Owain wrote:

An alternative could be to use a split load CU, with *all* the circuits off
the non-RCD main switch being RCBOs (say socket rings and lights), with the
RCD side being used for items where discrimination is less essential, eg
cooker, shower, immersion heater, radials to wall heaters, etc. (There would
be no whole-house RCD at all.)

Nice idea, but I don't think it will work for me in this case. I
currently have 5 circuits, 3 final rings, and two lighting circuits. No
radials, no electric heating, showers, immersion etc. I will be adding
one extra final ring, and one extra lighting circuit, and a dedicated
smoke alarm circuit. So the above suggestion would require 7 RCBOs. The
secondary problem would be that a split load CU with enough ways will
not fit in the available space. That then turns the job of "swapping"
CUs into a much bigger job of relocating and replacing the CU.

That would provide the advantages of system 3 on power and lighting
circuits, which are more likely to suffer high quiescent residual current
and also form the greatest danger/inconvenience in the event of a trip,
whilst reducing the number of RCBOs required. If the washing machine and
dishwasher are wired to radials on the RCD side, a separate kitchen ring
might not be required, so most houses would be sufficed by 2 socket rings
and 2 lighting radials only requiring 4 RCBOs.

I could add the new second floor sockets and lighting to the existing
circuits since I would still be within the permitted maximum area and
loadings. However I much prefer having the ability to isolate each floor
discretely.

The downside is that most split-loads only have about 60A available through
the RCD.

Which is all my supply is good for anyway. Having said that I will
probably install a 100A RCD to allow for the possibility of future
upgrades.


--
Cheers,

John.

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