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Topic Title: 3 wire TN-S
Topic Summary: 3 wire TN-S? Doesn't seem to fit the definition?
Created On: 09 February 2009 01:07 PM
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 09 February 2009 01:07 PM
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PG

Posts: 185
Joined: 17 October 2011

Can a 3 phase, 3 wire a.c. 400V system with the star point solidly earthed at the transformer (this is the only intentional connection with earth)be called TN-S if the neutral isn't distributed? It doesn't seem to comply with the definition or the pictures in Section 2 of the 17th.
If it isn't called TN-S what is it called?
Thanks
Regards
 09 February 2009 01:23 PM
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AJJewsbury

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I could be wrong, but it sounds like TN-S to me - especially if you have a metallic PE conductor connecting the 'installation' to the 'source' that isn't intended to carry any N current.

- Andy.
 09 February 2009 01:27 PM
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OMS

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I haven't got a copy to hand, but I recall IEC 60364-1 shows a TN-S System with or without a distributed neutral.

I'm not certain a DNO would actually offer you a 3 phase 3 wire supply (if private you could do almost what you like)

Regards

OMS

-------------------------
Failure is always an option
 09 February 2009 01:47 PM
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rocknroll

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Would'nt it be called just that "3 phase, 3 wire system with no distributed neutral" used for powering motors or any balanced load where a neutral is not required.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 09 February 2009 02:59 PM
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PG

Posts: 185
Joined: 17 October 2011

Thanks for the quick reply gents. I think that I'll go for TN-S.
OHMS. I noticed from page 10 of the regs that they had some sort of alignement with the IEC. Perhaps the IEE missed the 3 wire bit? I can't get hold of IECs.
Anyway,
Regards & Thanks again
 09 February 2009 05:05 PM
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rocknroll

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After searching through some foreign IEC documents similar to IEC 60364-1 I couldnt find a 3 wire TN-S but I did find this;

http://sparkyr.multiply.com/ph...bum%2F30%2FExtras%231

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 09 February 2009 05:15 PM
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OMS

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That looks suspiciously like an IEC figure RnR, 3 wire TN-S may well be Figure 31A3 - I'm away from my usual PC so can't access the document today.

Regards

OMS

-------------------------
Failure is always an option
 09 February 2009 05:24 PM
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rocknroll

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Gimme a while 10/15m and I will post that one 31A3

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 09 February 2009 05:29 PM
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Paul1966

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It would be TN-S if there is a fourth wire run as the protective earth back to the origin. If the installation is earthed just to a local rod then it would be TT. The fact that the neutral isn't distributed doesn't affect these basic definitions.

Is the star-point of the transformer solidly earthed? Yes. So the first letter is T.

Is the path for earth fault current a solid connection back to the neutral at the transformer? If no, then the second letter is T. If yes, then the second letter has to be N.

Assuming we've gotten as far as TN, is the conductor which carries the fault current a separate dedicated conductor, or a neutral conductor? As the neutral is not distributed, it must be separate.

So 3-wire with earth to local rod = TT.

3-wire plus a separate metallic earth conductor back to origin = TN-S.
 09 February 2009 05:36 PM
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OMS

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Yes, I had rather assumed that the 3 phases were actually contained in some metallic element such as a sheath - I think RnR's original sketch may have muddied the waters with TT (unintentionally of course).

OMS

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Failure is always an option
 09 February 2009 05:42 PM
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OMS

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Originally posted by: rocknroll

Gimme a while 10/15m and I will post that one 31A3



regards


Not certain my schoolboy french is up to the job but I think this is the one:

Figure 31A3 - Schéma TN-S avec conducteur de protection mis à la terre et pas de distribution du conducteur neutre dans l'ensemble du schéma


That seems to be with an earth but without a neutral

Regards

OMS

-------------------------
Failure is always an option
 09 February 2009 05:50 PM
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rocknroll

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Here;

http://sparkyr.multiply.com/ph...bum%2F30%2FExtras%232

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 09 February 2009 06:01 PM
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OMS

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That's the fella - TN-S it is then (assuming thet the 3 phases are accompanied by a protective conductor which may well be the sheath)

Regards

OMS

-------------------------
Failure is always an option
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