Working principle of MCB; Miniature arc circuit breaker

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Miniature Circuit Breakers (MCBs) – Working & Internal Parts

What is an MCB?

An MCB (Miniature Circuit Breaker) is an electromechanical device that protects an electrical circuit from over-current.
The over-current in a circuit may occur due to:

  • Short circuit

  • Overload

  • Faulty design



Unlike fuses, MCBs do not need replacement after tripping. Instead, they can be reset manually, offering better safety, convenience, and cost-effectiveness.
MCBs are widely used in domestic and industrial applications, particularly for lighting loads and small motor loads.

Working Principle of MCB

The working of an MCB is straightforward:

  • An MCB acts like a switch that turns off automatically when the current exceeds the permissible limit.

  • It is designed to protect against over-current and over-temperature faults.

How it works:

  • There are two contacts: fixed and movable.

  • When the current exceeds the set value, the solenoid pushes the movable contact to open → disconnecting the circuit.

  • To restore current flow, the MCB is manually reset.

  • For overheating protection, a bi-metallic strip bends and triggers the trip mechanism.

Response time of MCB:

  • For over-current: ~2.5 milliseconds

  • For overheating: 2 seconds – 2 minutes

Internal Parts of an MCB

A single-pole MCB (commonly used in households) has the following main parts:

  1. Frame/Enclosure – Provides insulation and protection.

  2. Contacts (Fixed & Movable) – Make or break the circuit.

  3. Arc Chute – Extinguishes arc during switching.

  4. Bi-metallic Strip – Trips during overheating.

  5. Solenoid/Electromagnet – Trips during short circuit or over-current.

  6. Operating Mechanism (Lever/Knob) – Allows manual ON/OFF operation.

  7. Terminal Connectors – For input and output wire connections.

📌 The internal diagram of an MCB (with casing removed) clearly explains these parts and their functions.

Advantages of MCB over Fuse

  • Reusable (no replacement needed)

  • High operational safety

  • Quick response

  • Easy reset after tripping

  • Cost-effective in long run

✅ Suggested Infographic Pack for Your Article

  1. MCB Working Flow Diagram (overload → solenoid trip → circuit open)

  2. Internal Parts Diagram (labeled cutaway view)

  3. MCB vs Fuse Comparison Table

  4. Response Time Chart (over-current vs overheating trip times)

  5. Application Box (domestic & industrial uses)


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