Overhead lines
Overview
 
Overhead lines or overhead wires are used to transmit electrical energy to tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

s, trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

es or trains at a distance from the energy supply point. These overhead lines are known variously as
  • Overhead contact system (OCS)—Europe, except UK and Spain
  • Overhead line equipment (OLE or OHLE)—UK
  • Overhead equipment (OHE) — UK, India, Pakistan and Malaysia
  • Overhead wiring (OHW)—Australia
  • Catenary—United States, India, UK, Singapore (North East MRT Line
    North East MRT Line
    The North East MRT Line is the third Mass Rapid Transit line in Singapore and the world's second longest fully underground, automated and driverless, rapid transit line after Singapore's Circle MRT Line. The line is 20 km long with 16 stations and operated by SBS Transit. Travelling from one end...

    ), Canada, France (:fr:Caténaire) and Spain.


In this article the generic term overhead line is used.

Overhead line is designed on the principle of one or more overhead wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

s or rails
Rail profile
The rail profile is the cross sectional shape of a railway rail, perpendicular to the length of the rail.In all but very early cast iron rails, a rail is hot rolled steel of a specific cross sectional profile designed for use as the fundamental component of railway track.Unlike some other uses of...

 (particularly in tunnels) situated over rail tracks
Rail tracks
The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...

, raised to a high electrical potential by connection to feeder stations at regular intervals.
Encyclopedia
Overhead lines or overhead wires are used to transmit electrical energy to tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

s, trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

es or trains at a distance from the energy supply point. These overhead lines are known variously as
  • Overhead contact system (OCS)—Europe, except UK and Spain
  • Overhead line equipment (OLE or OHLE)—UK
  • Overhead equipment (OHE) — UK, India, Pakistan and Malaysia
  • Overhead wiring (OHW)—Australia
  • Catenary—United States, India, UK, Singapore (North East MRT Line
    North East MRT Line
    The North East MRT Line is the third Mass Rapid Transit line in Singapore and the world's second longest fully underground, automated and driverless, rapid transit line after Singapore's Circle MRT Line. The line is 20 km long with 16 stations and operated by SBS Transit. Travelling from one end...

    ), Canada, France (:fr:Caténaire) and Spain.


In this article the generic term overhead line is used.

Overhead line is designed on the principle of one or more overhead wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

s or rails
Rail profile
The rail profile is the cross sectional shape of a railway rail, perpendicular to the length of the rail.In all but very early cast iron rails, a rail is hot rolled steel of a specific cross sectional profile designed for use as the fundamental component of railway track.Unlike some other uses of...

 (particularly in tunnels) situated over rail tracks
Rail tracks
The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...

, raised to a high electrical potential by connection to feeder stations at regular intervals. The feeder stations are usually fed from a high-voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

 electrical grid
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

.

Overview

Electric trains that collect their current from an overhead line system use a device such as a pantograph
Pantograph (rail)
A pantograph for rail lines is a hinged electric-rod device that collects electric current from overhead lines for electric trains or trams. The pantograph typically connects to a one-wire line, with the track acting as the ground wire...

, bow collector
Bow collector
A bow collector is one of the three main devices used on tramcars to transfer electric current from the wires above to the tram below. While once very common in continental Europe, it has now been largely replaced by the pantograph.-Origins:...

, or trolley pole
Trolley pole
A trolley pole is a tapered cylindrical pole of wood or metal, used to transfer electricity from a "live" overhead wire to the control and propulsion equipment of a tram or trolley bus. The use of overhead wire in a system of current collection is reputed to be the 1880 invention of Frank J....

. The device presses against the underside of the lowest wire of an overhead line system, the contact wire. The 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 trains (such as diesels
Diesel locomotive
A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...

) may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance
Structure gauge
The structure gauge, also called the minimum clearance outline, is the minimum height and width of tunnels and bridges as well as the minimum height and width of the doors that allow a rail siding access into a warehouse...

. Alternative electrical power transmission schemes for trains include third rail
Third rail
A third rail is a method of providing electric power to a railway train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost...

, ground-level power supply
Ground-level power supply
Ground-level power supply, also known as surface current collection and Alimentation par Sol is a modern method of third-rail electrical pick-up for street trams. It was invented for the Bordeaux tramway, which was constructed from 2000 and opened in 2003. Until 2011, this is the only place it is...

, batteries, and electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

.

