This article is about electric power conductors. For portable equipment, see power cord.
A power cable is an assembly of one 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 devices, mobile tools and machinery.
Power cable History
Early telegraph systems used the first forms of electrical cabling, transmitting tiny amounts of power. Gutta-percha insulation used on the first submarine cables was, however, unsuitable for building wiring use since it deteriorated rapidly when exposed to air.
The first power distribution system developed by Thomas Edison in 1882 in New York City 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.
Mass-impregnated paper-insulated medium 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 industry.
Cables consist of three major components: conductors, insulation, protective jacket. The makeup of individual cables varies according to application. The construction and material are determined by three main factors:
※ Working voltage, determining the thickness of the insulation;
※ Current-carrying capacity, determining the cross-sectional size of the conductor(s);
※ Environmental conditions such as temperature, water, chemical or sunlight exposure, and mechanical impact, determining the form and composition of the outer cable jacket.
Cables for direct burial or for exposed installations may also include metal armor in the form of wires spiralled around the cable, or a corrugated tape wrapped around it. The armor may be made of steel or aluminum, and although connected to earth ground is not intended to carry current during normal operation.
Power cables use stranded copper or aluminum conductors, although small power cables may use solid conductors (For a detailed discussion on copper cables, see: Copper wire and cable.).
The cable may include uninsulated conductors used for the circuit neutral or for ground (earth) connection.
The overall assembly may be round or flat. Non-conducting 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.
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. In nuclear industry applications the cable may have special requirements for ionizing radiation resistance. Cable materials may be specified not to produce large amounts of smoke if burned. Cables intended for underground use or direct burial in earth will have heavy plastic or metal, most often lead sheaths, or may require special direct-buried construction. When cables must run where exposed to mechanical impact damage, they may protected with flexible steel tape or wire armor, which may also be covered by a water resistant jacket.
The information is taken from Wikipedia, the free encyclopedia. For more wire and canle knowledge, Please visit www.qscable.com and www.huadongcable.com.

High voltage overhead transmission lines and high-voltage cables, what’s the difference?

The transmission line has two kinds, one is the overhead line the most common, it generally use the bare wire insulation, through the tower stands on the ground as support, the conductor insulator suspension in the tower; the other is the power cable, the cable which uses a special processing manufacturing and become, buried in the underground or laid in cable tunnel.

The transmission capacity and transmission distance power transmission line and voltage. Line voltage higher transport distance farther. According to the transmission distance and capacity to determine the voltage circuit and system required.

1 overhead transmission line routing overhead line tower transmission lines, conductors, insulators, set up above the ground.

Overhead transmission line

Good by the conductive metal wires are made, thick enough cross section (to maintain proper flow density) and large radius of curvature (to reduce corona discharge). EHV transmission is to use more split conductor. Overhead ground wire (also known as the lightning line) is arranged above the transmission line, used to protect the lines from lightning strikes. Transmission line important usually use two overhead ground wire. By a single suspension insulator string (or rod) insulators which are connected in series, to meet the insulation strength and mechanical strength requirements. The number of each string insulator is determined by the voltage level of transmission. The tower is made of steel or reinforced concrete, is the main supporting structure of overhead transmission line. Erection of overhead lines and convenient repair, low cost. Effect of overhead transmission lines to consider the temperature change, it is the strong storms, lightning, Yu Lin, ice, floods, wet fog and other natural conditions at design time. Overhead transmission line path also have enough ground corridor width and clearance.

The maximum transmission power transmission lines in the comprehensive consideration of technical, economic and other factors are identified, called the transmission capacity of the line. Transmission capacity in general and the transmission voltage is proportional to the square of the. Therefore, to improve the transmission voltage is the main technical means to realize large capacity and long distance transmission, but also a symbol of the development level of transmission technology. The current global (including the developed countries in Europe and America) is widely used in overhead lines as the main means of conveyance of electricity.

2 power cable line

Power cable by wire, insulation layer and protective layer with a single core, dual core and three core cable.

Underground lines used for overhead line construction difficult areas, such as transmission city or special cross section. At present, the cable transmission, mainly from the point of view of city landscape and the line security. But the cable line fault finding time and repair time is very long, to the normal operation of power grid reliability and electricity brings the serious influence. So in power grid construction, cable lines to replace all the overhead line or unable to realize.

Cable line features:
(1) power supply reliability.
(2) does not occupy the ground and space
(3) do not use the pole, saving wood, steel, cement
(4) the operation and maintenance are simple, save repair cost line.
(5) cable price expensive, hard line branch, cable construction process joint is complex, difficult to find the fault point, not timely processing of the accident.

