On 5 Mar, 03:23, John Rumm wrote:
wrote:

Don't take this the wrong way, but in common with some of the other
posters, I get the impression from some of your descriptions that you
might want to brush up on some of the finer details before taking on
this job, as there are a number of subtleties that can trap the unprepared!

Might I suggest a copy of the IEE On-Site guide as a basic starting
point? Amazon link:

http://tinyurl.com/32yb6h

I wish to have a split load CU, with the usual circuits on the RCD
side and lighting/freezer on the other side. Is this ok ? and do I
have to leave the ELCD in the circuit or can I connnect the new CU
direct to the meter ?

As Andy suggested two CUs is often a simpler way to do a TT system -
100mA trip RCD in one and 30mA trip in the other. It is often better to
split out outside circuits yet again to save the risk of having them
trip the house circuits. If you are having multiple CUs then a single
master switch positioned before a henley block is a good way to fan out
the meter tails to each CU. That way you still have an overall "off"
switch should it be needed in a hurry.

Finally I want to run separate RCD circuits to the shed (back garden)
and garage (front garden) from the spare ways in the CU. Should these
be on the RCD side or the other ?

My solution in your situation was to have two CUs for the house, and
then a another dedicated time delayed one for outbuildings (which in
turn have their own RCDs). The outbuildings do not have split load CUs,
but instead have non maintained emergency lighting where appropriate.

--
Cheers,

John.

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Could I also add in that if the run to the external CU's is lengthy,
then it is desirable to put in another earth rod at that CU. Rods in
parallel will improve the system earthing.

Also before you go upgrading the earth wire, bear in mind that the
earth resistance is quite high and the voltage drop across it will far
exceed that across the earth lead, unless excessively long. If you
search on this NG you will find a method of measuring the earth
resistance - unfortunately I stored the procedure as a .txt file and
can't point you to it. Post back if you can't find it and I will
paste it up.

Rob