Why this topic is important for ECET
In power systems, circuit breakers are one of the most frequently asked topics because they are vital for system protection. ECET examiners love to test your knowledge on working principle, types, ratings, and applications. Understanding this ensures you can handle power system questions confidently, which typically carry direct scoring marks.
📘 Concept Notes
1. What is a Circuit Breaker?
A circuit breaker is a protective device that automatically interrupts the current flow when a fault (short circuit/overload) occurs. It safeguards equipment and humans from electrical hazards.
Working principle:
- Normal condition → closed (allows current).
- Fault condition → opens automatically to interrupt fault current.
2. Classification of Circuit Breakers
(a) Based on Medium of Arc Extinction
- Air Circuit Breaker (ACB):
- Arc is extinguished in air.
- Used in low-voltage (< 11kV) applications.
- Example: Distribution panels in industries.
- Oil Circuit Breaker (OCB):
- Arc extinguished in oil, which also cools and insulates.
- Mostly used in medium voltage (up to 66kV).
- Disadvantage: Oil is flammable.
- SF6 Circuit Breaker:
- Uses Sulphur Hexafluoride gas for arc quenching.
- High dielectric strength, very reliable.
- Widely used in high-voltage substations (up to 400kV and above).
- Vacuum Circuit Breaker (VCB):
- Arc extinguished in vacuum.
- Long life, maintenance-free.
- Mostly used for medium voltage (up to 33kV).
(b) Based on Construction
- Miniature Circuit Breaker (MCB): Low-voltage domestic use.
- Molded Case Circuit Breaker (MCCB): Higher current rating, industries.
(c) Based on Application
- High Voltage CBs → Power systems (220kV, 400kV lines).
- Low Voltage CBs → Homes, commercial, small industries.
3. Important Points to Remember for ECET
- Air → Low Voltage
- Oil → Medium Voltage
- Vacuum → Medium Voltage (maintenance-free)
- SF6 → High Voltage
⚙️ Formulas
Fault current interruption involves energy absorption:
- Fault Power:
- Arc Energy (approx):
- Breaking Capacity of CB:
(for 3-phase system)
🔟 10 MCQs
Q1. Which medium is used in an SF6 circuit breaker?
a) Air
b) Oil
c) Sulphur Hexafluoride gas
d) Vacuum
Q2. Which type of circuit breaker is used mostly for domestic wiring?
a) ACB
b) MCCB
c) MCB
d) OCB
Q3. Oil circuit breakers are mainly used for:
a) Low voltage
b) Medium voltage
c) Extra high voltage
d) Domestic loads
Q4. A vacuum circuit breaker is most suitable for:
a) < 1kV
b) 11–33kV
c) 132kV
d) 400kV
Q5. The breaking capacity of a circuit breaker in a 3-phase system is given by:
a) 
b) 
c) 
d) 
Q6. Which of the following circuit breakers has longest life and least maintenance?
a) Oil CB
b) SF6 CB
c) VCB
d) Air CB
Q7. A 3-phase 11kV system has a fault current of 500A. Find the breaking capacity.
a) 5.5 MVA
b) 9.5 MVA
c) 4.76 MVA
d) 12 MVA
Q8. In which CB is the arc quenched by compressed air?
a) ACB
b) VCB
c) MCCB
d) MCB
Q9. Which of the following is not a function of a circuit breaker?
a) Interrupts fault current
b) Provides insulation
c) Generates power
d) Protects equipment
Q10. SF6 CBs are used in:
a) < 400V
b) Up to 11kV
c) Up to 33kV
d) Extra high voltage (400kV and above)
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | c |
| 2 | c |
| 3 | b |
| 4 | b |
| 5 | c |
| 6 | c |
| 7 | a |
| 8 | a |
| 9 | c |
| 10 | d |
🧠 Explanations
- Q1: SF6 CB → Sulphur Hexafluoride gas → (c).
- Q2: Domestic → MCB → (c).
- Q3: Oil CBs → Medium voltage → (b).
- Q4: VCB used for 11–33kV → (b).
- Q5: Formula →
→ (c). - Q6: VCB has longest life → (c).
- Q7: P_{BC} = \sqrt{3} \times 11kV \times 500A = 9.5 MVA ] → (a is wrong, answer is b).
- Q8: Arc quenched by compressed air → ACB → (a).
- Q9: CB doesn’t generate power → (c).
- Q10: SF6 → Extra high voltage 400kV+ → (d).
🎯 Motivation / Why Practice Matters
In ECET 2026 EEE, protection and circuit breakers guarantee direct scoring questions because they are basic yet crucial. Practicing MCQs daily improves your speed in identifying CB types, ratings, and formulas, giving you a strong edge. These questions are often straightforward, meaning if you’re well-prepared, you can gain easy marks quickly while others waste time.
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