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#133915 - 10/05/02 02:48 PM System grounding
tdhorne Offline
Member

Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
Is one of the current carrying conductors of the building wiring system grounded in UK practice or not.
--
Tom
_________________________
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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#133916 - 10/05/02 03:21 PM Re: System grounding
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Tom,

If power is derived from a private generator plant, then the answer is "Maybe."

But as far as regular PoCo service is concerned, it's a definite yes. Our Electricity Supply Regulations have required the neutral to be solidly grounded ("earthed" in British parlance) since at least 1930.

In the case of the old TN-S distribution system, there was generally just one ground connection at the xfmr. In the PME (Protective Multiple Earthing) or TN-C-S system, which has become increasingly widespread, there are multiple ground connections along the route.

(See the diagrams here. )


[This message has been edited by pauluk (edited 10-05-2002).]

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#133917 - 10/05/02 03:45 PM Re: System grounding
David UK Offline
Member

Registered: 10/03/02
Posts: 134
Loc: Inverness, Scotland
Hi Tom,
In UK wiring systems the neutral wire (current carrying conductor) is always earthed (grounded) at the supply source by the supply co. The only systems of supply that I have ever seen here are designated:
TT (local earth rod), TN-S (separate wire or armouring back to transformer) & TNC-S (known as PME in UK, neutral & earth combined in a single conductor on the power supply side & separated in consumer's installation).
Paul posted some 1st class diagrams on these systems, recently.
Any other systems would only be permitted as privately operated systems, not connected to the public mains.

David


[This message has been edited by David UK (edited 10-05-2002).]

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#133918 - 10/05/02 08:46 PM Re: System grounding
tdhorne Offline
Member

Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
Paul or David
It would appear from your diagrams that the neutral conductor of the utility supply is grounded only at the transformer rather than being earthed at the customer service equipment in three of those systems. The PME system looks a lot like the normal practice here with the neutral conductor bonded to earth at the service disconnecting means. The system that you call PME has come under some criticism here because of the unknown effect of stray currents on human and domestic animal health. There is growing evidence that the stray currents have many negative effects on farm and ranch live stock.

Is the third system with RCD protection very common? It would appear that it keeps the neutral aloof from earth at all points on the customer premises.
--
Tom
_________________________
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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#133919 - 10/06/02 12:58 AM Re: System grounding
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
David,
Thanks for the endorsement, but we must also give plenty of credit to our American friend Nick for taking the time to convert my rough sketches into the nice neat graphics you see on that link.

Tom,
I didn't put the new-style designations on the diagrams at the time, as most of you will be unfamiliar with them in the States and I thought it might add to the confusion, however for reference: #1 is TN-S, #2 is TN-C-S, and #3 is TT.

The TN-S system was installed widely in built-up urban areas where underground armored cables were employed. Such services are still to be found by the thousands in towns here.

The TT system is very common in rural areas with overhead distribution. When originally installed, many of these distribution networks also had the neutral grounded only at the star-point of the xfmr. (The RCD/GFI protection is standard now, but up until the 1970s the arrangement shown in diagram #4 was more common for residential services.)

The TN-C-S, or PME system has been in use since the 1930s, but was originally used only in certain rural areas where local soil conditions made it difficult to obtain a low-impedance ground connection. (David, I imagine this would have been the case in many areas of the Scottish Highlands ???)

In recent years, PME has gradually become more widespread, and many rural distribution networks have had the extra ground rods added along the route. Here in Eastern Electricity's region at least, all the old overhead networks have now been converted to PME.

Many buildings still have their original TT arrangement, but it means that any service can be converted to PME now if required.

As you said, our PME/TN-C-S system is the same basic arrangement as the standard U.S. service. The only real difference is the location of the neutral-ground bond at the house -- Before the meter here, at the main panel in the States.

The possibiliy of parallel paths with PME raise the same sort of arguments as you have in America. Our Regs., for example, specify larger size bonding conductors where PME is used to allow for the possibility of an open neutral on the service.

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#133920 - 10/06/02 06:20 AM Re: System grounding
tdhorne Offline
Member

Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
 Quote:
As you said, our PME/TN-C-S system is the same basic arrangement as the standard U.S. service. The only real difference is the location of the neutral-ground bond at the house -- Before the meter here, at the main panel in the States.


Although the service disconnecting means is often located in the lighting and appliance panel of the residence this is not always true in US practice. The Southern Building Code used to require an exterior disconnect for all residential buildings. This led to many installations that are very much like your PME diagram with a separate disconnect on the outside of the house. The biggest difference being that the metering equipment is nearly always on the supply/utility side of the service disconnecting means. The US NEC permits the connection of the Grounding Electrode Conductor at any point between the splice of the drop neutral to the service entrance conductors and the service disconnecting means. Some utilities still require that the connection be made at the service head to improve the lightning protection of the service entry conductors.
--
Tom
_________________________
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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#133921 - 10/08/02 10:31 AM Re: System grounding
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Tom,
That would also be a similar case for mobile homes under the NEC, wouldn't it? Separate disconnect and a 4-wire connection to the main panel in the trailer, right?

The other main difference on grounding here is that our Regs. would not allow a range or dryer to be grounded to the neutral (I know this is no longer allowed under the '02 NEC).

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#133922 - 10/08/02 11:13 AM Re: System grounding
tdhorne Offline
Member

Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
 Quote:
pauluk WROTE:
Tom,
That would also be a similar case for mobile homes under the NEC, wouldn't it? Separate disconnect and a 4-wire connection to the main panel in the trailer, right?

The other main difference on grounding here is that our Regs. would not allow a range or dryer to be grounded to the neutral (I know this is no longer allowed under the '02 NEC).


Yes Paul, you are right on both items.
--
Tom
_________________________
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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