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Which of the following factors should be considered in the design of transmission lines against lightning with ground wire?
Mechanical strength of ground wire
Clearance between the line conductor and ground wire
Clearance between line conductor and earth
All of above
All of above
The design of transmission lines against lightning strikes using ground (shield) wires requires a holistic approach to ensure the shielding effectiveness and insulation coordination. All the listed factors—mechanical integrity, clearance for insulation, and shielding geometry—are critical to preventing flashovers and structural failure.
The design of transmission lines against lightning strikes using ground (shield) wires requires a holistic approach to ensure the shielding effectiveness and insulation coordination. All the listed factors—mechanical integrity, clearance for insulation, and shielding geometry—are critical to preventing flashovers and structural failure.
S=kIp — simplified shielding angle vs stroke current relation
Vb=Ipeak×Rtower+Ltowerdtdi — back-flashover voltage across insulator
Ground wires act as a Faraday cage to intercept direct lightning strikes, preventing them from hitting phase conductors. The design must ensure: (1) Mechanical strength to withstand wind and ice loads, (2) Sufficient 'mid-span clearance' to prevent a lightning-induced arc from bridging the gap between the shield and phase conductor (back-flashover), and (3) Proper clearance from earth to ensure safety and insulation integrity.
Ground wires are placed at the top of the tower to provide a shielding angle typically between 20° and 30°.
Mechanical strength must account for tension, vibration, and sag to prevent contact with phases.
Mid-span clearance is vital because lightning surges increase the potential difference between the ground wire and the phase conductor.
The surge impedance of the tower and the footing resistance determine the peak voltage at the tower top.
Provides a preferred path for lightning current to earth.
Reduces the probability of direct strikes to phase conductors.
Increases the tower height and weight, leading to higher capital costs.
Potential for back-flashover if tower footing resistance is high.
High Voltage (HV) and Extra High Voltage (EHV) transmission lines.
Protection of substations from direct atmospheric discharge.
The 'shielding angle' is the angle between the vertical line through the ground wire and the line connecting the ground wire to the outermost phase conductor.
If the footing resistance is too high, the lightning current will cause the tower top potential to rise rapidly, causing a back-flashover to the line conductor despite the ground wire.
D is correct — All listed factors (mechanical strength, mid-span clearance, and ground-conductor clearance) are fundamental to the safety and reliability of a transmission line during a lightning event.
Always remember that high tower footing resistance is the primary cause of lightning-induced outages on lines equipped with shield wires.