EARTH WIRE

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Ever wondered why there an extra line on top of every transmission line we see.? Ever thought why we use any insulators for that line.?

One among the level of protection for the power system is this Earth / Shield wire. As for the power system(OH Line), biggest enemy is the lightning surge. Even though we install arrestors, it solely cannot complete the work. For increased rate of protection we use earth wire on top of the transmission line.
The overhead earth wire or ground wire is the form of lightning protection using a conductor or conductors. It is attached from support to support above the transmission line and well grounded at regular interval. The earth wire intercepts the direct lightning strikes, which would strike the phase conductors. The ground wire has no effect on switching surges.

When the lightning strikes an earth wire at mid-span, waves are produced which travel in opposite directions along the line. The waves reach the adjoining tower, which passes them to earth safely. The earth wire is effective only when the resistance between the tower foot and earth is sufficiently low.
 If the resistance between them is not low and the earth wire or tower will be struck by the lighting, then the lighting will be raised to the very high potential, which will cause a flash over from the tower to one or more phase conductors. Such a flashover is known as back flashover.
The back flash over only occurs when the product of the tower conductor and tower impedance exceeds the insulation levels of the line. It can be minimized by reducing tower footing resistance using driven rods and counterpoises where soil resistivity is high.

The counterpoise is the conductor buried in the ground. The wire is usually made up of galvanised steel. The counterpoise for an overhead terminal consists of a special ground terminal that reduces the surge impedance of the ground connection and increases the coupling between the ground wire and the conductor.

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