The busbar's material composition and cross-sectional size determine the maximum current it can safely carry. Busbars can have a cross-sectional area of as little as , but
electrical substations may use metal tubes in diameter or more as busbars.
Aluminium smelters use very large busbars to carry tens of thousands of
amperes to the
electrochemical cells that
produce aluminium from molten
salts. Busbars are produced in a variety of shapes, including flat strips, solid bars and rods, and are typically composed of
copper,
brass or
aluminium as solid or hollow tubes. Some of these shapes allow
heat to dissipate more efficiently due to their high
surface area to
cross-sectional area ratio. The
skin effect makes
50–60 Hz AC busbars more than about thickness inefficient, so hollow or flat shapes are prevalent in higher-current applications. A hollow section also has higher
stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor
electrical switchyards. A busbar must be sufficiently rigid to support its own weight, and forces imposed by mechanical
vibration and possibly
earthquakes, as well as accumulated
precipitation in outdoor exposures. In addition,
thermal expansion from temperature changes induced by
ohmic heating and ambient temperature variations, and
magnetic forces induced by large currents, must be considered. To address these concerns, flexible bus bars, typically a sandwich of thin conductor layers, were developed. They require a structural frame or cabinet for their installation. Mechanical forces generated by
fault currents, which can momentarily reach hundreds of thousands of amperes, must also be considered. Distribution boards split the electrical supply into separate circuits at one location. Busways, or bus ducts, are long busbars with protective covers. Rather than branching from the main supply at one location, they allow new circuits to branch off anywhere along the busway. A busbar may be either supported on insulators, or wrapped in insulation. They are protected from accidental contact either by a metal earthed enclosure or by elevation out of normal reach. Insulated bus bars are used in busways listed to UL 857 standards. Power
neutral busbars may also be insulated because it is not guaranteed that the potential between power neutral and safety grounding is always zero.
Earthing (safety grounding) busbars are typically bare and bolted directly onto any metal chassis of their enclosure. They may be enclosed in a metal housing, in the form of a bus duct or busway, segregated-phase bus, or
isolated-phase bus. Busbars may be connected to each other and to electrical apparatus by bolting, clamping or welding. Joints between high-current bus sections often have precisely machined matching surfaces that are
silver-plated to reduce
contact resistance. At
extra high voltages (more than 300 kV) in outdoor buses,
corona discharge around the connections becomes a source of
radio-frequency interference and
power loss, so special connection fittings designed for those voltages are used. UW_Ebingen_Sammelschienen.jpg|flexible busbar Kändelweg Sammelschienen.jpg|rigid busbar ==See also==