And the fuse for the circuit absolutely should not be the limiter, the RCCB should trip WAY before the main fuse.

While that certainly SHOULD be the case, in the US at least while RCCBs (we call them GFCIs) are generally required in wet areas and perhaps for new construction, in most older houses the majority of circuits don’t have any sort of ground fault protection other than the fuse/breaker. In my current house we have them on only two outlets - one in a bathroom and one in the kitchen.