How Does Overhead Line Work?

Above-ground structure for bulk transfer and distribution of electricity This article is about power lines for general transmission of electrical power. For overhead lines used to power road and rail vehicles, see Overhead line. "Power line" redirects here. For other uses, see Power line (disambiguation).

An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy along large distances. It consists of one or more conductors (commonly multiples of three) suspended by towers or poles. Since the surrounding air provides good cooling, insulation along long passages, and allows optical inspection, overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy.

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Construction

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Towers for support of the lines are made of wood (as-grown or laminated), steel or aluminum (either lattice structures or tubular poles), concrete, and occasionally reinforced plastics. The bare wire conductors on the line are generally made of aluminum[1] (either plain or reinforced with steel, or composite materials such as carbon and glass fiber), though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises. A major goal of overhead power line design is to maintain adequate clearance between energized conductors and the ground so as to prevent dangerous contact with the line, and to provide reliable support for the conductors, resilience to storms, ice loads, earthquakes and other potential damage causes.[2] Today,[when?] some overhead lines are routinely operated at voltages exceeding 765,000 volts between conductors, with even higher voltages possible in some cases.

Classification by operating voltage

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Overhead power transmission lines are classified in the electrical power industry by the range of voltages:

• Low voltage (LV) ' less than Volts, used for connection between a residential or small commercial customer and the utility.
• Medium voltage (MV; distribution) ' between Volts (1 kV) and 69 kV, used for distribution in urban and rural areas.
• High voltage (HV; subtransmission less than 100 kV; subtransmission or transmission at voltages such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.
• Extra high voltage (EHV; transmission) ' from 345 kV, up to about 800 kV,[3][page needed] used for long distance, very high power transmission.
• Ultra high voltage (UHV) ' higher than 800 kV. The Financial Times reported UHV lines are a "game changer", making a global electricity grid potentially feasible. StateGrid said that compared to conventional lines, UHV enables the transmission of five times more power, over six times the distance.[4]

Structures

[edit] See also: Transmission tower

Structures for overhead lines take a variety of shapes depending on the type of line. Structures may be as simple as wood poles directly set in the earth, carrying one or more cross-arm beams to support conductors, or "armless" construction with conductors supported on insulators attached to the side of the pole. Tubular steel poles are typically used in urban areas. High-voltage lines are often carried on lattice-type steel towers or pylons. For remote areas, aluminum towers may be placed by helicopters.[5][6] Concrete poles have also been used.[2] Poles made of reinforced plastics are also available, but their high cost restricts application.

Each structure must be designed for the loads imposed on it by the conductors.[2] The weight of the conductor must be supported, as well as dynamic loads due to wind and ice accumulation, and effects of vibration. Where conductors are in a straight line, towers need only resist the weight since the tension in the conductors approximately balances with no resultant force on the structure. Flexible conductors supported at their ends approximate the form of a catenary, and much of the analysis for construction of transmission lines relies on the properties of this form.[2]

A large transmission line project may have several types of towers, with "tangent" ("suspension" or "line" towers, UK) towers intended for most positions and more heavily constructed towers used for turning the line through an angle, dead-ending (terminating) a line, or for important river or road crossings. Depending on the design criteria for a particular line, semi-flexible type structures may rely on the weight of the conductors to be balanced on both sides of each tower. More rigid structures may be intended to remain standing even if one or more conductors is broken. Such structures may be installed at intervals in power lines to limit the scale of cascading tower failures.[2]

Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands. Each structure may be stabilized considerably by the use of guy wires to counteract some of the forces applied by the conductors.

Power lines and supporting structures can be a form of visual pollution. In some cases the lines are buried to avoid this, but this "undergrounding" is more expensive and therefore not common.

For a single wood utility pole structure, a pole is placed in the ground, then three crossarms extend from this, either staggered or all to one side. The insulators are attached to the crossarms. For an "H"-type wood pole structure, two poles are placed in the ground, then a crossbar is placed on top of these, extending to both sides. The insulators are attached at the ends and in the middle. Lattice tower structures have two common forms. One has a pyramidal base, then a vertical section, where three crossarms extend out, typically staggered. The strain insulators are attached to the crossarms. Another has a pyramidal base, which extends to four support points. On top of this a horizontal truss-like structure is placed.

A grounded wire is sometimes strung along the tops of the towers to provide lightning protection. An optical ground wire is a more advanced version with embedded optical fibers for communication. Overhead wire markers can be mounted on the ground wire to meet International Civil Aviation Organization recommendations.[7] Some markers include flashing lamps for night-time warning.

Circuits

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A single-circuit transmission line carries conductors for only one circuit. For a three-phase system, this implies that each tower supports three conductors.

A double-circuit transmission line has two circuits. For three-phase systems, each tower supports and insulates six conductors. Single phase AC-power lines as used for traction current have four conductors for two circuits. Usually both circuits operate at the same voltage.

In HVDC systems typically two conductors are carried per line, but in rare cases only one pole of the system is carried on a set of towers.

In some countries like Germany most power lines with voltages above 100 kV are implemented as double, quadruple or in rare cases even hextuple power line as rights of way are rare. Sometimes all conductors are installed with the erection of the pylons; often some circuits are installed later. A disadvantage of double circuit transmission lines is that maintenance can be difficult, as either work in close proximity of high voltage or switch-off of two circuits is required. In case of failure, both systems can be affected.

The largest double-circuit transmission line is the Kita-Iwaki Powerline.

• A single-circuit 138 kV line (top) with distribution wires (bottom)
• A double-circuit line
• Parallel single-circuit lines
• Four circuits on one tower line
• six circuits of three different types
• Various powerlines (110/220 kV) in Germany with double and quadruple circuits

Insulators

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Insulators must support the conductors and withstand both the normal operating voltage and surges due to switching and lightning. Insulators are broadly classified as either pin-type, which support the conductor above the structure, or suspension type, where the conductor hangs below the structure. The invention of the strain insulator was a critical factor in allowing higher voltages to be used.

