Bushing (electrical)

In electric power, a bushing is an insulated device that allows an electrical conductor to pass safely through a grounded conducting barrier such as the case of a transformer or circuit breaker.

All materials carrying an electric charge generate an electric field. When an energized conductor is near any material at earth potential, it can cause very high field strengths to be formed, especially where the field lines are forced to curve sharply around the earthed material. The bushing controls the shape and strength of the field and reduces the electrical stresses in the insulating material.

A bushing must be designed to withstand the electrical field strength produced in the insulation, when any earthed material is present. As the strength of the electrical field increases, leakage paths may develop within the insulation. If the energy of the leakage path overcomes the dielectric strength of the insulation, it may puncture the insulation and allow the electrical energy to conduct to the nearest earthed material causing burning and arcing.

A typical bushing design has a 'conductor', (usually of copper or aluminium, occasionally of other conductive materials), surrounded by insulation, except for the terminal ends.

In the case of a busbar, the conductor terminals will support the busbar in its location. In the case of a bushing, a fixing device will also be attached to the insulation to hold it in its location. Usually, the fixing point is integral or surrounds the insulation over part of the insulated surface. The insulated material between the fixing point and the conductor is the most highly stressed area.

The design of any electrical bushing must ensure that the electrical strength of the insulated material is able to withstand the penetrating 'electrical energy' passing through the conductor, via any highly stressed areas. It must also be capable of enduring, occasional and exceptional high voltage moments as well as the normal continual service withstand voltage, as it is the voltage that directs and controls the development of leakage paths and not current.

Insulated bushings can be installed either indoor, or outdoor, and the selection of insulation will be determined by the location of the installation and the electrical service duty on the bushing.

For a bushing to work successfully over many years, the insulation must remain effective both in composition and design shape and will be key factors in its survival. Bushings can therefore vary considerably in both material and design style.

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Wikipedia