IET logo
 
IET
Decrease font size
Increase font size
Topic Title: TN-S or TN-CS any difference in practice ?
Topic Summary:
Created On: 19 June 2008 09:24 AM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 19 June 2008 09:24 AM
User is offline View Users Profile Print this message



broadgage

Posts: 1244
Joined: 07 August 2007

I am of course well aware of the supposed difference between TN-S (earth and nuetral seperate throughout) and TN-CS (earth and neutral combined up to the point of supply, and then seperate)

However on watching cable jointers working, I wonder if it acrually makes any difference in practice.

The supply to a small shop recently failed due to a defective joint between the main and the service cable.
The main was 4 core with an earthed sheath, and the service cable was 2 core with an earthed sheath.
The supply was restored by cutting away all damaged cable, and jointing with a short length of 3 core main, the earthed sheath and the neutral core of the existing cable being connected together to the combined earth and neutral of the new cable.
A short length of new single core concentric service cable was jointed to the main and to the short remaining length of the existing service cable.
The distance between the meter/service fuse and the joint was about 2M.
Does it really make any difference, if the earth and neutral are combined up to the service fuse, or up to a joint only 2M away?
 19 June 2008 10:09 AM
User is offline View Users Profile Print this message


Avatar for GeoffBlackwell.
GeoffBlackwell

Posts: 3425
Joined: 18 January 2003

Good question - our supply authority colleagues will tell us that TN-S distributors could be converted to TN-C-S at any time. (OK not in the IOM Alan ).

In the past they have indicated that the cable used for TN-S distribution is no longer available in quantity and so conversion is often the only practical solution when systems are upgraded or repaired.

So TN-C-S spreads its net and the much loved TN-S is rapidly disappearing - so what, does it really matter?
TN-C-S is here to stay so we had better get used to it .

Now on that point, how do you feel about TN-C - illegal is it not - but how many installations with long standing (like since they were constructed but not tested) neutral to earth faults (often 'dead shorts') are there in daily use.

One other point - the often quoted 21 ohms for a TT supply is actually for TT derived from a TN-C-S distributor (Engineering Recommendation P23 1981). If it is not a TN-C-S distributor the value could be much higher .

Regards

Geoff Blackwell
 19 June 2008 11:40 AM
User is offline View Users Profile Print this message



broadgage

Posts: 1244
Joined: 07 August 2007

I have met a TN-C instalation about 30 years ago at a large country house, the lighting circuits were in single core rubber insulated, lead sheathed cable, with the sheath serving as a combined neutral and earth.

I believe that the instalation was originaly supplied at 110/120 volt DC from private generating plant,AFAIK, TN-C is permitted used thus.
However when I saw the installation it was connected to the public supply at 240 volts!
The same building also had a number of three phase TN-C sub mains consisting of small flat three core rubber insulated lead sheathed cables with the sheath being used as a combined neutral and earth. Looked like about 2.5mm on 30 amp semi-enclosed fuses.
 19 June 2008 01:32 PM
User is offline View Users Profile Print this message



unshockable

Posts: 828
Joined: 18 May 2007

http://www2.theiet.org/Publish...uestions_answered.pdf
above is a Geoff Cronshaw article on earthing systems. "A TN-S system, shown in fig. 1 has the neutral of the source of energy connected with earth at one point only, at or as near as is reasonably practicable to the source.."

When I see a supply that presents as TN-S but I am stepping over the trench in the street outside do I treat and write it as TN-S or not? The DNO(EDF) often has no records or is unable to tell me. Am I worrying about nothing as the lead sheath will(?) keep the internal metalwork voltage down in the case of a broken neutral?

Discombobulated as Zs would say.

Simon
 19 June 2008 08:10 PM
User is offline View Users Profile Print this message



alancapon

Posts: 5698
Joined: 27 December 2005

Ok then, time to confuse everyone.

We are all used to TT, TNS and TNC-S as supply types.

I'll start with TNC-S. As everyone knows, the customer is provided with a connection to the System Neutral to use as their main earth. The Neutral will be earthed at the star point of the supply transformer, again at the end of the main, at or beyond the connection of the furthest customer on that cable. In addition, earths are added to the neutral along the main at appropriate points, to reduce the System Neutral to earth potential under normal conditions, and to limit the potential on the System Neutral compared to earth in the event of a break occurring on the Neutral conductor back towards the supply transformer.

