Overhead power line#Ground wires
The ground wire is a grounded, electrically conductive cable which is above high-voltage overhead lines stretched as catching device for protection against direct lightning strikes. Colloquially also equipotential bonding conductors for grounding conductive moving parts (eg cabinet doors to the cabinet ) are referred to as earth wire.
According to international statistics result about 65 % of all line outages from lightning strikes. The use of guard wires in addition to the grounding of the mast systems an economically sensible measure to limit the shutdowns to an acceptable level. As a rule, high-voltage lines not with operating voltages below 50 kV, those are almost always equipped with operating voltages above 50 kV with a ground wire. The lightning protection effect of the ground wire is, like with most lightning protection lobbies of buildings primarily to the fact that form at atmospheric discharges by the high edge field strength immediately above the earth wire corona discharges. These discharges preferentially lead to points and edges to partial ionization of the surrounding air through peak discharge, which strikes one as a result the onset of lightning is more likely in the earth wire in the conductors of the power line. Ground wires are, therefore, designed to increase the presently desired edge field strength not as a bundle conductors, but always as a single rope and attached to the top mast. If greater demands are placed on the flash protection are on the masts two ground wires on the outer sides of an additional beam, the Erdseiltraverse, or to two Erdseilspitzen in V-shape, laid the Erdseilhörnern.
The earth wires also act by direct impact in the mast as additional Ableitanlage, since part of the current is dissipated through it and to the adjacent poles and thence to soil. The potential rise through the grounding resistance of the mast and the ground thus will be less, and there is a lower probability of formation of dangerous step voltages around the mast foot and rear -overs from the mast to the conductors.
Also in the determination of the K- factor, the ground wires are to be included with. The zero-sequence impedance of a transmission line is significantly reduced by the use of guard wires.
In a fiber optic cable shield wires messaging is often integrated. In some lines that were laid up in 1985 by the then energy supply Swabia (today EnBW), such a telephone cable is scalloped laid on the ground wire. For outdoor fiber optic cables however, can lead to thermal damage to the fiber optic direct lightning strikes.
Some HVDC overhead lines, the ground wire also serves as an electrical connection of the converter to the ground electrode, which is possible because of the very low-impedance ground over this. In this case, the ground wire must be isolated attached to the mast and may be connected only by the surge arrester with him to avoid electrochemical corrosion of the mast due to lasting current flow.
Insulated ground wires are also found in high voltage lines in the former Soviet Union, because they are used for the transmission of carrier frequency signals.
However, even with normal overhead lines, the ground wire to the tension towers is often attached to an insulator on the pole, which is but bridged normally. This form of attachment, it is possible to control the earth contact resistance.
In railways, a ground wire along the track on the outside of the overhead line masts is attached. In this case, it is not intended primarily as a lightning protection device, but is an additional safety precaution in the event that (eg during construction or maintenance work ) the grounding of a mast is damaged. About the OPGW each catenary mast is connected to its neighbors. In addition, the ground wire improves the return flow of current to the feeding sub-station, which is usually by rail.