During the process of using electricity, there are issues related to electrical safety. In electrical equipment, such as motors, cables, and household appliances, one of the normal operating conditions is the insulation level of the insulation materials, which is the value of the insulation resistance. When exposed to heat and moisture, the insulation materials age. Their insulation resistance decreases. This can lead to electrical leakage or short circuit accidents in the electrical equipment. To prevent such accidents, it is necessary to frequently measure the insulation resistance of various electrical equipment. To determine whether the insulation level meets the requirements of the equipment.
The measurement of ordinary resistors usually involves two methods: low-voltage measurement and high-voltage measurement. However, since the insulation resistance values are generally quite high (typically at the megohm level), the measurement results obtained at low voltages cannot accurately reflect the actual insulation resistance values under high-voltage conditions. Digital megohmmeters are also known as insulation resistance meters. They are the most commonly used instruments for measuring insulation resistance. When measuring insulation resistance, they inherently have a high-voltage power supply, which distinguishes them from resistance measurement instruments. Digital megohmmeters are convenient and reliable for measuring insulation resistance, but if used improperly, they can introduce unnecessary errors to the measurement. We must use digital megohmmeters correctly to measure insulation resistance.
When a digital megohmmeter is in operation, it generates high voltage by itself, and the object being measured is an electrical device. Therefore, it must be used correctly; otherwise, it could lead to personal or equipment accidents. Before use, the following preparations must be made:
(1) Before measurement, the power supply of the equipment to be tested must be cut off and discharged to the ground through a short circuit. It is strictly forbidden to conduct measurements while the equipment is still powered on to ensure the safety of personnel and the equipment.
(2) For equipment that may generate high voltage electricity, such a possibility must be eliminated before measurement can be carried out.
(3) The surface of the measured object should be clean to reduce contact resistance and ensure the accuracy of the measurement results.
(4) Before measurement, it is necessary to check whether the digital megohmmeter is in normal working condition. The main points to check are “0″ and “∞”. When the digital megohmmeter is short-circuited, it should point to the “0″ position; when it is open-circuited, it should point to the “∞” position.
(5) When using the digital megohmmeter, it should be placed on a stable and firm surface, and kept away from large external current conductors and external magnetic fields.
After completing the above preparations, the measurement can be carried out. During the measurement, it is also necessary to pay attention to the correct wiring of ∞; otherwise, it will cause unnecessary errors or even mistakes.
The connection terminals of the digital megohmmeter are three in total: one is “L” which is the line terminal, one is “E” which is the ground terminal, and the other is “G” which is the shielding terminal (also known as the protection ring). Generally, the measured insulation resistance is connected between the “L” and “E” terminals. However, when the insulation surface of the measured object has severe leakage, the shielding ring of the measured object or the part that does not need to be measured must be connected to the “G” terminal. In this way, the leakage current will directly flow back to the negative terminal of the generator through the shielding terminal “G” to form a circuit, and will not flow 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 properly connected. This is because when the air humidity is high or the insulation surface of the cable is not clean, the surface leakage current will be very large. To prevent the influence of leakage current on the internal insulation measurement of the measured object, a metal shielding ring is generally added to the outer surface of the cable, which is connected to the “G” terminal of the megohmmeter.
When using a digital megohmmeter to measure the insulation resistance of electrical equipment, it is essential to ensure that the “L” and “E” terminals are not connected in reverse. The correct connection method is as follows: the “L” terminal is connected to the conductor of the equipment under test, the “E” terminal is grounded to the housing of the equipment, and the “G” terminal is connected to the insulation part of the equipment under test. If the “L” and “E” terminals are connected in reverse, the leakage current flowing through the insulation inside and on its surface will gather through the housing to the ground. Then, it flows into the measuring coil through “L”, causing the “G” terminal to lose its shielding effect and resulting in significant errors in the measurement.
Furthermore, since the insulation between the internal leads at the “E” end and the housing is lower than that between the “L” end and the housing, when using a digital megohmmeter on the ground and using the correct wiring method, the insulation resistance between the “E” end and the instrument housing, as well as the insulation resistance between the housing and the ground, is equivalent to a short circuit, and will not cause errors. However, when the “L” and “E” terminals are connected in reverse, the insulation resistance of the “E” end to ground is in parallel with the measured insulation resistance, causing the measurement result to be smaller and resulting in a significant error in the measurement.
Thus, it can be seen that in order to accurately measure the insulation resistance of electrical equipment and other items, the digital megohmmeter must be used correctly; otherwise, the accuracy and reliability of the measurement will be lost.
Post time: Apr-21-2026