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Topic Title: Typical values for TNS, TNCS, TT systems
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Created On: 20 September 2005 06:00 AM
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 20 September 2005 06:00 AM
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deleted_appdw

Posts: 61
Joined: 24 April 2005

Hi, could anyone give me some typical values of Ze that you would expect to find on the following:-

1. TNS system
2. TNCS system
3. TT system

thanks, appdw
 20 September 2005 09:08 AM
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deleted_beenthere

Posts: 111
Joined: 24 May 2005

TNS 0.8 TNC-S 0.35 TT 20
 20 September 2005 09:50 AM
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deleted_marrynewland

Posts: 17
Joined: 09 September 2005

Dear Appdw,
The value depends on params of cirkuit breakers.
All values of Ze you can find in software WinElectric.
Open new project and choos Loop impedance (e.g. TN-C).
By press [enter] you can fill every record and automaticly
software will be calculate Ze. Software compare Ze to Zs and
evaulates [Positive/Nagative].
In WinElectric there is library of breakers included
their characteristic (current/time)

This is link to WinElectric Link removed .

Best regards,
Marry
 20 September 2005 11:53 AM
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gkenyon

Posts: 4478
Joined: 06 May 2002

quote:

Originally posted by: marrynewland
Dear Appdw,
The value depends on params of cirkuit breakers.
All values of Ze you can find in software WinElectric.
Open new project and choos Loop impedance (e.g. TN-C).
By press [enter] you can fill every record and automaticly
software will be calculate Ze. Software compare Ze to Zs and
evaulates [Positive/Nagative].
In WinElectric there is library of breakers included
their characteristic (current/time)

This is link to WinElectric www.daslsystems.com .

Best regards,
Marry
How does Ze vary with the parameters of Circuit Breakers?:

quote:

Hi, could anyone give me some typical values of Ze that you would expect to find on the following:-
Beenthere's answer is the one I'd expect to this question.

The values are documented in EA Recommendation P23/1.

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
 20 September 2005 01:38 PM
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OMS

Posts: 19602
Joined: 23 March 2004

These tend to be maximum values, they will vary significantly from place to place. If you need them for design purposes measure them at the origin of if new build speak to the DNO and come up with a more realistic vaule dependant on the network characteristics.

Not sure how Ze is influenced by the choice of circuit breaker, however the opposite would certainly be true.

Regards

OMS

-------------------------
Failure is always an option
 20 September 2005 02:17 PM
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gkenyon

Posts: 4478
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quote:

Originally posted by: OMS
These tend to be maximum values, they will vary significantly from place to place. If you need them for design purposes measure them at the origin of if new build speak to the DNO and come up with a more realistic vaule dependant on the network characteristics.
First, there's the issue of the DNO being able to change the arrangements (and therefore increase or decrease Ze and inversely Ipsc) at any time - so certainly for domestic and other small (100A or less) supplies, there's a school of thought that you should design for worst-case.

Second, the values quoted are only valid for supplies up to 100A. Similar values for supplies 100A to 200A (but advised to check by measurement - characteristics less likely to change).

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
 20 September 2005 02:49 PM
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deleted_marrynewland

Posts: 17
Joined: 09 September 2005

Dear OMS,

Maybe my way is wrong but I think that:

Ze = Un/Ia [Ohm]

Un - nominal voltage in [V] (e.g. 230V),
Ia = In x K in [A] where:
In - nominal current of breaker (e.g. 10A, 16A)
K - factor interpreted from table of current/time characteristic (example of characterictic:
Link removed/library/bifs/1064.PDF)

Please, correct me if my way is wrong.

Best regards,
Marry
 20 September 2005 03:46 PM
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OMS

Posts: 19602
Joined: 23 March 2004

Graham,

I agree, the point I was trying to make (badly) was that the Ze would vary and the broad assumption of Typical valuesw for design purposes is not a sound practice. For a domestic or small installation perspective I suggest that most circuits are selected rather than designed so a typical upper value is probably OK. I feel the big risk occurs when these values are applied to larger installations particularly if the correlation beteweeen a Low Ze and a High PSCC are not taken into account - hence my suggestion that enquiry coupled with calculation is a safer solution.

Marry,

The link does not appear to be functional however I think your calculations would derive the maximum impedance applicable for that device to operate under prescribed conditions. It would not provide you with the Ze of an installation. Apologies if we have misenterperated your comments

Regards

OMS

-------------------------
Failure is always an option
 20 September 2005 07:57 PM
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deleted_appdw

Posts: 61
Joined: 24 April 2005

Thanks everyone,
the reason i asked was to answer a past paper exam question that asked for typical values. Your replys were very usefull.
thanks appdw
 21 September 2005 01:45 PM
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AJJewsbury

Posts: 11374
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Marry's link seemed to have a stray ")" in it - seems to work better without: http://www.bussmann.com/library/bifs/1064.PDF

I'm not sure where Marry got "Ze = Un/Ia [Ohm]" from, the closest I've been able to find (not having the luxury of a searchable CD version of the regs!) is Zs <= Uo/Ia from regulation 413-02-08. If that's the case, there seem to be a couple of points of confusion:

1. It's based on Zs, not Ze. (Zs = Ze + R1 + R2)
2. It's not defining what the value of Zs is, it's stating a maximum limitfor Zs if a certain condition is to be satisfied (i.e. using an overcurrent device to automatically disconnect the supply in the event of a phase - cpc fault).

