A bushing is an insulating device that introduces a live conductor into an electrical device or through a wall. The former is called an electrical sleeve, and the latter is called a wall sleeve. The casing structure is generally composed of three parts: conductor (guide rod), insulator and metal flange. The conductor passes along the axis of the cylindrical insulator, while the metal ring flange is mounted outside the insulator and grounded. The casing belongs to an insulating structure with a strong vertical electric field component, and the electric field strength at the metal flange is very large, which is easy to produce corona discharge and slip flash discharge along the surface of the dielectric. The radial electric field strength between the flange and the guide rod is also very high, and it is easy to break down the insulating medium. In addition to the use of a single solid insulating material for the casing of 35kV and below, a variety of insulating materials or uniform electric field measures are often used to make the axial and tangential electric field distribution tend to be uniform.