Compliant?

Circuits supplied from the transformer secondary will not have RCD protection... Is the transformer output SELV?

The one mm beyond the 3A fuse is fine, any 1mm before that fuse really  needs to see a PSSC that will clear the 40A breaker before the 1mm2 over heats.

Depending on the zone the transformer secondary wiring is in , the transformer may need to be SELV or can be  just PELV . In any case I'm assuming transformer and pump are a kit, and as such it falls to maker's instructions,  I'm also assuming the pump operated of a voltage less than 50 ?

So wthout that extra info, probably OK, but needs checking.

How is the 6mm cable installed?  It could be too small for the B40 cb anyway.  The 1mm cable should be the other side of the 3A fuse.  The 2.5m run of 1mm is not protected and if there was a fault on this section the EFLI would be too high for timely operation of the B40 cb

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.

Circuits supplied from the transformer secondary will not have RCD protection... Is the transformer output SELV?

Doesn't necessarily have to be SELV or even ELV - the bathroom RCD regulation only applies to circuit that are protected by ADS (the 701.411.3.3 numbering means it's a adjustment to regulation 411.3.3 - which is in the ADS only section, 411). The output could be 230V separated (section 413) for example, and still not require 30mA RCD protection even it it was under BS 7671 (which is it probably isn't anyway as the equipment standard probably takes precedence).


  - Andy.

15m of 6mm2, surely?

I was working on the basis that 1mm² T&E (with a 1mm² c.p.c.) would have a resistance of 36.2mΩ/m - so 90.5mΩ for 2.5m. 6mm² with a 2.5mm² c.p.c has a resistance of 10.49mΩ/m - so you need about 8.6m worth to have the same increase in Zs. (All resistance figures at 20°C but the proportions will be the same at other temperatures.)


If 6mm² T&E had a full sized c.p.c. then yes it would be closer to 14.7m.


  - Andy.

If 6mm² T&E had a full sized c.p.c. then yes it would be closer to 14.7m.

Ah yes, silly me.

The easy fix is to avoid questions about the length of cable before the 3A fuse, is to change  the 40A breaker for something smaller - for a 3A fuse, almost anything would do, 6A, 10A 16A - whatever is available .

Yes Andy I was thinking of 434.2.1 (i) and (ii).


(i) The 1.0mm2 not exceeding 3m in length.


(ii) "Be installed in such a manner as to reduce the risk of fault to a minimum." The cable is inaccessible as it runs under a tank stand and out of reach or risk of being stood on or mechanically damaged. 


(iii) Just what requirement is meant here? Run in metal containment perhaps?


Z.