Armoured Cable
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In electrical power distribution, armoured cable usually means steel wire armoured cable (SWA) which is a hard-wearing
power cable A power cable is an electrical cable used specifically for transmission of electric energy, electrical power. It is an assembly of one or more electrical conductors, usually held together in a single bundle with an insulator (electricity), insu ...
designed for the supply of mains electricity. It is one of a number of armoured electrical cables – which include 11 kV Cable and 33 kV Cable – and is found in underground systems, power networks and cable ducting. Aluminium can also be used for armouring, and historically iron was used. Armouring is also applied to
submarine communications cable A submarine communications cable is a cable laid on the seabed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables were laid beginning in the 1850s and car ...
s.


Construction

The typical construction of an SWA cable can be broken down as follows: * Conductor: consists of plain stranded copper (cables are classified to indicate the degree of flexibility. Class 2 refers to rigid stranded copper conductors as stipulated by British Standard BS EN 60228:2005) * Insulation:
Cross-linked polyethylene Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, in ...
(XLPE) is used in a number of power cables because it has good water resistance and excellent electrical properties. Insulation in cables ensures that conductors and other metal substances do not come into contact with each other. * Bedding:
Polyvinyl chloride Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons of ...
(PVC) bedding is used to provide a protective boundary between inner and outer layers of the cable. * Armour: Steel wire armour provides mechanical protection, which means the cable can withstand higher stresses, be buried directly and used in external or underground projects. The armouring is normally connected to earth and can sometimes be used as the circuit protective conductor ("earth wire") for the equipment supplied by cable. * Sheath: a black PVC sheath holds all components of the cable together and provides additional protection from external stresses. The PVC version of SWA cable, described above, meets the requirements of both British Standard BS 5467 and
International Electrotechnical Commission The International Electrotechnical Commission (IEC; ) is an international standards organization that prepares and publishes international standards for all electrical, electronics, electronic and related technologies. IEC standards cover a va ...
standard IEC 60502. It is known as SWA BS 5467 Cable and it has a voltage rating of 600/1000 V. SWA cable can be referred to more generally as mains cable, armoured cable, power cable and booklet armoured cable. The name power cable, however, applies to a wide range of cables including 6381Y, NYCY, NYY-J and 6491X Cable.


Aluminium wire armoured cable

Steel wire armour is only used on multicore versions of the cable. A
multicore cable A multicore cable is a type of electrical cable that combines multiple signals or power feeds into a single jacketed cable. The term is normally only used in relation to a cable that has more cores than commonly encountered. Not all cables with mu ...
, as the name suggests, is one where there are a number of different cores. When cable has only one core, aluminium wire armour (AWA) is used instead of steel wire. This is because the aluminium is non-magnetic. A magnetic field is produced by the current in a single core cable. This would induce an electric current in the steel wire, which could cause overheating.


Use of armour for earthing

The use of the armour as the means of providing earthing to the equipment supplied by the cable (a function technically known as the circuit protective conductor or CPC) is a matter of debate within the electrical installation industry. It is sometimes the case that an additional core within the cable is specified as the CPC (for instance, instead of using a two core cable for line and neutral and the armouring as the CPC, a three core cable is used) or an external earth wire is run alongside the cable to serve as the CPC. Primary concerns are the relative conductivity of the armouring compared to the cores (which reduces as the cable size increases) and reliability issues. Recent articles by authoritative sources have analysed the practice in detail and concluded that, for the majority of situations, the armouring is adequate to serve as the CPC under UK wiring regulations.


SWA BS 6724 cable

The construction of an SWA cable depends on the intended use. When the power cable needs to be installed in a public area, for example, a Low Smoke Zero Halogen (LSZH) equivalent, called SWA BS 6724 Cable must be used. After the
King’s Cross fire The King's Cross fire occurred in 1987 at King's Cross St Pancras tube station in London, England, causing 31 fatalities. It began under a wooden escalator before spreading into the ticket hall in a flashover. The fire began at approximately 1 ...
in London in 1987 it became mandatory to use LSZH sheathing on all London Underground cables – a number of the fatalities were due to toxic gas and smoke inhalation. As a result, LSZH cables are now recommended for use in highly populated enclosed public areas. This is because they emit non-toxic levels of Halogen and low levels of smoke when exposed to fire. SWA Cable BS 6724 – which meets the requirements of British standard BS 6724 – has LSZH bedding and a black LSZH sheath.


Use in telecommunications

Armoured cable is used for
submarine communications cable A submarine communications cable is a cable laid on the seabed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables were laid beginning in the 1850s and car ...
to protect against damage by fishing vessels and wildlife. Early cables carrying telegraph used iron wire armouring, but later switched to steel. The first of these was a cable across the
English Channel The English Channel, also known as the Channel, is an arm of the Atlantic Ocean that separates Southern England from northern France. It links to the southern part of the North Sea by the Strait of Dover at its northeastern end. It is the busi ...
laid by the
Submarine Telegraph Company The Submarine Telegraph Company was a British company which laid and operated submarine telegraph cables. Jacob and John Watkins Brett formed the English Channel Submarine Telegraph Company to lay the first cable across the English Channel. An un ...
in 1851. Many more telegraph, and later, telephone cables soon followed with multiple cores. Modern cables are
fibre-optic cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with p ...
s rather than copper cores. The first transatlantic fibre-optic cable was
TAT-8 TAT-8 was the 8th transatlantic communications cable and first transatlantic fiber-optic cable, carrying 280 Mbit/s (40,000 telephone circuits) between the United States, United Kingdom and France. It was constructed in 1988 by a consortium ...
in 1988.Bray, John, ''Innovation and the Communications Revolution: From the Victorian Pioneers to Broadband Internet'', p. 244, IET, 2002 .


See also

*
Electrical cable Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
*
Electrical wiring Electrical wiring is an electrical installation of Electrical cable, cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure. Wiring is subject to safety standards for design and in ...


References

{{reflist Electrical wiring Power cables