On 26/01/2019 22:05, Chris Green wrote:
John Rumm wrote:
On 26/01/2019 17:33, Chris Green wrote:
I'm running power to a shed, not a long feed from an existing CU in
the garage. It will be fed from an MCB (or RCBO) in the garage CU and
will have a small (2 circuit?) CU in the shed with one CB for power
and one for light.

So which end does one have the RCD protection?

Assuming you don't need the RCD protection to disconnect an earth fault
on the supply cable, then the RCD should be as close to the final
circuit as possible. That way you make it easier to discriminate between
faults, and stop for example losing the outbuilding lighting as a result
of a trip on a power circuit.

So two RCBOs in the garage/shed consumer unit in the 'destination' and
an MCB in the 'source', yes?


Sounds ok

The whole installation is protected by a time delayed RCD as required
by the rules.

Is this a TT earthing install then?

In which case you will need RCD protection for the submain as well. A
type S (time delay) RCD would then be appropriate for the head end, and
normal RCDs/RCBO for the shed.

We have quite a 'long' installation as we have a large house and lots
of outbuildings.

At the 'source' after the company fuse is a time delayed RCD. This
is in one of our garages and feeds a 'Henley Block' which connects to
a local 'garage and rest of the site' consumer unit and a fuse switch
which supplies the house.

ok so that gives you the earth fault protection on the submains.

With a very large install you may find some advantage with more than one
time delay RCD feeding the install in parts rather than as a whole (just
to get a bit more total leakage budget).

The house has a conventional CU with RCDs, RCBOs, etc.

The 'garage and rest of the site' CU has RCD protected power circuits
for our two garages, MCB protected circuits for the garages' lighting
and one (soon to be two) MCB protected circuits for further
outbuildings.

The existing 'further' outbuildings comprise a small stable where
there's a small CU fed by 2.5sq mm from the garage CU (20A MCB).
So what I'm aiming to add is another MCB in the garage CU to feed
another sub-CU with (as I said above) two RCBOs for power and light in
the new garage/shed.

OK, if it was all 'start from scratch' it might not be quite the same
but it's not too bad! :-)



--
Cheers,

John.

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