3The power frequency electric field, magnetic field overhead lines and cable lines

3.1 power frequency electric field voltage electric field, voltage, and the population size and line overhead line power frequency electric field intensity of the distance from the. According to HJ_T_24-1998 “500kV EHV power transmission and transformation project electromagnetic radiation environmental impact assessment of technical specifications”, residential power frequency electric field intensity of the implementation of 4kV/m standard, power grid construction project only by the environmental impact assessment, in full compliance with the standards to the construction.

The underground tunnel and cable buried deep in the 7-10 meters, the tunnel reinforced concrete and surface soil layer, the power frequency electric field on the ground surface is very small, and tend to the environment background value. Therefore, the power frequency electric field intensity of cable line can be ignored.

3.2 power frequency magnetic field

An electric current produces a magnetic field, the current size, and population size and line overhead line power frequency magnetic induction intensity of distance and other relevant. According to HJ_T_24-1998 “500kV EHV power transmission and transformation project electromagnetic radiation environmental impact assessment of technical specifications”, residential power frequency magnetic field strength of the implementation of 0.1mT standard. The cable lines due to shield tunnel reinforced structure, and away from the larger reason and the surface, the surface of the power frequency magnetic field intensity is generally not more than 0.003mT. Along with the increase of distance and cable tunnel level, power frequency magnetic field intensity decay rapidly, more environmental background value.

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The exact cause of XLPE insulated cable accident

The cable fault generated
A joint design and manufacturing process
B insulation aging metamorphism
C insulation
D cable overheat
E mechanical injury
F corrosion protection layer
G overvoltage
H material defects
According to the report, the following reasons XLPE cable accident:
A water tree deterioration
B copper shield fault zone
C copper shield grounding fault
D cable sheath fault
E core shielding layer of uneven thickness
1 water tree degradation
It is the main reason of XLPE insulated cable accident, about 71% of the accidents, mostly occurs in the natural deterioration.
2 copper shield fault zone
In the copper shielding cable with one end grounded, when the copper shield with fracture, non grounded at one end of the copper shield with a non ground state, the copper shield with the induction high voltage, the high voltage if the rupture site discharge, often cause the destruction of insulation. Is shown schematically in Figure fracture site discharge.
The characteristics of fault zone is the copper shield:
(1) Single core cable  than the three core cable accident.
(2) from the production to the ravages of time, from a few weeks to a few years.
(3) the conductor resistance fracture site increases to the thousands of Europe, not the protection of non ground side cable a flashover to ground.
(4) fracture site discharge fire, smoke, fire may have caused serious.
3 copper shield grounding fault
XLPE copper cable shield grounding fault has been the scene of the attention. For example, a region of the XLPE cable are often adopted in buried, the terminal head of copper shielding wire and steel armor wires are led, grounding wire section were no less than 25mm2 and 10mm2, from thermal glove leads should be insulated from each other, through the above two improvements, there are conditions in the terminal head of regular measurement of steel armor insulation resistance and steel armor on the copper shield, can indirectly reflect the cable outer sheath, no damage, which can determine the cable damping.
Detection of copper cable shielding and grounding, a change point in the terminal side of the insulating resistance of 0.01M. Cable laying is shown schematically in figure.
Further detection, fault point location away from the substation 1973m 4 cable connection. The 4 joint digs, the joint, the outer sheath are respectively opened inspection, found the cause of the copper shield grounding is the inner, outer sheath of joint sealing lax, steel armor and copper shield has the moisture. Aiming at the fault reason, the joints were sufficient moisture use a blowtorch, the copper shield in the interface is disconnected, respectively, both sides of the joint copper shield on telemetry insulation resistance, test result is: substation side is 4.5M Ω, the terminal side of 5M. Due to timely treatment, to avoid accidents
4 cable sheath fault
XLPE single core cable can run safely and reliably, and the protective layer can be closely related to the safe and reliable operation. By the end of the cable sheath grounding, shield of the cable insulation requirements must be good. When the cable shield grounding, operation of cable sheath will be subjected to an alternating magnetic field, will be in the corrugated aluminum sheath voltage induced, directly to the ground terminal and the cable sheath insulation bad produced “circulation”. “Circulation” to make corrugated aluminum layer heating, and the transmission capacity is reduced to 30% to 40%; and the metal protecting layer serious burn, burn after the protective layer will enable the cable main insulation is exposed outside, and the underground (or air) of water or moisture contact, so that the insulating layer is destroyed, eventually lead to the breakdown of insulation.
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