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T/F: two earth wires for double circuit

Short answer

FALSE. The number of earth wires is determined by the required shielding angle, not solely by double‑circuit configuration. A single earth wire often provides sufficient protection (shielding angle ≤30°) for double‑circuit lines up to 220 kV. Two wires are selected for wide EHV towers or high‑lightning regions.

For a given amount of power to be transmitted over a given distance, state whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: For a double circuit transmission line, it is always preferred to have two earth wires.

FALSE.

The number of earth wires is decided by the shielding angle needed to protect all phase conductors from direct lightning strokes, usually 20°–30°. It is not decided only by whether the line is single or double circuit.

  • A double-circuit line normally has a vertical configuration: three cross-arms on each side of a slender tower. The overall width is small, so one earth wire at the tower peak can often give a shielding angle of 30° or less to the outermost conductors. Many 66 kV, 132 kV and 220 kV double-circuit lines therefore use a single earth wire.
  • Two earth wires are used on a double-circuit line only when the cross-arms are wide (EHV, 400 kV and above). They are also used where lightning activity is high and a smaller shielding angle (or negative shielding) is wanted, or where footing resistance is poor.

So two earth wires are not always preferred. The choice depends on the geometry, the voltage and the lightning level. Two wires are used where they are needed.

✓ Answer checked by Claude Opus 5.5 (high effort)

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