At the end of the 19th century, the limited electrical strength of telegraph-style pin insulators limited the voltage to no more than 69,000 volts. Up to about 33 kV (69 kV in North America) both types are commonly used.[2] At higher voltages only suspension-type insulators are common for overhead conductors.

Insulators are usually made of wet-process porcelain or toughened glass, with increasing use of glass-reinforced polymer insulators. However, with rising voltage levels, polymer insulators (silicone rubber based) are seeing increasing usage.[8] China has already developed polymer insulators having a highest system voltage of kV and India is currently developing a kV (highest system voltage) line which will initially be charged with 400 kV to be upgraded to a kV line.[9]

Suspension insulators are made of multiple units, with the number of unit insulator disks increasing at higher voltages. The number of disks is chosen based on line voltage, lightning withstand requirement, altitude, and environmental factors such as fog, pollution, or salt spray. In cases where these conditions are suboptimal, longer insulators must be used. Longer insulators with longer creepage distance for leakage current, are required in these cases. Strain insulators must be strong enough mechanically to support the full weight of the span of conductor, as well as loads due to ice accumulation, and wind.[10]

Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few milliamperes) passes through the insulator. This warms the surface slightly and reduces the effect of fog and dirt accumulation. The semiconducting glaze also ensures a more even distribution of voltage along the length of the chain of insulator units.

Polymer insulators by nature have hydrophobic characteristics providing for improved wet performance. Also, studies have shown that the specific creepage distance required in polymer insulators is much lower than that required in porcelain or glass. Additionally, the mass of polymer insulators (especially in higher voltages) is approximately 50% to 30% less than that of a comparative porcelain or glass string. Better pollution and wet performance is leading to the increased use of such insulators.

Insulators for very high voltages, exceeding 200 kV, may have grading rings installed at their terminals. This improves the electric field distribution around the insulator and makes it more resistant to flash-over during voltage surges.

Conductors

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The most common conductor in use for transmission today is aluminum conductor steel reinforced (ACSR). Also seeing much use is all-aluminum-alloy conductor (AAAC). Aluminum is used because it has about half the weight of a comparable resistance copper cable (though larger diameter due to lower specific conductivity), as well as being cheaper.[2] Copper was more popular in the past and is still in use, especially at lower voltages and for grounding.

While larger conductors lose less energy due to lower electrical resistance, they are more costly than smaller conductors. An optimization rule called Kelvin's Law (named for Lord Kelvin) states that the optimum size of conductor for a line is found when the cost of the energy wasted in the conductor is equal to the annual interest paid on that portion of the line construction cost due to the size of the conductors. The optimization problem is made more complex by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.[2][11]

Since a conductor is a flexible object with uniform weight per unit length, the shape of a conductor strung between two towers approximates that of a catenary. The sag of the conductor (vertical distance between the highest and lowest point of the curve) varies depending on the temperature and additional load such as ice cover. A minimum overhead clearance must be maintained for safety. Since the temperature and therefore length of the conductor increase with increasing current through it, it is sometimes possible to increase the power handling capacity (uprate) by changing the conductors for a type with a lower coefficient of thermal expansion or a higher allowable operating temperature.

Two such conductors that offer reduced thermal sag are known as composite core conductors (ACCR and ACCC conductor). In lieu of steel core strands that are often used to increase overall conductor strength, the ACCC conductor uses a carbon and glass fiber core that offers a coefficient of thermal expansion about 1/10 of that of steel. While the composite core is nonconductive, it is substantially lighter and stronger than steel, which allows the incorporation of 28% more aluminum (using compact trapezoidal-shaped strands) without any diameter or weight penalty. The added aluminum content helps reduce line losses by 25 to 40% compared to other conductors of the same diameter and weight, depending upon electric current. The carbon core conductor's reduced thermal sag allows it to carry up to twice the current ("ampacity") compared to all-aluminum conductor (AAC) or ACSR.

The power lines and their surroundings must be maintained by linemen, sometimes assisted by helicopters with pressure washers or circular saws which may work three times faster. However this work often occurs in the dangerous areas of the Helicopter height'velocity diagram,[12][13][14] and the pilot must be qualified for this "human external cargo" method.[15]

Bundle conductors

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For transmission of power across long distances, high voltage transmission is employed. Transmission higher than 132 kV poses the problem of corona discharge, which causes significant power loss and interference with communication circuits. To reduce this corona effect, it is preferable to use more than one conductor per phase, or bundled conductors.[16]

Bundle conductors consist of several parallel cables connected at intervals by spacers, often in a cylindrical configuration. The optimum number of conductors depends on the current rating, but typically higher-voltage lines also have higher current. American Electric Power[17] is building 765 kV lines using six conductors per phase in a bundle. Spacers must resist the forces due to wind, and magnetic forces during a short-circuit.

Bundled conductors reduce the voltage gradient in the vicinity of the line. This reduces the possibility of corona discharge. At extra high voltage, the electric field gradient at the surface of a single conductor is high enough to ionize air, which wastes power, generates unwanted audible noise and interferes with communication systems. The field surrounding a bundle of conductors is similar to the field that would surround a single, very large conductor'this produces lower gradients which mitigates issues associated with high field strength. The transmission efficiency is improved as loss due to corona effect is countered.

Bundled conductors cool themselves more efficiently due to the increased surface area of the conductors, further reducing line losses. When transmitting alternating current, bundle conductors also avoid the reduction in ampacity of a single large conductor due to the skin effect. A bundle conductor also has lower reactance, compared to a single conductor.

While wind resistance is higher, wind-induced oscillation can be damped at bundle spacers. The ice and wind loading of bundled conductors will be greater than a single conductor of the same total cross section, and bundled conductors are more difficult to install than single conductors.

Ground wires

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Overhead power lines are often equipped with a ground conductor (shield wire, static wire, or overhead earth wire). The ground conductor is usually grounded (earthed) at the top of the supporting structure, to minimize the likelihood of direct lightning strikes to the phase conductors.[18] In circuits with earthed neutral, it also serves as a parallel path with the earth for fault currents. Very high-voltage transmission lines may have two ground conductors. These are either at the outermost ends of the highest cross beam, at two V-shaped mast points, or at a separate cross arm. Older lines may use surge arresters every few spans in place of a shield wire; this configuration is typically found in the more rural areas of the United States. By protecting the line from lightning, the design of apparatus in substations is simplified due to lower stress on insulation. Shield wires on transmission lines may include optical fibers (optical ground wires/OPGW), used for communication and control of the power system.

