when you say you off MCB, is it the :

(A)


or

(B)

i never had a problem with external fuse blown,
but i encountered a problem last month regarding my RCCB (the one in picture B).

power trip due to wallsocket faulty, but all MCB, RCCB, ELCB not tripped.
was puzzled at first, but when i tried turning ON/OFF the MCB & RCCB, the power is back on again.

it happened again afterwards, during a thunderstorm (and lightning),
no power due to lightning strike, but all MCB (picture A), RCCB & ELCB not tripped.

did some checking, and i assumed that my RCCB is faulty: there was no power in the house,
but the RCCB switch is in its 'ON" position (internal mechanism i guess).

changed my RCCB and it solved my problem, so far.

do you check your RCCB regularly (press the 'TEST' button to see if it can trip)?

if it works (the RCCB can trip using TEST button), then RCCB might not be at fault.
if it doesn't work, then why not try changing your RCCB first, and observe if the external fuse still blow afterwards.

like what MadhavanR mentioned, i also had the experience that normally the external fuse
can just cabut/replace to do anything to your main fuse box.. seldom that we'd have it getting blown (multiple times some more).

since you've changed everything in the mains box,
then most likely there's something wrong with your house wiring
or probably the cable in-between the main lines - connected to your mains box.

is it a new house? are you the first owner/occupant?