📘 Circuit Breaker – Brief Theory Notes (GATE/PSU)
A Circuit Breaker (CB) is a mechanical switching device capable of making, carrying and breaking current under normal and fault conditions.
🔹 1️⃣ Definition
A Circuit Breaker automatically interrupts fault current to protect electrical equipment from damage.
- Make current
- Carry current
- Break current
🔹 2️⃣ Need for Circuit Breaker
During fault:
- Very high current flows
- Overheating occurs
- Mechanical stress increases
- Insulation may fail
Circuit breaker isolates the faulty section quickly.
🔹 3️⃣ Working Principle
- Relay detects fault
- Trip signal sent to breaker
- Contacts separate
- Arc forms
- Arc is extinguished safely
🔹 4️⃣ Important Ratings
✔ Breaking Capacity
Maximum RMS current breaker can interrupt safely.
Breaking Capacity = √3 × V × Isc
✔ Making Capacity
Peak current breaker must withstand at the instant of closing.
Making Capacity = 2.55 × Isc
✔ Short Time Rating
Current breaker can withstand for short duration (usually 1–3 seconds).
🔹 5️⃣ Types of Circuit Breakers
| Type | Application |
|---|---|
| Oil Circuit Breaker | Medium Voltage |
| Air Blast CB | High Voltage |
| SF₆ Circuit Breaker | Very High Voltage |
| Vacuum Circuit Breaker | Medium Voltage |
🔹 6️⃣ Arc Formation
When contacts separate, a high-temperature plasma arc forms. The breaker must extinguish this arc.
- High dielectric strength medium
- Rapid contact separation
- Cooling and deionization
🔹 7️⃣ Short Circuit MVA Concept
SC MVA = Base MVA / Zth (pu)
Higher short circuit MVA → Stronger breaker required.
🎯 GATE Key Points
- Making capacity > Breaking capacity
- Fault current depends on system impedance
- Breaker rating must exceed system fault level
- Fast clearing improves stability
Circuit Breaker = Fast & Safe Fault Isolation
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