This article does not cover regenerative braking
Regenerative brake
A regenerative brake is an energy recovery mechanism which slows a vehicle or object down by converting its kinetic energy into another form, which can be either used immediately or stored until needed...

, where the traction motor
Traction motor
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ....

s act as generators to retard movement and return power to the overhead.

Construction

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 above by a second wire known as the (US & Canada) or catenary (UK). This wire approximates the natural path of a wire strung between two points, a catenary curve
Catenary
In physics and geometry, the catenary is the curve that an idealised hanging chain or cable assumes when supported at its ends and acted on only by its own weight. The curve is the graph of the hyperbolic cosine function, and has a U-like shape, superficially similar in appearance to a parabola...

, 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. The messenger wire is supported regularly at structures, by a pulley
Pulley
A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...

, link, or clamp
Clamp (tool)
A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure...

. The whole system is then subjected to a mechanical tension
Tension (mechanics)
In physics, tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. As tension is the magnitude of a force, it is measured in newtons and is always measured parallel to the string on which it applies...

.

As the contact wire makes contact with the pantograph, the carbon insert on top of the pantograph is worn down. Going around a curve, the "straight" wire between supports will cause the contact wire to cross over the whole surface of the pantograph as the train travels around the curve, causing uniform wear and avoiding any notches. On straight track, the contact wire is zigzag
Zigzag
A zigzag is a pattern made up of small corners at variable angles, though constant within the zigzag, tracing a path between two parallel lines; it can be described as both jagged and fairly regular....

ged slightly to the left and right of centre at each successive support so that the pantograph wears evenly.

The zigzagging of the overhead line is not required for trams using trolley poles or for trolleybuses.

Depot areas tend to have only a single wire and are known as simple equipment. 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 wire.
  • Compound equipment uses a second support wire, known as the auxiliary, between the messenger wire and the contact wire. Droppers support the auxiliary from the messenger wire, and additional droppers support the contact wire from the auxiliary. The auxiliary wire can be constructed of a more conductive but less wear-resistant metal, increasing the efficiency of power transmission.


Dropper wires traditionally only provide physical support of the contact wire, and do not join the catenary and contact wires electrically. Contemporary systems use current-carrying droppers, which eliminate the need for separate wires.

The present transmission system originated about 100 years ago. A simpler system was proposed in the 1970s by the Pirelli Construction Co consisting of a single wire embedded at each support for 2.5 metres (8.2 ft) of its length in a clipped extruded aluminum beam with the wire contact face exposed. With a somewhat higher tension than used before clipping the beam yielded a deflected profile for the wire which could be easily handled at 250 miles per hour (111.8 m/s) by a pneumatic servo pantograph with only 3 G accelerations.

For tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

ways there is often only a contact wire and no messenger wire.

Where there is limited clearance to accommodate wire suspensions systems such as in tunnels, the overhead wire may be replaced by rigid overhead rail. This was done when the overhead line was raised in the Simplon Tunnel to accommodate taller rail vehicles.

Parallel overhead lines

An electrical circuit requires at least two conductors. Trams and railways use the overhead line as one side of the circuit and the steel rails as the other side of the circuit. For a trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

 there are no rails to send the return current along—the vehicles use rubber tyres and the normal road surface. Trolleybuses use a second parallel overhead line for the return, and two trolley-poles, one contacting each overhead wire. The circuit is completed by using both wires.

Germany

In Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 there are special overhead power lines for single phase AC traction current with a frequency of 16⅔ hertz. All operate at a voltage of 110 kV (the voltage of the railway overhead lines is 15 kV) and have four conductor cables for two circuits. As a rule at traction current lines, a single-level arrangement of conductor cables
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

 is used.

A traction current pylon is a type of electricity pylon
Electricity pylon
A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...

 with at least one electric circuit for traction current. For traction current lines with four circuits (eight conductor cables) usually two-level arrangements of conductors are used, in which one pylon crossbar carries four conductor cables. For traction current lines used to supply high-speed rail tracks, three-level arrangements of conductors are employed; thereby four conductor cables are mounted on the lowest crossbar, and two on the upper crossbars. The three-level arrangement is also used for traction current lines with six electric circuits (12 conductor cables).

There are other overhead line pylons with crossbars for 110 kV traction voltage. For example the power supplies of rapid transit
Rapid transit
A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic. Rapid transit systems are typically located either in underground tunnels or on...

 railways. Additionally there are pylons that transmit both electric power for railway traction current as well as three-phase alternating current for the public power grid.

