A lite explanation of why this topic is important for ECET
Circuit breakers are one of the most important protection devices in electrical power systems. They safeguard equipment and people by interrupting fault currents quickly. In ECET 2026, questions from circuit breakers are common under Power Systems – Protection. Understanding types, working principles, and ratings helps students answer both theoretical and numerical problems.
📘 Concept Notes
1. Definition of Circuit Breaker
A circuit breaker is a switching device that can make or break a circuit under normal conditions and automatically break it under abnormal conditions like short-circuit, overload, or fault.
Key Functions:
- Detect fault (with relays)
- Interrupt current safely
- Prevent damage to system
2. Arc Formation and Interruption
When contacts separate, current continues to flow through an arc (ionized path). The circuit breaker’s job is to extinguish the arc quickly.
- Arc energy depends on
losses. - Interruption methods: cooling, lengthening, high-pressure medium, dielectric recovery.
3. Types of Circuit Breakers
(a) Air Circuit Breaker (ACB)
- Uses air at atmospheric pressure as arc quenching medium.
- Voltage: up to 11kV.
- Applications: LT systems, industrial plants.
(b) Oil Circuit Breaker (OCB)
- Uses insulating oil (mineral oil) for arc extinction.
- Arc vaporizes oil → hydrogen gas generated → cools arc.
- Applications: Medium voltage systems.
(c) SF6 Circuit Breaker
- Uses sulfur hexafluoride gas (excellent dielectric).
- Voltage: up to 765kV.
- Advantages: high efficiency, silent, less maintenance.
(d) Vacuum Circuit Breaker (VCB)
- Arc is quenched in vacuum.
- Voltage: up to 66kV.
- Advantages: compact, long life, eco-friendly.
(e) Miniature Circuit Breaker (MCB)
- Used for small current protection (<100A).
- Domestic applications, replaces fuses.
(f) Molded Case Circuit Breaker (MCCB)
- For higher ratings than MCB (up to 1600A).
- Used in distribution panels.
4. Important Ratings
- Rated Voltage
- Rated Current
- Breaking Capacity
→ maximum fault current it can interrupt - Making Capacity
→ maximum current it can close onto
⚙️ Formulas
- Breaking Capacity:
- Making Current:
- Arc Power:
🔟 10 MCQs
Q1. The main function of a circuit breaker is:
a) To regulate current
b) To protect the system by interrupting fault currents
c) To improve power factor
d) To step up voltage
Q2. In an oil circuit breaker, arc extinction takes place due to:
a) Cooling effect of hydrogen gas
b) Magnetic blowout
c) Dielectric strength of vacuum
d) Thermal ionization
Q3. Which circuit breaker is best suitable for extra-high voltage (>220kV) transmission?
a) ACB
b) SF6 CB
c) OCB
d) VCB
Q4. Vacuum circuit breakers are generally used up to:
a) 11kV
b) 33kV
c) 66kV
d) 400kV
Q5. A 3-phase breaker is rated 11kV, 20kA. Find breaking capacity in MVA.
a) 220 MVA
b) 380 MVA
c) 450 MVA
d) 381 MVA
Q6. The making current of a circuit breaker is generally:
a) Equal to breaking current
b) 1.2 times breaking current
c) 2.55 times breaking current
d) Double breaking current
Q7. MCB is generally used for:
a) Transmission lines
b) Domestic installations
c) EHV substations
d) Distribution transformers
Q8. SF6 circuit breaker is popular because:
a) SF6 is a good dielectric and arc quenching medium
b) It is cheapest
c) No maintenance required at all
d) It increases voltage
Q9. The maximum current a breaker can close onto a fault is called:
a) Breaking capacity
b) Making capacity
c) Rated current
d) Short-time current
Q10. An 11kV circuit breaker has a breaking current of 25kA. Find its MVA capacity.
a) 250 MVA
b) 450 MVA
c) 476 MVA
d) 500 MVA
✅ Answer Key
| Q.No | Answer |
|---|---|
| 1 | b |
| 2 | a |
| 3 | b |
| 4 | c |
| 5 | d |
| 6 | c |
| 7 | b |
| 8 | a |
| 9 | b |
| 10 | c |
🧠 Explanations
- Q1: Circuit breakers interrupt fault currents → (b).
- Q2: OCB extinguishes arc by hydrogen gas cooling → (a).
- Q3: EHV systems (>220kV) → SF6 CB → (b).
- Q4: VCB normally used up to 66kV → (c).
- Q5:
→ (d). - Q6: Making current = 2.55 × breaking current → (c).
- Q7: MCB = domestic installations → (b).
- Q8: SF6 is superior dielectric & arc quenching medium → (a).
- Q9: By definition → making capacity = max current breaker can close onto → (b).
- Q10:
→ (c).
🎯 Motivation / Why Practice Matters
In ECET 2026, questions on protection systems like circuit breakers are both theory-based and numerical. Speed matters because students must quickly recall formulas like breaking capacity and making current. Practicing MCQs daily helps improve accuracy and confidence. Mastering circuit breakers ensures you’re ready for tougher power system questions in the exam.
📲 CTA
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