Write short notes on MCB.

MCB is an electromagnetic device which operates and disconnects the circuit when the current reaches a pre-determine value. It is normally operated at 1.25 times its rated current. It is an overcurrent protection device. Overcurrent may result from short circuit or overload. MCB is better than fuse since it does not require replacement once the overload is detected. In simple terms MCB is a switch which turns off when the current passing through it passes the maximum allowable limit. It is basically operated on two type of working principles.

  

Thermal operation (overload protection): 

Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated & deflects by bending. As the bimetal bends, it pulls the trip bar which open the breakers contact. Time required for bimetal to bend varies inversely with current.

Magnetic operation (short circuit protection): 

When short circuit occurs the rising current passes through coils creating a magnetic field that attracts the movable contact towards the fixed contact. This causes the opening of breaker contacts. It takes time around 5 milliseconds.