition, mainly check its "0" and "∞" points. That is, shake the handle to make the motor reach the rated speed. The megohmmeter should point at the "0" position when it is short-circuited, and it should point at the "∞" position when it is open-circuited.

(5) When using the megohmmeter, it should be placed in a stable and solid place, away from large external current conductors and external magnetic fields.

After completing the above preparations, you can proceed with the measurement. When measuring, you must also pay attention to the correct wiring of the megohmmeter, otherwise unnecessary errors or even errors will occur.

There are three terminals of the megohmmeter: one is "l" which is the line terminal, one "e" is the ground terminal, and the other "g" is the shielding terminal (also called the protection ring). Generally, the insulation resistance to be measured is connected. Between the "l" and "e" terminals, 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 megger. 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, because when the air humidity is high or the cable insulation When the surface is not clean, the leakage current on the surface will be very large. In order to prevent the measured object from affecting its internal insulation measurement due to leakage, a metal shielding ring is generally added to the outside of the cable to match the "g" of the megohmmeter. "The ends are connected.

When using a megohmmeter 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 enclosure 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 megohmmeter is placed on the ground and used in the correct wiring method, the "e" terminal will and the insulation resistance of the shell to the ground, which 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 be biased. Small, it will bring large errors to the measurement.

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