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Andrew Gabriel

In article .com,
writes:

In preparation for an extension to my house, I wish to replace my
consumer unit (wired fuse type) with a larger one with spare ways for
the circuits in the extension. The CU replacement will be fairly
straightforward but I have a couple of items I wish to check first.

The power reaches the house by way of a long overhead cable, running
underground from the very last post to our house. Incoming is a fat
black cable which splits to two smaller but still fat cables running
into the company fuse and a henley(?) block.

From these two blocks I have fat red and black cables to the meter,
then same large red & black to a Chilton Current Operated Earth
Leakage Current Detector. Finally cables run from the ELCD to the
Consumer unit. A separate earth cable runs from the CU to an earth
spike. (I will replace this with a much fatter earth cable at the
same time as it looks a little thin to me)

This is a TT installation. Current Operated Earth Leakage Detector
is the original name for an RCD -- does it actually say that on it,
as Chilton are better known (to me at least) as a manufacturer of
Voltage Operated Earth Leakage Detectors, which is a previous
generation device. What rating is written on it?
The earth wiring you describe is however consistent with an RCD.

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 you have a TT installation, all your circuits will need to be
RCD protected. It would be worth asking your supplier if you can
be easily converted to TN-C-S in where this would not be the case,
but that depends if the local supply network has been converted to
to Protective Multiple Earthing.

If you remain TT (own earth rod), you can still use a split CU,
but both sides will need RCD protection. One is protected at 30mA
for the circuits where protection against electrocution is required
or desirable, and the other is protected at 100mA (and can be time-
delayed) where protection against electrocution is not required (it
has to be time-delayed if the 30mA side is daisy-chained off it).
If you have the space available, I would consider doing these as 2
separate CU's, which gives you slightly more flexibility.

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 ?

You generally don't want circuits running outdoors sharing an RCD
with indoor circuits, as outdoor faults are more common (moisture
ingress), and could leave indoor circuits unusable. Socket outlets
in your shed and garage should be RCD protected as they're likely
to be used to power outdoor appliances. This really implies they
should be on the 100mA RCD side and protected by an additional 30mA
RCD. If you were going the separate CU route, then add a third
dedicated 30mA RCD protected CU for them. There are lots of options
here, depending what you will be running in there (a freezer, for
example?) and how dry they are. If you do end up sharing an RCD
between indoor and outdoor circuits, do at least include a 2-pole
isolating switch in the outdoor feeds so you can isolate a faulty
circuit.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]