---
title: "Lightning protection schemes"
subject: "Switchgear and Protection"
chapter: 5
key: "semester_6/switchgear_and_protection:lightning-protection-schemes"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/lightning-protection-schemes
site: BCT Engineering
---

# Lightning protection schemes

Switchgear and Protection · Chapter 5: System Earthing

- Asked in: 2079 Chaitra, 5 marks
- Video: [Utility overvoltage protection with surge arresters | Eaton PSEC](https://www.youtube.com/watch?v=EBUb_Gphda8) — Eaton
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/lightning-protection-schemes
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-5

## Question

Explain in detail about the need and different types of lightning protection scheme.

## Short answer

Lightning protection is needed because strokes produce multi-megavolt surges exceeding insulation levels, causing flashovers, equipment damage, outages, and fires. Schemes include earthing screens and shield wires to intercept direct strokes, lightning masts for substations, metal-oxide arresters to divert travelling waves, and surge absorbers to reduce wave steepness at equipment terminals.

## Answer

**Lightning protection** is the set of devices that intercept a lightning stroke or divert the resulting surge safely to earth so that system insulation, equipment and people are not damaged.

#### Need for lightning protection
- A lightning stroke carries 10–200 kA with very steep front (1–10 μs), producing surges of several MV on lines, far above insulation levels (BIL).
- Without protection it causes **insulator flashover**, puncture of transformer and machine windings, damage to substation equipment, prolonged outages, fires and danger to life.
- Nepal's hilly terrain has high thunderstorm activity, so lines and substations are frequently hit.

#### Types of lightning protection schemes
1. **Earthing screen (for substations):** a network of earthed copper/steel wires stretched over the substation, connected to the earth mat. Direct strokes hit the screen and go to earth. It does not protect against surges coming in along the lines.
2. **Overhead ground (shield) wires (for lines):** earthed wires above phase conductors, bonded at each tower. They shield conductors against direct strokes within a 30°–45° protective angle; low tower-footing resistance (below about 10 Ω) avoids back flashover.
3. **Lightning conductors/masts:** vertical rods or masts with good earthing protect buildings and outdoor yards within a protective cone.
4. **Lightning arresters (surge diverters):** protect equipment from travelling waves that reach terminals. Types:
   - **Rod gap:** two rods set at a gap; cheap but no follow-current control.
   - **Horn gap:** arc rises on horn-shaped electrodes and extinguishes itself; with series resistor.
   - **Multi-gap:** series of small gaps with resistors.
   - **Expulsion type:** fibre tube; gases blow out the arc; used on distribution lines.
   - **Valve type (SiC):** series gaps with non-linear silicon carbide resistor blocks.
   - **Metal oxide (ZnO) arrester:** gapless, highly non-linear; now the standard at all voltages.
5. **Surge absorbers:** reduce steepness of the incoming wave before it reaches transformer windings.

```text
   Shield wire
  ===========  (earthed at tower)
  |  a  b  c |
  |  |  |  | |
  Tower    Line --+--> Substation
                  |
               [Arrester] --- earth mat
```
A complete scheme uses shield wires and earthing screens against direct strokes, and arresters at line entrances and at transformer terminals against travelling waves, all connected to a low-resistance earth.

## Check yourself

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

1. Why is lightning protection needed?
   A. Strokes give surges of several MV, far above insulation levels (BIL), causing flashover and puncture
   B. Strokes only cause small voltage dips
   C. Lightning lowers the frequency
   D. To raise the line impedance
   Answer: A. A stroke of 10-200 kA with a steep front can destroy insulation and equipment. (Key point: Need)

2. What is the limitation of an earthing screen?
   A. It cannot protect against direct strokes
   B. It does not protect against surges entering along the lines
   C. It needs no earth
   D. It only works on lines
   Answer: B. The screen takes direct strokes in the yard, so arresters are still needed for travelling waves. (Key point: Earthing screen)

3. What does a valve-type (SiC) arrester consist of?
   A. A fibre tube with gas expulsion
   B. Series gaps with non-linear silicon carbide resistor blocks
   C. A single rod gap
   D. A gapless ZnO column
   Answer: B. The gaps and non-linear blocks discharge the surge and limit follow current. (Key point: Valve type arrester)

4. Which arrester is now standard at all voltages?
   A. Rod gap
   B. Horn gap
   C. Metal oxide (ZnO) gapless arrester
   D. Expulsion type
   Answer: C. ZnO arresters are gapless and highly non-linear, giving the best protection. (Key point: Standard arrester)

## Related in Switchgear and Protection

- [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7
- [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3×
- [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1
- [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7
- [Need for protection and causes of faults](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aneed-protection-fault-causes) — Switchgear and Protection, chapter 1
- [Substation protection against direct strokes](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asubstation-direct-stroke-protection) — Switchgear and Protection, chapter 5

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