Jump to content

Power cable

From Wikipedia, the free encyclopedia
This is an old revision of this page, as edited by Fabartus (talk | contribs) at 02:56, 1 July 2008 (→Named cable types: tweak prior adds). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Template:Two other uses

A electrical power cable found commonly in modern European houses. The cable consists of 3 wires (2 wires + 1 grounding) and is double-insulated

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. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed. Flexible power cables 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. [1] Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls power project. Oil-impregnated 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.[2]

Construction

Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses.[3] Large single insulated conductors are also sometimes called power cables in the industry.[4]

Cables 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 power cables must be flexible, the copper or aluminum conductors are made of stranded wire, although very small power 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,[5], 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-voltage cables use cross linked polyethylene (XLPE) for insulation.

A hybrid cable will include conductors for control signals or may also include optical fibers for data.

Named cable types

Common types of general-purpose cables used by electricians are defined by national or international regulations or codes, which define and regulate the various wire alloys that may make up a cable, it's insulation type and characteristics, and chemical or radiological resistance properties.

Commonly-used types of power cables are often known by a "shorthand" name, and nowadays virtually all contain a bare (naked) center wire which is connected to earth ground and is for safe-guarding local junction and plug boxes with an earth ground for consumer and fire safety. In the later case, in theory, a fireman's axe cutting through a ground equipped cable will guarantee a short circuit and blow the fuse or breaker, so the whole circuit becomes dead, and shock-from-water risks are minimized. For consumer safety, the use is somewhat outdated, as it was made mandatory in most jurisdictions when most all household appliances had metallic outer cases, and weren't double-insulated. Plastic housings and routine double insulation has reduced but not eliminated the safety rationales incorporated in law. Three prong power outlets, and plug-cords however require the third ("naked ground) wire and national electrical codes are unlikely to change back.

For examples of official names and shorthand names, first take USA's NEC type NM-B (Non-Metallic, variant B, which coding refers to the insulation properties coating each working wire, the sheath, and the temperature and radiological (See UV following) characteristics of the plastic insulators), often referred to as "Romex"TM[6], is a cable made of solid copper wires with a nonmetallic plastic jacket containing a waxed paper wrapped inner group of at least a pair of plastic insulated service wires and the now ubiquitous naked ground wire that is characteristic of modern electrical cables. The most common Romex/NM-B cable thus has three wires: the return wire (called "neutral", usually colored white), and the wire providing power to the load, which is nearly always black—and the naked ground wire. NM-B "two wire" cables are available in different wire gages, providing a range of current carrying capacities at expected maximal ambient temperatures in a cable run—and the type services (provides distribution for) most loads of most buildings and their needs.

RomexTM wire
label type		 count of
insulated wires		 Colors
(normal)		 total
wires		 wire gauge
rating
12-2 NMB 2 W + B three AWG 12
14-2 NMB 2 W + B three AWG 14
12-3 NMB 2 W + B + R four AWG 12
14-3 NMB 2 W + B + R four AWG 14
"AWG" means American Wire Gauge, see table here for current capacities derated for various maximum expected temperatures within a cable run or conduit. Other commonly used wire gauges for common electric services ndistribution range from AWG-4 through AWG-10.

The next most common "Romex" variant (sometimes called a "Three way" cable, after it's application in lighting) has the return (white/neutral) and two power sourcing wires: the first is always black (and always present) whereas the second conductor (and third insulated wire), is usually colored red (sometimes yellow or blue, but only rarely— such colors must be special ordered costing extra money) and a uninsulated or "naked" copper wire for fire safety. This type is generally used for multiple switching locations of a common or shared lighting need (For example, at either end of a long commercial office suite, hallway, or on two floors for the stairways lighting, and are called a "three-way circuit", a well entrenched North-American misnomer the various British cultures don't repeat—for similar mysterious reasons three or more switches directly controlling a light require one or more "four-way switches" plus a pair of "three-way switches". See table of switch types here, and diagrams in the sections below that). Type NM-B cables with three insulated conductors are also suitable for larger two-phase loads like larger air conditioners or baseboard electric heating, where the neutral acts as a return for both voltage phases).

Thus for NEC NM-B series/RomexTM cables, at least two insulated wires plus the naked copper ground wire are always present. In general, NEC NM-B/Romex wire is not suitable for enclosure with other wiring in conduits, especially outdoor conduits susceptible to solar heating, which requires a higher temperature plastic insulation such as "THHN" (stranded, sold in rolls of single conductor in many colors).

Another common nickname is type UF (underground feeder) which is also possessed of a nonmetallic protective sheath but uses a moisture- and sunlight-resistant construction suitable for direct burial in the earth or where exposed to sunlight, or in wet, dry, or corrosive locations. "UF" uses an sheath that is a flexible solid about the insulated wires instead of the waxed paper wrapping, is generally heavier, less flexible than NM-B, and is radiologically rated against breakdown from ultra-violet light wavelengths.

A few other common named types are: Type "AC" which is a fabricated assembly of insulated conductors in a flexible metallic armor, made by twisting an interlocking metal strip around the conductors. This is a descendant of type "BX", an early genericized trademark of the General Electric company which was used from before and during World War II, designating a particular design of armored cable. The "BX" name is commonly applied loosely to all flexible metal armored cables including those purpose run with individual wires through a flexible metal conduit of similar appearance.

In Canada, type TECK cable, with a flexible aluminum or steel armor and overall flame-retardant 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.

Mineral-insulated copper-clad cable (type MI) is a fire-resistant cable using magnesium oxide as an insulator. It is used in demanding applications requiring superior heat resistance such as fire alarms and oil refineries.

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" (North American usage) or "flex" (United Kingdom)[citation needed]. 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 weeks. Very flexible power cables are used in automated machinery, robotics, and machine tools. 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.

See also

References

  1. ^ Underground Systems Reference Book, Edison Electric Institute, New York, 1957, no ISBN
  2. ^ R. M. Black The History of Electric Wires and Cables, Peter Pergrinus, London 1983 ISBN 0 86341 001 4
  3. ^ Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07013932-6, sections 2-13 through 2-84
  4. ^ Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07020974-X pg. 18-85
  5. ^ Underground Systems
  6. ^ "Romex"TM - named by the Rome Wire Company, now a trademark of Southwire Company [1]
Wikimedia Commons has media related to Power cables.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Power_cable&oldid=222786722"