Tensioning

Catenary wires are kept at a mechanical tension because the pantograph causes oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...

s in the wire and the wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 must travel faster than the train to avoid producing standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

s that would cause wire breakage. Tensioning the line makes waves travel faster.

For medium and high speeds, the wires are generally tensioned by means of weights or occasionally by hydraulic tensioners. Either method is known as auto-tensioning (AT), or constant tension and ensures that the tension in the equipment is virtually independent of temperature. Tensions are typically between 9 and 20 kN (2,023.3 and 4,496.2 lbf) per wire. Where weights are used, they slide up and down on a rod or tube attached to the mast, to prevent the weights from swaying.

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. Here the tension is generally about 10 kN (2,248.1 lbf). This type of equipment will sag on hot days and hog on cold days.

Where AT is used, there is a limit to the continuous length of overhead line which may be installed. This is due to the change in the position of the weights with temperature as the overhead line expands and contracts. This movement is proportional to the tension length, that is, the distance between anchors. This leads to the concept of maximum tension length. For most 25 kV OHL equipment in the UK, the maximum tension length is 1970 m.

An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length will be free to move along track. To rectify this issue, a midpoint anchor (MPA), close to the centre of the tension length, restricts movement of the messenger 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; otherwise, they are anchored to the typical vertical catenary poles or portal catenary supports. Therefore, a tension length can be seen as a fixed centre point, with the two half tension lengths expanding and contracting with temperature.

Most overhead systems include a brake to stop the wires from unravelling completely should a wire break or tension be lost for any other reason. German systems usually use a single large tensioning pulley 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, lifts 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 a wire breaks or tension is otherwise lost, the pulley falls back toward the mast, and one of its teeth will jam 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 other braking mechanisms, usually with multiple smaller pulleys in a block and tackle
Block and tackle
A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.The pulleys are assembled together to form blocks so that one is fixed and one moves with the load...

 arrangement.

Section Break

To allow maintenance to sections of the overhead line without having to turn off the entire system, the overhead line system is broken into electrically separated portions known as sections. Sections often correspond with tension lengths as described above. The transition from section to section is known as a section break and is set up so that the locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

's pantograph is in continuous contact with the wire.

For bow collectors and pantographs, this is done by having two contact wires run side by side over a length about four wire supports: a new one dropping down and the old one rising up, allowing the pantograph to smoothly transfer from one to the other. The two wires don't touch (although the bow collector/pantograph is briefly in contact with both wires). In normal service, the two sections are electrically connected (to different substations if at or near the halfway mark between them) but this can be broken for servicing.

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
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

 or trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

 must turn off the power when the trolley pole passes through, to prevent arc damage to the insulator.

Pantograph equipped locomotives may never run through a section break when one side is de-energized. Of course the locomotive would then become trapped, but as it passes the section break, the pantograph will briefly short the two catenary lines together. If the opposite line is de-energized, this voltage transient may trip supply breakers. If the line is under maintenance, personnel injury may occur as the catenary is suddenly energized. Even if the catenary is properly grounded, the arc generated across the pantograph will likely cause damage to the pantograph, the catenary insulator, or both.

Neutral Section (Phase Break)

Sometimes on a larger electrified railway, tramway or trolleybus system, it is necessary to power different areas of track from different power grids, the synchronisation of the phases of which cannot be guaranteed. (Sometimes the sections are powered with different voltages or frequencies.) There may be mechanisms for having the grids synchronised on a normal basis but events may cause desynchronisation. This is no problem for DC systems but, for AC systems, it is highly undesirable to connect two unsynchronised grids. A simple section break is insufficient to guard against this as the pantograph briefly connects both sections.

Instead, a neutral section or phase break is used. This consists of two section breaks back-to-back so that there is a short section of overhead line that belongs to neither grid. If the two grids are synchronised this stretch of line is energised (by either supply) and trains run through it normally. If the two supplies are not synchronised, the short isolating section is disconnected from the supplies, leaving it electrically dead, ensuring that the two grids cannot be connected to each other.

The sudden loss, and then reconnection to the supply over the neutral section might cause damage to the locomotive if it was drawing power - so special signs are set up to warn the crew. When synchronisation is lost and the phase break is deenergised, the train's operator should put the controller (throttle) into neutral and coast through an isolated phase break section.

