What is a megohmmeter? Functions and precautions for use of a megohmmeter

When exposed to heat and moisture, the insulation material ages, causing the insulation resistance to decrease, resulting in electrical equipment leakage or short circuit accidents. 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. Since the insulation resistance value is generally high (generally megohm level), the measured value at low voltage cannot reflect the true insulation resistance value working under high voltage conditions. Also called an 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.

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 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: the "l" wire terminal is connected to the conductor of the equipment under test, The "e" ground terminal is connected to the grounded 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 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.