---
title: "T/F: two earth wires for double circuit"
subject: "Transmission and Distribution System Design"
chapter: 3
key: "semester_8/transmission_and_distribution_system_design:tf-double-circuit-two-earth-wires"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-double-circuit-two-earth-wires
site: BCT Engineering
---

# T/F: two earth wires for double circuit

Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design

- Asked in: 2069 Bhadra (old course), 4 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-double-circuit-two-earth-wires
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3

## Question

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.

## 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.

## Answer

**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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Is it always preferred to have two earth wires on a double circuit line?
   A. True: for every voltage
   B. True: always for D/C
   C. False: double circuits need none
   D. False: the number depends on shielding angle and geometry
   Answer: D. Many 66-220 kV D/C lines use one earth wire. (Key point: Verdict)

2. Why can one earth wire suffice on a double-circuit line?
   A. Vertical slender tower: one wire at the peak gives 30 degrees or less to the outer conductors
   B. Lightning never hits D/C
   C. Phases are horizontal
   D. The tower is wide
   Answer: A. The narrow width keeps shielding angle small. (Key point: Vertical configuration)

3. When are two wires used on a double-circuit line?
   A. Wide cross-arms (EHV), high lightning activity or poor footing resistance
   B. Always below 66 kV
   C. Never
   D. Only when cost is no concern
   Answer: A. They are used where needed. (Key point: When two wires)

## Related in Transmission and Distribution System Design

- [T/F: earth wire choice depends on circuit and voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-earth-wire-choice-circuit-voltage) — Transmission and Distribution System Design, chapter 3
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