---
title: "T/F: V-type string for 400 kV lines"
subject: "Transmission and Distribution System Design"
chapter: 3
key: "semester_8/transmission_and_distribution_system_design:tf-v-string-400kv-mechanical"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-v-string-400kv-mechanical
site: BCT Engineering
---

# T/F: V-type string for 400 kV lines

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-v-string-400kv-mechanical
- 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: The major reason that V-type insulator string configurations are used for a transmission line of 400 kV and above is to improve the mechanical strength of the string.

## Short answer

FALSE. V‑strings are used at 400 kV and above primarily to eliminate insulator swing, which reduces required air clearance, tower window, and right‑of‑way. The improved mechanical strength is a secondary benefit.

## Answer

**FALSE.**

The main reason for using V-strings at 400 kV and above is **electrical clearance and economy, not mechanical strength**.

- An I-string at 400 kV is long, about 3.5–4 m. Under wind it can swing up to about 45°, and the swung conductor would come too close to the tower. A very long cross-arm and a large tower window would be needed.
- In a V-string the two legs hold the conductor fixed in the transverse direction, so there is practically no swing. The required clearance is therefore met with:
  - shorter cross-arms and a smaller tower window,
  - closer phase spacing,
  - a narrower right-of-way, and lighter and cheaper towers.

The two legs of a V-string do share the load, so its mechanical strength is higher. That is only a side benefit. When higher strength alone is needed, a double I-string is used instead.

## Check yourself

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

1. Is the major reason for V-strings at 400 kV the mechanical strength?
   A. True: they reduce corona
   B. False: they are used to add discs
   C. True: V-strings are stronger, which is the reason
   D. False: it is electrical clearance and economy
   Answer: D. Strength is only a side benefit. (Key point: Verdict)

2. Why is an I-string a problem at 400 kV?
   A. It is too short
   B. It cannot hold the conductor
   C. It lacks discs
   D. It is long (3.5-4 m) and can swing up to 45 degrees, bringing the conductor close to the tower
   Answer: D. Large swing demands a long cross-arm and big window. (Key point: Swing)

3. What does fixing the conductor with V-string legs allow?
   A. Higher swing
   B. More discs
   C. Longer cross-arms
   D. Shorter cross-arms, closer phase spacing, narrower right-of-way
   Answer: D. Practically no swing means smaller, cheaper towers. (Key point: Benefit)

## Related in Transmission and Distribution System Design

- [400 kV lines: configuration, V-string, earth wires](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ajustify-400kv-config-vstring-earthwire) — Transmission and Distribution System Design, chapter 4
- [Insulator discs per string, 180 MW over 200 km](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-discs-180mw-200km) — Transmission and Distribution System Design, chapter 3
- [Classify insulators; string insulator configurations](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aclassify-insulators-string-configuration) — Transmission and Distribution System Design, chapter 3
- [T/F: conductor configuration for 400 kV lines](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-vertical-configuration-400kv) — Transmission and Distribution System Design, chapter 4, asked 2×
- [Basic configurations of insulator string](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-string-configurations) — Transmission and Distribution System Design, chapter 3
- [T/F: % power loss equals % energy loss](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-power-loss-energy-loss-same) — Transmission and Distribution System Design, chapter 6

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- [Significance of transmission voltage level, 132 vs 400 kV](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Avoltage-level-132kv-vs-400kv) — Power System Analysis I, chapter 1
- [True/false: cross-arm length and string efficiency](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atf-cross-arm-string-efficiency) — High Voltage Engineering, chapter 5
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