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Miniature Circuit Breaker (MCB): A Crucial Safety Device in Electrical Systems

Electricity plays a vital role in our daily lives. However, electrical faults such as overloads and short circuits can cause serious damage. To prevent this, a Miniature Circuit Breaker (MCB) is used. It is a safety device designed to protect electrical circuits from damage due to excess current.

What is an MCB?

An MCB is an automatic switch that trips and disconnects the circuit when excessive current flows through it. Unlike traditional fuses, an MCB does not need replacement after tripping. It can simply be reset, making it more convenient and reliable.

Current Ratings of MCB

MCBs are available in different current ratings. The common ratings include:

1, 2, 4, 6, 10, 16, 20, 25, 32, 63, and up to 125 Amperes.

How Does an MCB Work?

An MCB operates using two main principles:

  • Thermal Operation (Overload Protection)

A bimetallic strip inside the MCB bends when heated due to continuous overcurrent.

This bending causes the contacts to separate, disconnecting the circuit and preventing overheating.

  • Magnetic Operation (Short Circuit Protection)

In case of a short circuit, a solenoid coil creates a strong magnetic force.

This force instantly separates the contacts, cutting off the current flow to prevent damage.

 Parts of an MCB

An MCB consists of several important components:


  • Actuator Lever – Used to manually turn the MCB on or off and reset it after tripping.
  • Actuator Mechanism – Moves the contacts together or apart.
  • Contacts – Allow current to pass when touching and break the flow when separated.
  • Lower and Upper Terminals – Connect the MCB to the electrical circuit.
  • Bimetallic Strip – Bends due to heat from an overload, causing the circuit to break.
  • Solenoid – Instantly trips the MCB in case of a short circuit.
  • Calibration Screw – Adjusts the thermal tripping point after manufacturing.
  • Arc Divider/Extinguisher – Reduces the electrical arc when the circuit is interrupted, preventing damage to the contacts.
Types of MCBs and Their Trip Characteristics

MCBs are classified based on their tripping characteristics, which determine how quickly they trip under different fault conditions.


  • Type B MCBs are used for residential and light commercial applications where lower inrush currents are expected.
  • Type C MCBs are commonly used in commercial and industrial settings where moderate inrush currents occur (e.g., motors and fluorescent lighting).
  • Type D MCBs are designed for heavy industrial equipment with high inrush currents, such as transformers and large motors.
Short Circuit Rating of an MCB

The short circuit rating of an MCB is measured in kilo amperes (KA) and represents its ability to handle a short circuit condition.
  • 6KA MCBs are suitable for household and small commercial applications.
  • 10KA or higher MCBs are used in industrial electrical boards where higher fault levels may occur.

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