Air cable
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An air cable is an insulated cable usually containing all conductors required for an electrical transmission system or a telecommunication line, which is spun between poles or pylones. Further air cables of fiberoptic basic are also available. As air cables are completely insulated there is no danger of electrical shocks when touching them and there is no requirement for mounting them with insulators on pylons and poles. A further advantage of air cables is, that they require a less right of way than overhead lines for same purpose and that there is less danger of interference as they can be designed as shielded cables ( for telecommunication purposes) or that in case of pylon failure or hit by a fallen tree the line is still functional, if insulation is not destroyed by this incident. Also reparation may be possible in many cases without interruption.
As air cables are installed on pylons or poles, they may be cheaper to install than underground cables as no work for digging is required, which can be very expensive at rocky areas.
[edit] Use
Air cables are mostly used for telecommunication systems or for power transmissions with voltages below 1000 volts, although air cable systems for voltages around 10000 volts were also realized, especially for the supply of farms, waterworks, transmitters and other facilities outside urban areas. A further common use is the replacement of overhead telecommunication lines for example along railway lines by air cables as they can be installed on existing poles and make the facility more reliable. Telecommunication systems running along power lines or aerial tramways are often built as air cables as they can be easily installed on the pylons respectively aerial tramway support towers. However these cables must be designed for higher forces as span widths are longer. At powerlines the air cable can serve also as ground conductor on the top of the pylon. It can be also installed in form of a separate rope on the conductor. A special method was used at former EVS ( now EnBW)in Germany until Mid of 1980ies, where the air cable was installed like a garland on the ground conductor or an auxiliary rope.
For reasons of electromagnetic interference air cables running along powerlines are most often of fibreoptic types. As these are dielectric, it is even possible to install them directly in the conductors, which is also sometimes realized. Air cables are also used sometimes for power transmission from the transmitter building to the antenna at radio stations.
