---
title: "T/F: flashover voltage vs puncture voltage"
subject: "Transmission and Distribution System Design"
chapter: 3
key: "semester_8/transmission_and_distribution_system_design:tf-puncture-before-flashover"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-puncture-before-flashover
site: BCT Engineering
---

# T/F: flashover voltage vs puncture voltage

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

- Asked in: 2074 Magh, 1+3 marks; 2071 Bhadra, 1+3 marks; 2071 Magh, 1+3 marks (asked 3 times)
- 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-puncture-before-flashover
- 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. Justify your answer with a brief explanation: The insulators are designed so that it will punctured before it gets flashover.

## Short answer

FALSE. Insulators are designed to flash over before puncture because puncture causes permanent damage, whereas flashover is self-clearing. Puncture strength is kept well above flashover voltage, with a safety factor of about 1.3–1.5 for suspension discs.

## Answer

**FALSE.** Insulators are designed so that they **flash over before they puncture**.

- **Flashover:** an arc through the air over the insulator surface. The insulator is not damaged (perhaps slight glazing burn) and service resumes after the breaker recloses.
- **Puncture:** breakdown through the porcelain/glass body. The disc is permanently damaged and must be replaced.
- Therefore the puncture strength is kept well above the flashover voltage. The ratio
$$
\text{Safety factor} = \frac{\text{Puncture strength}}{\text{Flashover voltage}}
$$
  is about 10 for pin insulators and 1.3–1.5 or more for suspension discs (impulse).

## Check yourself

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

1. Are insulators designed to puncture before flashover?
   A. Yes
   B. No, to flash over before puncture
   C. Only pin insulators
   D. Only suspension discs
   Answer: B. Flashover is harmless to the insulator; puncture is permanent damage, so flashover strength is kept lower. (Key point: Verdict)

2. What is the consequence of puncture?
   A. Service resumes after reclosing
   B. Disc is permanently damaged and must be replaced
   C. Only slight glazing burn
   D. Nothing
   Answer: B. Puncture is breakdown through the body of the insulator, causing permanent damage. (Key point: Consequence)

3. What is the safety factor of an insulator?
   A. Flashover voltage / puncture strength
   B. Puncture strength / flashover voltage
   C. Rated voltage / BIL
   D. Impulse / dry voltage
   Answer: B. Safety factor = puncture strength / flashover voltage, about 10 for pin insulators. (Key point: Safety factor)

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