At some HVDC converter stations, the ground wire is used also as the electrode line to connect to a distant grounding electrode. This allows the HVDC system to use the earth as one conductor. The ground conductor is mounted on small insulators bridged by lightning arrestors above the phase conductors. The insulation prevents electrochemical corrosion of the pylon.

Medium-voltage distribution lines may also use one or two shield wires, or may have the grounded conductor strung below the phase conductors to provide some measure of protection against tall vehicles or equipment touching the energized line, as well as to provide a neutral line in Wye wired systems.

On some power lines for very high voltages in the former Soviet Union, the ground wire is used for PLC systems and mounted on insulators at the pylons.

Insulated conductors and cable

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Overhead insulated cables are rarely used, usually for short distances (less than a kilometer). Insulated cables can be directly fastened to structures without insulating supports. An overhead line with bare conductors insulated by air is typically less costly than a cable with insulated conductors.

A more common approach is "covered" line wire. It is treated as bare cable, but often is safer for wildlife, as the insulation on the cables increases the likelihood of a large-wing-span raptor to survive a brush with the lines, and reduces the overall danger of the lines slightly. These types of lines are often seen in the eastern United States and in heavily wooded areas, where tree-line contact is likely. The only pitfall is cost, as insulated wire is often costlier than its bare counterpart. Many utility companies implement covered line wire as jumper material where the wires are often closer to each other on the pole, such as an underground riser/pothead, and on reclosers, cutouts and the like.

Dampers

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Because power lines can suffer from aeroelastic flutter driven by wind, Stockbridge dampers are often attached to the lines to reduce the vibrations.

Compact transmission lines

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A compact overhead transmission line requires a smaller right of way than a standard overhead powerline. Conductors must not get too close to each other. This can be achieved either by short span lengths and insulating crossbars, or by separating the conductors in the span with insulators. The first type is easier to build as it does not require insulators in the span, which may be difficult to install and to maintain.

Examples of compact lines are:

• Lutsk compact overhead powerline
• Hilpertsau-Weisenbach compact overhead line

Compact transmission lines may be designed for voltage upgrade of existing lines to increase the power that can be transmitted on an existing right of way.[19]

Low voltage

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Low voltage overhead lines may use either bare conductors carried on glass or ceramic insulators or an aerial bundled cable system. The number of conductors may be anywhere between two (most likely a phase and neutral) up to as many as six (three phase conductors, separate neutral and earth plus street lighting supplied by a common switch); a common case is four (three phase and neutral, where the neutral might also serve as a protective earthing conductor).

Train power

[edit] Main article: Overhead line

Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains. Overhead line is designed on the principle of one or more overhead wires situated over rail tracks. Feeder stations at regular intervals along the overhead line supply power from the high-voltage grid. For some cases low-frequency AC is used, and distributed by a special traction current network.

Further applications

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Overhead lines are also occasionally used to supply transmitting antennas, especially for efficient transmission of long, medium and short waves. For this purpose a staggered array line is often used. Along a staggered array line the conductor cables for the supply of the earth net of the transmitting antenna are attached on the exterior of a ring, while the conductor inside the ring, is fastened to insulators leading to the high-voltage standing feeder of the antenna.

Use of area under overhead power lines

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Use of the area below an overhead line is limited because objects must not come too close to the energized conductors. Overhead lines and structures may shed ice, creating a hazard. Radio reception can be impaired under a power line, due both to shielding of a receiver antenna by the overhead conductors, and by partial discharge at insulators and sharp points of the conductors which creates radio noise.

In the area surrounding the overhead lines it is dangerous to risk interference; e.g. flying kites or balloons, using ladders or operating machinery.

Overhead distribution and transmission lines near airfields are often marked on maps, and the lines themselves marked with conspicuous plastic reflectors, to warn pilots of the presence of conductors.

Construction of overhead power lines, especially in wilderness areas, may have significant environmental effects. Environmental studies for such projects may consider the effect of bush clearing, changed migration routes for migratory animals, possible access by predators and humans along transmission corridors, disturbances of fish habitat at stream crossings, and other effects.

Aviation accidents

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General aviation, hang gliding, paragliding, skydiving, balloon, and kite flying must avoid accidental contact with power lines. Nearly every kite product warns users to stay away from power lines. Deaths occur when aircraft crash into power lines. Some power lines are marked with obstruction markers, especially near air strips or over waterways that may support floatplane operations. The placement of power lines sometimes use up sites that would otherwise be used by hang gliders.[20][21]

History

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The first transmission of electrical impulses over an extended distance was demonstrated on July 14, by the physicist Stephen Gray.[citation needed] The demonstration used damp hemp cords suspended by silk threads (the low resistance of metallic conductors not being appreciated at the time).

However the first practical use of overhead lines was in the context of telegraphy. By experimental commercial telegraph systems ran as far as 20 km (13 miles). Electric power transmission was accomplished in with the first high-voltage transmission between Munich and Miesbach (60 km). saw the construction of the first three-phase alternating current overhead line on the occasion of the International Electricity Exhibition in Frankfurt, between Lauffen and Frankfurt.

In the first 110 kV-overhead power line entered service followed by the first 220 kV-overhead power line in . In the s RWE AG built the first overhead line for this voltage and in built a Rhine crossing with the pylons of Voerde, two masts 138 meters high.