Where did you get it from Marry? (it wouldn't be the first time a mis-print in a text book or suchlike caused much confusion!)

- Andy.
 21 September 2005 05:51 PM
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deleted_marrynewland

Posts: 17
Joined: 09 September 2005

Dear AJJewsburry,

thank you for correct link.

Regarding symbols Zs/Ze Un/Uo I used wrong marks.
The formulas you can find in IEC 60364-6-61.

I woludn't be so sure that using "typical value" of Ze is correct.
Time of acting of fuse has to the same (e.g. lower than 400ms) so
'Ze' will be different for installation where there is fuse 63A G and
different where there is 16A F.
If the impadance is measure near to transformer requirements and
measured values will be different than if it's make in rooms on the top
of high-rise. Installation system is still the same e.g. TN-S.

Sorry for my English...I'm not English.

Best regards,
Marry
 21 September 2005 09:30 PM
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briandoherty

Posts: 303
Joined: 08 May 2004

Marry,

Don't worry about your English; I think you'll find that we British are generally less apable at other languages than you are at ours!!

Anyway, you seem to be answering the question "What is the highest Ze allowable" (for a particular fuse or circuit breaker) but the actual question was "What is the typical Ze found in a real installation".

Brian


-------------------------
Regards,

Brian
 21 September 2005 09:46 PM
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stevesimpson

Posts: 351
Joined: 03 September 2005

Apable? Yes we are sometimes not very apable at our own language!!! HE he sorry i could not resist.

-------------------------
Yours, Steve Simpson
 21 September 2005 10:04 PM
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briandoherty

Posts: 303
Joined: 08 May 2004

I'm sure we're all capable of being apable with our own lingo....

-------------------------
Regards,

Brian
 21 September 2005 11:25 PM
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mapj1

Posts: 2630
Joined: 22 July 2004

Note, the 20 ohms of a TT system could be all at the supplier's end where they spike down the nuetral of the transformer. Not in a large system, which willl be tied down at many points, where it could be ohm or so, but quite likely one or 2 houses on a pole-pig transformer of mature vintage, as found "in the sticks".
In which case no extra number of spikes at the consumer end will reduce it much. Also in some dry areas, or with samdy soil, an earth rod may need to be augmented by copper strips or bare wires laid into trenches.
TT is often needed (or used to be ) for radio mast type installations, where there was a wish to not waste expensive transmitter power and keep the RF earth curents off the supply cables, and avoid bringing in interference at the same time.
This is not so important for balanced antennas or VHF & microwave works.
regards Mike.

-------------------------
regards Mike
 22 September 2005 01:40 PM
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AJJewsbury

Posts: 11374
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Marry -
I can't say I'm familiar with IEC 60364, but it sounds to be along the same lines as our BS 7671. Maybe there's a slight difference in the symbols used though. According to BS 7671, Zs and Ze are different:

Zs = 'earth fault impedance' (which is what I think you are talking about), but
Ze = 'that part of the earth fault impedance which is external to the installation' (which is what the original question asked about).

In a simple installation, Ze is a feature of the supplier's transformer, supply phase conductors and return earth path only - nothing to do with any circuit conductors within the installation. If you like Ze = Zs only at the output terminals of the supplier's electricity meter - before any of the installation's fuses or circuit breakers. Ze is starting point for calculating Zs for individual circuits (e.g. at design stage - often by adding the resistance of the phase (R1) and protective (R2) conductors).

I hope that makes things a little clearer!

- Andy.
 22 September 2005 07:38 PM
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deleted_marrynewland

Posts: 17
Joined: 09 September 2005

Dear Andy,
thank you for your explanations.
I'll try to again review my knowledge.

Regards,
Marry
 22 September 2005 08:36 PM
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stateit

Posts: 2157
Joined: 15 April 2005

And the values vary enormously.
The house I was at earlier today (TN-S) was very near the transformer, and recorded a very healthy Ze of 0.19ohm.
Unfortunately, the voltage was not too healthy, at a high 250v.

-------------------------
S George
http://www.sg-electrical.com
 27 December 2007 10:26 AM
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Inrush

Posts: 703
Joined: 19 December 2007

Hi

Can anyone provide me with typical ELI and PSCC values (or provide reference to a suitable engineering recommendations) for three phase TN-C-S supplies with BS 88 cut-out fuses listed below, we are doing some work involving the installation of large three phase supplies.

The DNO are being rather unhelpful and quoting a Ze of 0.35 and a PSCC of 16kA, these values are normally quoted for supplies which have a maximum cut-out fuse of 100 Amps, and are not realistic for such large supplies.

160 A
200 A
250 A
315 A

Any help would be appreciated.
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