How is the measurement of insulation resistance and absorption ratio conducted?

Insulation resistance, denoted as R, is typically determined by applying a voltage of 2500V for 60 seconds and measuring the resulting value using a megohmmeter with a maximum range of 10000 MQ. Due to variations in the stability of insulation resistance arising from differences in product geometry, structure, material, and moisture content within the insulation, there is no standardized value for R. To provide a rough constraint on R and depict its relationship with temperature,

the absorption ratio (Ro/Rs) is the ratio of insulation resistance measured during direct current application for 60 seconds and 15 seconds (for large transformers, measurements are also required for 10 minutes and 1 minute, known as the polarization index). Since the current passing through any insulating material subjected to direct voltage consists of three types: charging, absorption, and leakage currents, the first two currents decay and are influenced by the moisture level of the insulation. Therefore, the values of Ro and Rs are different. For products with voltages of 60kV and above, Rø/R₁s should be greater than 1.5, otherwise, the insulation may be susceptible to moisture.

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