Editing Overhead power line (section)

==Structures==
[[File:ЛЭП 10кВ 01.jpg|thumb|172px|Wooden pylon of medium-voltage overhead line 10 kV]]
[[File:Overhead power lines in Dnipro, Ukraine.jpg|thumb|172px|330 and 150&nbsp;kV high-voltage overhead power lines]]

{{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 [[Utility pole|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 [[Electricity pylon|steel towers]] or pylons. For remote areas, aluminum towers may be placed by [[helicopter]]s.<ref>{{cite web|url=http://www.verticalmag.com/news/article/PoweringUp|archiveurl=https://web.archive.org/web/20151004113042/http://www.verticalmag.com/news/article/PoweringUp|title=Powering Up - Vertical Magazine - The Pulse of the Helicopter Industry|archivedate=4 October 2015|work=verticalmag.com |accessdate=4 October 2015 |url-status=live}}</ref><ref>{{YouTube|GBWHUdPQCH8|[[Sunrise Powerlink]] Helicopter Operations}}</ref> Concrete poles have also been used.<ref name=Fink78/> 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.<ref name=Fink78/> 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.<ref name=Fink78/>

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.<ref name=Fink78/>

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-wire|guy wires]] to counteract some of the forces applied by the conductors.
[[Image:Einebenenleitung.jpg|thumb|Low-profile power lines near an [[airfield]]. Note the red and white transmission towers and [[overhead wire marker]]s (also known as aerial marker balls) on the right.]]
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 insulator]]s 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 fiber]]s for communication. [[Overhead wire marker]]s can be mounted on the ground wire to meet [[International Civil Aviation Organization]] recommendations.<ref>{{cite web|url=http://www.avaids.com/icao.pdf|title=Chapter 6. Visual aids for denoting obstacles|date=2004-11-25|work=Annex 14 Volume I Aerodrome design and operations|publisher=[[International Civil Aviation Organization]]|quote=6.2.8 ... spherical ... diameter of not less than 60 cm. ... 6.2.10 ... should be of one colour.|accessdate=1 June 2011|archive-url=https://web.archive.org/web/20070205130600/http://www.avaids.com/icao.pdf|archive-date=5 February 2007}}</ref>
Some markers include [[Balisor|flashing lamps]] for night-time warning.

===Circuits===
A ''single-circuit transmission line'' carries conductors for only one circuit. For a [[Three-phase electric power|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 [[Railway electrification system|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]].

<gallery widths="230" heights="230">
File:Single circuit overhead power line with distribution lines.png|A single-circuit 138 kV line (top) with distribution wires (bottom)
File:Electric Sails.jpg|A double-circuit line
File:Img0289SCE 500kV lines close.JPG|Parallel single-circuit lines
File:Hamilton Beach Pylon (2).JPG|Four circuits on one tower line
File:Wernau Double Pylon1.JPG|six circuits of three different types
File:Stromtrasse bei Mannheim-Seckenheim.jpg|Various powerlines (110/220 kV) in Germany with double and quadruple circuits
</gallery>