What is a MCB? Structure, operating principle and application

MCB of Fuji Electric

MCB is one of the indispensable devices in the switchgear system, can be considered as a protection device of the whole system. Currently, MCB is no stranger to users and has become one of the widely used devices from home to enterprise.

What is a MCB?

MCB is an abbreviation of Miniature Circuit Breaker, usually has nominal current, low overload current and low short-circuit current (1-> 125A, 10kA, 6Ka, 4,5Ka)



What is the structure of an MCB ?

A complete MCB has structure including: Contact, MCB cutting drive mechanism. All of these components are the key components that make up a complete MCB before they reach the users.


MCB is usually composed of two levels of contact and three contacts.

The two levels of contact are:

  • Main contact
  • Arc.

Three contacts include:

  • Main contact
  • Auxiliary contact
  • Arc.

The contact operates according to the principle of closing the first closed arc contact circuit, followed by the auxiliary contact and finally the main contact.

Conversely, when the main contact circuit breaker will do the task to open first, the secondary contact open later, will eventually be electric arc.

The Cutting drive mechanism

MCB cutting drive mechanism is also one of the indispensable parts to create its complete.

There are two popular ways of MCB cutting drive mechanism, manual and electromechanical.

Manual control of the cutting drive mechanism is performed with MCBs that do not require automation. The opposite is the electromechanical cutting drive mechanism in MCBs that require automatic control in civil & industrial applications.

What is the function of the MCB?

Safety for users is always a top concern of the manufacturer and this is also the function of MCB.

  • Manual power on / off
  • Automatically cut off power when overloaded / short-circuit occurs
  • Safety protection for people and electrical equipment

-> See more: The difference between MCB and MCCB

What is the operation principle of the MCB ?

Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated and deflects by bending.

This deflection of bimetallic strip releases a mechanical latch.  As this mechanical latch is attached with the operating mechanism, it causes to open the miniature circuit breaker contacts, and the MCB turns off thereby stopping the current to flow in the circuit. To restart the flow of current the MCB must be manually turned ON. This mechanism protects from the faults arising due to overcurrent or overload.

But during short circuit condition, the current rises suddenly, causing electromechanical displacement of plunger associated with a tripping coil or solenoid. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts.

An MCB is very simple, easy to use and is not generally repaired. It is just easier to replace. The trip unit is the main part, responsible for its proper working. There are two main types of trip mechanism. A bi-metal provides protection against overload current and an electromagnet provides protection against short-circuit current.

MCB of Fuji Electric

MCB of Fuji Electric

MCB operation

If the circuit is overloaded for a long time, the bi-metallic strip becomes overheated and deformed. This deformation of Bi-metallic strip causes, displacement of latch point. The moving contact of the MCB is arranged by means of spring pressure, with this latch point, that a little displacement of latch causes, release of spring and makes the moving contact to move for opening the MCB.

The current coil or trip coil is placed so that during short circuit fault the magneto-motive force (mmf) of the coil causes its plunger to hit the same latch point and make the latch to be displaced. Again,

when operating lever of the miniature circuit breaker is operated by hand, that means when MCB goes off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in the same manner.

It may be due to deformation of a bi-metallic strip, or increased mmf of a trip coil or maybe a manual operation, the same latch point is displaced and same deformed spring is released, which ultimately responsible for movement of the moving contact.

When the moving contact separated from fixed contact, there may be a high chance of arc. This arc then goes up through the arc runner and enters arc splitters and is finally quenched.

When we switch it on, we reset the displaced operating latch to its previous on position and the MCB is ready for another switch off or trip operation.

Application of MCB

Like many other switchgears, MCB is used very popular in the lives of people. Currently, MCB is being widely used in both industrial and civil constructions. More common when MCB is installed at homes, large projects: hotels, restaurants and apartments … to help protect the safety of people anytime, anywhere.

Which is the best MCB provider?

The situation of fake goods or Counterfeiters is widespread. Gullible consumers are at risk of wasting money. It is necessary to have a reputable and quality address, ensuring a clear origin in the market.

Hao Phuong Corp is one of the companies specializing in distributing MCB product lines of the Fuji Electric brand.

Hao Phuong will definitely be a reliable address for you to send your trust.

Read more: The difference between MCB and MCCB