⭐ Top 25 Circuit Breaker Interview Questions & Answers
(For Technician + Electrical Engineer)
1. What is a Circuit Breaker?
Answer:
A circuit breaker is an automatic electrical switch that stops the flow of current during overload or short circuit conditions, protecting the system from damage.
2. How does a Circuit Breaker work?
Answer:
It operates on two mechanisms:
- Thermal mechanism → Detects overload
- Magnetic mechanism → Detects short circuit
When a fault occurs, the breaker trips and disconnects the power supply.
3. What is the difference between MCB and MCCB?
Answer:
- MCB: Used for small loads (homes/offices), fixed settings
- MCCB: Used for heavy loads (industries), adjustable settings, higher current capacity
4. What is RCCB?
Answer:
RCCB (Residual Current Circuit Breaker) detects leakage current and provides protection against electric shock. It does not protect against overload or short circuit.
5. Difference between ELCB and RCCB?
Answer:
- ELCB: Senses earth voltage (old technology)
- RCCB: Senses leakage current (modern and more accurate)
6. What is a Short Circuit?
Answer:
When current flows through an unintended low-resistance path, producing extremely high current instantly, it is called a short circuit.
7. What is an Overload Condition?
Answer:
When the connected load exceeds the rated capacity of the circuit and causes overheating, it is called overload.
8. What are the internal parts of an MCB?
Answer:
- Bimetal strip
- Magnetic coil
- Arc chamber
- Contacts
- ON/OFF mechanism
9. Causes of Circuit Breaker Tripping?
Answer:
- Overload
- Short circuit
- Loose connections
- Faulty appliances
- Earth leakage
- Water entry
10. What is Breaking Capacity?
Answer:
The maximum current a breaker can safely interrupt, e.g., 6kA, 10kA.
If fault current exceeds this, the breaker may get damaged.
11. What do B, C, and D Curves of MCB indicate?
Answer:
- B Curve: Light loads (homes)
- C Curve: Motor loads, moderate surge
- D Curve: Heavy motors, high inrush current
12. What does a Magnetic Trip Unit do?
Answer:
It detects short circuit current and trips the breaker within milliseconds.
13. What is the function of Thermal Trip Unit?
Answer:
It detects overload current and trips after a time delay (slow operation).
14. What is an Arc Chute?
Answer:
When the breaker trips, an electric arc is formed. The arc chute cools and splits this arc to extinguish it safely.
15. Difference between MCB and Fuse?
Answer:
- MCB is reusable; fuse must be replaced.
- MCB provides accurate tripping; fuse wire melts.
- MCB is safer and more convenient.
16. Why are Adjustable Settings used in MCCB?
Answer:
In industries, load varies frequently. Adjustable overload and short circuit settings provide more accurate protection.
17. Where is ACB used?
Answer:
Used in high current applications such as:
- Industries
- LT panels
- Commercial buildings
18. Voltage range of VCB (Vacuum Circuit Breaker)?
Answer:
Used in medium voltage systems (3.3 kV to 33 kV).
Arc extinction is easy in a vacuum.
19. What is an SF6 Circuit Breaker?
Answer:
A breaker used in high-voltage systems where SF6 gas quenches the arc.
Highly reliable with low losses.
20. Difference between Contact Rating and Interrupt Rating?
Answer:
- Contact rating: Continuous current carrying capacity
- Interrupt rating: Capability to interrupt fault current
21. Why does an MCB trip repeatedly?
Answer:
Common reasons:
- Faulty appliance
- Short circuit
- Loose wiring
- Overload
- Weak breaker
- Incorrect rating selection
22. What is Earth Leakage Current?
Answer:
When electricity flows through leakage paths like human body, wet surfaces, or earth, it is called earth leakage current. RCCB detects this.
23. What is Trip-Free Mechanism?
Answer:
Even if someone tries to manually switch ON the breaker, it will not stay ON if a fault exists. This safety feature is called trip-free mechanism.
24. Why do we use 2-pole or 4-pole breakers?
Answer:
- 2-pole: Disconnects phase + neutral
- 4-pole: Disconnects 3-phase + neutral
This ensures safety and complete isolation.
25. Factors to consider while selecting a Circuit Breaker?
Answer:
- Load current
- Voltage rating
- Breaking capacity
- Trip curve
- Number of poles (1P, 2P, 3P, 4P)
- Application type (home/industry)
- Brand reliability
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