I've looked in the FAQ's etc but am puzzled whether my electricity supply
has TN-S or TN-C-S
earthing. I thought that TN-S had the earth supplied by the incoming cable
sheath, and that TN-C-S
had the earth supplied by tapping an earth off the neutral core as it enters
the service head thingy.

Now then, my system has an incoming cable with a central red insulated core
surrounded by 16 black insulated 2.5mm2 wires. The red core is used as live,
12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which would
happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Andy.

"andrewpreece" wrote in message
...
I've looked in the FAQ's etc but am puzzled whether my electricity supply
has TN-S or TN-C-S
earthing. I thought that TN-S had the earth supplied by the incoming cable
sheath, and that TN-C-S
had the earth supplied by tapping an earth off the neutral core as it
enters
the service head thingy.

Now then, my system has an incoming cable with a central red insulated
core
surrounded by 16 black insulated 2.5mm2 wires. The red core is used as
live,
12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would
happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Andy.



I would say TNC-S

"andrewpreece" wrote in message
...
I've looked in the FAQ's etc but am puzzled whether my electricity supply
has TN-S or TN-C-S
earthing. I thought that TN-S had the earth supplied by the incoming cable
sheath, and that TN-C-S
had the earth supplied by tapping an earth off the neutral core as it
enters
the service head thingy.

Now then, my system has an incoming cable with a central red insulated
core
surrounded by 16 black insulated 2.5mm2 wires. The red core is used as
live,
12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would
happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Andy.


If your not sure phone the supplier up, they should tell you

Dave Jones wrote:


12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would happen to an earthed cable sheath anyway.

I've no experience of supply cables to speak of; but it sounds more PME
(TN-C-S) than TN-S, simply because - going on your description - there'd
be no way for the installer to distinguish between the 12 cores used to
provide N and the 4 used for E. For all we know the cores you describe
are bound together where the final supply to your place tees off the
feeder in the road...

But Dave has it most right when he says,

If you're not sure phone the supplier up, they should tell you

The "should" here refers to the fact that they have a legal obligation
to tell you what earthing arrangement you have, what the maximum earth
loop impedance of the supply is, and what the maximum prospective
short-circuit fault current is. (Don't expect those last two figures to
be consistent with a 240 (or 230)V supply and Mr Ohm's most excellent
Law, btw: they're both worst-case figures from opposite ends of the same
spectrum, and suppliers allegedly most often say "0.35ohm" (for TN-C-S)
or "0.8ohm" (for TN-S) (which would imply L-to-E fault currents -
admittedly not exactly the same as a PSC, but damn close for PME at
least - of about 690A and 300A respectively), while simultaneously
quoting PSCs of 10kA or 16kA (the Conventional Maximum figures).

Of course, getting through to anyone in "engineering" at an electricity
supply company in these days of micro-privatised
sub-sub-outsource-contracted Competitive Electricity Supply is a
patience-stretching experience...

Stefek

"Stefek Zaba" wrote in message
...
Dave Jones wrote:


12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an
earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would happen to an earthed cable sheath anyway.

I've no experience of supply cables to speak of; but it sounds more PME
(TN-C-S) than TN-S, simply because - going on your description - there'd
be no way for the installer to distinguish between the 12 cores used to
provide N and the 4 used for E. For all we know the cores you describe are
bound together where the final supply to your place tees off the feeder in
the road...

But Dave has it most right when he says,

If you're not sure phone the supplier up, they should tell you
The "should" here refers to the fact that they have a legal obligation to
tell you what earthing arrangement you have, what the maximum earth loop
impedance of the supply is, and what the maximum prospective short-circuit
fault current is. (Don't expect those last two figures to be consistent
with a 240 (or 230)V supply and Mr Ohm's most excellent Law, btw: they're
both worst-case figures from opposite ends of the same spectrum, and
suppliers allegedly most often say "0.35ohm" (for TN-C-S) or "0.8ohm" (for
TN-S) (which would imply L-to-E fault currents - admittedly not exactly
the same as a PSC, but damn close for PME at least - of about 690A and
300A respectively), while simultaneously quoting PSCs of 10kA or 16kA (the
Conventional Maximum figures).

Of course, getting through to anyone in "engineering" at an electricity
supply company in these days of micro-privatised
sub-sub-outsource-contracted Competitive Electricity Supply is a
patience-stretching experience...

Stefek

The cable sounds as if it is a split concentric cable. This is generally
used to provide a TN-S supply. This is not PME. The only way to be certain
is to contact your local network operator and ask them.

Stephen Dawson wrote:


Whilst you ar all talking, tell me what I have.

I have a substation in the garden. A coaxial cable comes from that with
the neutral pretty much attached to the Earth at the transformer, and my
earth busbar goes to a stake I drove into the ground about 5ft deep.

What do I have, and what the **** difference does it make to anything?

I am not sure, but I think that the earth and nuetral are also joined in
the meter cabinet

"andrewpreece" wrote in message
...
I've looked in the FAQ's etc but am puzzled whether my electricity supply
has TN-S or TN-C-S
earthing. I thought that TN-S had the earth supplied by the incoming cable
sheath, and that TN-C-S
had the earth supplied by tapping an earth off the neutral core as it
enters
the service head thingy.