In , the first 345 kV line was put into service by American Electric Power in the United States. In Germany in the first 380 kV overhead power line was commissioned (between the transformer station and Rommerskirchen). In the same year the overhead line traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1. This was used as the model for the building of the Elbe crossing 2 in the second half of the s which saw the construction of the highest overhead line pylons of the world. Earlier, in , the first 380 kV line was put into service in Sweden, in  km (625 miles) between the more populated areas in the south and the largest hydroelectric power stations in the north. Starting from in Russia, and also in the USA and Canada, overhead lines for voltage of 765 kV were built. In overhead power line was built in Soviet Union between Kokshetau and the power station at Ekibastuz, this was a three-phase alternating current line at kV. In , in Japan the first powerline designed for kV with 2 circuits were built, the Kita-Iwaki Powerline. In the building of the highest overhead line commenced in China, the Yangtze River Crossing, its two 346.5 m (1,137 ft) high suspension towers beginning service in .[22]

In the 21st century, replacing steel with carbon fiber cores (advanced reconductoring) became a way for utilities to increase transmission capacity without increasing the amount of land used.

Mathematical analysis

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An overhead power line is one example of a transmission line. At power system frequencies, many useful simplifications can be made for lines of typical lengths. For analysis of power systems, the distributed resistance, series inductance, shunt leakage resistance and shunt capacitance can be replaced with suitable lumped values or simplified networks.

Short and medium line model

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A short length of a power line (less than 80 km) can be approximated with a resistance in series with an inductance and ignoring the shunt admittances. This value is not the total impedance of the line, but rather the series impedance per unit length of line. For a longer length of line (80'250 km), a shunt capacitance is added to the model. In this case it is common to distribute half of the total capacitance to each side of the line. As a result, the power line can be represented as a two-port network, such as with ABCD parameters.[23]

The circuit can be characterized as

Z = z l = ( R + j ω L ) l {\displaystyle Z=zl=(R+j\omega L)l}

where

• Z is the total series line impedance
• z is the series impedance per unit length
• l is the line length
• ω {\displaystyle \omega \ } is the sinusoidal angular frequency

The medium line has an additional shunt admittance

Y = y l = j ω C l {\displaystyle Y=yl=j\omega Cl}

where

• Y is the total shunt line admittance
• y is the shunt admittance per unit length

• Short length of power line
• Medium length of power line

See also

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• Aerial cable
• Conductor marking lights
• CU project controversy
• Overhead cable
• Overhead line
• Raptor conservation
• Third rail
• Operation Outward
• Powerline river crossings in the United Kingdom
• Wireless monitoring of overhead power lines

References

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Cable that provides power to electric railways, trams, and trolleybuses This article is about the transmission of electrical power to road and rail vehicles. For transmission of bulk electrical power to general consumers, see Electric power transmission. For powerlines mounted on pylons, see Overhead power line. For lines carrying information, see Overhead cable.

An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, electric multiple units, trolleybuses or trams. The generic term used by the International Union of Railways for the technology is overhead line.[1] It is known variously as overhead catenary, overhead contact line (OCL), overhead contact system (OCS), overhead equipment (OHE), overhead line equipment (OLE or OHLE), overhead lines (OHL), overhead wiring (OHW), traction wire, and trolley wire.

An overhead line consists of one or more wires (or rails, particularly in tunnels) situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regularly spaced intervals along the track. The feeder stations are usually fed from a high-voltage electrical grid.

Overview

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Electric trains that collect their current from overhead lines use a device such as a pantograph, bow collector or trolley pole. It presses against the underside of the lowest overhead wire, the contact wire. Current collectors are electrically conductive and allow current to flow through to the train or tram and back to the feeder station through the steel wheels on one or both running rails. Non-electric locomotives (such as diesels) may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance. Alternative electrical power transmission schemes for trains include third rail, ground-level power supply, batteries and electromagnetic induction.

Vehicles like buses that have rubber tyres cannot provide a return path for the current through their wheels, and must instead use a pair of overhead wires to provide both the current and its return path.

Construction

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To achieve good high-speed current collection, it is necessary to keep the contact wire geometry within defined limits. This is usually achieved by supporting the contact wire from a second wire known as the messenger wire or catenary. This wire approximates the natural path of a wire strung between two points, a catenary curve, thus the use of "catenary" to describe this wire or sometimes the whole system. This wire is attached to the contact wire at regular intervals by vertical wires known as "droppers" or "drop wires". It is supported regularly at structures, by a pulley, link or clamp. The whole system is then subjected to mechanical tension.

As the pantograph moves along under the contact wire, the carbon insert on top of the pantograph becomes worn with time. On straight track, the contact wire is zigzagged slightly to the left and right of the centre from each support to the next so that the insert wears evenly, thus preventing any notches. On curves, the "straight" wire between the supports causes the contact point to cross over the surface of the pantograph as the train travels around the curve. The movement of the contact wire across the head of the pantograph is called the "sweep".

Additional reading:
The Ultimate Buyer's Guide for Purchasing 25 Pin Connector

Link to zhuhaicable

The zigzagging of the overhead line is not required for trolley poles. For tramways, a contact wire without a messenger wire is used.

Depot areas tend to have only a single wire and are known as "simple equipment" or "trolley wire". When overhead line systems were first conceived, good current collection was possible only at low speeds, using a single wire. To enable higher speeds, two additional types of equipment were developed:

• Stitched equipment uses an additional wire at each support structure, terminated on either side of the messenger/catenary wire.
• Compound equipment uses a second support wire, known as the "auxiliary", between the messenger/catenary wire and the contact wire. Droppers support the auxiliary from the messenger wire, while additional droppers support the contact wire from the auxiliary. The auxiliary wire can be of a more conductive but less wear-resistant metal, increasing transmission efficiency.

Earlier dropper wires provided physical support of the contact wire without joining the catenary and contact wires electrically. Modern systems use current-carrying droppers, eliminating the need for separate wires.

The present transmission system originated about 100 years ago. A simpler system was proposed in the s by the Pirelli Construction Company, consisting of a single wire embedded at each support for 2.5 metres (8 ft 2 in) of its length in a clipped, extruded aluminum beam with the wire contact face exposed. A somewhat higher tension than used before clipping the beam yielded a deflected profile for the wire that could be easily handled at 400 km/h (250 mph) by a pneumatic servo pantograph with only 3 g acceleration.[citation needed]

Parallel overhead lines

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An electrical circuit requires at least two conductors. Trams and railways use the overhead line as the positive terminal of the circuit and the steel rails as the negative terminal of the circuit. For a trolleybus or a trolleytruck, no rails are available for the return current, as the vehicles use rubber tyres on the road surface. Trolleybuses use a second parallel overhead line for the return, and two trolley poles, one contacting each overhead wire. (Pantographs are generally incompatible with parallel overhead lines.) The circuit is completed by using both wires. Parallel overhead wires are also used on the rare railways with three-phase AC railway electrification.

