nd just to mop up another of your points, the RCD main trip does not
have any overload protection; in your case this will be provided by
the leccy board fuse. The main RCD looks for earth faults only.

So in a 'normal' instalation with a 80 amp RCD and a 100 amp company fuse
nothing stops the RCD being overloaded by up to 20 amps?

Jaime