Grunff wrote:

I was asking whether it would really be ok to fuse the circuit at 40A
without knowing how the 6mm cable was routed. I personally feel
uncomfortable using 6mm cable with anything more than a 32A breaker in
any case, because you just don't know what might happen in the future
(e.g. someone slaps a load on insulation on top).

The fuse rating's a bit irrelevant here, provided that it still provides
short circuit fault and earth fault protection. It's the _load_ current
in relation to the cable rating that matters. Increasing the fuse
rating from 30 A to 40 A won't increase the current flowing through the
cable and therefore won't increase the conductor temperature.

The higher rated fuse will result in a higher final conductor
temperature after fault clearance, but such heating is essentially
adiabatic and isn't significantly affected by thermal insulation round
the cable.

So it's the earth fault loop impedance you need to check, not what's
happening in the loft.

--
Andy