DIYbanter - View Single Post - To export an earth or to TT from an RCBO?

ARW

On 02/09/2019 08:51, wrote:
On 01/09/2019 14:54, ARW wrote:
On 31/08/2019 09:57, wrote:
On 30/08/2019 19:49, ARW wrote:
On 30/08/2019 19:08, wrote:
I have a couple of external sockets that are fed from an RCBO which
also provides power (via about 30m of buried 3-core SWA cable) to a
class II Well pump and controller. The SWA armour and earth
conductor are connected to the house earth solely by the CPC in the
T&E cable. I'm wondering about putting a small weather-proof CU at
the Well so I can use its supply for a remote socket, garden
lights, and maybe a gate controller. The question is whether to TT
the remote CU (but the SWA will still have to be fed from the RCBO)
or whether to add a 10mm earth cable between the MET and the SWA
armour so I can use the exported earth. The house has a dedicated
TN-C-S supply from a 2-wire overhead feed, but I have fitted an
additional ground rod so (hopefully) the loss of the supply neutral
should not cause a safety issue.

What would the experts do?

You cannot make a TT supply from a 30mA RCBO.

I can see a possible issue with discrimination, but if the RCBO (or
RCD) is just measuring L-N current imbalance then why would there be
a functional problem?

Other than discrimination it would work.

So, the question becomes: which is the lesser of the two evils, to
export an earth or to have an RCBO and RCD in series? I'm leaning
towards the latter because it's in the nuisance category, whereas
exporting the earth leads to a possible (but unlikely in this case)
safety issue.
Hmmm, further thought ... if I were to export the earth I would need to
lift a lot of floorboards to run an earth wire back to the MET from the
SWA termination, but these are the same boards I would have to lift to
run a dedicated T&E cable back to a spare MCB in the CU. My conscience
is now telling me that I should run the T&E and do a "proper" TT job
from a dedicated MCB.

This DIY group has been useful yet again. In asking and discussing a
question it forced me to think about the problem in a different way.

Why is the SWA RCD protected?

And you do not have to export the earth as a main equipotential bond
unless there are extraneous conducive parts.

--
Adam