Measurement steps for insulation resistance: Measurement preparation work
When the object under test is connected to a power source, it should be stopped from operating before measurement. The power supply should be cut off. It is strictly prohibited to conduct the test while the object is energized. Otherwise, not only will the insulation resistance not be accurately measured, but the megohmmeter will also be damaged and other accidents such as electric shock to personnel and other incidents will occur.
For the tested object after the power is disconnected, after confirming that the equipment is de-energized, it is necessary to short-circuit the tested points or to ground them to fully discharge the charges. Especially for equipment with capacitors and those with large capacitance, such as large motors, long cables, and large-capacity transformers, the discharge time needs to be longer, usually about two to three minutes. In some cases, it is necessary to short-circuit the tested points and related parts for a period of time before opening the equipment, wait for a while before short-circuiting again, and repeat this process several times until it is confirmed that there are no remaining charges, then the measurement can be carried out.
Some of the tested objects may not be directly related to the power supply. However, if there are high-voltage, high-current conductors or factors that can emit electromagnetic field signals within or near the object, these influencing factors must also be eliminated. In summary, only when the tested object itself is not electrified and is impossible to be electrified due to induction from other power sources can the insulation resistance measurement be carried out to ensure the safety of personnel and instruments and the accuracy of the test results.
To ensure the accuracy of the test results, the measurement part of the tested object must be cleaned. For example, the areas near the tested points and the surfaces of the tested objects should be wiped clean, without any dirt or moisture, to avoid affecting the accuracy of the measurement due to leakage. For the test points on the tested object where test lines are connected, they also need to be kept clean. All dust, oil stains and rust should be removed to reduce the contact resistance and ensure the reliability of electrical contact.
Connection rules for megohmmeter
The three terminals “L” (line), “E” (ground), and “G” (shield) on the megohmmeter should be correctly connected to the object being measured. They must not be reversed randomly, otherwise it will cause significant errors in the measurement and even make it impossible to measure. Usually, the “L” terminal is connected to the high potential (conductive working part) of the object being measured, the “E” terminal is connected to the casing or low potential of the object, and the “G” terminal is connected to the protective shielding part or other parts that do not participate in the measurement of the object.
When measuring the insulation resistance to ground of electrical equipment or power lines, the “L” terminal should be connected to the conductive working part of the object being measured, and the “E” terminal should be connected to its casing or ground wire to avoid the influence caused by stray currents in the ground. For example, when measuring the insulation resistance to ground of an electric motor, the “L” terminal can be connected to the terminal of the motor, and the “E” terminal can be connected to the casing of the motor; when measuring the insulation resistance to ground of a power line, the “L” terminal should be connected to the conductor of the tested line, and the “E” terminal should be connected to the ground wire.
When the surface of the tested insulator is not very clean or has high humidity and other unfavorable factors, the surface leakage current may be relatively large, which will seriously distort the test results.
In order to eliminate the influence of surface current, the shielding layer of the object under test or the parts that do not need to be tested must be connected to the “G” terminal. In this way, the leakage current will flow directly from the “G” terminal back to the negative terminal of the generator inside the megohmmeter to form a circuit. This fundamentally eliminates the influence of surface leakage current. For example, when testing the insulation resistance of a coaxial cable, in order to eliminate the test error caused by the surface leakage of the cable insulation layer, the “L” terminal should be connected to the core conductor of the cable, the “E” terminal should be connected to the cable shielding layer (metal mesh, metal foil or metal tube), and the “G” terminal should be wound around the inner layer insulation of the cable (such as the inner PE core or the inner PE sheath).
For the measurement of three-phase four-wire power cables
For the measurement of the insulation resistance between U phase and V phase, W phase, N line and the casing, connect the wire of U phase at the “L” terminal, connect the V phase, W phase, N line to the cable casing with bare wires, then connect them to the “E” terminal. After winding several loops of bare wire around the outer surface of the insulation layer of U phase, connect to the “G” terminal. Follow this connection method to measure the insulation resistance of U phase. After discharging the cable, measure the insulation resistance of V phase, W phase and N line to the ground using the same method.
For the measurement of insulation resistance of power transformers (such as oil-immersed self-cooled type)
First, short-circuit the high-voltage terminals A, B, C and the low-voltage terminals a, b, c, N of the transformer with bare copper wires respectively. Then, wipe clean the porcelain sleeves of the transformer’s high and low-voltage porcelain bushings. Next, wrap 2 to 3 turns of bare copper wire around each porcelain skirt of the porcelain bushings. Finally, connect the wires on the high and low-voltage porcelain bushings respectively. When measuring the insulation resistance between the high-voltage winding and the low-voltage winding as well as the housing, connect the “L” terminal to the high-voltage winding of the transformer, the “E” terminal to the low-voltage winding and the housing, and the “G” terminal to the wire on the high-voltage porcelain bushing. When measuring the insulation resistance between the low-voltage winding and the high-voltage winding as well as the housing, connect the “L” terminal to the low-voltage winding of the transformer, the “E” terminal to the high-voltage winding and the housing, and the “G” terminal to the wire on the low-voltage porcelain bushing.
Post time: Jul-14-2026