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__________ is the resistance offered by the metal parts of the tower + the ground resistance.
Tower Resistance
Tower Earthing Resistance
Tower footing resistance
Transmission Line Resistance
Tower footing resistance
Tower footing resistance is defined as the combined effective resistance offered by the metallic structural members of the transmission tower and the ground/earth resistance at the tower base. It plays a critical role in determining the surge impedance and the lightning impulse withstand capability of transmission towers.
IEEEStd 80 / IS 3043:1987
Tower footing resistance is defined as the combined effective resistance offered by the metallic structural members of the transmission tower and the ground/earth resistance at the tower base. It plays a critical role in determining the surge impedance and the lightning impulse withstand capability of transmission towers.
RtfтАЛ=RmetalтАЛ+RgroundтАЛ тАФ Tower Footing Resistance formula
VtowerтАЛ=IlightningтАЛ├ЧRtfтАЛ тАФ Voltage rise at top of tower during lightning stroke
When a lightning stroke hits a transmission tower or an overhead ground wire, a massive surge current flows down through the tower structure to the earth. If the tower footing resistance is high, the voltage drop (V=I├ЧRtfтАЛ) across the tower structure becomes extremely high, causing a potential difference across the insulator strings that can exceed their flashover voltage, leading to a back flashover from the tower body to the phase conductors.
To prevent back flashover, tower footing resistance should ideally be kept below 10┬а╬й.
In high-resistivity soils (rocky areas), counterpoise wires (buried earth conductors) are connected to the tower legs to reduce footing resistance.
Low tower footing resistance significantly reduces lightning-induced outage rates.
Protects insulator strings from severe voltage stress during back flashovers.
Achieving low footing resistance in rocky or dry soil can be economically challenging and labor-intensive.
High-voltage (HV) and Extra-High-Voltage (EHV) overhead power transmission lines.
Substation lightning protection structures and grounding mats.
Target resistance value: Below 10┬а╬й for standard transmission towers (100тАУ220┬аkV systems).
Option A (Tower Resistance) refers loosely to the metallic resistance of the frame structure alone.
Option B (Tower Earthing Resistance) refers only to the earth electrode contact resistance, omitting structural components.
Option D (Transmission Line Resistance) refers to the conductor resistance per unit length of the line itself.
C is correct тАФ Tower footing resistance is the combined total resistance offered by the metallic structural members of the tower together with the ground resistance at the tower base.
In competitive exams like GATE and ESE, remember that counterpoise earthing is specifically used to lower 'Tower Footing Resistance' in rocky terrains to prevent back flashover, not for normal load balancing.