I imagine that you don't understand the sorts of currents that can flow during a dead-short 'live to neutral' or 'live to earth'?
And that you probably don't have any correct test equipment?
As assuming an MCB, 'that has just done exactly what it is designed to do', is faulty, isn't my first logical step..
and the MCB can be checked very quickly and easily with an approved voltage tester.
Swapping circuits/mcb's is not the way to test or identify faults..
There are various possibilities that would need correct testing to identify the exact cause..
It's connected wrong..
Cables, (e.g. joint in j/box), damaged elsewhere due to high fault current melting/welding something..
Switch damaged elsewhere due to high fault current melting/welding something..
etc..
If you have no test equipment to do correct dead testing prior to re energising the circuit,
then you probably need to employ someone who does.