Types of wires

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In the Soviet Union the following types of wires/cables were used.[2] For the contact wire, cold drawn solid copper was used to ensure good conductivity. The wire is not round but has grooves at the sides to allow the hangers to attach to it. Sizes were (in cross-sectional area) 85, 100, or 150 mm2. To make the wire stronger, 0.04% tin might be added. The wire must resist the heat generated by arcing and thus such wires should never be spliced by thermal means.

The messenger (or catenary) wire needs to be both strong and have good conductivity. They used multi-strand wires (or cables) with 19 strands in each cable (or wire). Copper, aluminum, and/or steel were used for the strands. All 19 strands could be made of the same metal or a mix of metals based on the required properties. For example, steel wires were used for strength, while aluminium or copper wires were used for conductivity.[3] Another type looked like it had all copper wires but inside each wire was a steel core for strength. The steel strands were galvanized but for better corrosion protection they could be coated with an anti-corrosion substance.

In Slovenia, where 3 kV system is in use, standard sizes for contact wire are 100 and 150 mm2. The catenary wire is made of copper or copper alloys of 70, 120 or 150 mm2. The smaller cross sections are made of 19 strands, whereas the bigger has 37 strands. Two standard configurations for main lines consist of two contact wires of 100 mm2 and one or two catenary wires of 120 mm2, totaling 320 or 440 mm2. Only one contact wire is often used for side tracks.[4]

In the UK and EU countries, the contact wire is typically made from copper alloyed with other metals. Sizes include cross-sectional areas of 80, 100, 107, 120, and 150 mm2. Common materials include normal and high strength copper, copper-silver, copper-cadmium, copper-magnesium, and copper-tin, with each being identifiable by distinct identification grooves along the upper lobe of the contact wire. These grooves vary in number and location on the arc of the upper section.[5] Copper is chosen for its excellent conductivity, with other metals added to increase tensile strength. The choice of material is chosen based on the needs of the particular system, balancing the need for conductivity and tensile strength.

Tensioning

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Catenary wires are kept in mechanical tension because the pantograph causes mechanical oscillations in the wire. The waves must travel faster than the train to avoid producing standing waves, which could break the wire. Tensioning the line makes waves travel faster, and also reduces sag from gravity.

For medium and high speeds, the wires are generally tensioned by weights or occasionally by hydraulic tensioners. Either method is known as "auto-tensioning" (AT) or "constant tension" and ensures that the tension is virtually independent of temperature. Tensions are typically between 9 and 20 kN (2,000 and 4,500 lbf) per wire. Where weights are used, they slide up and down on a rod or tube attached to the mast, to prevent them from swaying. Recently, spring tensioners have started to be used. These devices contain a torsional spring with a cam arrangement to ensure a constant applied tension (instead of varying proportionally with extension). Some devices also include mechanisms for adjusting the stiffness of the spring for ease of maintenance.

For low speeds and in tunnels where temperatures are constant, fixed termination (FT) equipment may be used, with the wires terminated directly on structures at each end of the overhead line. The tension is generally about 10 kN (2,200 lbf). This type of equipment sags in hot conditions and is taut in cold conditions.

With AT, the continuous length of the overhead line is limited due to the change in the height of the weights as the overhead line expands and contracts with temperature changes. This movement is proportional to the distance between anchors. Tension length has a maximum. For most 25 kV OHL equipment in the UK, the maximum tension length is 1,970 m (6,460 ft).[6]

An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length is free to move along the track. To avoid this a midpoint anchor (MPA), close to the centre of the tension length, restricts movement of the messenger/catenary wire by anchoring it; the contact wire and its suspension hangers can move only within the constraints of the MPA. MPAs are sometimes fixed to low bridges, or otherwise anchored to vertical catenary poles or portal catenary supports. A tension length can be seen as a fixed centre point, with the two half-tension lengths expanding and contracting with temperature.

Most systems include a brake to stop the wires from unravelling completely if a wire breaks or tension is lost. German systems usually use a single large tensioning pulley (basically a ratchet mechanism) with a toothed rim, mounted on an arm hinged to the mast. Normally the downward pull of the weights and the reactive upward pull of the tensioned wires lift the pulley so its teeth are well clear of a stop on the mast. The pulley can turn freely while the weights move up or down as the wires contract or expand. If tension is lost the pulley falls back toward the mast, and one of its teeth jams against the stop. This stops further rotation, limits the damage, and keeps the undamaged part of the wire intact until it can be repaired. Other systems use various braking mechanisms, usually with multiple smaller pulleys in a block and tackle arrangement.

Breaks

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Lines are divided into sections to limit the scope of an outage and to allow maintenance.

Section break

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To allow maintenance to the overhead line without having to turn off the entire system, the line is broken into electrically separated portions known as "sections". Sections often correspond with tension lengths. The transition from section to section is known as a "section break" and is set up so that the vehicle's pantograph is in continuous contact with one wire or the other.

For bow collectors and pantographs, this is done by having two contact wires run side by side over the length between 2 or 4 wire supports. A new one drops down and the old one rises up, allowing the pantograph to smoothly transfer from one to the other. The two wires do not touch (although the bow collector or pantograph is briefly in contact with both wires). In normal service, the two sections are electrically connected; depending on the system this might be an isolator, fixed contact or a Booster Transformer. The isolator allows the current to the section to be interrupted for maintenance.

On overhead wires designed for trolley poles, this is done by having a neutral section between the wires, requiring an insulator. The driver of the tram or trolleybus must temporarily reduce the power draw before the trolley pole passes through, to prevent arc damage to the insulator.

