--- title: "T/F: double earth wire for 400 kV single circuit" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:tf-400kv-single-circuit-double-earth" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-400kv-single-circuit-double-earth site: BCT Engineering --- # T/F: double earth wire for 400 kV single circuit Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2068 Bhadra (old course), 5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-400kv-single-circuit-double-earth - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3 ## Question 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. ## 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. ## Answer **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In general, is double earth wire selected for a 400 kV single circuit line? A. False B. True only for 66 kV C. True D. False because one is always enough Answer: C. The wide horizontal configuration needs two wires. (Key point: Verdict) 2. Why is one centre earth wire insufficient? A. Phase spacing is 11-12 m horizontal, so the wire would have to be very high for a 20-30 degree angle B. The line is short C. Phases are vertical D. Earth wire has no role Answer: A. This would be uneconomical. (Key point: Configuration) 3. Which is a benefit of two earth wires? A. Lower tower strength B. Lower surge impedance, better back-flashover performance, and mutual shielding of middle phase C. Higher outage rate D. No OPGW use Answer: B. They also reduce lightning outages worth the cost at EHV. 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