Introduction and Working of Miniature Circuit Breaker

MCBs or Miniature Circuit Breakers are electromechanical devices which protect an electrical circuit from an overcurrent as well as overload condition. MCB is a better than Fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCB can be easily reset and thus offers improved operational safety and greater convenience without incurring large operating cost.

In the case of tripping because of a current overload, MCBs are very easy to reset, which is much simpler to do that having to replace blown fuses. MCBs also trip to stop the current from flowing much faster than fuses can operate.

WORKING OF MINIATURE CIRCUIT BREAKER (MCB)

The MCB has two different operation used in the internal body. The two operations are described as thermal effect of over current and second one due to electromagnetic effect of over current.
Thermal trip(Over Load)
Magnetic trip(short circuit)

The principal of operation of miniature circuit breaker is simple. A miniature circuit breaker functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. In simple way MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit.
There are two contacts in MCB.
Fixed contact,
Moving contact

When the current exceeds its pre-determined value, solenoid forces the moveable contact to open and the MCB turns off and discontinuing the current to flow in the circuit. To resume the flow of current; the MCB needs to be turned on manually. This protects the circuit from faulty current flowing due to overload and over current.
To protect against fault arising due to over heating or increase in temperature a bi-metallic strip is used. The thermal action of miniature circuit breaker is achieved with a bimetallic strip whenever continuous over current flows through MCB, the bimetallic strip is heated and bends the trip. This deflection of bimetallic strip release latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts.
But during short circuit condition, there is sudden rising of current; it causes electromechanical movement of plunger connected with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing instant release of latch mechanism subsequently open the circuit breaker contacts.
The tripping time of MCBs is 2.5 millisecond, when the short circuit takes place. In case of thermal action it may take 2 seconds to 2 minutes for the MCB to trip.