CIRCUIT BREAKERS || ELECTRICAL NOTES

 

CIRCUIT BREAKERS – GATE LEVEL NOTES (POWER SYSTEM PROTECTION)

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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.

Circuit Breakers in Electrical Engineering

A circuit breaker is an automatic electrical switch designed to protect electrical circuits from damage caused by overcurrent, overload, or short circuit. It interrupts the current flow when a fault is detected.

Unlike fuses, circuit breakers can be reset and reused.

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2. BASIC WORKING PRINCIPLE

When fault occurs:

Relay detects fault → sends signal → Circuit breaker opens → Arc forms → Arc extinguished → Fault isolated

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3. ARC FORMATION IN CIRCUIT BREAKER

When contacts separate, current continues due to ionized air → arc forms.

Arc must be extinguished quickly.

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ARC INTERRUPTION PRINCIPLE

Two methods:

1. High Resistance Method

Increase arc resistance by:

• Lengthening arc
• Cooling arc
• Reducing ionization

Used in DC circuit breakers.

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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.

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4. CURRENT ZERO CONCEPT (VERY IMPORTANT FOR GATE)

AC current:

$$i = I_{max} \sin(\omega t)$$

Current becomes zero every:

$$\frac{1}{2f}$$

For 50 Hz:

$$= \frac{1}{100} = 0.01 \text{ sec}$$

Arc extinguishes at current zero.

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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.

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Condition for successful interruption:

Dielectric strength > restriking voltage

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6. TYPES OF CIRCUIT BREAKERS (VERY IMPORTANT FOR GATE)

Based on arc extinguishing medium:

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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

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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

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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.

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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.

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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

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8. IMPORTANT TERMS

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Breaking Capacity (MOST IMPORTANT)

Maximum fault current breaker can safely interrupt.

Unit:

MVA

Formula:

$$Breaking\ Capacity = \sqrt{3} V I_{breaking}$$

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Making Capacity

Maximum current breaker can close safely.

$$Making\ current = 2.55 \times I_{breaking}$$

(VERY IMPORTANT FOR GATE)

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Short Circuit Current

Fault current flowing during fault.

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Rated Voltage

Maximum voltage breaker designed for.

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Rated Current

Maximum current breaker can carry safely.

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9. OPERATING TIME OF CIRCUIT BREAKER

Total time:

$$Total = Relay\ time + Breaker\ time$$

Typical CB time:

0.05 to 0.1 sec

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10. ARC QUENCHING METHODS SUMMARY

Method	Used in
Oil	Oil CB
Air	Air CB
SF₆ gas	SF₆ CB
Vacuum	Vacuum CB

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11. SF₆ CIRCUIT BREAKER – WHY BEST (VERY IMPORTANT THEORY)

SF₆ has:

High dielectric strength

Fast arc quenching

Electronegative property → absorbs electrons

No fire hazard

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12. VACUUM CIRCUIT BREAKER – KEY POINT

Vacuum has highest dielectric strength.

Arc extinguishes very fast.

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13. GATE IMPORTANT FORMULAS SUMMARY

Breaking capacity:

$$\sqrt{3}$$

Making current:

$$2.55 \times I_{breaking}$$

Current zero time (50 Hz):

$$0.01 \text{ sec}$$

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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

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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

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🔹 Why Circuit Breakers Are Needed

They protect against:

• Overload – When current exceeds the rated capacity for a long time

• Short Circuit – Direct connection between phase and neutral/earth

• Earth Fault – Leakage current to ground

• Arc Fault – Dangerous sparking in wiring

Without protection, these faults can cause:

• Fire hazards

• Equipment damage

• Electric shock

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🔹 Main Parts of a Circuit Breaker

1. Contacts – Carry current (open/close the circuit)

2. Arc extinguisher – Extinguishes arc formed during opening

3. Operating mechanism – Manual or automatic switching

4. Trip unit – Detects fault and trips breaker

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🔹 Types of Circuit Breakers

1️⃣ Low Voltage Circuit Breakers (Up to 1 kV)

• MCB (Miniature Circuit Breaker)
  Used in homes and small offices.

• MCCB (Molded Case Circuit Breaker)
  Used in industries, higher current rating.

• RCCB (Residual Current Circuit Breaker)
  Protects against earth leakage.

• ELCB (Earth Leakage Circuit Breaker)
  Older version of RCCB.

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2️⃣ High Voltage Circuit Breakers

Used in power transmission systems:

• Oil Circuit Breaker

• Air Blast Circuit Breaker

• SF₆ Circuit Breaker

• Vacuum Circuit Breaker

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🔹 Working Principle

When a fault occurs:

1. Fault current increases.

2. The trip mechanism senses abnormal current.

3. Breaker contacts separate.

4. Arc forms between contacts.

5. Arc extinguishing medium (air/oil/SF₆/vacuum) stops the arc.

6. Circuit is interrupted.

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🔹 Comparison: MCB vs Fuse

Feature	MCB	Fuse
Reusable	Yes	No
Speed	Fast	Very fast
Cost	Higher	Lower
Maintenance	Easy	Needs replacement