Now then, my system has an incoming cable with a central red insulated
core
surrounded by 16 black insulated 2.5mm2 wires. The red core is used as
live,
12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would
happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Sounds like standard concentric cable. The stripped cores are seperate from
the black insulated neutral cores so the "earthing system" really depends on
just how the connection of your bit is made into the underground supply
system. Thus it could be either TN-S or TN-C-S and you ought to ask your
regional supply company who made the initial connection to the distribution
system for a definitive answer. We can only guess from afar

The Natural Philosopher wrote:


Whilst you ar all talking, tell me what I have.

I have a substation in the garden. A coaxial cable comes from that with
the neutral pretty much attached to the Earth at the transformer, and my
earth busbar goes to a stake I drove into the ground about 5ft deep.

What do I have, and what the **** difference does it make to anything?

I am not sure, but I think that the earth and nuetral are also joined in
the meter cabinet

Having got the previous one probably wrong (the estimable Mr Stumbles
tells us the std concentric cable *does* make it easy to distinguish the
N cores from the E ones, so it really really is an ask-the-supplier
one), let's see if I can cock this one up too.

Given "e and n also joined in the meter cabinet", it's pretty definite
that your supplier is trying to give you TN-C-S, or PME. Your extra
earth stake either does naff-all, if it's supplementary to the
supplier's earth (other than creating an extra path for faults to earth
in your local area, which may not be what you want!) or have created a
TT supply, without an assurance that the earth resistance is low enough
or (I assume) the usual "well we'll put a 100mA RCD across the whole
lot" response.

The FAQ describes the earthing types and their effect. PME means you
need to be a tad more cautious in exporting the house earth to shed,
remote garij, and the like. TT means your supplier leaves you to sort
out a local earth, and these days you need to supplement with the
aforementioned whole-system RCD.

On Wed, 9 Mar 2005, The Natural Philosopher wrote:

I have a substation in the garden. A coaxial cable comes from that with the
neutral pretty much attached to the Earth at the transformer, and my earth
busbar goes to a stake I drove into the ground about 5ft deep.

What do I have, and what the **** difference does it make to anything?

I am not sure, but I think that the earth and nuetral are also joined in the
meter cabinet

If the earth and neutral are combined in a single conductor right up to
the supply head then it is TN-C-S.

If there is no metallic connection between your main earth terminal and
the neutral of the supply transformer (i.e. you have a metal stake in the
ground as your sole earth connection) then it is TT.

It makes a difference because in the case of a TT supply, there is a good
chance that the earth loop impedance will be sufficiently high that a
live-earth fault in the customer's installation will not trip a protective
device (circuit breaker) quickly enough or indeed at all. It is possible
that "earthed" metalwork could be humming away at voltages significantly
above true earth potential. To mitigate against this a Residual Current
Device must be installed covering the entire installation.

--
Alistair Riddell - BOFH

"John" wrote in message
...

"andrewpreece" wrote in message
...
I've looked in the FAQ's etc but am puzzled whether my electricity
supply
has TN-S or TN-C-S
earthing. I thought that TN-S had the earth supplied by the incoming
cable
sheath, and that TN-C-S
had the earth supplied by tapping an earth off the neutral core as it
enters
the service head thingy.

Now then, my system has an incoming cable with a central red insulated
core
surrounded by 16 black insulated 2.5mm2 wires. The red core is used as
live,
12 of the 16 black cores are used as neutral, and four of the black
cores
have had their insulation stripped and they have been directed to an
earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which
would
happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Sounds like standard concentric cable. The stripped cores are seperate
from
the black insulated neutral cores so the "earthing system" really depends
on
just how the connection of your bit is made into the underground supply
system. Thus it could be either TN-S or TN-C-S and you ought to ask your
regional supply company who made the initial connection to the
distribution
system for a definitive answer. We can only guess from afar



Thanks for the replies folks! I see now that what I thought was a simple
question depends on
things that might be buried in the ground i.e. how far back along my
concentric cable are the four
earthy cores connected to the 12 neutral ones - if back at the substation
( which is only one house away ) then it's TN-S, but if it is nearer than
that, or multiply connected, then it's TN-C-S (PME).

Andy.

Stephen Dawson wrote:

The cable sounds as if it is a split concentric cable. This is generally
used to provide a TN-S supply. This is not PME.

Agreed. It's not usual though to be able to see the termination of a
service cable; it's normally sealed inside the cut-out. Would the OP
care to post a photograph somewhere?

--
Andy

Alistair Riddell wrote:

On Wed, 9 Mar 2005, The Natural Philosopher wrote:

I have a substation in the garden. A coaxial cable comes from that
with the neutral pretty much attached to the Earth at the transformer,
and my earth busbar goes to a stake I drove into the ground about 5ft
deep.

What do I have, and what the **** difference does it make to anything?

I am not sure, but I think that the earth and nuetral are also joined
in the meter cabinet


If the earth and neutral are combined in a single conductor right up to
the supply head then it is TN-C-S.

If there is no metallic connection between your main earth terminal and
the neutral of the supply transformer (i.e. you have a metal stake in
the ground as your sole earth connection) then it is TT.

It makes a difference because in the case of a TT supply, there is a
good chance that the earth loop impedance will be sufficiently high that
a live-earth fault in the customer's installation will not trip a
protective device (circuit breaker) quickly enough or indeed at all. It
is possible that "earthed" metalwork could be humming away at voltages
significantly above true earth potential. To mitigate against this a
Residual Current Device must be installed covering the entire installation.

Ah. Thanks for that. All makes total sense now.