By hvhipot 
Modern life is changing rapidly, and people cannot do without electricity for a moment. There are electrical safety issues in the process of using electricity, such as in electrical equipment such as motors, cables, household appliances, etc. One of their normal operations is the degree of insulation of their insulation material, which is the value of insulation resistance. When exposed to heat and moisture, insulation materials age. Its insulation resistance decreases. As a result, electrical equipment leakage or short circuit accidents may occur. To avoid accidents, it is required to regularly measure the insulation resistance of various electrical equipment. Determine whether its insulation level meets the requirements of the equipment. There are usually two methods for measuring ordinary resistors: low voltage measurement and high voltage measurement. And the insulation resistance is generally high in value (usually in the megaohm range). The measured value at low voltage cannot reflect the true insulation resistance value under high voltage conditions. A megohmmeter is also called an insulation resistance meter. It is the most commonly used instrument for measuring insulation resistance. It has a high-voltage power supply when measuring insulation resistance, which is the difference between it and resistance measuring instruments. A megohmmeter is convenient and reliable for measuring insulation resistance. But if used improperly, it will bring unnecessary errors to the measurement. We must use a megohmmeter to measure the insulation resistance correctly.
A megohmmeter generates high voltage during operation, and the measuring object is electrical equipment, so it must be used correctly, otherwise it may cause personal or equipment accidents. Before use, the following preparations should be made:
(1) Before measurement, the power supply of the tested equipment must be cut off and short circuited to ground for discharge. It is not allowed to measure with the equipment live 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 tested object should be cleaned 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 checking its “0” and “∞” points. Shake the handle to reach the rated speed of the motor. The megohmmeter should indicate the “0” position when short circuited and the “∞” position when open circuited.
(5) When using a megohmmeter, it should be placed in a stable and secure place, and kept away from large external current conductors and magnetic fields. After completing the above preparations, the measurement can be carried out. During the measurement, it is also important to pay attention to the correct wiring of the megohmmeter, otherwise it will cause unnecessary errors or even mistakes.
There are three terminals for a megohmmeter: one is the “L” terminal, one is the ground terminal, and the other is the shielding terminal (also known as the protective ring). Generally, the measured insulation resistance is connected between the “L” and “E” terminals. However, when the surface leakage of the measured insulation is severe, the shielding ring 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 circuit, without passing through the measuring mechanism (moving coil) of the megohmmeter. This fundamentally eliminates the influence of surface leakage current. It is particularly important to note that when measuring the insulation resistance between the cable core and the outer surface, the shielding terminal “G” must be connected properly, because when the air humidity is high or the insulation surface of the cable is not clean, the surface leakage current will be large. To prevent the influence of the measured object on its internal insulation measurement due to leakage, a metal shielding 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, it is important to note that the “L” and “E” terminals cannot be connected in reverse. The correct connection method is to connect the “L” terminal to the conductor of the tested equipment, the “E” ground terminal to the equipment casing, and the “G” shield terminal to the insulation part of the tested equipment. If “L” and “E” are connected in reverse, the leakage current flowing through the insulation body and surface will be collected by the outer shell to the ground, and then flow into the measuring coil through “L” from the ground, causing “G” to lose its shielding effect and causing significant measurement errors. In addition, because the insulation degree between the internal leads of the “E” end and the shell is lower than that between the “L” end and the shell, when the megohmmeter is used on the ground and the correct wiring method is used, the insulation resistance between the “E” end and the instrument shell and the shell to ground is equivalent to a short circuit and will not cause errors. However, when the “L” and “E” are connected in reverse, the insulation resistance between the “E” end and the ground is parallel to the measured insulation resistance, resulting in a smaller measurement result and a larger measurement error.
From this, it can be seen that in order to accurately measure the insulation resistance of electrical equipment, it is necessary to use the megohmmeter correctly. Otherwise, the accuracy and reliability of the measurement will be lost.