The working principal of MCB is very simple. An MCB functions by interrupting the continuity of electrical flow through the circuit when a fault is detected. In other word, MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit. Generally the MCB are designed to protect against over current and over temperature faults (over heating).
There are two contacts in the MCB for each phases. One contact is fixed and the other one moveable. When the current in circuit exceeds the pre-defined limit, a solenoid forces the movable contact to open. Hence it disconnect from the fixed contact, the MCB turns off and thereby stopping the current flow. In order to restart the flow of current, the MCB shall be  manually turned on. This mechanism is used to protect circuit from the faults arising due to over current or over load in the circuit.

MCBs are generally designed to operate (means trip) within 2.5 millisecond when an over current fault arises. In case of temperature rise or over heating, it will take 2 seconds to 2 minutes for the MCB to trip, depending on MCB types

MCBs or Miniature Circuit Breakers are electromechanical protection devices which protect an electrical circuit from an overcurrent of any kind. The overcurrent, in an electrical circuit, may result from short circuit, overload, wrong device selection  or faulty design. An MCB is more or less same in function with fuse, but 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 causing large operational cost.