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Overhead PowerlineAn 'overhead powerline is an electric line, which uses as conductors non-isolated conductors. Overhead powerlines must possess a minimum height of 4 meters (for voltagesunder 1000 V, for higher tensions the distance to ground of an overhead powerine must be larger) over the soil for safety reasons (danger of electrical shocks). They are hung up at insulators on pylons. For overhead lines with voltages under 50 kV are used wood -, concrete -, steel tube and steel framework pylons. For votages over 50kV at least in Europe steel framework pylons are usually used. However for lines of the 110 kV voltage level increasingly steel tube pylons are used. It is permissible in principle in Germany and Austria to build overhead powerlines which would run through under a valley bridge. One finds a such undercrossing at the Koersch Valley bridge close Esslingen at the Neckar. Power transmission As insulators hanging or standing insulators are used for voltages under approx. 50kV. First can bear higher forces, the latters offer an additional security, since in the case of an insulator break the conductor cable falls on the pylon. An additional security against fracture can be obtained by use of two (or more) parallel insulators. For voltages over 50kV only hanging long staff insulators are used. As insulator material glass or ceramic(s) is usually used. For voltages over 200kV frequently suspension insulators are used, consisting of two to four long staff insulators (e.g. two-chain). Recently for insulators for voltages over 100kV also insulators from high-strength plastic are used in particular. Conductor cables of overhead lines consist of copper, Aldrey and group hurrying of steel and aluminum. The latters have a higher electrical conductivity than copper cables despite larger cross section with same weight and with high voltage transmission lines preferentially are therefore used. For voltages over 110 kV frequently, in order to avoid corona losses, bundle conductors so used mentioned. Bundle consuctors consist of several conductor cables connected by means of measuring rods. For 220 kV lines two-bundles are usually used, for 380 kV lines usually three or four. Overhead powerlines with operating voltages over 50 kV (in some cases also under it) are usually equipped with a ground conductor. A ground conductor is an electrically conductive rope fastened to the top of the pylon, which is to protect the line against lightning strikes. In the ground conductor often still another fiber-optic cable is embedded, which can be used to the data communication. These fiber-optic cables are used not only by the power suppliers, but also by telecommunications offerers. Sometimes for higher requirements in terms of the lightning protection high voltage transmission lines are equipped with 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 earth cross bar for the ground conductors. Overhead lines are not to be confounded with overhead cables. Overhead cables are insulated cable layn on pylons. Overhead cables can be fastened, since they are isolated, without insulators to the pylon. In addition, for voltages over 50 kV today and in foreseeable time the overhead line is as a rule the most economical form of the line, because overhead powerlines can at winter time, when the current consumption is very high, be loaded with high currents, because of the cooling by surrounding air very easily cooled. Telecommunication An overhead line is in the telecommunication technology a line run on telephone poles, whose wires are not isolated. In order to fasten it to the masts, insulators from glass, ceramic(s) or plastics are used. Short-circuits between the wires are avoided by the fact that a minimum distance between them is kept. The telephone network in Germany was reequipped under high costs on ground cable (and occasionally also overhead cable), since overhead lines are often disturbed by influences of the weather: Storms upset masts, which can freeze and because of their weight tear wires, etc.. Overhead lines for the purpose of the transmission of news disappeared in Germany and western Continental Europe in the meantime nearly completely. One finds it only occasionally for lines of the course-internal telephone network along not-electrified secondary railway lines. Underdeveloped countries, in addition, countries as for example the USA and Great Britain hands, large parts of their lines to customers implemented still as overhead lines. Because of the antenna effect of overhead lines these can catch also amateur radio and CB radio. While the normal telephone service is actually not impaired of it, however a DSL transmission of can disturbed if radio and DSL use the same frequency range. Further applications A special form of the overhead line are the overhead lines and third rails of electric railways, because these must be equipped for the withdrawal by electricity by the current collector of rail-mounted vehicles. Also for the supply of transmitting antennas, in particular of antennas for very strong transmitters for long -, central and short wave are used occasionally overhead lines. For this a staggered array line is often used. At a staggered array line the conductor cables for the supply of the earth net of the transmitting antenna on the exterior of a ring are attached, while the inside one the ring, to insulators fastens, which runs under high voltage standing feeder of the antenna Security tips In the surrounding field of overhead lines (and also of radio towers, in particular of mast antennas) it is forbidden and dangerous letting kites or chain balloons ascend since by the line, in particular in the damp condition dangerous rivers to flow to be able. Also caution is in particular required in handling long bars or leaders with low-hanging overhead lines, in particular if these are from electrically conductive material. Under overhead lines one is not to refuel motor vehicles or to fill containers over with combustible liquid. History The first overhead line of the world was built on July 14th, 1729 by the physician Stephen Gray, in order to show that one can transfer electricity. It used damp hemp cords, which were fastened to bean bars as a leader. However there were first practical uses of overhead lines in the context of the telegrafie. 1882 were accomplished the first overhead line transmission with high voltage between Munich and bad brook, whereby direct current with a tension was used by 2kV. 1891 took place the building of the first three-phase alternating current overhead line on the occasion of the international electricity exhibition in Frankfurt/Main between Lauffen and Frankfurt/Main. 1912 went the first 110kV-overhead powerline and 1923 the first 220kV-overhead powerline into service. In the 20's the RWE AG built the first overhead line net for this voltage, which was appropriate for 380kV partially already (north south line) and to also the 1926 built Rhine crossing high with Voerde with two masts 138 meters belonged. 1957 went into Germany the first 380kV-overhead powerline into enterprise (between the transformer station high-hit a corner 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 as model for the carrying pylon and was up to the building of the Elbe crossing 2 in the second half of the 70's the highest overhead line pylons of the world. Starting from 1967 in Russia, the USA and Canada overhead lines for voltage of 765kV were built. 1982 were built in Russia between Elektrostaland the power station Ekibastusz a three-phase alternating current line with 1200kV. 1987 took place in China the building of the highest overhead line masts of the Perl river overhead line crossing.
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