Single vs double earth wire selection

The earth (shield/ground) wire is strung above the phase conductors to intercept lightning strokes and drain them to earth through the towers. The choice between one and two earth wires depends on how well the phase conductors are shielded.

Criteria

1. Shielding angle: the angle between the vertical through the earth wire and the line joining the earth wire to the outermost phase conductor. It should be about 30° (20–30° at EHV and in high-lightning areas). If one earth wire on the tower peak cannot give this angle to all phases, two are used.
2. Conductor configuration:
   • Vertical or triangular formation (narrow tower; 66–132 kV S/C and D/C) → a single earth wire at the peak is enough.
   • Horizontal formation (220 kV and 400 kV S/C, wide spacing) → two earth wires, one over each outer phase. The middle phase, lying between the two wires, is shielded from both sides, so a larger shielding angle of up to about 60° is acceptable for it (roughly, the spacing between the earth wires should not exceed about 3.5–4 times their height above the middle conductor).
3. System voltage and importance: EHV lines (≥ 220 kV) and important evacuation or interconnection lines use two earth wires for better shielding and a lower outage rate.
4. Lightning intensity (isokeraunic level): in areas with many thunderstorm days (for example the hilly regions of Nepal), double earth wires reduce shielding failures.
5. Tower height and width: taller and wider towers attract more strokes and need better coverage.
6. Tower footing resistance: two earth wires share the stroke current and lower the effective surge impedance, which reduces back-flashover.
7. Cost: a second earth wire adds conductor, peak steel and tower loading, so it is used only when needed.

  Single earth wire       Two earth wires
        o                  o           o
       /|\                 |\         /|
   30°/ | \              30| \       / |
     o  |  o               o    o    o
     o  |  o            (horizontal phases)
     o  |  o
  (vertical D/C)