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).

Which fuse? the one beside the meter?

- Your's is 3 phase or single phase
- What is usually turned on? like the incident this morning?

If I am not mistaken, the fuse beside the meter is slow blow fuse (if its the one beside the meter), it take hell lots of current or high spark of voltage to blow that fused (shorting is one of the method). Or long use of overloading to blow that fused (Long as in > few second).

<Assuming your internal wiring are working> your's situation, the outside fuse blow at random time and internal fused box not trip, and internal Main switch (also fused protected) not blow / trip.  Could happen if somethings / animal move a loose wire in between the meter and fused box inside.

Did you off all the MCB before insert the fuse?

What you can do is isolated the problem to find out where it happen.

Before insert the fuse again, off all the MCB. off the rccb and take out the main fuse beside the rccb.

If the main fuse doesn't blow, than return back the main fuse. Than the rccb. Than 1 by 1 ON back the mcb. Till the fuse blow again.

Than you can know where it happen.