---
title: "Precautions for VCB as generator breaker"
subject: "Power Plant Design"
chapter: 4
key: "semester_8/power_plant_design:vcb-as-gcb-precaution"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/vcb-as-gcb-precaution
site: BCT Engineering
---

# Precautions for VCB as generator breaker

Power Plant Design · Chapter 4: Electric System Design of a Power Plant

- Asked in: 2074 Magh, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/vcb-as-gcb-precaution
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4

## Question

What particular precaution should be taken while using vacuum circuit breaker (VCB) as a generator circuit breaker (CB)?

## Short answer

Install surge arresters and RC surge suppressors at generator terminals to limit overvoltages from current chopping. Ensure the breaker is type-tested for generator duty per IEEE C37.013 / IEC 62271-37-013 to handle high DC offset and delayed current zeros. Use low-chopping contact material and add TRV capacitors if needed.

## Answer

A **generator circuit breaker (GCB)** must interrupt very high currents with a large DC component and a fast-rising TRV. A **vacuum circuit breaker (VCB)** chops current sharply, so special precautions are needed.

#### Precautions

- **Surge protection against current chopping:** install **surge arresters** (metal oxide) and **RC surge suppressors / surge capacitors** at the generator and transformer terminals, because chopping and re-ignition create high $L\,di/dt$ overvoltages on the inductive winding.
- **Check DC component and delayed current zeros:** the breaker must be type-tested to IEEE C37.013 / IEC 62271-37-013 for generator duty, since the DC offset can delay current zeros.
- **TRV control:** add capacitors to reduce the rate of rise of recovery voltage.
- Rate it for continuous current with proper cooling, and check vacuum integrity periodically.

## Check yourself

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

1. What does a VCB do that creates a hazard on generator duty?
   A. Fails to interrupt
   B. Chops current before natural zero causing L di/dt overvoltage
   C. Overheats on no-load
   D. Leaks gas
   Answer: B. Chopping into inductive windings produces overvoltages and re-ignitions. (Key point: Problem)

2. What should be installed at generator terminals with a VCB?
   A. Fuses
   B. Series resistor
   C. Neutral reactor
   D. Surge arresters and RC surge suppressors
   Answer: D. They limit chopping and re-ignition overvoltages. (Key point: Protection)

3. Which duty must the breaker be type-tested for?
   A. Isolator duty
   B. Motor starting only
   C. Generator duty (IEEE C37.013 / IEC 62271-37-013) with DC offset
   D. Capacitor switching only
   Answer: C. DC component can delay current zeros, so generator-duty testing is required. (Key point: Type test)

## Related in Power Plant Design

- [Vacuum versus SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Avcb-vs-sf6) — Power Plant Design, chapter 4
- [VCB as GCB and limits versus SF6](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Avcb-gcb-limitations-sf6) — Power Plant Design, chapter 4
- [Choice of generator circuit breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Agenerator-circuit-breaker-choice) — Power Plant Design, chapter 4, asked 2×
- [Design: 310 m head plant, HV breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2080-310m-hvcb) — Power Plant Design, chapter 4, asked 3×
- [Quotation for HV breaker of a 48 MW plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahv-breaker-quotation-48mw) — Power Plant Design, chapter 4

## Related in other subjects

- [Vacuum circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-construction) — Switchgear and Protection, chapter 6, asked 2×
- [Precautions while applying cement mortar](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aprecautions-applying-cement-mortar) — Civil Engineering Materials, chapter 6, asked 3×
- [Short circuit MVA of feeder circuit breaker](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afeeder-cb-short-circuit-mva) — Power System Analysis II, chapter 3, asked 2×
- [Reactor to protect circuit breaker of small station](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactor-protect-circuit-breaker) — Power Plant Equipment, chapter 4