Components and Working Principle of MCB

MCB stands for Miniature Circuit Breaker which is widely used in home and industries. A Miniature Circuit Breaker (MCB) is a small circuit breaker that is used for residential and industrial applications.

As every circuit breaker, it has both a switching function (it allows the connection / disconnection of the circuit where it is installed) but also a protection function (it must interrupt the circuit in the event of overcurrents (overloads and short-circuits) so that further damage in the installation is prevented). This second function is the most important.

Some basic features of the MCBs are the following:

  • Modular design.
  • Simple use and maintenance.
  • Standardized time-current curves.
  • They have a fixed thermal and a fixed magnetic trip setting.
  • Usually, they do not have secondary trip units

Basic Components and Operation Principle:

Figure below shows the basic components of a MCB.


Thermal / Magnetic Trip Units:

Typically, MCBs use an electromechanical (Thermal / Magnetic) trip unit to open the breaker contacts during an overcurrent event as shown in figure below. The thermal trip unit is temperature sensitive and the magnetic trip unit is current sensitive. Both units act independently and mechanically with the breaker’s trip mechanism to open the breaker’s contacts.

Thermal Trip Unit:

The thermal trip unit protects against overload currents. The thermal unit is based on a bimetal element located behind the circuit breaker trip bar and is part of the breaker’s current carrying path. When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts. The time required for the bimetal to bend and trip the breaker varies inversely with the current. Because of this, the tripping time becomes quicker as current increases in magnitude. Overload protection is applicable to any installation, conductor, or component which can be subjected to low-magnitude but long- time over-currents. Low-magnitude, long-time over-currents can be dangerous because they reduce the life of the electrical installation, conductor, and components and if left unchecked could result in fire.

Magnetic Trip Unit:

The Magnetic trip unit protects against a short circuit. The magnetic trip unit is comprised of an electromagnet and an armature. When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature. The hammer trip is pushed against the movable contact and the contacts are opened.

Arc Chamber:

The arch chamber, by the action of the arc runners and arc chute, limits and dissipate the arc energy during the interruption of an overload or short circuit event.

Operating Mechanism:

It is the set of springs, levers and other internal elements that enable the opening of the contacts as well as to perform their independent closing. It is a trip-free mechanism. This means that the tripping action works independently of the handle position for safety purposes.

Advantages / Drawbacks of MCB:



  • Simple maintenance.
  • Modular constitution.
  • Easy to be used by non-expert people.


  • Limited features (in comparison with normal Circuit Breakers).
  • Costly when compared with fuse.

Thank you!

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