On the Pennsylvania Railroad
Pennsylvania Railroad
The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

 in America, phase breaks were indicated by a position light signal face with all eight radial positions filled by lenses and no center light. When the phase break was active (that is when the catenary sections were out of phase), all lights were lit. The position light signal aspect was originally devised by the Pennsylvania Railroad
Pennsylvania Railroad
The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

 but was continued by its successor Amtrak
Amtrak
The National Railroad Passenger Corporation, doing business as Amtrak , is a government-owned corporation that was organized on May 1, 1971, to provide intercity passenger train service in the United States. "Amtrak" is a portmanteau of the words "America" and "track". It is headquartered at Union...

 and has been adopted by Metro North
Metro north
Metro North can refer to either of* Metro-North Railroad, a commuter railroad serving parts of New York and Connecticut in the United States* Dublin Metro#Metro North, a branch of the proposed Dublin Metro, in Dublin, Ireland...

. Metal signs were also hung from the catenary supports with the letters PB created by a pattern of drilled holes.

Transnet
Transnet
Transnet SOC Ltd is a large South African rail, port and pipeline company, headquartered in the Carlton Centre in Johannesburg. It was formed as a limited company on April 1, 1990. A majority of the company's stock is owned by the Department of Public Enterprises, or DPE, of the South African...

 Freight Rail in South Africa has permanent magnets between the rails at both sides of the neutral section where two phases are separated. These are detected by equipment on the locomotive, which disconnect and reconnect power from the pantographs.

Dead Section

A special category of phase break was also developed in American practice, primarily by the Pennsylvania Railroad. Since its traction power network was centrally supplied, and only segmented by abnormal conditions, phase breaks were normally not active. On the other hand, phase breaks which were always activated came to be known as Dead Sections. They often were to separate boundaries between power systems (for example, the Hell's Gate Bridge boundary between Amtrak and Metro North's electrification systems), which would never be in-phase. Since a dead section is, by definition, always dead, no special signal aspect was developed to warn engineers of its presence. A simple metal sign with DS in drilled-hole letters was hung from the catenary supports.

Crossings

Tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

s draw their power from a single overhead wire at about 500 to 750 V, while trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

es draw their power from two overhead wires at a similar voltage. Because of that, at least one of the trolleybus wires must be insulated from tram wires. This is usually solved by the trolleybus wires running continuously through the crossing, with the tram conductors
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

 a few centimetres lower. Close to the junction on each side, the wire merges 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. This is 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
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

 material extending 20 or 30 mm below.

Until 1946, there was a level crossing in Stockholm
Stockholm
Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 between the railway south of Stockholm Central Station and a tramway line. The tramway operated on 600-700 V DC and the railway on 15 kV AC
15 kV AC
The AC railway electrification system is used in Germany, Austria, Switzerland, Sweden and Norway. The high voltage enables high power transmission with low losses powering traction motors available since the beginning of the 20th century...

. In the Swiss village of Suhr, the WSB tramway operating at 1,200 V DC crossed the SBB line at 15 kV AC
15 kV AC
The AC railway electrification system is used in Germany, Austria, Switzerland, Sweden and Norway. The high voltage enables high power transmission with low losses powering traction motors available since the beginning of the 20th century...

. Some crossings between tramway/light rail and railways are still extant in Germany. In Zürich
Zürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...

, Switzerland the VBZ
Verkehrsbetriebe Zürich
Verkehrsbetriebe Zürich is a public transport operator in the Swiss city of Zurich, and is wholly owned by the city. Previously known as the Städtische Strassenbahn Zürich , the organisation was founded in 1896 and adopted its current name in 1950...

 trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

 line 32 has a level crossing with the 1,200 V DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 railway to mount Uetliberg
Uetliberg
The Üetliberg is a mountain in the Swiss plateau, part of the Albis chain, rising to 873 m . The Uetliberg offers a panoramic view of the entire city of Zurich and the Lake of Zurich. There is also a hotel in the name of "Uto Kulm" on this small mountain...

; at many places in the town, trolleybus lines cross the tramway. In some cities, trolleybuses and trams have shared the same positive (feed) wire. In such cases, a normal trolleybus frog can be used.

Another system that has been used is to coincide section breaks with the crossing point so that the crossing is electrically dead.

Australia

Many cities had trams and trolleybuses both using trolley pole current collection. 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
Melbourne
Melbourne is the capital and most populous city in the state of Victoria, and the second most populous city in Australia. The Melbourne City Centre is the hub of the greater metropolitan area and the Census statistical division—of which "Melbourne" is the common name. As of June 2009, the greater...