With TNS, the star point of the supply transformer is earthed. A separate conductor is provided back to the supply transformer to connect to the customer's earth transformer. Virtually all TNS supplies are provided on lead mains, the lead sheath / steel armouring effectively acts like a large earth rod, equivalent to the length of the cable.

With TT, the star point of the supply transformer is earthed. No earth connection is provided to the customer.

I hope everyone agrees up to this point.

Now the confusion starts. There are variations of TT and TNS which use "MEN" - a Multiply Earthed Neutral. There are a number of reasons for doing this. On a longish overhead single-phase spur, earths may need adding to hold down the neutral potential (the same resistance that casuses "volt drop" on the phase will also give a rise in voltage on the neutral). This may also be required if the loading on a three-phase line is not well balanced. MEN may also be used in preparation for TNC-S. If ground conditions are poor at the supply transformer, MEN may also be required to reduce neutral to earth voltage if significant earth currents are flowing back to the supply transformer.

Whilst the use of MEN may look like TNC-S and to be honest it is certainly heading in that general direction, it is likely that the number and location of earths on the System Neutral do not comply with the much more stringent conditions required for offering TNC-S. This is one of the reasons that earthing type (apart from TT which is obvious) must be determined by enquiry, not by measurement.


Regards,

Alan.
 19 June 2008 08:18 PM
User is offline View Users Profile Print this message



intrinsic4225B

Posts: 1618
Joined: 30 September 2004

Aren't there also two flavours of TN-C-S, Alan?

-------------------------
Ross Currie TMIET
 19 June 2008 08:35 PM
User is offline View Users Profile Print this message



alancapon

Posts: 5698
Joined: 27 December 2005

Originally posted by: intrinsic4225B
Aren't there also two flavours of TN-C-S, Alan?


I can't think of a second, but then we don't do TNC-S at the moment.


Regards,

Alan.
 19 June 2008 08:39 PM
User is offline View Users Profile Print this message



intrinsic4225B

Posts: 1618
Joined: 30 September 2004

Alan,

I once once seen a reference to 'two forms of TN-C-S' in a publication I was flicking through briefly.

One was 'Protective Multiple Earthed (PME)' and the other was I think 'Protective Neutral Bond (PNB)'. I unfotunately cannot remember the details of the second system now.

I've never seen anything again on PNB, but then it was a reasonably old publication I was looking at, so it may now be obsolete?

-------------------------
Ross Currie TMIET
 19 June 2008 08:42 PM
User is offline View Users Profile Print this message


Avatar for GeoffBlackwell.
GeoffBlackwell

Posts: 3425
Joined: 18 January 2003

AFAIK PNB is usually only used for a single customer i.e. they are the only connection to the transformer.

Regards

Geoff Blackwell
 19 June 2008 08:54 PM
User is offline View Users Profile Print this message



alancapon

Posts: 5698
Joined: 27 December 2005

I agree with Geoff. PNB does exist, and is only used where a transformer exclusively supplies a single customer, where the overcurrent protection is provided at the customer's switchboard. In this case, to avoid unnecessary joints and to improve the earth loop impedance, it is permissible to make the transformer star-point to earth connection at the customer's end of the supply cable. It is therefore (I believe) a variation of TNS, as this will be the only connection to earth on the transformer secondary.


Regards,

Alan.
 20 June 2008 02:56 PM
User is offline View Users Profile Print this message



markengland

Posts: 32
Joined: 05 December 2005

Alan/Geoff,

Is PNB then a good candidate for the system in use at the installation referred to in this thread to which you both contributed?

Mark
 20 June 2008 03:28 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 18917
Joined: 23 March 2004

Yes - it would appear so.

As Geoff and Alan pointed out PNB is usually limited to a single consumer or where that consumer has ownership of the transformer (ie they receive thier supply at high voltage).

