A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. MCBs improve on this functionality as they are usually not destroyed during over-current so are reusable.
source/image(PrtSc): Learn Engineering
The MCB works on two principle I) Electromagnetic Effect, II) Thermal Effect.Under normal working condition the current flow through incoming terminal, fixed & moving contacts,solenoid , bimetallic strip(bends to predefined level),outgoing terminals.Watch the video from Learn Engineering for more info:
I) Electromagnetic Effect:-Whenever the condition of SHORT CIRCUIT occurs then their is heavy current flow through solenoids. due to sudden change electromagnetic field of solenoid the plunger is attracted upwards which in turn releases the latch mechanism and contacts get open.the arc formed is extinguished in arc chamber.
II) Thermal Effect:-Whenever the condition of OVER LOAD occurs in the electrical circuit, then the low magnitude low current flows which leads to bimetallic strip bends further than predefined level due to which latch mechanism is released and contacts gets open. the arc formed is extinguished in arc chamber./quora