# TNS system



## bennysecond

Which system is the standard now in new residential construction in UK?
1:Is that TNS or TNC-S. 
2:Is it required to have main RCD or only moist places RCD (30mA)?
3:Is the neutral wire (IEC = blue color) also protected by breaker?

Thanks,


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## frank

Hi Benny

TNC (pme) is the prefferd standard but it is not a gauranteed option. Much depends upon the supply company. 
RCD (rcbo) are mandatory at the upstream terminal.
The neutral of the main incomer together with the live must be protected as also must be the same cables if a local upstream local bus decice is chosen.

Good practice currently is to install a seperate rcbo (rcd with integral current overload to each circuit leaving the distribution board.

Frank


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## bennysecond

Thanks for info.

Ad. 1: TNC really, Are you sure it is not a TNC-S.
How can you use RCD on TNC? 
Ad. 2: So I assume that you have double breaker on f.e. 220V outlets circuit:
http://www.google.ca/imgres?q=Legra...143&start=0&ndsp=31&ved=1t:429,r:17,s:0,i:133

and 4P when you have some 3 phase unit with 220 necessary:
http://www.yeniiletisim.com.tr/?act=urunDetay&id=177&/legrand-605673-xg-6ka-c-curve-4p-32a-mcb


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## great68

frank said:


> Hi Benny
> 
> TNC (pme) is the prefferd standard but it is not a guaranteed option. Much depends upon the supply company.
> RCD (rcbo) are mandatory at the upstream terminal.
> The neutral of the main incomer together with the live must be protected as also must be the same cables if a local upstream local bus device is chosen.
> 
> Good practice currently is to install a separate rcbo (rcd with integral current overload to each circuit leaving the distribution board.
> 
> Frank


 
Most of this is incorrect.
There is no such thing as TNC as a service. A TNC-S supply is normally provided nowadays. I dont think TNS is ever offered anymore. Some new supplies are still TT
RCD protection is NOT mandatory on any TN supply, but it is on a TT
Any current model BS(British Standard) RCD is a double pole device but BS RCBO's (Combined overcurrent & Rcd) are generally single pole.
The neutral incomer must NOT be protected, other than the fact a RCD monitors the flow through L & N and will break both conductors. Many years ago the neutral would be fused, but that is banned now. 
There are many arguments here between electricians that say a TT supply must have double pole RCD protection, but that is not in the regs

Various regulations for different purposes say you have to fit 30ma RCD protection, so most domestic installs have this done. But it is not always needed. When needed the regs say you have to fit more than one RCD (to avoid nuisance tripping) so most cheapskates fit 2. I try to fit RCBO's to each circuit, but many times its to expensive.

For anyones info,
TT is L & N supply only and you have to supply your own earth (Ground) 
TNS is ( but not normally available for new service) L, N & E supply with all 3 conductors returning to the local substation.
TNCS is a L, N & E supply but the earth is joined to the neutral at the supply head/main cut out in the property, so only a 2 wire supply. A TNCS supply is derived from a PME (Protective multiple earth) service where the neutral is staked and also jointed to the neutral at multiple points


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## great68

bennysecond said:


> Thanks for info.
> 
> Ad. 1: TNC really, Are you sure it is not a TNC-S.
> How can you use RCD on TNC?
> Ad. 2: So I assume that you have double breaker on f.e. 220V outlets circuit:
> http://www.google.ca/imgres?q=Legra...143&start=0&ndsp=31&ved=1t:429,r:17,s:0,i:133
> 
> and 4P when you have some 3 phase unit with 220 necessary:
> http://www.yeniiletisim.com.tr/?act=urunDetay&id=177&/legrand-605673-xg-6ka-c-curve-4p-32a-mcb


 
You have shown European breakers not generally found in the UK

Single pole devices are all thats needed for a single phase supply. The local isolation must however break all conductors (not earth) with a min of a 3mm gap.
4 pole breakers are not used. A 3 phase service would use a 3 pole breaker, You should not break the Neutral. 4 pole are needed for a 3 phase RCD of course as the N is monitored


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## great68

This is a good UK website with lots of pictures and technical drawings, http://www.tlc-direct.co.uk/Main_Index/index.html

For circuit breakers click on consumer units ( the stupid name given to a single phase domestic distribution board) and you will see many breakers. Note that all RCBO's say double pole because the neutral passes through, but they only break the Live conductor.
The majority of domestic supplies are single phase, 230V, 60-100A. I have recently finished 3 large houses, both with 3 phase 400V, 100A per phase

Edit, I was refering to single module RCBO's as being single pole switching, 2 module units generally are double pole although seldom used. Double pole switching, single pole rcbo's are just being introduced now.