From the transformer output terminals there will be (as a minimum) 4 conductors ( 3 phases and neutral)terminating at the incoming ptotective device of the panel and at the neutral bar. There will be a dirty great link between the N and E bars - effectively the protective neutral bond.

After this point the system reverts to seperate earth and neutral. There will also be an earth bond to the transformer can to maintain potentials. Essentially all that has happened is the Star point connection to earth has been made at the switchboard not at the transformer.

The cables between the TX and the switchboard are usually protected by a restricted earth fault scheme or a form of unit protection. The protective neutral earth bond may also be a contactor (or in fact a series of contactors ) if the system is complicated by the use of several input transformers coupled with several generators (usually all but one bond is opened to prevent circulating earth currents)

Multiple earthed neutrals are also not that uncommon on private networks that would at first glance appear to be TT - large opencast sites or strip mining operations, although there would be a preference for IT systems generally


Regards

OMS

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

Edited: 20 June 2008 at 03:29 PM by OMS
 23 June 2008 02:49 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 10963
Joined: 13 August 2003

our supply authority colleagues will tell us that TN-S distributors could be converted to TN-C-S at any time.

Indeed. Given it seems impractical to check every consumer's installation in these circumstances, who's responsibility would it be if a house, correctly bonded to TN-S, requirements were to burn down due to inadequate main bonding CSAs when subjected to diverted N currents? (e.g. due to an open circuit CNE).
In the past they have indicated that the cable used for TN-S distribution is no longer available in quantity and so conversion is often the only practical solution when systems are upgraded or repaired.

But aren't cables produced to distributors' (collective) requirements? (how many other people use copper armoured single core aluminium cables?) Isn't that rather a self-satisfying prophesy, and therefore just as easily reversed, if they put their minds to it?

- Andy.
 23 June 2008 03:32 PM
User is offline View Users Profile Print this message


Avatar for GeoffBlackwell.
GeoffBlackwell

Posts: 3425
Joined: 18 January 2003

Indeed. Given it seems impractical to check every consumer's installation in these circumstances, who's responsibility would it be if a house, correctly bonded to TN-S, requirements were to burn down due to inadequate main bonding CSAs when subjected to diverted N currents? (e.g. due to an open circuit CNE).


I don't know, but I would say that such an occurrence would be rare. Open circuit supply TN-C-S neutrals are rare enough, and I have not heard of any reports of serious damage. If it happened it would difficulty to verify the cause.

But aren't cables produced to distributors' (collective) requirements? (how many other people use copper armoured single core aluminium cables?) Isn't that rather a self-satisfying prophesy, and therefore just as easily reversed, if they put their minds to it?


No doubt this was true when the decision was made to use cables constructed from more modern materials but it would be difficult to source cables constructed to old designs now. New cables could be made that reproduce the same features but I expect this would be far too expensive.

The decision to use TN-C-S was made a long time ago and whether we agree with it or not 'we are where we are'.

TN-S is not some wonder system, it has disadvantages - one being that the earth path can fail and this could go unnoticed for years. Customers tend to complain if a TN-C-S neutral fails .

There are those who 'hate' TN-C-S with a passion - I have worked with them . I could never see the problem myself, after all - any system you care to mention has advantages and disadvantages.

One such person was instrumental in having TN-C-S banned from petrol filling station forecourt supplies. The reason - he was concerned about stray neutral currents and the like.

Well is banning TN-C-S the answer to this problem. Discuss

Notes:
1) Many forecourts are close to buildings that have steel frames and are supplied via TN-C-S distributors.
2) The person I am referring to was the same one that - during my job interview - convinced me that we should bond lightning protection systems because we could not guarantee that they would be separate from our installation bonding.

Regards

Geoff Blackwell
 23 June 2008 09:51 PM
User is offline View Users Profile Print this message



chrishaworth

Posts: 247
Joined: 08 March 2003

My standard system used to be, check the Ze then interchange the earth and neutral probes and check again. If it was lower, then it's TNS, if it's the same, TNCS. However, the LEC is now undergrounding at a great rate and Ze readings are changing almost daily, so I'm sticking everything down as TNCS with the greater fault current reading as both PFC and PSCC and the lower loop reading as Ze. Shoot me down in flames.

Cheers
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.