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
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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: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory
4555 Overlook Avenue, SW Washington, DC 20375-5337
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.
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.
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.
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