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T/F: double earth wire for 400 kV single circuit

Short answer

TRUE. A 400 kV single-circuit line uses a wide horizontal phase arrangement, so one earth wire cannot provide the required shielding angle economically. Two earth wires protect all phases, reduce lightning outage rate, and improve back-flashover performance.

State whether the following statement is TRUE or FALSE and give reasons briefly: In general, for a single circuit transmission line of 400 kV, double earth wire is selected.

TRUE.

A 400 kV single-circuit line normally uses a horizontal conductor configuration. The three phases are on one level, with a large spacing of about 11–12 m between adjacent phases.

  • The outer phases are far from the tower centre. One earth wire at the centre would have to be placed very high to give the needed shielding angle of about 20°–30°, which is uneconomical.
  • Two earth wires placed above or slightly outside the outer phases give good shielding to all three conductors with low peaks. The middle phase is shielded from both sides (mutual shielding).
  • At EHV a lightning tripout causes a heavy loss of transmitted power. The lower lightning outage rate from two wires is worth their cost.
  • Two earth wires also lower the surge impedance of the earth-wire system. This improves back-flashover performance, and the wires can carry OPGW communication.

So, in general, two earth wires are selected for a 400 kV single-circuit line, as in standard IS practice.

✓ Answer checked by Claude Opus 5.5 (high effort)

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