Pantograph-equipped locomotives must not run through a section break when one side is de-energized. The locomotive would become trapped, but as it passes the section break the pantograph briefly shorts the two catenary lines. If the opposite line is de-energized, this voltage transient may trip supply breakers. If the line is under maintenance, an injury may occur as the catenary is suddenly energized. Even if the catenary is properly grounded to protect the personnel, the arc generated across the pantograph can damage the pantograph, the catenary insulator or both.

Neutral section (phase break)

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Sometimes on a larger electrified railway, tramway or trolleybus system, it is necessary to power different areas of track from different power grids, without guaranteeing synchronisation of the phases. Long lines may be connected to the country's national grid at various points and different phases. (Sometimes the sections are powered with different voltages or frequencies.) The grids may be synchronised on a normal basis, but events may interrupt synchronisation. This is not a problem for DC systems. AC systems have a particular safety implication in that the railway electrification system would act as a "Backdoor" connection between different parts, resulting in, amongst other things, a section of the grid de-energised for maintenance being re-energised from the railway substation creating danger.

For these reasons, Neutral sections are placed in the electrification between the sections fed from different points in a national grid, or different phases, or grids that are not synchronized. It is highly undesirable to connect unsynchronized grids. A simple section break is insufficient to guard against this as the pantograph briefly connects both sections.[7]

In countries such as France, South Africa, Australia and the United Kingdom, a pair of permanent magnets beside the rails at either side of the neutral section operate a bogie-mounted transducer on the train which causes a large electrical circuit-breaker to open and close when the locomotive or the pantograph vehicle of a multiple unit passes over them.[8] In the United Kingdom equipment similar to Automatic Warning System (AWS) is used, but with pairs of magnets placed outside the running rails (as opposed to the AWS magnets placed midway between the rails). Lineside signs on the approach to the neutral section warn the driver to shut off traction power and coast through the dead section.

A neutral section or phase break consists of two insulated breaks back-to-back with a short section of line that belongs to neither grid. Some systems increase the level of safety by the midpoint of the neutral section being earthed. The presence of the earthed section in the middle is to ensure that should the transducer controlled apparatus fail, and the driver also fail to shut off power, the energy in the arc struck by the pantograph as it passes to the neutral section is conducted to earth, operating substation circuit breakers, rather than the arc either bridging the insulators into a section made dead for maintenance, a section fed from a different phase, or setting up a Backdoor connection between different parts of the country's national grid.

On the Pennsylvania Railroad, phase breaks were indicated by a position light signal face with all eight radial positions with lenses and no center light. When the phase break was active (the catenary sections out of phase), all lights were lit. The position light signal aspect was originally devised by the Pennsylvania Railroad and was continued by Amtrak and adopted by Metro North. Metal signs were hung from the catenary supports with the letters "PB" created by a pattern of drilled holes.

Dead section

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A special category of phase break was developed in America, primarily by the Pennsylvania Railroad. Since its traction power network was centrally supplied and only segmented by abnormal conditions, normal phase breaks were generally not active. Phase breaks that were always activated were known as "Dead Sections": they were often used to separate power systems (for example, the Hell's Gate Bridge boundary between Amtrak and Metro North's electrifications) that would never be in-phase. Since a dead section is always dead, no special signal aspect was developed to warn drivers of its presence, and a metal sign with "DS" in drilled-hole letters was hung from the catenary supports.

Gaps

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Occasionally gaps may be present in the overhead lines, when switching from one voltage to another or to provide clearance for ships at moveable bridges, as a simpler alternative for moveable overhead power rails. Electric trains coast across the gaps. To prevent arcing, power must be switched off before reaching the gap and usually the pantograph would be lowered.

Overhead conductor rails

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Given limited clearance such as in tunnels, the overhead wire may be replaced by a rigid overhead rail. An early example was in the tunnels of the Baltimore Belt Line, where a Π section bar (fabricated from three strips of iron and mounted on wood) was used, with the brass contact running inside the groove.[9] When the overhead line was raised in the Simplon Tunnel to accommodate taller rolling stock, a rail was used. A rigid overhead rail may also be used in places where tensioning the wires is impractical, for example on moveable bridges. In modern uses, it is very common for underground sections of trams, metros, and mainline railways to use a rigid overhead wire in their tunnels, while using normal overhead wires in their above ground sections.

In a movable bridge that uses a rigid overhead rail, there is a need to transition from the catenary wire system into an overhead conductor rail at the bridge portal (the last traction current pylon before the movable bridge). For example, the power supply can be done through a catenary wire system near a swing bridge. The catenary wire typically comprises messenger wire (also called catenary wire) and a contact wire where it meets the pantograph. The messenger wire is terminated at the portal, while the contact wire runs into the overhead conductor rail profile at the transition end section before it is terminated at the portal. There is a gap between the overhead conductor rail at the transition end section and the overhead conductor rail that runs across the entire span of the swing bridge. The gap is required for the swing bridge to be opened and closed. To connect the conductor rails together when the bridge is closed, there is another conductor rail section called "rotary overlap" that is equipped with a motor. When the bridge is fully closed, the motor of the rotary overlap is operated to turn it from a tilted position into the horizontal position, connecting the conductor rails at the transition end section and the bridge together to supply power.[10]

Short overhead conductor rails are installed at tram stops as for the Combino Supra.[11]

Crossings

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Trams draw their power from a single overhead wire at about 500 to 750 V DC. Trolleybuses draw from two overhead wires at a similar voltage, and at least one of the trolleybus wires must be insulated from tram wires. This is usually done by the trolleybus wires running continuously through the crossing, with the tram conductors a few centimetres lower. Close to the junction on each side, the tram wire turns into a solid bar running parallel to the trolleybus wires for about half a metre. Another bar similarly angled at its ends is hung between the trolleybus wires, electrically connected above to the tram wire. The tram's pantograph bridges the gap between the different conductors, providing it with a continuous pickup.

Where the tram wire crosses, the trolleybus wires are protected by an inverted trough of insulating material extending 20 or 30 mm (0.79 or 1.18 in) below.

