Electrical Insulator

a device for the electrical insulation and mechanical connection of parts of electrical installations that are at different electrical potentials. An insulator consists of a dielectric (the insulator itself) and parts used to mount it (fittings). Porcelain and glass are most frequently used to manufacture insulators. Insulators made from steatite, high-grade porcelain, and other materials with low dielectric loss are used in radio engineering apparatus and in other high-frequency installations.

The design and dimensions of insulators are determined by the mechanical loads applied to them, by the voltage of the installation, and by operating conditions. Insulators on power transmission lines and in open bus-and-switch structures of electric power plants and substations are exposed to the action of atmospheric precipitation, which is particularly dangerous if the ambient air is strongly contaminated. To increase the flashover voltage (surface discharge) in such insulators, the outside surfaces are made with a complex contour, which increases the flashover path. Pin insulators are used in power transmission lines with voltages of 6–35 kilovolts (kV). In transmission lines with higher voltages, strings of suspension insulators are used. The number of insulators in a string is determined by the rated line voltage. Support insulators of pin construction are used in open bus-and-switch structures for installation of buses or of equipment under voltage. For very high voltages (up to 220 kV) such insulators are assembled in columns, one on top of another.

Partition insulators, or bushings, are used to carry a high potential through a grounded surface (for example, the top of the transformer body). The porcelain body of a bushing, which is filled with transformer oil, is divided by cylindrical barriers made from a solid dielectric. Such a design provides the required electrical insulation between a current-carrying rod and a flange. The lower part of the insulator is located within the transformer body and therefore can be made smaller than the upper part located in open air. Insulators for devices operating in enclosed spaces are made from bakelite or porcelain and have a much simpler contour for the outside surface (for example, flange-type support insulators).