The dc cable is made of bare copper insulation. Insulating materials are non-conductive materials under allowed voltage, but not absolutely non-conductive materials, under a certain external electric field strength, will also occur conduction, polarization, loss, breakdown and other processes, and long-term use will occur aging. Its resistivity is very high, usually in the range of 10 ~ 10Ω·m. For example, in a motor, the insulating material around the conductor isolates the turns and is isolated from the grounded stator core to ensure the safe operation of the motor.