Until , a level crossing in Stockholm, Sweden connected the railway south of Stockholm Central Station and a tramway. The tramway operated on 600'700 V DC and the railway on 15 kV AC. In the Swiss village of Oberentfelden, the Menziken'Aarau'Schöftland line operating at 750 V DC crosses the SBB line at 15 kV AC; there used to be a similar crossing between the two lines at Suhr but this was replaced by an underpass in . Some crossings between tramway/light rail and railways are extant in Germany. In Zürich, Switzerland, VBZ trolleybus line 32 has a level crossing with the 1,200 V DC Uetliberg railway line; at many places, trolleybus lines cross the tramway. In some cities, trolleybuses and trams shared a positive (feed) wire. In such cases, a normal trolleybus frog can be used.

Alternatively, section breaks can be sited at the crossing point, so that the crossing is electrically dead.

Australia

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Many cities had trams and trolleybuses using trolley poles. They used insulated crossovers, which required tram drivers to put the controller into neutral and coast through. Trolleybus drivers had to either lift off the accelerator or switch to auxiliary power.

In Melbourne, Victoria, tram drivers put the controller into neutral and coast through section insulators, indicated by insulator markings between the rails.

Melbourne has several remaining level crossings between electrified suburban railways and tram lines. They have mechanical switching arrangements (changeover switch) to switch the V DC overhead of the railway and the 650 V DC of the trams, called a Tram Square.[12] Several such crossings have been grade separated in recent years as part of the Level Crossing Removal Project.

Greece

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Athens has two crossings of tram and trolleybus wires, at Vas. Amalias Avenue and Vas. Olgas Avenue, and at Ardittou Street and Athanasiou Diakou Street. They use the above-mentioned solution.

Italy

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In Rome, at the crossing between Viale Regina Margherita and Via Nomentana, tram and trolleybus lines cross: tram on Viale Regina Margherita and trolleybus on Via Nomentana. The crossing is orthogonal, therefore the typical arrangement was not available.

In Milan, most tram lines cross its circular trolleybus line once or twice. Trolleybus and tram wires run parallel in streets such as viale Stelvio, viale Umbria and viale Tibaldi.

Multiple overhead lines

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Some railways used two or three overhead lines, usually to carry three-phase current. This is used only on the Gornergrat Railway and Jungfrau Railway in Switzerland, the Petit train de la Rhune in France, and the Corcovado Rack Railway in Brazil. Until , it was widely used in Italy. On these railways, the two conductors are used for two different phases of the three-phase AC, while the rail was used for the third phase. The neutral was not used.

Some three-phase AC railways used three overhead wires. These were an experimental railway line of Siemens in Berlin-Lichtenberg in (length 1.8 kilometres (1.1 mi)), the military railway between Marienfelde and Zossen between and (length 23.4 kilometres (14.5 mi)) and an 800-metre (2,600 ft)-long section of a coal railway near Cologne between and .

On DC systems, bipolar overhead lines were sometimes used to avoid galvanic corrosion of metallic parts near the railway, such as on the Chemin de fer de la Mure.

All systems with multiple overhead lines have a high risk of short circuits at switches and therefore tend to be impractical in use, especially when high voltages are used or when trains run through the points at high speed.

The Sihltal Zürich Uetliberg Bahn had two lines with different electrification. To be able to use different electric systems on shared tracks, the Sihltal line had its overhead wire right above the train, whilst the Uetliberg line had its overhead wire off to one side. This configuration was used up until summer , since then the Uetliberg line has been switched to the standard 15kV 16.7 Hz configuration.[13]

Overhead catenary

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A catenary is a system of overhead wires used to supply electricity to a locomotive, tram (streetcar), or light rail vehicle that is equipped with a pantograph.

Unlike simple overhead wires, in which the uninsulated wire is attached by clamps to closely spaced crosswires supported by poles, catenary systems use at least two wires. The catenary or messenger wire is hung at a specific tension between line structures, and a second wire is held in tension by the messenger wire, attached to it at frequent intervals by clamps and connecting wires known as droppers. The second wire is straight and level, parallel to the rail track, suspended over it as the roadway of a suspension bridge is over water.

Catenary systems are suited to high-speed operations whereas simple wire systems, which are less expensive to build and maintain, are common on light rail or tram (streetcar) lines, especially on city streets. Such vehicles can be fitted with either a pantograph or trolley pole.

Electrification support structures

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Overhead line equipment may be supported above the running lines by a range of different methods. Support structures are often designed to allow mechanically independent registration (MIR) which refers to a setup where each contact and catenary wire for each track is mechanically independent from the adjacent wire runs. Support structures are required to provide support and registration to the OLE wires. In OLE terminology, support refers to vertical position of catenary wire (and of the contact wire via the catenary wire), while registration refers to the horizontal position of both catenary and contact wires. The metal parts the provide OLE registration are typically designed to adjust in the vertical plane as the pantograph moves under it. The generic types of support structures are summarised below.[14]

• Single Cantilever
  • The most basic and common type of OLE structure that supports and registers one wire run above one track.
• Double Cantilever
  • Similar to the previous but with two cantilever arms adjacent to each other on one mast. Often used where two wire runs converge.
• Back to Back Cantilever
  • Two wire runs over two tracks supported and registered by one mast placed in the centre of the tracks with cantilever arms attached to opposite sides. Frequently used on tram and light rail systems but can appear on heavy rail lines.
• Two Track Cantilever (TTC)
  • Two wire runs over two tracks supported by one mast with a boom structure extending over to the second track. TTCs typically provide mechanically independent registration, but subtypes exists called the "span-wire two track cantilever" which has both registration arms mechanically linked. TTCs are often used where there are poor ground conditions or obstructions on one side of a two track railway. They are also sometimes used to minimised piling since only one track must be taken out of service for this phase of construction.
• Portals
  • Also known generically as "gantries", portals are large structures with masts on either side and a fixed steel beam between them. They are frequently used on sections of railway line with more than two tracks. Portals typically provide mechanically independent registration, however variants exist with registration span-wires that mechanically link adjacent wire runs. Portals are sometimes used on two track railways over bridges and viaducts or where ground conditions are poor. This is because portals inflict fewer rotational forces on their foundations.
• Headspans
  • An alternative method to supporting multi-track areas, headspans consist of a mast at either side of the railway and various cables running horizontally between the two masts (called span wires) to support and register all wire runs at tension. Because the tension of all wire runs and span wires are required simultaneously to hold up the OLE wires, headspans by definition do not provide mechanically independent registration and a failure of one OLE or span wire will bring all wire runs out of geometric limits. Headspans are cheaper and less obtrusive than portals and so are well suited to low speed complex multitrack areas like stations, station approaches, depots and sidings, and areas where visual intrusion is an important consideration. They are also capable of supporting tracks with speeds up to 200km/h (125mph) but provide significant reliability disadvantages over portals or TTCs for this application.