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🔹 Important Ratings

• Rated Voltage

• Rated Current

• Breaking Capacity (kA)

• Tripping Curve (B, C, D type for MCB)

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🔹 Applications

• Residential wiring

• Industrial panels

• Power stations

• Transmission lines

• Motor protection

Basic Internal Diagram (MCB)

        ┌──────────────────────────┐
        │        CIRCUIT BREAKER   │
        │                          │
Line ───┤  Fixed Contact           │
        │      ●───────●  Moving Contact
        │            \             │
        │             \  Arc       │
        │              \  Chute    │
        │                          │
        │  Bimetal Strip (Overload)│
        │                          │
        │  Magnetic Coil (Short Ckt)│
        │                          │
Load ───┴──────────────────────────┘

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🔹 Main Parts & Their Functions

1️⃣ Fixed Contact

• Stationary part connected to the supply (Line).

2️⃣ Moving Contact

• Opens or closes the circuit.

• Connected to operating mechanism.

3️⃣ Arc Chute

• When contacts open, an arc forms.

• Arc chute divides and cools the arc to extinguish it safely.

4️⃣ Bimetal Strip (Thermal Protection)

• Protects against overload.

• Made of two different metals.

• When heated by excess current → bends → trips breaker.

5️⃣ Magnetic Coil (Electromagnetic Protection)

• Protects against short circuit.

• High current creates strong magnetic field.

• Instantly pulls trip lever → breaker opens immediately.

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🔹 Working Explained Step-by-Step

✅ Normal Condition

• Current flows from Line → Fixed Contact → Moving Contact → Load.

• No tripping occurs.

🔥 Overload Condition

• Current slightly higher than rated.

• Bimetal strip heats up slowly.

• It bends and releases trip mechanism.

• Breaker opens (few seconds delay).

⚡ Short Circuit Condition

• Very high current flows suddenly.

• Magnetic coil produces strong magnetic force.

• Instant tripping (within milliseconds).

• Arc is extinguished in arc chute.

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🔹 High Voltage Circuit Breaker (Concept Diagram)

Example: Vacuum Circuit Breaker

 Line ──●   (Vacuum Chamber)   ●── Load
         \                      /
          \____ Arc in Vacuum __/

• Contacts open inside vacuum.

• No air → arc extinguishes quickly.

• Used in substations.

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🔹 Important Exam Points

✔ MCB works on thermal + magnetic principle
✔ Overload → Thermal trip
✔ Short circuit → Magnetic trip
✔ Arc quenching medium depends on breaker type

2️⃣ Working Principle (Power System Perspective)

When a fault occurs:

1. Fault detected by Protective Relay

2. Relay sends signal to Trip Coil

3. Trip coil energizes → releases latch

4. Moving contact separates from fixed contact

5. Arc forms between contacts

6. Arc is extinguished by suitable medium

7. Circuit is interrupted

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3️⃣ Arc Formation Theory (Important for B.Tech Exams)

When contacts separate:

• Current does not immediately become zero.

• Ionization of medium occurs.

• Arc plasma is formed.

Arc Voltage Equation:

$$V = A + \frac{B}{I}$$

Where:

• A = Constant

• B = Constant

• I = Current

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4️⃣ Arc Extinction Methods

(A) High Resistance Method

• Increase arc resistance

• Cooling

• Lengthening arc

• Splitting arc

Used in:

• Low voltage breakers

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(B) Low Resistance (Current Zero) Method

• Used in AC systems

• Arc extinguishes at natural current zero

• Medium regains dielectric strength quickly

Used in:

• High voltage circuit breakers

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5️⃣ Types of High Voltage Circuit Breakers

🔹 Oil Circuit Breaker

• Arc quenched in oil

• Oil acts as insulation + cooling medium

🔹 Air Blast Circuit Breaker

• High-pressure air blows arc

🔹 SF₆ Circuit Breaker

Uses Sulfur hexafluoride

Advantages:

• High dielectric strength

• Non-flammable

• Fast operation

🔹 Vacuum Circuit Breaker

• Arc extinguishes in vacuum

• Very short arc duration

• Used up to 33 kV

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6️⃣ Important Ratings (Very Important for Exams)

🔸 Breaking Capacity

$$\text{Breaking Capacity} = \sqrt{3} \times V \times I_{breaking}$$

🔸 Making Capacity

$$I_{making} = 2.55 \times I_{rms}$$

🔸 Short Circuit Current

$$I_{sc} = \frac{V}{Z}$$

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7️⃣ Performance Parameters

• Arc Voltage

• Restriking Voltage

• Recovery Voltage

• Rate of Rise of Restriking Voltage (RRRV)

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8️⃣ Comparison (Vacuum vs SF₆)

Feature	Vacuum CB	SF₆ CB
Medium	Vacuum	SF₆ Gas
Maintenance	Low	Moderate
Cost	Lower	Higher
Dielectric Strength	High	Very High
Used up to	33 kV	400 kV+

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9️⃣ Viva Questions (B.Tech Level)

1. What is restriking voltage?

2. Define RRRV.

3. Why SF₆ is preferred over air?

4. What is current chopping?

5. Why vacuum breaker is suitable for frequent operation?