The resistance offered to the lightning current when the tower is directly exposed to lightning is called tower footing resistance. It is the value of footing resistance measured at 50 Hz. The performance of the line purely depends upon the value of resistance during lightning. The resistance is the function of soil resistivity, critical breakdown gradient of soil, length of driven grounds, and surge currents.
The voltage and the current transmitted into the tower will depend upon the surge impedance of the tower and ground impedance (i.e., the tower-footing resistance) of the tower. Taking all the factors into consideration such as wave shape, the magnitude of lightning current striking the tower, surge impedance, potential wave at the top of the tower, footing impedance, etc.
The value of tower-footing resistance should be always low. If the tower-footing resistance is low, the chances of occurrence of flashover are eliminated and the potential at the top of the tower is reduced. Always tower-footing resistance is connected in parallel with ground rods or counterpoise wires.
If the tower-footing resistance is low, it helps in controlling the lightning overvoltages on the lines effectively with the help of ground wires. The tower-footing resistance lies between 10-20Ω. For medium-voltage transmission lines, it is less than 10Ω. Thus in order to keep the tower-footing resistance low some special arrangements are made.
Reduction of Tower Footing Resistance :
The tower resistance is reduced by two methods, either by driving rods near the tower and connecting them to the tower base or by burying counterpoise wires into the ground and connecting them to the tower base.
The driven rods are made up of galvanized iron or copper weld. This method is best suited in the locations where the soil is free from rocks i.e., it is best suited in sandy soil. Sometimes driven galvanized iron pipes are also used. Below shows the driven rod in the ground, connected to the base of the tower.
The counterpoise wires method of reducing tower-footing resistance is extensively used compared to driven rods due to its simple implementation. The wires are usually made up of copper, aluminum, galvanized iron, or stranded cable and are arranged either radially (continuous) or tower-to-tower (non-continuous) as shown below.
In a continuous counterpoise wire arrangement, the wire is buried underground along the line route and is connected to each tower base. This arrangement is more efficient than non-continuous counterpoise wires.
Current is drawn into the counterpoise from 1.5 to 2.5 km away from the point of the stroke and fed into the stroke by means of the counterpoise wire and the overhead ground wires.
The concentration of the current at any tower is thus reduced. The reduced current at any tower causes a reduced voltage drop at the tower base, and thus the tower potential is held to a value below the flashover voltage of the line insulators.
By the above methods, the lightning current is reduced, thereby reducing the voltage at the base of the tower. Therefore, the potential value at the base of the tower is lower than the flashover voltage and helps in neutralizing them to the ground. Hence, tower-footing resistance is recommended to be low always.