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Topic Title: TN-C-S Supply Agricultural Premises
Topic Summary: What to do
Created On: 24 December 2012 02:58 PM
Status: Post and Reply
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 24 December 2012 02:58 PM
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Brunokid

Posts: 146
Joined: 13 August 2005

A new supply is installed to an stable block but it a TN-C-S


705.411.4 A TN-C system shall not be used.
 24 December 2012 03:11 PM
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typiod

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Joined: 09 February 2008

TT then.
 24 December 2012 03:14 PM
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Brunokid

Posts: 146
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Sorry last half of the message got lost.

Should I just disregard the the existing earth terminal and TT it with an earth electrode or go back to the supplier?
 24 December 2012 03:27 PM
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daveparry1

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TT it and ignore the tncs earth terminal i'd say,

Dave.
 24 December 2012 03:34 PM
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John Peckham

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BS7671 forbids the use of TNC not TN-C-S in A&H. It does say PME is not recommended. Not all TN-C-S is PME but all PME is TN-C-S.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 24 December 2012 06:07 PM
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Brunokid

Posts: 146
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Originally posted by: John Peckham

BS7671 forbids the use of TNC not TN-C-S in A&H. It does say PME is not recommended. Not all TN-C-S is PME but all PME is TN-C-S.


Thanks for your informative reply John

I have always said that the best part to BS7671 is 'Part 2 Definitions'

As of page 35, it does indeed say 'a TN-C system and TN-C-S system are two different things'

The suppliers cut-out is clearly labeled 'PME'

705.415.2.1 Unless a metal grid is laid in the floor, the use of PME earthing facility as a means of earthing the installation is not recommended.

I guess that pretty much answers my question.

Thanks again

Bruno
 24 December 2012 06:15 PM
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John Peckham

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If this is an academic question then you have your answer. If it is a practical question then you are going to TT the installation if you are going to have horses coming in to contact with exposed or extraneous conductive parts. The same for humans but they area lower risk than horses.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 24 December 2012 06:29 PM
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Brunokid

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Originally posted by: John Peckham

If this is an academic question then you have your answer. If it is a practical question then you are going to TT the installation if you are going to have horses coming in to contact with exposed or extraneous conductive parts. The same for humans but they area lower risk than horses.


So why exactly don't we use PME?

I sort of half understand it, animals with 4 legs etc
 24 December 2012 07:17 PM
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John Peckham

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As the exposed conductive parts and extraneous conductive parts are all connected to the MET and the MET is connected to the suppliers neutral, depending on load and cable length, there will be a potential difference between the supply earth and the general mass of earth. Small voltages may not be detectable to humans but horses standing on 4 legs with metal shoes touching say a metal drinker will experience a voltage which would stop them drinking which could cause them to develop colic which could lead to death. Also the loss of the incoming neutral would cause all the exposed parts to rise up to 230V and kill the horses and maybe humans.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 24 December 2012 09:24 PM
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alancapon

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Originally posted by: Brunokid
. . . So why exactly don't we use PME? . . .

You need to remember what a PME "Earth" actually is. Effectively the supplier are unable to provide an earth connection for you, but have allowed you to connect your earth terminal to the System Neutral instead. The System Neutral is a current carrying conductor, and is subject to voltage rise with current flow, in exactly the same was as the phase is subject to a voltage drop.

It does not take many volts to kill a horse - a recent example being the problem at Newbury Racecourse in February 2011.

Regards,

Alan.
 24 December 2012 10:03 PM
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slittle

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Doesn't do Reindeer a lot of good either :-)

TT every time for any four legged beast in my book.

Remembering to keep the TT electrode well away from any PME electrodes



Stu
 25 December 2012 05:42 AM
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ebee

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I always feel happier when I see a good old TNS supply.
Although that might be a false sense of safety.

TNC-S (PME) does give me some concerns.

TT , in theory, appears safe but two things worry me 1/ Integrity/stability of earth rods and 2/ failure rate of RCDs.
Dual earth rods & cascading RCDs help to mitigate this a bit.

