Those who conduct electrical tests know that "shaking insulation" is the first item of every preventive test - transformers, cables, motors, and transformers must have their insulation resistance measured before operation and during regular maintenance. But many people are not clear about the true meaning of this test: what does the value of insulation resistance really mean? Why use high voltage of several thousand volts to measure?

The physical significance of insulation resistance

Any actual insulation material (such as transformer oil paper, cable polyethylene, motor slot insulation, etc.) is not an ideal insulator, and there will be small leakage currents flowing through under high voltage. Insulation resistance is the ratio of applied voltage to leakage current at a specified voltage, measured in megaohms (M Ω), gigaohms (G Ω), and even teraohms (T Ω).

The higher the insulation resistance, the smaller the leakage of the insulation material at that voltage, and the better the insulation performance; As the insulation ages, becomes damp, or experiences local damage, the leakage current increases and the insulation resistance decreases. The decrease in insulation resistance before equipment breakdown is often the most direct precursor to insulation degradation.

Why use high voltage measurement

Insulation resistance is nonlinear - at low voltages, the leakage of insulation materials is extremely small, and measuring with the ohm range of a multimeter will yield an approximately infinite value, making it impossible to detect any problems. The true leakage state of insulation can only be exposed when it is close to the actual operating voltage (or higher test voltage).

Transformers use 5000V or 10000V insulation resistance testers, high-voltage cables use 2500V or 5000V, and large generator stator windings use 5000V or above - the principle for selecting the test voltage is "equivalent to the actual operating voltage".