Power CableA power cable is an assembly of two or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed. Flexibles are used for portable and mobile tools and machinery. History Early telegraph systems were the first forms of electrical cabling but transmitted only small amounts of power. Gutta-percha insulation used for the first transatlantic cables was unsuitable for building wiring use since gutta-percha deteriorated rapidly when exposed to air. The first power distribution system developed by Thomas Edison used copper rods, wrapped in jute and placed in rigid pipes filled with a bituminous compound. Although vulcanized rubber had been patented by Charles Goodyear in 1844, it was not applied to cable insulation until the 1880s, when it was used for lighting circuits. Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls power project. Oil-impregenated paper-insulated high voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.
Construction Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses. Large single insulated conductors are also sometimes called power cables in the trade. They consist of three major components, namely conductors, insulations, protection. The constructional detail of individual cables will vary according to their application. The construction and material are determined by three main factors: Working voltage, which determines the thickness and composition of the insulation; Current carrying capacity, which determines the cross-section size of the conductors; Environmental conditions such as temperature, chemical or sunlight exposure, and mechanical impact, which determines the form and composition of the cable jacket enclosing conductors. Since they must be flexible, the copper or aluminum conductors are made of stranded wire, although very small cables may use solid conductors. The cable may include uninsulated conductors used for the circuit neutral or for ground (earth) connection. The overall assembly may be round or flat. Filler strands may be added to the assembly to maintain its shape. Special purpose power cables for overhead or vertical use may have additional elements such as steel or Kevlar structural supports. For circuits operating at 2,400 volts between conductors or more, a conductive shield may surround each conductor. This equalizes electrical stress on the cable insulation. This technique was patented by Martin Hochstadter in 1916, and so the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to earth ground at one or both ends of each length of cable. Some power cables for outdoor overhead use may have no overall sheath. Other cables may have a plastic or metal sheath enclosing all the conductors. The materials for the sheath will be selected for resistance to water, oil, sunlight, underground conditions, chemical vapors, impact, or high temperatures. Cables intended for underground use or direct burial in earth will have heavy plastic or lead sheaths, or may require special direct-buried construction. Where cables must run where exposed to impact damage, they are protected with flexible steel tape or wire armor, which may also be covered by a water resistant jacket. Cables for high-voltage (more than 65,000 volts) power distribution may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminium or lead jacket or sheath. The oil is kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. Newer high-voltge cables use cross linked polyethylene (XLPE) for insulation
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