---
title: "Causes and protection against overvoltage"
subject: "Switchgear and Protection"
chapter: 5
key: "semester_6/switchgear_and_protection:overvoltage-protection-methods"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/overvoltage-protection-methods
site: BCT Engineering
---

# Causes and protection against overvoltage

Switchgear and Protection · Chapter 5: System Earthing

- Asked in: 2071 Bhadra, 8 marks; 2077 Chaitra, 5 marks (asked 2 times)
- Video: [Causes of OverVoltages | Unit 4 PS II | Powersytems2 JNTU Btech EEE](https://www.youtube.com/watch?v=TvPySFGVxDs) — Instant Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/overvoltage-protection-methods
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-5

## Question

Define causes of overvoltage and explain methods of overvoltage protection of electrical equipments.

## Short answer

Overvoltage causes include external lightning strokes (direct, induced, electrostatic) and internal switching surges, arcing grounds, insulation failure, resonance, and Ferranti effect. Protection methods: shield wires and masts intercept direct strokes; metal-oxide arresters divert surges; surge absorbers reduce wave steepness; neutral grounding eliminates arcing grounds; switching control and insulation coordination limit overvoltages; low earth resistance ensures safe discharge.

## Answer

An **overvoltage** is a voltage, normally a short-duration surge, above the highest normal operating voltage of the system. Insulation must be protected from it by shielding, by diverting the surge to earth and by controlling switching.

#### Causes of overvoltage
**External:**
- Direct lightning stroke on conductors, towers or substation.
- Induced lightning (bound charge released when a nearby cloud discharges).
- Electrostatic build-up on lines due to wind, dust and atmospheric changes.

**Internal:**
- Switching surges: closing/opening long lines, switching capacitor banks (restrike), current chopping of transformer magnetising current.
- Arcing grounds in isolated-neutral systems.
- Insulation failure (sudden earth fault).
- Resonance and ferro-resonance.
- Load rejection and Ferranti effect.

#### Methods of overvoltage protection
1. **Ground (earth/shield) wires:** bare conductors run above the phase conductors and earthed at every tower. They intercept direct strokes and shield the line within a protective angle (about 30°). Low tower-footing resistance is needed to avoid back flashover.
2. **Lightning conductors / masts / shield wires over substations:** tall rods or overhead wires earthed through a good earth mat protect outdoor equipment from direct strokes.
3. **Lightning (surge) arresters:** connected between line and earth near equipment. They act as an open circuit at normal voltage, conduct during a surge to divert it to earth, then reseal. Types: rod gap, horn gap, expulsion type, valve type (SiC) and **metal-oxide (ZnO) gapless arresters**, now standard.
4. **Rod gaps and horn gaps:** simple spark gaps across insulators; cheap back-up, but they cause a short circuit until the CB trips.
5. **Surge absorbers:** devices that flatten the steep front of travelling waves before they reach transformer or machine windings, e.g. a capacitor between line and earth, an inductor with a parallel resistor in series with the line, or the **Ferranti surge absorber** (an air-cored inductor inside an earthed metal sheath, whose eddy-current losses dissipate the surge energy).
6. **Neutral grounding:** solid, resistance or Peterson coil grounding removes arcing ground overvoltages.
7. **Control of switching surges:** CBs with pre-insertion/opening resistors, synchronised (point-on-wave) switching, restrike-free breakers (vacuum, SF6), shunt reactors to limit Ferranti rise.
8. **Insulation coordination:** insulation levels (BIL) of equipment are chosen higher than the protective level of arresters by a safety margin (about 20–25%).
9. **Good earthing:** low earth-mat and tower-footing resistance so that surge current is discharged safely.

```text
 Line ---+-------------+---- Transformer
         |             |
      [Arrester]   [Surge absorber]
         |             |
        ===           ===
 Shield wire above line, earthed at each tower
```
A combination of shield wires against direct strokes and metal-oxide arresters near every important item of equipment gives the most reliable protection.

## Check yourself

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

1. What is the role of an overhead ground (shield) wire?
   A. It limits switching surges
   B. It intercepts direct strokes and shields conductors within a protective angle of about 30 degrees
   C. It absorbs the steepness of travelling waves
   D. It grounds the neutral
   Answer: B. Earthed at every tower, it takes the stroke instead of the phase conductors; low footing resistance avoids back flashover. (Key point: Ground wire)

2. How does a lightning arrester behave?
   A. Conducts always to earth
   B. Open circuit at normal voltage, conducts to divert the surge to earth, then reseals
   C. Short circuits the line until the CB trips
   D. Only blocks switching surges
   Answer: B. It diverts the surge only while the voltage is high and then restores insulation. (Key point: Arrester action)

3. What does insulation coordination require?
   A. BIL of equipment lower than the arrester protective level
   B. BIL of equipment above the arrester protective level by a margin of about 20-25%
   C. No relation between BIL and arrester
   D. Arresters rated at twice the BIL
   Answer: B. A safety margin ensures the arrester operates before the equipment insulation is stressed. (Key point: Insulation coordination)

4. Which measure controls switching surges?
   A. Earthing screen
   B. Pre-insertion or opening resistors, synchronised switching and restrike-free breakers
   C. Larger tower footing
   D. Horn gaps on transformers
   Answer: B. These reduce restrikes and the voltage transients when closing or opening lines and capacitor banks. (Key point: Switching surge control)

## Related in Switchgear and Protection

- [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
- [Overvoltage causes; solid grounding merits](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aovervoltage-causes-solid-grounding) — Switchgear and Protection, chapter 5, asked 2×
- [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
- [Static overvoltage and undervoltage relays](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-over-under-voltage-relay) — Switchgear and Protection, chapter 8
- [Earth fault protection: 20 MVA alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-20mva-5-ohm) — Switchgear and Protection, chapter 7
- [Causes of PT ratio and phase errors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apt-ratio-phase-angle-error) — Switchgear and Protection, chapter 2

## Related in other subjects

- [Protection against ESD](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aesd-protect) — Instrumentation II, chapter 6, asked 4×
- [Causes of switching overvoltage; inductive current interruption](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aswitching-overvoltage-inductive-current) — High Voltage Engineering, chapter 3
- [Overvoltage from interrupting low inductive current](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Alow-inductive-current-interruption) — High Voltage Engineering, chapter 3
- [Causes, disadvantages and improvement of low pf](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Alow-power-factor-causes-methods) — Utilization of Electrical Energy, chapter 6, asked 2×