, Victoria
Victoria (Australia)
Victoria is the second most populous state in Australia. Geographically the smallest mainland state, Victoria is bordered by New South Wales, South Australia, and Tasmania on Boundary Islet to the north, west and south respectively....

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

Melbourne has four level crossing
Level crossing
A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...

s between electrified suburban railways and tram lines. They have complex switching arrangements to separate the 1,500 V DC overhead of the railway and the 650 V DC of the trams, called an overhead square. Proposals have been put forward which would see these crossings grade separated or the tram routes diverted.

Queensland uses 25 kV AC overhead traction with booster transformers in the Brisbane suburban area and auto transformers elsewhere.

Western Australia (Perth city) uses 25 kV AC overhead traction with booster transformers.

Greece

In Athens
Athens
Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...

, there are two crossings between tram
Athens Tram
The Athens Tram is public tram network system serving Athens. It is constructed, owned and operated by Tram S.A. , subsidiary company of Attiko Metro S.A. ....

 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.

From the opening of the tram system in the summer of 2004, trams and trolleybuses in the direction of Pagrati shared the same exclusive lane, about 400m long, on the far right side of Vas. Olgas Avenue, with tram and trolleybus wires side-by-side above a narrow lane of road. The trolleybus wires were on the far right of the lane, away from the trams' (very wide) pantographs. Trolleybus drivers were required to drive very slowly because the trolley poles were extended to their limits. A change of route for trolleybuses was implemented in mid-2005, ending this arrangement.

Italy

In Milan
Milan
Milan is the second-largest city in Italy and the capital city of the region of Lombardy and of the province of Milan. The city proper has a population of about 1.3 million, while its urban area, roughly coinciding with its administrative province and the bordering Province of Monza and Brianza ,...

, most of the city's tram lines
Trams in Milan
The Milan tramway network is an important part of the public transport network of the city and comune of Milan, in the region of Lombardy, northern Italy.In operation since 1876, the network is presently about long...

 cross its circular trolleybus line
Trolleybuses in Milan
The Milan trolleybus system forms part of the public transport network of the city and comune of Milan, in the region of Lombardy, northern Italy...

 once or twice, so crossings between overhead tram and trolleybus wires are quite commonplace. Trolleybus and tram wires run parallel in some streets, like viale Stelvio and viale Tibaldi.

Multiple overhead lines

There are and were some railways that used two or three overhead lines, usually to carry three-phase
Three-phase
In electrical engineering, three-phase electric power systems have at least three conductors carrying voltage waveforms that are radians offset in time...

 current to the trains. Nowadays, three-phase AC current is used only on the Gornergrat Railway
Gornergratbahn
The Gornergratbahn is a nine-kilometre metre-gauge mountain rack railway, with Abt rack system. It leads from Zermatt, Switzerland , up to the Gornergrat...

 and Jungfraujoch Railway
Jungfraubahn
The Jungfraubahn is an gauge rack railway electrified at 3-phase 1,125 volts 50 Hertz, which runs 9 kilometres from Kleine Scheidegg to the highest railway station in Europe at Jungfraujoch...

 in Switzerland, the Petit train de la Rhune
Petit train de la Rhune
The Petit train de la Rhune is a metre gauge rack railway in France at the western end of the Pyrenees. It links the Col de Saint-Ignace, some to the east of Saint-Jean-de-Luz, to the summit of the La Rhune mountain...

 in France, and the Corcovado Rack Railway
Corcovado Rack Railway
The Corcovado Train is a mountain railway in Rio de Janeiro in Brazil, from Cosme Velho to the summit of the Corcovado Mountain at an altitude of . The summit is known for its statue of Christ the Redeemer and its views over the city and beaches of Rio....

 in Brazil; until 1976, it was widely used in Italy. On these railways, the two conductors of the overhead lines 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 1898 (length: 1.8 kilometres), the military railway between Marienfelde and Zossen between 1901 and 1904 (length: 23.4 kilometres) and an 800-metre-long section of a coal railway near Cologne, between 1940 and 1949.

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 of multiple overhead lines have the disadvantage of 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
Sihltal Zürich Uetliberg Bahn
The Sihltal Zürich Uetliberg Bahn – commonly abbreviated to SZU – is a railway company and transport network in the canton of Zürich in Switzerland...

 is the result of a merge of two railways with different electification. To be able to use different electic systems on shared tracks one of the railways (Sihltalbahn) has overhead wire right above the train, and the other line (Uetlibergbahn) has overhead wire a bit off to one side.

