How its work MCB ? Working of MCB

How it works MCB ? Working of MCB

So do you know how it works  MCB ? Working of MCB. Name MCB stands for (Miniature Circuit Breaker). Basics of the Miniature Circuit Breaker.


MCB use to protect your electrical wiring and your electrical appliances.  MCB will break or isolate circuit automatically in faulty condition. It will protect your electrical wiring and appliances against short circuit and over load. In many places like your house, hotel, mall there in distribution box you may see mini switch is called MCB as shown in image. You can simply ask it is mini circuit breaker.

How its work MCB ? Working of MCB

Working Principle

Short circuit and overload are most common fault in the electrical circuit. MCB designed to protect against short circuit and overload fault. In MCB two types of protection are available one is magnetic protection and second is thermal protection. Magnetic protection is used to protect against the short circuit fault. Thermal protection is used to protect against overload fault.

What is short circuit fault

Most common reason of short circuit fault is an insulation failure. In short circuit fault phase conductor short with neutral conductor. During the faulty condition potential level of the circuit is nearest to zero and current level will be extremely high of at infinite level.

Because of extremely high current during short circuit fault. That high current will be passes through the normal conductor. It will create high heat and melt a conductor or fire a insulation of wire.

What is overload fault

In Overload fault current gradually increased from its rated current. It may damage the switching device or machine.

Short Circuit Protection

To protect circuit from the short circuit fault in MCB magnetic protection used. In MCB electromagnetic plunger is used to isolate circuit in faulty condition. But electromagnetic circuit will not operate in normal condition. In turns of electromagnetic coil thick wire is used and few turns. Reason why to use thick wire and less turns in coil it will operate at very high current.

How its work MCB ? Working of MCB

When phase conductor contacted with neutral conductor then short circuit fault occured. During short circuit fault potential level of the circuit is nearest to zero. Because of phase and neutral conductor shorted. But it will generate extremely high current. When this extremely high current passes throught the electromagnetic coil, it will built a magnetic force. Because of magnetic field plunger will hit the latch and circuit will be disconnected. This way circuit protected from the short circuit fault.

Overload Protection

Overload protection is also important. Same as short circuit protection MCB also designed to protect against overload protection. In electrical circuit P=I2R, P (power), I (current), R ( resistance). In conductor because of I2R losses heat will be generated. In conductor resistance is fixed but current will be variable factor. So when current in the circuit changed power will changed and also with respect to power temperature also changed.

So during overload fault current increased with respect to load. Because of variation in temperature metal will expand and shrink. It is depends on the conductivity of the metal. In MCB bimetallic strip used to protect against overload protection. Bimetallic strip is made of two different metals both metal have a different conductivity, One metal more conductive then other. Commonly combination of iron and copper metal used to made bimetallic strip.

How its work MCB ? Working of MCB

During overload I2R losses increased as a resultant temperature of bimetallic strip also increased. Copper is more conductive then iron, so iron plate expands more then copper plate as a resultant bimetallic strip band and it will hit the latch and circuit will be disconnected.

How its work MCB ? Working of MCB

Construction of MCB

  • Outer Body
  • In coming/ Outgoing Terminal
  • Bimetallic Strip
  • Latch Mechanism
  • Arc Chutes and Arc Runner
  • Arc Chutes Holder
  • Knob
  • Solenoid
  • Plunger
  • Fixed Contact and Moving Contact

How its work MCB ? Working of MCB


Named body as a body of MCB, but you can say also a housing of the MCB. Where all parts of the MCB  placed in proper manner. It is made out from fire proof plastic material.

Incoming and Outgoing Terminal

In MCB In coming and outgoing terminals are used to connect circuit connections  to the MCB. Where Power supply connected to the incoming terminal and load connected to the out going terminal. As shown in image outgoing terminal connected to the bimetallic strip and Incoming terminal connected to the moving contact of the MCB.

Bimetallic Strip

Bimetallic strip is made of combination of iron and copper or a different melting point metals. It is used to protect against overload fault in MCB. When higher current flows compare to rated current from the circuit. Because of I2R losses Bimetallic strip will heat and it will bands and hit the latch and latch will be operated. MCB tripped and protect the circuit.

Latch Mechanism

In other terms you can say heart of MCB. It will trip the MCB when it get commands from the Bimetallic strip or a solenoid in faulty condition. The latch mechanism connected is also connected to the knob.

Arc Chutes and Arc Runner

In faulty condition latch mechanism open the contacts of the MCB.  When moving contact and fix contacts are separated in running condition high amount of arc produced. If arc will not quenched it may fire the body of MCB. That’s why arc runner and arc chutes are designed to overcome the effect of the arc in MCB. Arc runners are made from steel of high melting point metal.

Arc chute holder

Arc chute holder designed to hold the arc runner in respective manner condition.


Knob of MCB is directly connected to the latch mechanism. Knob used to On/Off the MCB in healthy condition. During faulty condition latch mechanism trip the MCB and knob will be in down condition. After fault clearing of circuit using knob you can On the MCB.


As a bimetallic strip used to protect against overload protection. In case of Short circuit fault extreme high current passes through the solenoid and solenoid actuate the plunger and plunger hit the latch mechanism. As a resultant latch mechanism open the contact of the MCB and isolate the faulty circuit.


Plunger is a main part of solenoid when extreme high current passes through the solenoid. Then solenoid produce magnetic field and because of magnetic field, plunger will be actuated ans it will hit the latch mechanism of the MCB. Circuit it will isolate the circuit.

Fixed contact and Moving Contact

As you can see in image fix and moving contact is placed near to the incoming terminal and below the arc quenching section. Here one is fixed contact and other one is moving contact in the faulty condition moving contact will move and isolate the circuit, But fix contact can’t move. At the time of contact separation high amount of arc produced that will be quenched by the arc quenching section. If arc will not quenched in time then it may fire the MCB.

List of tests performed on MCB

  • Physical verification and Dimension Test
  • Heat Run Test
  • 28 Days Test
  • Contact Resistance Test
  • Mechanical operation Test
  • Tripping test
  • Short circuit test

This are the major tests to be performed on the MCB. In next blog we will discuss about a different test to be performed on MCB.

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