Tunnels, low overbridges and other location specific features (retaining walls, adjacent rockfaces etc) frequently require bespoke OLE structures that may incorporate some features of the generic types above.

Overhead catenary systems in the United States

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The Northeast Corridor in the United States has catenary over the 600 miles (970 km) between Boston, Massachusetts and Washington, D.C., for Amtrak's inter-city trains. Commuter rail agencies including MARC, SEPTA, NJ Transit, and Metro-North Railroad utilize the catenary to provide local service.

In Cleveland, Ohio, the interurban/light rail lines and the heavy rail line use the same overhead wires, due to a city ordinance intended to limit air pollution from the large number of steam trains that passed through Cleveland between the east coast and Chicago. Trains switched from steam to electric locomotives at the Collinwood railyards about 10 miles (16 km) east of Downtown and at Linndale on the west side. When Cleveland constructed its rapid transit (heavy rail) line between the airport, downtown, and beyond, it employed a similar catenary, using electrification equipment left over after railroads switched from steam to diesel. Light and heavy rail share trackage for about 3 miles (4.8 km) along the Cleveland Hopkins International Airport Red (heavy rail) line, Blue and Green interurban/light rail lines between Cleveland Union Terminal and just past East 55th Street station, where the lines separate.

Part of Boston's Blue Line through the northeast suburbs uses overhead lines, as does the Green Line.

The Yellow Line on the Chicago "L" used an overhead catenary system for the west half of the route, transitioning to third rail for the east half. This was discontinued in when the entire route was converted to third rail.

On the San Francisco peninsula in California, the Caltrain commuter rail system completed the installation of an overhead contact system (OCS) in , to prepare for the conversion of its 160-year old San Francisco to San José Peninsula Corridor to fully-electrified revenue service in September .

Height

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The height of the overhead line can create hazards at level crossings, where it may be struck by road vehicles. Warning signs are placed on the approaches, advising drivers of the maximum safe height.

The wiring in most countries is too low to allow double stack container trains. The Channel Tunnel has an extended height overhead line to accommodate double-height car and truck transporters. China and India operate lines electrified with extra height wiring and pantographs to allow for double stack container trains.[15][16][17]

Problems with overhead equipment

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Overhead lines may be adversely affected by strong winds causing wires to swing.[18] Power storms can knock the power out with lightning strikes on systems[19] with overhead wires, stopping trains following a power surge.

During cold or frosty weather, ice may coat overhead lines. This can result in poor electrical contact between the collector and the overhead line, resulting in electrical arcing and power surges.[20] Ice coatings also add extra weight, as well as increase their surface area exposed to wind, consequently increasing the load on the wires and their supports.

Lines may sag during hot weather and if a pantograph gets entangled, this can result in a dewirement. Similarly, in very cold weather they may contract and snap.

The installation of overhead lines may require reconstruction of bridges to provide safe electrical clearance.[21]

Overhead lines, like most electrified systems, require a greater capital expenditure when building the system than an equivalent non-electric system. While a unelectrified railway line requires only the grade, ballast, ties and rails, an overhead system also requires a complex system of support structures, lines, insulators, power-control systems and power lines, all of which require maintenance. This makes non-electrical systems more attractive in the short term, although electrical systems can pay for themselves eventually. Also, the added construction and maintenance cost-per-mile makes overhead systems less attractive on already existing long-distance railways, such as those found in North America, where the distances between cities are typically far greater than in Europe. Such long lines require enormous investment in overhead line equipment, which private rail companies are unlikely to be interested in, and major difficulties confront energizing long portions of overhead wire on a permanent basis, especially in areas where energy demand already outstrips supply.

Many people consider overhead lines to be "visual pollution", due to the many support structures and complicated system of wires and cables that fill the air. Such considerations have driven the move towards replacing overhead power and communications lines with buried cables where possible. The issue came to a head in the UK with the Great Western Main Line electrification scheme, especially through the Goring Gap. A protest group with their own website has been formed.[22]

The valuable copper conductor can also be subject to theft, as for example the Lahore-Khanewal line in Pakistan and the Gweru-Harare section of line in Zimbabwe.

History

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The first tram with overhead lines was presented by Werner von Siemens at the International Exposition of Electricity in Paris: the installation was removed after that event. In October , the first permanent tram service with overhead lines was on the Mödling and Hinterbrühl Tram in Austria. The trams had bipolar overhead lines, consisting of two U-pipes, in which the pantographs hung and ran like shuttles. From April to June , Siemens had tested a similar system on his Electromote, an early precursor of the trolleybus.

Much simpler and more functional was an overhead wire in combination with a pantograph borne by the vehicle and pressed at the line from below. This system, for rail traffic with a unipolar line, was invented by Frank J. Sprague in . From it was used at the Richmond Union Passenger Railway in Richmond, Virginia, pioneering electric traction.

See also

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References

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Further reading

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• Cooper, B.K. (February'March ). "Catenaries and Contact Wires". Rail Enthusiast. EMAP National Publications. pp. 14'16. ISSN -561X. OCLC .
• "Garry Keenor ' Overhead Line Electrification for Railways" .
• "Trans Power Guide". Archived from the original on -07-16 .
• Liudvinavičius, Lionginas; Dailydka, Stasys (1 January ). "The Aspects of Catenary Maintenance of Direct Current (DC) and Alternating Current (AC)". Procedia Engineering. 134: 268'275. doi:10./j.proeng..01.007. ISSN -.