When the suppliers alter position of an existing TNS they often join E & N together and redeclare that supply as PME. Trouble is other existing supplies on the same branch still thought to be TNS have the possible problems of effectively being PME too.

John`s statement "Not all TN-C-S is PME but all PME is TN-C-S." might be referring to the good old PME/PNB debate (I`m still not 100% convinced either way on this - seems to revolve around the EN link being at star point immediately on transformer or a bit away or a bit further away or a lot further away or far away.).

Anyway , happy turkey day everybody

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 25 December 2012 10:04 AM
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daveparry1

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I sometimes wonder about those things too Ebee, also as I see quite a lot of TT in my area it amuses me a bit when I get say 60 ohms Ra and then see Zs's of around 0.20 ohms etc when the bonding is connected due to somewhere nearby being tncs and sharing a metal gas/water supply pipe!

Merry xmas to all,

Dave.
 03 January 2013 06:46 AM
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Al

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Originally posted by: alancapon

[
. . . So why exactly don't we use PME? . . .


You need to remember what a PME "Earth" actually is. Effectively the supplier are unable to provide an earth connection for you, but have allowed you to connect your earth terminal to the System Neutral instead. The System Neutral is a current carrying conductor, and is subject to voltage rise with current flow, in exactly the same was as the phase is subject to a voltage drop.

Q]Originally posted by: Brunokid

I think you would say they are not supplying a seperate earth to you, it is not correct to say that the neutral provided does not have a good connection to earth.

Also for TT what happens if the earth is broken as is often raised about what if the neutral is broken? The 120mm Neutral looks more robust than 6mm earth.

Also (again). What is the voltage on the earth conductor when connected to a 60 ohm earth rod?

food for thought


Regards,



Alan.
 03 January 2013 07:35 AM
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ebee

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what you really need to look at is not voltage of system earth to "true earth" but voltage of system earth compared to local earth and the gradient from local earth to "true earth" .

If you take, under fault conditions, the voltage at your rod or plate or tape and measure it per 100mm as you step away from it and make a graph you will have a picture of a voltage mountain (or voltage molehill) .

Giving the PD at difference wheelbases

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 03 January 2013 09:11 AM
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AJJewsbury

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Also (again). What is the voltage on the earth conductor when connected to a 60 ohm earth rod?

It depends on how much current is flowing.

On a TT system under normal circumstances, it'd be just leakage current - perhaps 10 or 20mA - so we might say around 1.2V.

On a TN-C-S system the "protective" conductor might have a much lower resistance (say around 0.1 Ohms), but it's often carrying a lot more current (as it's carrying the N current for the installation, perhaps other installations too within the distribution network) - if it was carrying 100A then we'd be looking at 10V above true earth - over 8 times as high as our TT example.

During a L-PE fault all bets are off of course, and touch voltages could well be lethal - that's when we rely on disconnection times (sub 0.4 or 0.2 s) for safety.
- Andy.
 04 January 2013 07:47 AM
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Al

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Originally posted by: AJJewsbury

Also (again). What is the voltage on the earth conductor when connected to a 60 ohm earth rod?


It depends on how much current is flowing.



On a TT system under normal circumstances, it'd be just leakage current - perhaps 10 or 20mA - so we might say around 1.2V.



On a TN-C-S system the "protective" conductor might have a much lower resistance (say around 0.1 Ohms), but it's often carrying a lot more current (as it's carrying the N current for the installation, perhaps other installations too within the distribution network) - if it was carrying 100A then we'd be looking at 10V above true earth - over 8 times as high as our TT example.



During a L-PE fault all bets are off of course, and touch voltages could well be lethal - that's when we rely on disconnection times (sub 0.4 or 0.2 s) for safety.

- Andy.


All bets cant really be off as this is the situation we want to design for and we dont just rely on time thats why the time permitted for the fault to persist is dependant on the touch/step voltage.

Al
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