Overhead catenary

A catenary is a system of overhead wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

s used to supply electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 to a locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

, streetcar, or light rail
Light rail
Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...

 vehicle which is equipped with a pantograph
Pantograph (rail)
A pantograph for rail lines is a hinged electric-rod device that collects electric current from overhead lines for electric trains or trams. The pantograph typically connects to a one-wire line, with the track acting as the ground wire...

.

Unlike simple overhead wires, in which the uninsulated wire or cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

 is attached by clamps to closely spaced crosswires, themselves supported by line poles, catenary systems use at least two wires. One wire, called the catenary or messenger wire, is hung at a specific tension between line structures. A second wire is held in tension
Tension (mechanics)
In physics, tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. As tension is the magnitude of a force, it is measured in newtons and is always measured parallel to the string on which it applies...

 by the messenger wire, and is attached to it at frequent intervals by clamp
Clamp (tool)
A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure...

s and connecting wires. The second wire is straight and level, parallel to the rail tracks, suspended over it as the roadway of a suspension bridge
Suspension bridge
A suspension bridge is a type of bridge in which the deck is hung below suspension cables on vertical suspenders. Outside Tibet and Bhutan, where the first examples of this type of bridge were built in the 15th century, this type of bridge dates from the early 19th century...

 is over water.

Simple wire installations are common in light rail applications, especially on city
City
A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.For example, in the U.S...

 street
Street
A street is a paved public thoroughfare in a built environment. It is a public parcel of land adjoining buildings in an urban context, on which people may freely assemble, interact, and move about. A street can be as simple as a level patch of dirt, but is more often paved with a hard, durable...

s, while more expensive catenary systems are especially suited to high-speed operations.

The Northeast Corridor
Northeast Corridor
The Northeast Corridor is a fully electrified railway line owned primarily by Amtrak serving the Northeast megalopolis of the United States from Boston in the north, via New York to Washington, D.C. in the south, with branches serving other cities...

 in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 features electrified catenary over a 600-mile or 1000 km distance between Boston
Boston
Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...

, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

 and Washington, D.C.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....

, providing power for Amtrak
Amtrak
The National Railroad Passenger Corporation, doing business as Amtrak , is a government-owned corporation that was organized on May 1, 1971, to provide intercity passenger train service in the United States. "Amtrak" is a portmanteau of the words "America" and "track". It is headquartered at Union...

's high-speed Acela Express
Acela Express
The Acela Express is Amtrak's high-speed rail service along the Northeast Corridor in the Northeast United States between Washington, D.C., and Boston via Baltimore, Philadelphia, and New York...

 and other trains. Several commuter rail agencies, including MARC
MARC Train
MARC , known prior to 1984 as Maryland Rail Commuter Service, is a regional rail system comprising three lines in the Baltimore-Washington Metropolitan Area. MARC is administered by the Maryland Transit Administration , a Maryland Department of Transportation agency, and is operated under contract...

, SEPTA
Southeastern Pennsylvania Transportation Authority
The Southeastern Pennsylvania Transportation Authority is a metropolitan transportation authority that operates various forms of public transit—bus, subway and elevated rail, commuter rail, light rail, and electric trolley bus—that serve 3.9 million people in and around Philadelphia,...

, NJ Transit
New Jersey Transit
The New Jersey Transit Corporation is a statewide public transportation system serving the United States state of New Jersey, and New York, Orange, and Rockland counties in New York State...

, Metro-North utilize the catenary to provide local service along the Northeast Corridor.

In Cleveland, Ohio
Cleveland, Ohio
Cleveland is a city in the U.S. state of Ohio and is the county seat of Cuyahoga County, the most populous county in the state. The city is located in northeastern Ohio on the southern shore of Lake Erie, approximately west of the Pennsylvania border...

 the interurban/light rail
Blue and Green Lines (Cleveland)
The Blue Line, Green Line, and Waterfront Line are the interurban/light rail component of the RTA Rapid Transit, a rapid transit rail system in greater Cleveland and Shaker Heights, Cuyahoga County, Ohio...

 lines use overhead wires, and the heavy rail
Red Line (Cleveland)
The Red Line is a rapid transit line of the RTA Rapid Transit in Cleveland, Ohio, running from Cleveland Hopkins International Airport northeast to Tower City in downtown Cleveland, then east and northeast to Windermere...

 line also uses overhead wires, instead of a third rail
Third rail
A third rail is a method of providing electric power to a railway train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost...

