Under the action of DC voltage, the resistivity of non-conductive or minimally conductive substances is generally greater than 1010 Ω · m. The main function of insulating materials is to isolate live conductors of different potentials in electrical equipment, so that the current can flow in a certain path, and can also play a role in mechanical support and fixation, as well as arc extinguishing, heat dissipation, energy storage, moisture-proof, mildew-proof or improve the potential distribution of the electric field and protect the conductor. Therefore, the insulating material is required to have as high insulation resistance, heat resistance, and moisture resistance as possible, and also to have a certain mechanical strength.

In order to prevent the damage of the insulation performance of the insulation material from causing accidents, the insulation material must meet the performance indicators specified in the national standards. There are many performance indicators of insulating materials, and the characteristics of various insulating materials are also different. The main performance indicators of commonly used insulating materials are breakdown strength, heat resistance, insulation resistance and mechanical strength.
(1) breakdown strength. Under the action of electric field strength higher than a certain value, the insulating material will be damaged and lose its insulation performance. This phenomenon is called breakdown. The electric field strength when the insulation material is broken down is called the breakdown strength, and the unit is kV/mm.
(2) Heat resistance. When the temperature increases, the resistance, breakdown strength, mechanical strength and other properties of the insulating material will be reduced. Therefore, it is required that the insulation material can work for a long time at a specified temperature and ensure reliable insulation performance. Different components of the insulation material heat resistance degree is different, heat resistance grade can be divided into Y, A, E, B, F, H, C and so on 7 grades, and for each grade of insulation material provides the maximum limit working temperature.