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#135744 - 01/30/03 01:18 PM Description of earthing arrangements
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Hi!

I typed down (most of) what I know of the different earthing systems (TT, TN etc.).

Most information I've found has been rather brief, and I have done some thinking on my own. (Dangerous!) Have I understood everything correctly?

I've put it on the web here

[This message has been edited by C-H (edited 01-30-2003).]

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#135745 - 01/31/03 05:38 AM Re: Description of earthing arrangements
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
That looks like a good outline of the different systems. Just a couple of extra points:

IT systems are not allowed for public suplies in Britain. I know that Norway has some such systems, but I don't know if they are prohibited for public supplies elsewhere in Europe.

For TN-C-S in the U.K., there are not only extra bonding requirements for water pipes etc. within buldings, but the neutral is also connected to extra earth rods at regular intervals along the distribution route (hence the British name Protective Multiple Earthing). I assume that this is also done in Europe ?????

It's a little different in America where they tend (at least for residential) to have a larger number of smaller transformers compared to here.

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#135746 - 01/31/03 06:15 AM Re: Description of earthing arrangements
Belgian Offline
Member

Registered: 10/10/02
Posts: 177
Loc: antwerp
Here the IT system is used for operating theaters in hospitals.

Mainly we use TT systems, but we also have TN systems for big commercial installations.

We are also always (even in TT systems) required to bond all metal parts of a building. What's more, we also have to make a seperate bonding in all bathrooms.

[This message has been edited by Belgian (edited 01-31-2003).]

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#135747 - 01/31/03 06:34 AM Re: Description of earthing arrangements
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Belgian:

Can you describe the IT system used in hospitals?

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#135748 - 01/31/03 12:11 PM Re: Description of earthing arrangements
Texas_Ranger Offline
Member

Registered: 12/17/01
Posts: 2343
Loc: Vienna, Austria
TN-C-S and TT in Austria, I think TT has been used far more often than TN-C-S in the past (take the nice added ground in many older installation - short red or yellow/green wire from receptacle to the next water line which is used as a ground rod, did that quite often when it was still legal). Now TN-C-S systems are put in the following way: At the point where the PoCo wires enter the house there's the PEN wire under the incoming terminal and two wires (N/PE) under the house side terminal. Simple. Was done that way when we got a new house services in '98 or something like that, prior all grounding was done by a wire from the water service to the main fuse panel. At this point however it had to go as we had our service cable (from the 60ies, dating from the DC to AC change) replaced and the water service being changed from lead to plastic. (lots of leaks and an incredible lead value in the water).

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#135749 - 02/01/03 10:35 AM Re: Description of earthing arrangements
Belgian Offline
Member

Registered: 10/10/02
Posts: 177
Loc: antwerp
C-H:

I havn't personally ever seen a IT system. They need special measuring devices to keep track of faults. This device is installed between neutral and earth.
What exactly do you want me to describe? How a IT system works or a schema/plan?

Well, according to my knowledge, the neutral of the transfo is Isolated from earth, (that's what the I stands for) . The mass of appliances are connected to earth. The fault of first fase is of no danger but has to be repaired before that a fault of second fase occurs, so that there is no short circuit.

In hospitals they also have a special backup generator.

This system is also used in industrial installations which have grave implications if machines have to be stopped. e.g. The chemical sector...

Here is a link to more info, but it's in French: http://194.2.245.4/fr_pdf_princ.htm?BTKP225

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#135750 - 02/01/03 11:07 AM Re: Description of earthing arrangements
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
You just answered what I wanted to know. Thanks!

If there are any Americans reading this: I know there is a 480V "ungrounded" system used "over there". It would seem to be an IT system. When and why is this used?

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#135751 - 02/01/03 07:12 PM Re: Description of earthing arrangements
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
C-H, here is a fairly recent paper on US practices. http://www.neiengineering.com/papers/paper1JN.pdf

The general consensus seems to be against ungrounded systems, while using high-resistance-grounded systems to limit resonant- and transient-overvoltage incidents. The idea is to alarm on first ø-g fault, so an “orderly shutdown” can proceed—limiting production loss. If an effort isn’t made to repair soon after the first fault, then a second ø-g fault can easily take out two overcurrent devices—and two pieces of equipment.

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