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Home > Archive > Electrical Engineering > February 2006 > MV Multi-Grounded Neutral Systems
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MV Multi-Grounded Neutral Systems
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| J. B. Wood 2006-02-17, 12:21 pm |
| Hello, all, especially those who engineer medium voltage AC power
distribution systems. I came across an interesting paper that examines
stray current issues and also suggests a remedy. The paper can be found
at
http://www.mikeholt.com/documents/s...Final5-3-03.pdf
and I am not an author nor do I know the authors. I am curious as to the
take on this paper by power systems engineers/technicians. The
safety/health aspects in the paper are intriquing but are not discussed
in my copy of McGraw-Hill's "Standard Handbook for Electrical Engineers."
Thanks for your time and comment. Sincerely,
John Wood (Code 5550) e-mail: wood@itd.nrl.navy.mil
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337
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| Andrew Gabriel 2006-02-17, 8:21 pm |
| In article <wood-1702061041090001@jbw-mac.itd.nrl.navy.mil>,
wood@itd.nrl.navy.mil (J. B. Wood) writes:
> Hello, all, especially those who engineer medium voltage AC power
> distribution systems. I came across an interesting paper that examines
> stray current issues and also suggests a remedy. The paper can be found
> at
>
> http://www.mikeholt.com/documents/s...Final5-3-03.pdf
>
> and I am not an author nor do I know the authors. I am curious as to the
> take on this paper by power systems engineers/technicians. The
> safety/health aspects in the paper are intriquing but are not discussed
> in my copy of McGraw-Hill's "Standard Handbook for Electrical Engineers."
> Thanks for your time and comment. Sincerely,
Multi-Grounded Neutral Systems are used in some areas in the UK.
They are known as PME (Protective Multiple Earthing) systems in
the UK, and TN-C-S [1] which is the European harmonised name.
The PME advantage is a lower fault current loop impedance and
hence faster operation of fault current protective devices.
The main risk from it is that of a broken PEN (Protective Earth
and Neutral combined conductor), which results in downstream
installations tending to float to the live voltage levels in
their PEN conductors and exposed metalwork. UK law requires
very high integrity PEN conductors to minimise this risk.
PME was first used in Austrialia. The other earthing systems in
use in the UK are TN-S, and TT (older rural supplies).
I scanned very quickly through that paper (too long to read details).
We only use PME for some 240V supplies AFAIK, not for the higher
voltage networks (although note that we routinely feed 240V supplies
very much further than you do your 120-0-120V supplies). The max drop
you're likely to see along such a distribution circuit is probably
around 10V, half of which will be across the PEN conductor. Such a
conductor might have, say, 10 earthing points, so you're looking at
something like 0.5V between them. This is between wiring support poles.
The voltage between the feet of cattle is going to be less than a tenth
of this as they don't span anywhere near the distance between earthing
points (which at a very quick glance seemed to be overlooked by the
author, but maybe I just missed it). So this would seem to be
insignificant in our case at least.
One related problem which has caused death of cattle in UK is where
you have a HV line crossing an area and one of the support insulators
has started leaking. This can cause the pole to generate a significant
electric field strength across the ground, resulting in the death of
nearby large quadrapedal animals.
[1] TN-C-S is from the French, Terre Neutre (grounded neutral),
Combiné (combined) in supply, Séparé (separated) in the
installation.
--
Andrew Gabriel
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| I agree completely with the proposal that the neutral conductor should be
insulated throughout.
Found the author's revelation that the utilities in North America have been
using the customer's conductor's and water piping and well systems as part
of their distribution systems particularly insightful. Not only is the
danger constantly present but corrosion of these systems from being
electrically connected to the utility is probably also significant over
time.
Fred
"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:43f65cb7$0$1170$5a6aecb4@news.aaisp.net.uk...
> In article <wood-1702061041090001@jbw-mac.itd.nrl.navy.mil>,
> wood@itd.nrl.navy.mil (J. B. Wood) writes:
>
> Multi-Grounded Neutral Systems are used in some areas in the UK.
> They are known as PME (Protective Multiple Earthing) systems in
> the UK, and TN-C-S [1] which is the European harmonised name.
> The PME advantage is a lower fault current loop impedance and
> hence faster operation of fault current protective devices.
> The main risk from it is that of a broken PEN (Protective Earth
> and Neutral combined conductor), which results in downstream
> installations tending to float to the live voltage levels in
> their PEN conductors and exposed metalwork. UK law requires
> very high integrity PEN conductors to minimise this risk.
>
> PME was first used in Austrialia. The other earthing systems in
> use in the UK are TN-S, and TT (older rural supplies).
>
> I scanned very quickly through that paper (too long to read details).
> We only use PME for some 240V supplies AFAIK, not for the higher
> voltage networks (although note that we routinely feed 240V supplies
> very much further than you do your 120-0-120V supplies). The max drop
> you're likely to see along such a distribution circuit is probably
> around 10V, half of which will be across the PEN conductor. Such a
> conductor might have, say, 10 earthing points, so you're looking at
> something like 0.5V between them. This is between wiring support poles.
> The voltage between the feet of cattle is going to be less than a tenth
> of this as they don't span anywhere near the distance between earthing
> points (which at a very quick glance seemed to be overlooked by the
> author, but maybe I just missed it). So this would seem to be
> insignificant in our case at least.
>
> One related problem which has caused death of cattle in UK is where
> you have a HV line crossing an area and one of the support insulators
> has started leaking. This can cause the pole to generate a significant
> electric field strength across the ground, resulting in the death of
> nearby large quadrapedal animals.
>
> [1] TN-C-S is from the French, Terre Neutre (grounded neutral),
> Combiné (combined) in supply, Séparé (separated) in the
> installation.
>
> --
> Andrew Gabriel
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| Thomas D. Horne, FF EMT 2006-02-20, 4:21 pm |
| Fred wrote:
> I agree completely with the proposal that the neutral conductor should be
> insulated throughout.
>
> Found the author's revelation that the utilities in North America have been
> using the customer's conductor's and water piping and well systems as part
> of their distribution systems particularly insightful. Not only is the
> danger constantly present but corrosion of these systems from being
> electrically connected to the utility is probably also significant over
> time.
>
> Fred
Quite some time ago a joint committee of the American Water Works
Association and the National Electrical Code code making panel on
grounding researched the issue and found that the corrosive affect of AC
current was minimal.
That doesn't make the multi grounded neutral a good thing.
--
Tom Horne
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