Hello almasee
My explanation on how they work :
An electric current requires two wires to complete the circuit : From source via the in wire to the load and from there via the out wire back to the electrical source.
The current in these two wires should always be identical. When an electrical leak to earth occures, these two currents are no longer identical.
In the MCB both currents are run through 2 small coils, mounted in such way that the magnetic fields produced are annihilating each other. A small mechanical switch mounted near these coils trips however when the fields are no longer identical, and therefore result in an electromagnetical field. (earthleak protection)
The system is further wired in such a way that by abnormal high currents the switch also trips (shortcircuit protection).
Note that MCB's have a max. current above which tripping occures.
===
Miniature circuit breakers : from a DIYnot topic on MCB's (Link). See also the long list of comments !
===
Another website with interesting information on MCB's is this
Electronics on Line (Link)