We should probably earth each tower of an electrical transmission line. We must measure footing resistance of each tower. We should take the footing resistance of tower in dry season before stringing the earth wire and/or OPGW (where OPGW is applicable). In any circumstances footing resistance of the tower will not be more than 10 ohms.
We should use either pipe earthing or counterpoise for earthing of the electrical transmission line tower. The tower earthing lug should extend beyond the concrete base of the tower leg. We also use lug connector in case of counterpoise earthing. Logically we should pipe earthing to any of the four legs of a tower but practically we should provide earthing to that leg which is specifically marked for providing earthing. Generally the leg members of that leg is marked with capital letter A. It’s a normal practice to avoid mistakes by tower erection gang. In case of river crossing and railway crossing towers we provide earthing at diagonally opposite two legs of a tower.
Let us now discuss these two types of earthing one by one.
Pipe Earthing of Electrical Transmission Tower
In case of pipe earthing system we use galvanised steel pipe of diameter 25 mm and of length 3 meters. We bury the pipe vertically in the soil in such a manner that the top of the pipe shall be 1 meter below the ground level. Where the tower stands on rock we have to bury the earthing pipe in damp soil available nearer to the tower.
Then we connect the tower leg with the pipe with the help of galvanized steel tape of suitable cross section. In that case we have to bury the steel tape in a groove cut on the rock and adequately protect the steel tape from damage.
In case of pipe earthing system we fill surroundings of the pipe with alternating layers of charcoal and salt, which maintain the surrounding soil of the pipe moist. A details pictorial representation of a pipe earthing is down here below.
We use 10.97 mm dia galvanized wire for the purpose of counterpoise earthing of electrical transmission tower. Here we connect the galvanized wire with the leg of the tower with the help of galvanized lug and the galvanized lug is fitted with a tower leg with the help of 16 mm dia nut and bolts. The steel wire used for the purpose must be of minimum 25 meters in length. The wire is buried tangentially under the ground of minimum 1 meter depth from the ground level. Here four legs of a tower are connected together with counterpoise earth wire burried below the ground level of 1 meter depth as already been told.