Electrotechnical equipment insulation both resistive and capacitive properties, for some equipment, such as cables, capacitors, transformers, motors and other capacitance is very large. In the theoretical analysis, the insulation structure is often viewed as a parallel RC equivalent circuit (Figure 1a). However, the insulation of many devices is multilayer, and the RC value of each layer cannot be the same. For ease of analysis, an equivalent circuit with a double-layer dielectric (Fig. 1b) is often used to represent this. Absorption ratio measurement is to check whether there is any abnormality in the relative relationship between the equivalent resistance and capacitance of the insulator in order to judge the insulation condition. As can be seen from Fig. 1, when just closing K, the ammeter indicates mainly the charging current of the capacitor, and it is necessary to wait for the capacitor to be fully charged before the ammeter indicates the leakage current determined by the insulation resistance. From the start to stabilize, the current is decaying with time, known as the absorption phenomenon, that is, the insulation resistance is increasing with time, and finally reached a stable value. The length of time to stabilize, determined by the time constant RC or R1R2 (C1 + C2) / (C1 + R2). The time constant of dry insulation is large, the absorbed current decays slowly, and the insulation resistance rises slowly. Moisture insulation has a small time constant, the absorbed current decays quickly, and the insulation resistance reaches stability in a short time.