The design of high voltage bushings

In all electrical equipment operating at a potential over about 2000 volts the problem of the penetration of walls, normally at ground potential, by current-carrying conductors is one which must be solved. The normal solution is a bushing which will provide the necessary conductor isolation and frequently will also perform the function of a gas or oil seal. A well-designed bushing will perform its function with little measureable dielectric loss and will not distort the circuit with an objectionable discharge at operating voltage levels. While discharge inception above nominal operating voltage is usually the point of reference, it is not the sole operational criterion. The possible presence of transient overvoltages, even for short periods of time, may cause the inception of discharge. It is therefor mandatory in a bushing that the discharge extinction voltage be above the operating level in order that transient induced discharges may not be continued at the operating voltage level.