. This was due to a city ordinance intended to limit air pollution from the large number of steam trains passing through the Cleveland between the east coast and Chicago. Trains switched from steam to overhead catenary electric locomotives at the Collinwood Rail Yards about 10 miles (16.1 km) east of Downtown Cleveland and similarly at Linndale on the west side. When Cleveland constructed its rapid transit (heavy rail) line between the airport, Downtown Cleveland and beyond it employed similar overhead catenary technologies that the railroads used, and were able to utilize railroad electrification equipment left over after railroads switched from steam to diesel locomotives. Consequently, light and heavy rail public transit systems share trackage for about 3 miles (4.8 km) along the Cleveland Hopkins International Airport
Cleveland Hopkins International Airport
Cleveland Hopkins International Airport is a public airport located nine miles southwest of the central business district of Cleveland, a city in Cuyahoga County, Ohio, United States. The airport lies just within the city limits of Cleveland...

 Red (heavy rail) line
Red Line (Cleveland)
The Red Line is a rapid transit line of the RTA Rapid Transit in Cleveland, Ohio, running from Cleveland Hopkins International Airport northeast to Tower City in downtown Cleveland, then east and northeast to Windermere...

, Blue and Green interurban/light rail lines
Blue and Green Lines (Cleveland)
The Blue Line, Green Line, and Waterfront Line are the interurban/light rail component of the RTA Rapid Transit, a rapid transit rail system in greater Cleveland and Shaker Heights, Cuyahoga County, Ohio...

 between Cleveland Union Terminal and just past East 55th Street station, where the heavy- and light-rail line tracks separate.

Part of the Blue Line
Blue Line (MBTA)
The Blue Line is one of four subway lines of the Massachusetts Bay Transportation Authority serving Downtown, East Boston and the North Shore. It runs from northeast to southwest, extending from Wonderland station in Revere, Massachusetts to Bowdoin station near Beacon Hill in Boston...

, running through suburbs northeast of Boston, Massachusetts, uses overhead power lines.

Height

The height of overhead wiring can create hazards at level crossing
Level crossing
A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...

s, where it may be struck by road vehicles. The wiring in most countries is too low to allow double stack container trains. The Channel Tunnel
Channel Tunnel
The Channel Tunnel is a undersea rail tunnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is deep...

 has an extended height overhead line to accommodate double-height car and truck transporters. India is proposing a network of freight only lines, which would almost certainly be electrified with extra height wiring and pantographs that can reach it.

Technical advances lower running costs

The introduction of supercapacitor
Supercapacitor
An electric double-layer capacitor , also known as supercapacitor, supercondenser, electrochemical double layer capacitor, or ultracapacitor, is an electrochemical capacitor with relatively high energy density. Their energy density is typically hundreds of times greater than conventional...

s has promised to drop electrical running costs for trains powered by overhead lines or third rail
Third rail
A third rail is a method of providing electric power to a railway train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost...

s. Kinetic braking energy is reclaimed by storing electrical energy in supercapacitors onboard the vehicle. This stored energy is used when accelerating the train, when high current is needed. The supplementing supercapacitors reduce current drawn through the electrical supply during acceleration and puts less strain on the distribution system.

Later developments locate banks of supercapacitors at track side. All trains on the system can then use the stored energy in the supercapacitors to supplement the energy drawn through a third rail or overhead wires. Trackside location reduces vehicle weight and creates more onboard space. However, such locations would require additional equipment to charge the supercapacitors from the overhead line voltage and to generate supplementary power at the voltage and frequency of the overhead line from the stored energy.

Claimed energy reduction is around 30%. Electric railway systems can be more competitive and a real economical alternative to automobiles.

The technology can be used equally well for diesel electric locomotives, where 25% to 40% reduction in energy consumption is claimed, however only onboard location of supercapacitor banks is feasible. (This technology equally applies to road vehicles that use electric motors for propulsion, such as hybrid cars and buses.) Any electrical equipment that requires regular braking can reduce operating costs using supercapacitors. Reduced operating costs of elevators on underground railways would be a great benefit to operators and adding to their economic competitiveness.

An additional benefit is that emissions from generating plants and diesel-electric locomotives will be decreased.

