The bushing is hollow, allowing a conductor to pass along its centre and connect at both ends to other equipment. Bushings are often made of wet-process fired porcelain, and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing.
The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation. Bushings for medium-voltage and low-voltage apparatus may be made of resins reinforced with paper. The use of polymer bushings for high voltage applications is becoming more common. The largest high-voltage bushings made are usually associated with high-voltage direct-current converters.
Bushings sometimes fail due to partial discharge degradation in the insulation. There is at present great interest in the electricity supply industry in monitoring the condition of high voltage bushings.