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Which of the following will occur if the value of Tower footing resistance (Rt) is too high?
Flash-Over
Back Flash-Over
Puncher
None of these
Back Flash-Over
If the tower footing resistance (RtтАЛ) is high, the potential at the top of the tower rises significantly during a lightning strike because the lightning current (I) must dissipate through this high resistance. This surge in potential difference between the tower top and the transmission line conductor exceeds the insulation strength, leading to a discharge known as a back flash-over.
If the tower footing resistance (RtтАЛ) is high, the potential at the top of the tower rises significantly during a lightning strike because the lightning current (I) must dissipate through this high resistance. This surge in potential difference between the tower top and the transmission line conductor exceeds the insulation strength, leading to a discharge known as a back flash-over.
VtowerтАЛ=I├ЧRtтАЛ тАФ Potential rise of the tower during a surge
VdiffтАЛ=VtowerтАЛтИТVconductorтАЛ тАФ Potential difference across insulator string
When lightning strikes a tower, the voltage at the tower top is given by VtowerтАЛ=I├ЧRtтАЛ. If RtтАЛ is high, VtowerтАЛ becomes much greater than the voltage of the line conductor. If the potential difference exceeds the critical breakdown voltage of the insulators, the discharge flows from the tower back to the conductor (the reverse of a normal flashover), hence the term 'Back Flash-over'.
Tower footing resistance is critical for lightning protection on transmission lines.
Lowering RtтАЛ is achieved using counterpoise wires or driven ground rods.
Back flash-over is a major cause of circuit breaker tripping during thunderstorms.
The required RtтАЛ depends on the tower height and the soil resistivity of the region.
Lowering RtтАЛ prevents equipment damage.
Improves system reliability during severe weather.
High RtтАЛ leads to frequent line outages.
Costly to install deep grounding systems in rocky terrain.
High Voltage (HV) and Extra High Voltage (EHV) transmission line design.
Substation earthing system design.
Standard design practice is to keep tower footing resistance below 10тБ╗┬▓тБ░ ╬йin many power utility specifications.
A 'Puncher' refers to electrical breakdown through the solid dielectric of an insulator, whereas flash-over occurs over the surface of the insulator.
B is correct тАФ High tower footing resistance results in a high potential rise at the tower top relative to the line conductor, causing a back flash-over.
Always remember that in transmission lines, 'Flash-over' usually refers to external discharge, while 'Puncher' refers to internal puncture of the insulator material.