Why is it necessary to measure the absorption ratio of electrical equipment?

The absorption ratio is defined as follows: When operating a megohmmeter (insulation resistance tester), rotate the handle at a speed of 120 revolutions per minute. Record the insulation resistance reading at 15 seconds (R15) and then at 60 seconds (R60). The absorption ratio is calculated using the formula:

Absorption Ratio = R60 / R15, which should be greater than or equal to 1.3.

Measuring the absorption ratio helps determine whether the insulation of electrical equipment is damp. When insulation material is dry, the leakage current component is very small, and the insulation resistance is primarily determined by the charging (capacitive) current. At 15 seconds, the charging current is still relatively large, resulting in a smaller insulation resistance value (R15). By 60 seconds, due to the dielectric absorption characteristics of the insulation material, the charging current has significantly decayed, leading to a larger insulation resistance value (R60). Therefore, the absorption ratio is relatively high.

However, when the insulation is damp, the leakage current component increases significantly. The time-dependent charging current becomes less dominant, and the insulation resistance shows little change over time. As a result, R60 and R15 become very close, meaning the absorption ratio decreases.

Thus, the measured value of the absorption ratio can provide a preliminary assessment of whether the insulation of electrical equipment is damp.

The absorption ratio test is suitable for equipment with relatively large capacitance, such as motors and transformers, and should be interpreted in conjunction with the specific environmental conditions of the equipment. The general criterion is that if the insulation is not damp, the absorption ratio K ≥ 1.3. However, for equipment with very small capacitance (e.g., insulators), the insulation resistance reading stabilizes within just a few seconds and does not continue to rise—indicating no significant absorption effect. Therefore, performing an absorption ratio test on such small-capacitance equipment is unnecessary.

For high-capacity test specimens, relevant domestic and international standards specify that the Polarization Index (PI), defined as R10min / R1min, may be used instead of the absorption ratio test.

Temperature is inversely proportional to insulation resistance: higher temperatures result in lower insulation resistance and higher conductor resistance. Based on general experience, medium- and high-voltage cables are typically subjected to rigorous partial discharge and high-voltage tests before leaving the factory. Under normal conditions, the insulation resistance of medium-voltage cables can reach several hundred to over a thousand MΩ·km.