CIRCUIT BREAKERS – GATE LEVEL NOTES (POWER SYSTEM PROTECTION)
1. DEFINITION
A Circuit Breaker (CB) is a mechanical switching device used to:
- Make current (close circuit)
- Carry current (normal operation)
- Break current (during fault)
under normal and abnormal conditions.
It protects power system by isolating faulty section.
2. BASIC WORKING PRINCIPLE
When fault occurs:
Relay detects fault → sends signal → Circuit breaker opens → Arc forms → Arc extinguished → Fault isolated
3. ARC FORMATION IN CIRCUIT BREAKER
When contacts separate, current continues due to ionized air → arc forms.
Arc must be extinguished quickly.
ARC INTERRUPTION PRINCIPLE
Two methods:
1. High Resistance Method
Increase arc resistance by:
- Lengthening arc
- Cooling arc
- Reducing ionization
Used in DC circuit breakers.
2. Current Zero Method (MOST IMPORTANT FOR AC)
In AC, current naturally becomes zero every half cycle.
Arc extinguished at current zero.
Used in AC circuit breakers.
4. CURRENT ZERO CONCEPT (VERY IMPORTANT FOR GATE)
AC current:
Current becomes zero every:
For 50 Hz:
Arc extinguishes at current zero.
5. RESTRIKING VOLTAGE
Voltage appearing across breaker contacts immediately after arc extinction.
Two types:
1. Transient Recovery Voltage (TRV) (MOST IMPORTANT)
High frequency transient voltage.
2. Power Frequency Recovery Voltage
Normal voltage after transient dies out.
Condition for successful interruption:
Dielectric strength > restriking voltage
6. TYPES OF CIRCUIT BREAKERS (VERY IMPORTANT FOR GATE)
Based on arc extinguishing medium:
1. Oil Circuit Breaker
Uses oil as arc quenching medium.
Types:
- Bulk oil CB
- Minimum oil CB
Advantages:
Good insulation
Disadvantages:
Fire hazard
Slow operation
2. Air Circuit Breaker
Uses air for arc extinction.
Types:
- Air break CB
- Air blast CB
Air blast CB uses compressed air.
Advantages:
Fast operation
Disadvantages:
High maintenance
3. SF₆ Circuit Breaker (MOST IMPORTANT)
Uses SF₆ gas.
SF₆ = Sulphur Hexafluoride
Best circuit breaker.
Properties of SF₆:
- High dielectric strength
- Excellent arc quenching
- Electronegative gas
Advantages:
Very fast
Highly reliable
Used in high voltage systems.
4. Vacuum Circuit Breaker (VERY IMPORTANT)
Uses vacuum.
Arc extinguishes quickly because vacuum has no ionization.
Advantages:
Longest life
Low maintenance
Fast operation
Used up to 66 kV.
7. COMPARISON TABLE (VERY IMPORTANT FOR GATE)
| CB Type | Arc Medium | Voltage Level |
|---|---|---|
| Oil CB | Oil | Medium |
| Air CB | Air | Medium |
| SF₆ CB | SF₆ gas | High |
| Vacuum CB | Vacuum | Medium |
Best CB → SF₆ CB
8. IMPORTANT TERMS
Breaking Capacity (MOST IMPORTANT)
Maximum fault current breaker can safely interrupt.
Unit:
MVA
Formula:
Making Capacity
Maximum current breaker can close safely.
(VERY IMPORTANT FOR GATE)
Short Circuit Current
Fault current flowing during fault.
Rated Voltage
Maximum voltage breaker designed for.
Rated Current
Maximum current breaker can carry safely.
9. OPERATING TIME OF CIRCUIT BREAKER
Total time:
Typical CB time:
0.05 to 0.1 sec
10. ARC QUENCHING METHODS SUMMARY
| Method | Used in |
|---|---|
| Oil | Oil CB |
| Air | Air CB |
| SF₆ gas | SF₆ CB |
| Vacuum | Vacuum CB |
11. SF₆ CIRCUIT BREAKER – WHY BEST (VERY IMPORTANT THEORY)
SF₆ has:
High dielectric strength
Fast arc quenching
Electronegative property → absorbs electrons
No fire hazard
12. VACUUM CIRCUIT BREAKER – KEY POINT
Vacuum has highest dielectric strength.
Arc extinguishes very fast.
13. GATE IMPORTANT FORMULAS SUMMARY
Breaking capacity:
Making current:
Current zero time (50 Hz):
14. GATE IMPORTANT THEORY QUESTIONS
Best circuit breaker → SF₆
Fastest breaker → Vacuum CB
Most used in EHV → SF₆ CB
Arc extinction in AC → current zero method
15. ONE-PAGE REVISION SHEET
Circuit breaker → interrupts fault current
Arc forms during contact separation
Arc extinguished at current zero
Best CB → SF₆ CB
Making current:
2.55 × breaking current
Breaking capacity:
√3 VI
