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MV Multi-Grounded Neutral Systems

J

J. B. Wood

Jan 1, 1970
0
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/strayvoltage/pdf/MultiGroundedNeutralFinal5-3-03.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: [email protected]
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337
 
A

Andrew Gabriel

Jan 1, 1970
0
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/strayvoltage/pdf/MultiGroundedNeutralFinal5-3-03.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.
 
F

Fred

Jan 1, 1970
0
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 said:
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/strayvoltage/pdf/MultiGroundedNeutralFinal5-3-03.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.
 
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