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## bennysecond

Thanks for info,

PS. I always thought that UK is in Europe. Nice to learn new geography. :thumbsup:


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## great68

bennysecond said:


> Thanks for info,
> 
> PS. I always thought that UK is in Europe. Nice to learn new geography. :thumbsup:


Mmmmmmm, Technically it is.


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## frank

Great68

You must be a 'young wipersnapper' Quick to correct!An over reaction on your part My use of the term TNC(pme) would be read by all, including yourself to refer to TNC-S (pme) and is the common usage term amongst all my electrician pals. There is no other terminology begining with TNC that it could be confused with.
Use of RCD devices is mandatory but with exceptions.Check the 20amp rule.

In respect of the main incomer supply we are not refering to supply company mains heads but to the main incomer isolator within the distribution board - which must be 2 pole as must any local bus residual device feeding downstream circuits.
TNS supply systems are installed regularly by supply companies. I work in the industrial sector of the contracting industry and such supplies would be common above 60Kva


Frank


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## great68

frank said:


> Great68
> 
> You must be a 'young wipersnapper' Quick to correct!An over reaction on your part My use of the term TNC(pme) would be read by all, including yourself to refer to TNC-S (pme) and is the common usage term amongst all my electrician pals. There is no other terminology begining with TNC that it could be confused with.
> Use of RCD devices is mandatory but with exceptions.Check the 20amp rule.
> 
> In respect of the main incomer supply we are not refering to supply company mains heads but to the main incomer isolator within the distribution board - which must be 2 pole as must any local bus residual device feeding downstream circuits.
> TNS supply systems are installed regularly by supply companies. I work in the industrial sector of the contracting industry and such supplies would be common above 60Kva
> 
> 
> Frank


 
For your information i am 53 & have been in electrical contracting since i was 16. I still am learning & occasionally may make a mistake but this time its you that is wrong.
Remember Bennysecond asked about domestic supplies & i think you have got confused due to your normal industrial work, i have never seen a 60Kva domestic supply & i doubt you have. As stated in the regs there are TT or TN supplies but no TNC which could have meant TNS or TNCS , so i & Bennysecond did not read your TNC as being TNCS & both of us commented on it, confused ?
TNC & PME are distributors designations, not to be used on the consumers side. PME is not TNC-S.
RCD protection is NOT mandatory & never has been except when its a TT supply, Please give me the regulation numbers to back this up ?
I work to BS7671:2008 Amendment No1:2011 & so should all electricians in the UK. 
I have never heard of the 20A rule, are you confusing it with the need to RCD protect socket outlets under 20A ? which there are many exceptions to and NOT mandatory. 
Regarding the switching/breaking of the neutral conductor. I agree the main isolation switch has to break all conductors but the OP said breaker, not switch & there are no regulations to say this has to be double pole. In addition, if an installation needs RCD protection more than one RCD has to be fitted so this can not be a main switch. Again please tell me the regulation number ?
Your terminology 'Local bus residual device' is not something i have ever heard used which makes me think you rarely get involved with domesic new builds, which is what the OP asked about. 
In saying all of this i still think RCD's are a worthwhile addition and nearly always install them on domestics,even when not needed for the regs, but i 
must stick to BS7671 & not what i prefer when answering.


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## frank

Great

We have covered a lot of print space to come to this! I answered Bennys first question by saying TNC rather than TNC-S. Ok but there can be no confusion with any other system since there is no other designation begining 'TNC'. I am sure you use the term in your daily work. Since you wish to be technically precise then TNC-S is the correct designation.

I told benny that RCD units are mandatory. Unfortunatly they are. The regs are written in positive format (must be obeyed).The negative form follows and offers exemptions as to why the regulation may not be obeyed. Reg 411.3.3 staes that sockets having a rated current not exceeding 20 amps must have 30mA residual protection. Since we are talking about a domestic property that will have such outlets the rule is mandatory.

I said also that the neutral must form part of the protected circuit together with the live at the main incomer. This being the DP isolator protecting any sub circuits. Not the supply company head. I went on to include any local residual devices fixed to a bus bar feeding individual circiuts.

