Diverted neutral current

Have a look at your incoming cable. If it has the earth wired to the sheath
of the cable that's TNS. If the earth goes into the neutral terminal that's
TNCS (or a PME sticker). If you have an outdoor earth rod that's TT.

Post by Scott
What about an RCD? I thought the RCD/RCBO measured any difference in
current between the 'brown' and the 'blue' and therefore any leakage
to earth would immediately trip the RCD. I think I'll stick with
Morcambe and Wise before bed next time!

Yes. Particularly required on TT where the earth impedance can be 200 ohms
- the fault current wouldn't trip an MCB but it would trip an RCD. If next
door causes your earth to go live via the water pipe, your RCD won't help
but your earth bonding should shunt much of the current to your real earth.
On TT your neighbour should have an RCD that trips.

Theo

John Rumm

2024-12-23 16:16:14 UTC

Note that an earth rod can also be added to a TN-C-S install - it just
becomes another of the Multiple earths of PME.

If the neighbour has a disconnected neutral, but there is a path to your
neutral via theirs and yours equipotential bonding, then their RCD may
not see a fault since there will be a balance between live and what it
is seeing as neutral.
--
Cheers,

John.

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Scott

2024-12-23 17:00:28 UTC

On Mon, 23 Dec 2024 16:16:14 +0000, John Rumm

Note that an earth rod can also be added to a TN-C-S install - it just
becomes another of the Multiple earths of PME.

Would it not be safer to have an isolated section on the incoming
water pipe?

John Rumm

2024-12-23 21:43:58 UTC

Post by Scott
On Mon, 23 Dec 2024 16:16:14 +0000, John Rumm

Note that an earth rod can also be added to a TN-C-S install - it just
becomes another of the Multiple earths of PME.

Would it not be safer to have an isolated section on the incoming
water pipe?

Possibly, but were do you stop? Gas pipe, oil pipe, building metalwork,
that TT earth on an outbuilding submain etc. It is difficult to make
sure you catch all possible connections to an unintended earth.
--
Cheers,

John.

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Scott

2024-12-24 09:58:27 UTC

On Mon, 23 Dec 2024 21:43:58 +0000, John Rumm

Post by Scott
On Mon, 23 Dec 2024 16:16:14 +0000, John Rumm

Note that an earth rod can also be added to a TN-C-S install - it just
becomes another of the Multiple earths of PME.

Would it not be safer to have an isolated section on the incoming
water pipe?

I regret to say that in the seasonal spirit of Christmas I watched
another 'JW' YouTube video last night and he actually said the best
way to install a TT system for an existing property is to use a
plastic water pipe. I think he also said that a plastic gas pipe can
be used outdoors but not indoors.

John Rumm

2024-12-24 10:15:28 UTC

Post by Scott
On Mon, 23 Dec 2024 21:43:58 +0000, John Rumm

Post by Scott
On Mon, 23 Dec 2024 16:16:14 +0000, John Rumm

Note that an earth rod can also be added to a TN-C-S install - it just
becomes another of the Multiple earths of PME.

Would it not be safer to have an isolated section on the incoming
water pipe?

Yup plastic gas pipes outside are common in the distribution system, and
often right up to a premises these days, but less so after the meter on
the consumer side. Trackpipe (plastic coated corrugated steel) is quite
common though.
--
Cheers,

John.

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\=================================================================/

Andrew

2024-12-24 17:59:25 UTC

Post by Scott
On Mon, 23 Dec 2024 21:43:58 +0000, John Rumm

Post by Scott
On Mon, 23 Dec 2024 16:16:14 +0000, John Rumm

Note that an earth rod can also be added to a TN-C-S install - it just
becomes another of the Multiple earths of PME.

Yes. Particularly required on TT where the earth impedance can be
200 ohms
- the fault current wouldn't trip an MCB but it would trip an
RCD. If next
door causes your earth to go live via the water pipe, your RCD won't help
but your earth bonding should shunt much of the current to your real earth.
On TT your neighbour should have an RCD that trips.

Would it not be safer to have an isolated section on the incoming
water pipe?

Up to the premises, but *never* inside the boundary of a building
including uninhabited spaces like garages. At least that is the
case with many of the semis where I live. These have a pair of
semi-detached garages in between each pair of houses but separated
from the houses by a metre wide path. The gas meter is in the garage
and the yellow plastic terminates under the concrete apron in front
of the up/over door and then a run of 3/4 inch iron pipe runs under
the garage floor to the meter. Another run of 3/4 inch iron runs
from the meter, under the path into the house entering adjacent to
where the Baxi Bermuda back boiler was originally fitted.

Andy Burns

2024-12-24 19:56:19 UTC

Post by Andrew

Post by John Rumm
plastic gas pipes outside are common in the distribution system,
and often right up to a premises these days,

Up to the premises, but *never* inside the boundary of a building
including uninhabited spaces like garages.

As built, my house was plastic from the main in the footpath, to
"somewhere" in front of the house, then black iron rising in the
cupboard under the stairs where the meter was.

When Cadent replaced the main in the street they found the junction
between plastic and iron was under the built-on porch, so they had to
re-site the meter, coming up outside and in through the wall of the porch.

Then the meter and along the skirting in the hall a couple of feet, back
through the house wall into the garage along and then through the wall
again to under the stairs and rejoin my pipe that used to be on the
output side of the meter.

So externally it looks pretty clean, an 18" stub of white pipe up to a
plastic elbow (concealing a stopcock) and out of sight, I need to build
some sort of cupboard round the meter.

