Compliant?

1.0 mm2 T&E run 2.5 metres.

I guess you might have the 3m rule of 434.2.1 in mind (allowing fault protection devices to be downstream of the reduction of c.s.a.). That reg however also requires the installation method to reduce the risks of faults, fire and danger to persons to a minimum - I hardly think that a bit of T&E slung across the loft goes anywhere near meeting those criteria. So no justification in omitting/postponing fault protection as far as I can see.


1mm² on a B40 has no chance of being protected from faults by a B40 according to generic data - even at the minimum 3kA rating it would need to be 1.278mm² and nearer 2.5mm² for 6kA. Even pushing things by assuming the 1mm² conductors start off cold before a fault (k=143) things are still (marginally) on the wrong side (so trying to justify a lower initial conductor temperature due to the small load on the 1.0mm² isn't going to be a solution either). Manufacturer's data might give you a little more leeway, but I suspect not enough.


The 6mm² should be fine - unless circumstances are really odd it'll be protected from faults by the B40 and from overload by the 3A fuse.  2.5m of 1.0mm² will have a similar resistance to about 8.6m of 6mm² so unless the circuit length is already exceptionally long, Zs should be fine.


   - Andy.

1.0 mm2 T&E run 2.5 metres.

I guess you might have the 3m rule of 434.2.1 in mind (allowing fault protection devices to be downstream of the reduction of c.s.a.). That reg however also requires the installation method to reduce the risks of faults, fire and danger to persons to a minimum - I hardly think that a bit of T&E slung across the loft goes anywhere near meeting those criteria. So no justification in omitting/postponing fault protection as far as I can see.


1mm² on a B40 has no chance of being protected from faults by a B40 according to generic data - even at the minimum 3kA rating it would need to be 1.278mm² and nearer 2.5mm² for 6kA. Even pushing things by assuming the 1mm² conductors start off cold before a fault (k=143) things are still (marginally) on the wrong side (so trying to justify a lower initial conductor temperature due to the small load on the 1.0mm² isn't going to be a solution either). Manufacturer's data might give you a little more leeway, but I suspect not enough.


The 6mm² should be fine - unless circumstances are really odd it'll be protected from faults by the B40 and from overload by the 3A fuse.  2.5m of 1.0mm² will have a similar resistance to about 8.6m of 6mm² so unless the circuit length is already exceptionally long, Zs should be fine.


   - Andy.