Correct method for measuring insulation resistance

There are safety issues in the use of electricity in electrical equipment, such as motors, cables, and household appliances. Modern life is changing with each passing day, and people cannot live without electricity. One of the normal operations of these equipment is the insulation degree of their insulating materials, that is, the value of the insulation resistance. When heated and damp, the insulating materials age. Their insulation resistance decreases. This causes leakage or short circuit accidents in electrical equipment. In order to avoid accidents, it is required to measure the insulation resistance of various electrical equipment frequently. Determine whether the insulation degree meets the needs of the equipment. There are usually two ways to measure ordinary resistance: measuring at low voltage and measuring at high voltage. However, due to the general high value of insulation resistance (generally megohm level), the measured value at low voltage cannot reflect the true insulation resistance value working under high voltage conditions. It is also called an insulation resistance meter. It is the most commonly used instrument for measuring insulation resistance. It has a high voltage when measuring insulation resistance, which is the difference between it and the resistance measuring instrument. The megohmmeter is convenient and reliable for measuring insulation resistance. However, if used improperly, it will bring unnecessary errors to the measurement. We must use the megohmmeter correctly to measure insulation resistance.

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

(1) Before measurement, the power supply of the device under test must be cut off and short-circuited to the ground. Never measure with the device powered on to ensure personal and equipment safety.

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

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

(4) Before measuring, 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 to the "0" position when short-circuited and the "∞" position when open-circuited.

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

After completing the above preparations, you can start measuring. When measuring, you must also pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors or even mistakes.

There are three terminals on the megohmmeter: one is "l", which is the line terminal, one is "e", which is the ground terminal, and the other is "g", which is the shield terminal (also called the protection ring). Generally, the insulation resistance to be measured is connected between the "l" and "e" terminals. However, when the surface leakage of the insulator to be measured is serious, the shield ring of the object to be measured or the part that does not need to be measured must be connected to the "g" terminal. In this way, the leakage current will flow directly back to the negative terminal of the generator through the shield terminal "g" to form a loop, and will not flow through the measuring mechanism (moving coil) of the megohmmeter. 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 shield terminal "g" must be connected properly, because when the air humidity is high or the cable insulation surface is not clean, the surface leakage current will be very large. In order to prevent the influence of leakage on the internal insulation measurement of the object to be measured, a metal shield ring is generally added to the outer surface of the cable and connected to the "g" terminal of the megohmmeter.

When using a megohmmeter to measure the insulation resistance of electrical equipment, be sure to pay attention to the "l" and "e" terminals cannot be connected in reverse. The correct connection method is: the "l" line terminal is connected to the conductor of the device under test, the "e" ground terminal is connected to the grounded device shell, and the "g" shield terminal is connected to the insulation part of the device under test. If "l" and "e" are connected in reverse, the leakage current flowing through the insulator and the surface will be collected to the ground through the shell, and then flow into the measuring coil through the ground, so that "g" loses its shielding effect and causes a large error in the measurement. In addition, because the insulation degree between the internal lead of the "e" terminal and the shell is lower than the insulation degree between the "l" terminal and the shell, when the megohmmeter is placed on the ground and used, the insulation resistance of the "e" terminal to the instrument shell and the shell to the ground is equivalent to a short circuit when the correct wiring method is used, and no error will be caused. When "l" and "e" are connected in reverse, the insulation resistance of "e" to the ground is connected in parallel with the insulation resistance to be measured, which makes the measurement result smaller and causes a large error in the measurement.

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