Not as bad as next doors, who have converted their garage, so it had to
go up, over the porch, along the flat roof, along the whole length of
the former garage, round to the back of the house ... looks awful!

Harry Bloomfield Esq

2024-12-25 11:29:01 UTC

Post by Scott
I think he also said that a plastic gas pipe can
be used outdoors but not indoors.

Gas regulations! Plastic buried underground, is much less likely to be
melted by the heat from a fire, and less of an issue, even it it did. A
plastic gas pipe subjected to heat in a building, would be much more
serious.

David Wade

2024-12-23 13:30:14 UTC

OK I believe that in reality there are only two systems in use, TT and
TN-C-S. Whilst your system may appear to be TN-S I understand you should
treat it as TN-C-S as any repairs to the distribution system may result
in Earth and Neutral being combined.

On PME Neutral and Earth are combined .....

Dave

Andy Burns

2024-12-23 14:18:08 UTC

Post by Scott
What about an RCD? I thought the RCD/RCBO measured any difference in
current between the 'brown' and the 'blue' and therefore any leakage
to earth would immediately trip the RCD.

RCDs/RCBOs only care about an imbalance which is on your side of your
consumer unit, they won't trip due to a fault on the supplier's side.

John Rumm

2024-12-23 16:10:31 UTC

Details of the types and how to tell which you have here:

https://wiki.diyfaq.org.uk/index.php/Earthing_Types

Post by Scott
What about an RCD? I thought the RCD/RCBO measured any difference in
current between the 'brown' and the 'blue' and therefore any leakage
to earth would immediately trip the RCD.

Indeed...

However (most) RCDs have no earth connection, so if it is the earth
which is introducing a potential into your house, the RCD won't see it,
and even if it tripped, it would not disconnect the earth.

(which is why we have equipotential bonding)

Post by Scott
I think I'll stick with
Morcambe and Wise before bed next time!

Bring me sunshine :-)
--
Cheers,

John.

/=================================================================\
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| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

tony sayer

2024-12-23 23:59:04 UTC

In article <vkc21n$199f6$***@dont-email.me>, John Rumm <***@n
owhere.null> scribeth thus

https://wiki.diyfaq.org.uk/index.php/Earthing_Types

Post by Scott
What about an RCD? I thought the RCD/RCBO measured any difference in
current between the 'brown' and the 'blue' and therefore any leakage
to earth would immediately trip the RCD.

Indeed...
However (most) RCDs have no earth connection, so if it is the earth
which is introducing a potential into your house, the RCD won't see it,
and even if it tripped, it would not disconnect the earth.

John.. Do any RCD's have an earth connection i thought it was only the
very old e-RCD was it called?..

Post by John Rumm
(which is why we have equipotential bonding)

Post by Scott
I think I'll stick with
Morcambe and Wise before bed next time!

Bring me sunshine :-)

--
Tony Sayer

Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.

Andrew Gabriel

2024-12-24 00:53:04 UTC

Post by tony sayer
John.. Do any RCD's have an earth connection i thought it was only the
very old e-RCD was it called?..

Yes, some do. They can use it to trip on live/neutral reversal and
neutral disconnected. They can also use it to superimpose a low voltage
on the neutral wire. I'm racking my brains to remember why, but it might
have been to detect neutral/earth shorts even when no load is being
drawn. It was a standard feature of one of the RCD ICs.

--
Andrew Gabriel

Chris Green

2024-12-24 08:54:36 UTC

Post by tony sayer

Post by John Rumm
However (most) RCDs have no earth connection, so if it is the earth
which is introducing a potential into your house, the RCD won't see it,
and even if it tripped, it would not disconnect the earth.

John.. Do any RCD's have an earth connection i thought it was only the
very old e-RCD was it called?..

ELCB - Earth Leakage Circuit Breaker

--
Chris Green
·

John Rumm

2024-12-24 10:24:55 UTC

Post by Chris Green

Post by tony sayer

John.. Do any RCD's have an earth connection i thought it was only the
very old e-RCD was it called?..

ELCB - Earth Leakage Circuit Breaker

Yup earth connection for those is the whole MO, a different approach to
fault detection:

https://wiki.diyfaq.org.uk/index.php/ELCB

not to be confused with those current operated RCDs that also have earth
for detecting things like polarity reversal and other fault conditions.

(oddly, those makers that produce them, seem rather shy on promoting the
benefits / features)
--
Cheers,

John.

/=================================================================\
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\=================================================================/

John Rumm

2024-12-24 10:20:22 UTC

Post by tony sayer
owhere.null> scribeth thus

https://wiki.diyfaq.org.uk/index.php/Earthing_Types

Post by Scott
What about an RCD? I thought the RCD/RCBO measured any difference in
current between the 'brown' and the 'blue' and therefore any leakage
to earth would immediately trip the RCD.

Indeed...
However (most) RCDs have no earth connection, so if it is the earth
which is introducing a potential into your house, the RCD won't see it,
and even if it tripped, it would not disconnect the earth.

John.. Do any RCD's have an earth connection i thought it was only the
very old e-RCD was it called?..

You quite often see it on some brands of RCBO for example:

https://mksentrycircuitprotection.co.uk/product-item/residual-current-breakers-with-overcurrent-protection-rcbos/
--
Cheers,

John.

/=================================================================\
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\=================================================================/

tony sayer

2024-12-23 23:56:57 UTC