All electrically conducting materials generate an electric field when energised with a voltage. In the case of DC the field remains positive, in the case of AC, the field is alternating between positive and negative.
When an energized conductor passes near any part of material that is at earth potential, its electric field can be affected and distorted by the shape of the earthed material. Voltage from the conducting material can be attracted towards the earthed material, and if the electric field becomes of sufficient strength, this can lead to the formation of a leakage path.
When conductive material passes through a space without any earthed material the device is known as a busbar. When conductive material is insulated, preventing the passage of electrical energy through the insulation, and passes through a space without any earthed material, the device is known as an insulated busbar.
When the conductive material is insulated and passes through any earthed material, it is known an electrical bushing.
This significant operational difference requires the design of a bushing to be considerate of the electrical field strength produced in the insulation, when any earthed material is present. As the strength of the electrical field increases, it may be attracted to the earthed material and if sufficient voltage is present 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.
Read more about this topic: Bushing (electrical)
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