To export an earth or to TT from an RCBO?
 

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?

To export an earth or to TT from an RCBO?
 

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.


--
Adam

To export an earth or to TT from an RCBO?
 

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?

To export an earth or to TT from an RCBO?
 

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.

--
Adam

To export an earth or to TT from an RCBO?
 

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.

To export an earth or to TT from an RCBO?
 

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

To export an earth or to TT from an RCBO?
 

On 02/09/2019 18:02, ARW wrote:
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?
In "plan A" it was because there's an external socket (fed from an RCBO)
on the end of the house nearest to the Well - a convenient point to take
the SWA from. However, I've decided to "do it right" and lift the
floorboards to put in a dedicated T&E feed to the SWA from an MCB.

And you do not have to export the earth as a main equipotential bond
unless there are extraneous conducive parts.
Currently the pump and controller are class II, but if I have a small CU
at the tail of the SWA then I can't predict what might be connected in
the future. But that all goes away if it's TT'd.

To export an earth or to TT from an RCBO?
 

On 02/09/2019 19:01, wrote:
On 02/09/2019 18:02, ARW wrote:
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?
In "plan A" it was because there's an external socket (fed from an RCBO)
on the end of the house nearest to the Well - a convenient point to take
the SWA from. However, I've decided to "do it right" and lift the
floorboards to put in a dedicated T&E feed to the SWA from an MCB.

And you do not have to export the earth as a main equipotential bond
unless there are extraneous conducive parts.
Currently the pump and controller are class II, but if I have a small CU
at the tail of the SWA then I can't predict what might be connected in
the future. But that all goes away if it's TT'd.

I don't think I have seen a class II pump, but there is no reason why
they should not exist - the number of pumps fitted in my lifetime
(sump and pond only) is about 10.

I agree about the TT then. You can then pretty much fit whatever you want.

--
Adam