--- title: "Single vs double earth wire selection" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:single-double-earth-wire-criteria" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/single-double-earth-wire-criteria site: BCT Engineering --- # Single vs double earth wire selection Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2077 Chaitra, 6 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/single-double-earth-wire-criteria - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3 ## Question Discuss the criterion for single and double earth wire selection for a high voltage overhead transmission line. ## Short answer Single earth wire suffices when shielding angle ≤30° covers all phases, as in vertical/triangular configurations. Double earth wires are required for horizontal formation, EHV lines, high lightning areas, or wide towers; earth-wire spacing should not exceed 3.5–4 times height above the middle conductor. ## Answer 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. ```text 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) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the shielding angle criterion for earth wires? A. It is irrelevant; only cost matters B. About 30 degrees (20-30 at EHV); if one wire cannot achieve it for all phases, use two C. About 60 degrees for every line; always use one wire D. Exactly 90 degrees to the phase conductor Answer: B. Earth wires must shield every phase within the angle; otherwise a second wire is added. (Key point: Shielding angle) 2. Which configuration normally needs two earth wires? A. Triangular formation at 66 kV B. Vertical D/C narrow tower at 66-132 kV C. Any line below 33 kV D. Horizontal formation with wide spacing (220/400 kV single circuit) Answer: D. Wide horizontal layouts cannot be shielded by one wire at the peak. (Key point: Configuration) 3. How do two earth wires help with tower footing resistance problems? A. They only reduce cost B. They remove the need for earthing C. They share stroke current and lower effective surge impedance, reducing back-flashover D. They increase footing resistance Answer: C. Parallel wires reduce the surge impedance of the earth wire system and the tower top voltage. (Key point: Footing resistance) 4. Why is a second earth wire not used on every line? A. It makes shielding worse B. It increases the lightning outage rate C. It adds conductor, peak steel and tower loading cost, so it is used only when needed D. It is prohibited by standards below 400 kV Answer: C. The extra cost is justified only when the shielding angle or lightning performance requires it. 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