Since 2003, the Mannheim Stadtbahn in Germany has operated a light-rail vehicle using electric double-layer supercapacitors to store braking energy.

A number of companies are developing electric double-layer supercapacitor technology. Siemens AG
Siemens AG
Siemens AG is a German multinational conglomerate company headquartered in Munich, Germany. It is the largest Europe-based electronics and electrical engineering company....

 is developing mobile energy storage based on double-layer supercapacitors called Sibac Energy Storage Sitras SES, are developing stationary trackside version. The company Cegelec
Cegelec
Cegelec is a European company providing technological services to companies and public authorities. It has a workforce of 25,000 and has a presence in 30 countries. It has been a subsidiary of Vinci since 2010....

 is also developing an electric double-layer capacitor-based energy storage system.

History

In 1881 the first tram with overhead lines was presented by Werner von Siemens on the International Electric Exposition in Paris 1881 but the installation was removed after that event.
In October 1883, the first permanent tram service with overhead lines was started on Mödling and Hinterbrühl Tram
Mödling and Hinterbrühl Tram
Mödling and Hinterbrühl Tram or Mödling and Hinterbrühl Local Railway was an electric tramway in Austria, running from Mödling to Hinterbrühl, in the southwest of Vienna. The gauge was 1000 mm...

 in Austria. These trams had bipolar overhead lines, consisting of two U-pipes, in which the pantographs
Pantograph (rail)
A pantograph for rail lines is a hinged electric-rod device that collects electric current from overhead lines for electric trains or trams. The pantograph typically connects to a one-wire line, with the track acting as the ground wire...

 hung and ran like shuttles. In April to June 1882, Siemens had tested a similar system on his Electromote, an early percursor of the trolleybus
Trolleybus
A trolleybus is an electric bus that draws its electricity from overhead wires using spring-loaded trolley poles. Two wires and poles are required to complete the electrical circuit...

es.

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
Frank J. Sprague
Frank Julian Sprague was an American naval officer and inventor who contributed to the development of the electric motor, electric railways, and electric elevators...

 in 1888. Since 1889, it was used at the Richmond Union Passenger Railway
Richmond Union Passenger Railway
The Richmond Union Passenger Railway, in Richmond, Virginia, was the first practical electric trolley system, and set the pattern for most subsequent electric trolley systems around the world. It is an IEEE milestone in engineering....

 in Richmond, Virginia
Richmond, Virginia
Richmond is the capital of the Commonwealth of Virginia, in the United States. It is an independent city and not part of any county. Richmond is the center of the Richmond Metropolitan Statistical Area and the Greater Richmond area...

. That was the onset of worldwide use of electric traction.

See also

  • Bow collector
    Bow collector
    A bow collector is one of the three main devices used on tramcars to transfer electric current from the wires above to the tram below. While once very common in continental Europe, it has now been largely replaced by the pantograph.-Origins:...

  • Electro-diesel locomotive
    Electro-diesel locomotive
    An Electro-diesel locomotive is powered either from an electricity supply or by using the onboard diesel engine...

  • Frank J. Sprague
    Frank J. Sprague
    Frank Julian Sprague was an American naval officer and inventor who contributed to the development of the electric motor, electric railways, and electric elevators...

  • Lineman
  • List of current systems for electric rail traction

  • Metro-North Railroad
    Metro-North Railroad
    The Metro-North Commuter Railroad , trading as MTA Metro-North Railroad, or, more commonly, Metro-North, is a suburban commuter rail service that is run and managed by the Metropolitan Transportation Authority , an authority of New York State. It is the busiest commuter railroad in the United...

    , a US commuter railway serving New York
    New York
    New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

     and Connecticut
    Connecticut
    Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

    , using overhead wires, third rail
    Third rail
    A third rail is a method of providing electric power to a railway train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost...

    , and diesel locomotive
    Diesel locomotive
    A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...

    s.
  • Pantograph
    Pantograph (rail)
    A pantograph for rail lines is a hinged electric-rod device that collects electric current from overhead lines for electric trains or trams. The pantograph typically connects to a one-wire line, with the track acting as the ground wire...

  • Railway electrification system
    Railway electrification system
    A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...

  • Trolley pole
    Trolley pole
    A trolley pole is a tapered cylindrical pole of wood or metal, used to transfer electricity from a "live" overhead wire to the control and propulsion equipment of a tram or trolley bus. The use of overhead wire in a system of current collection is reputed to be the 1880 invention of Frank J....



External links

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