Power cable

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This article is about the conducting cord. For the fictional town, see Power Cable.

A 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.

Contents

[edit] 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 was applied to cables.

[edit] 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 called power cables in the trade.

Conductors are usually made of copper or aluminum wires, or may be composite conductors with steel strands at their core. Conductors are usually stranded for flexibility, but small cables may use solid conductors.

Conductors in a cable may be different sizes. Each conductor has its own electrical insulation. 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 2400 volts between conductors or more, a conductive shield surrounds 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 the intended application, and may be specially resistant 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 sheaths, or may be protected by a lead sheath. 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/sheath. The oil is kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation.

A hybrid cable will include conductors for control signals or may also include optical fibers for data. Besides data transmission, these optical fibres are used for distributed temperature monitoring in order to optimize the load/ampacity of the cable.

[edit] Named cable types

Common types of general-purpose cables used by electricians are defined by national or international regulations or codes. Commonly-used types of power cables are often known by a "shorthand" name. For example, NEC type NM-B (Non-Metallic, variant B), often referred to as RomexTM (named by the Rome Wire Company, now a trademark of Southwire Company [1]), is a cable with a nonmetallic jacket. UF (underground feeder) is also nonmetallic but uses a moisture- and sunlight-resistant construction suitable for direct burial in the earth or in interiors in wet, dry, or corrosive locations. Type AC is a fabricated assembly of insulated conductors in a flexible metallic armor, made by twisting an interlocking metal strip around the conductors. BX, an early genericized trademark of the General Electric company was used before and during WWII, designating a particular design of armored cable.

In Canada, type TECK cable, with a flexible aluminum or steel armour and overall flame-retardent PVC jacket, is used in industry for wet or dry locations, run in trays or attached to building structure, above grade or buried in earth. A similar type of cable is designated type MC in the United States.

Electrical power cables are often installed in raceways including electrical conduit, and cable trays, which may contain one or more conductors. Conduit may also be rigid or flexible, metallic or nonmetallic, and differentiation from cable may require some investigation of the contents at their termination.

Mineral Insulated Copper Clad cable (type MI) is a fire-resistant cable using magnesium oxide as an insulator. It is used in demanding applications such as fire alarms and oil refineries.

[edit] Ampacity and ampacity derating

The ampacity (current-carrying capacity) of any given cable is the maximum amount of current a cable can constantly carry without jeopardizing the integrity of the insulating/sheathing materials due to excess heat dissipation. The heat-dissipating ability of the conductor is reduced by the electrical insulation around the conductor. If the current in a conductor is too high, its insulation will be damaged long before the conductor melts. Depending on the type of insulating material, common maximum allowable temperatures are 60, 90, or 105 degrees Celsius.

The ampacity for a cable is thus based on conductor composition, insulator composition, conductor size, ambient temperature, and environmental conditions adjacent to the cable. In a long run of cable different conditions govern, and installation regulations specify that the most severe condition along the run governs the cable's rating. Cables run in wet or oily locations may carry a lower temperature rating than in a dry installation. Special calculations are necessary for multiple circuits in proximity in adjacent raceways. When multiple cables are bundled together, each contributes heat to the bundle and diminishes the amount of cooling air that can flow past the individual cables. The overall ampacity of the insulated conductors in a bundle of more than 3 must be derated, whether in a raceway or cable. Usually the de-rating factor is tabulated in the wiring regulations.

The allowed current in cables can be decreased (derated) when the cable is covered in fireproofing material.

[edit] Flexible cables

All cables are flexible, which allows them to be shipped to installation sites on reels or drums. Where applications require a cable to be moved repeatedly, more flexible cables are used. Small cables are called "cords" or "flex". Flexible cords contain finer stranded conductors, rather than solid, and have insulation and sheaths that are engineered to withstand the forces of repeated flexing. Heavy duty flexible power cords such as feeding a mine face cutting machine are carefully engineered -- since their life is measurable in (6) weeks! See "Power cord" and "Extension cable" for further description of flexible power cables. Other types of flexible cable include twisted pair, extensible, coaxial, shielded, and communication cable.

[edit] See also

[edit] References

Underground Systems Reference Book , Edison Electric Institute, New York, 1957, no ISBN

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