This ends the matter for me so we may have to disagree on this one. I may add that I never overstate an answer or give more that is asked and this may sometimes lead to misunderstanding,

Frank


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## great68

Benny asked if it is required to have a main RCD or only in moist places, You replied yes its mandatory which unfortunately is wrong.( I will answer that later)

Lets agree to disagree on your mandatory stand on final circuits as you have now posted 'mandatory with exceptions' (which does not make any sense at all)
If you wish to discuss RCD protection of individual circuits or areas, You mention 411.3.3 but fail to quote, 'exceptions (a) socket outlets for use under the supervision of skilled or instructed persons' (Well thats my house !)
or '(b) a specific labeled or otherwise suitably identified socket-outlet provided for connection of a particular of equipment'.
I carried out a house rewire for a man with OCD & every outlet was for a pacific use and labeled as such. There were no general use outlets as he said they were not needed. That job (was a pain in the neck) did not need RCD protection under (b) We did fit RCD's anyhow to my house and this job, but it was not mandatory.
Also BS7671 allows for departures, see 120.3. & BS7671 is not a statutory document



Back to the OP



bennysecond said:


> Which system is the standard now in new residential construction in UK?
> 1:Is that TNS or TNC-S.
> 2:Is it required to have main RCD or only moist places RCD (30mA)?
> 3:Is the neutral wire (IEC = blue color) also protected by breaker?
> 
> Thanks,


My opinion 
1) There is no standard but TNCS is the norm. TT is supplied on some jobs and TNS very rarely now
2) Only on a TT supply. The regulations say you should have more than one RCD so normal procedure is to fit a duel RCD board that has a main switch feeding 2, 30ma RCD'S or a main switch feeding 30ma RCBO's. The regulations do translate, for various reasons that the majority of domestic circuits need RCD protection so nearly all domestic jobs now have them.
Any circuits in 'special locations' such as bathrooms, kitchens, detached outbuildings need RCD's. Cables buried less than 50mm in walls and not mechanically protected need RCD's. Socket outlets not exceeding 20A need RCD's (see above )
3)I assume this breaker is the RCD ? RCD are double pole so yes. Even though RCBO's are generally single pole switching the neutral is being monitored so any short circuit to L or E or N should trip the circuit off. As most RCBO's are single pole it can cause problems (although rare) tripping RCBO's on other circuits & also can be time consuming when fault finding.

Thats the best i can help.


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## bennysecond

Thank you guys. I've got the picture.:thumbsup:


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## Mike D

As an American, this thread is fun. I am reading it with an English accent!


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## great68

Mike D said:


> As an American, this thread is fun. I am reading it with an English accent!


Although i find it fun reading the American stuff i cant get the accent yet :blink:


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## frank

I did say the 'last word' but you went on to give another treatise on the regulations. But technical aspects apart as I mentioned regulations are mandatory ( Positive) In the nature of things there will be reasons that obeying the law is not practical and for all reasonable purposes there will be exeptions ( Negative) Regulations require that socket outlets have residual protection. Positive/mandatory. But such protection is not always required where certain applications apply. Negative. In answer to the question therfore - rcd protection for sockets is mandatory. That you may have exceptions to this rule is of no consequence. It is against the law to ride a motor bike without a helmet. Mandatory. If you have religous reasons for not doing so you have an exception.Negative The answer to the question ' is the wearing of a helmet on a motor bike mandatory - then yes. It's the same thing. Unless of course we go on and on for hours giving more information that is asked for.Benny must be having great fun following this thread. BS7671 is as you say not a statutory document but it falls under the guidance of Specific Statutes and is by default mandatory. I know of no company that would depart from its guidlines for fear of falling foul of the law. And if it is not a default lawful requirement why bother with BS7671 at all? So if I am asked is it mandatory to follow BS7671 then I say yes to this answer too.


Frank


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## great68

Keep digging your hole Frank. 
The OP asked '2:Is it required to have main RCD or only moist places RCD (30mA)?' You replied 'RCD (rcbo) are mandatory at the upstream terminal.' WHICH IS WRONG. 

You, then mentioned sockets, nothing to do with the question. You then follow up with ramblings about motorbikes and laws which is of no significance as the regulations are NOT STATUTORY where as the motorbike ones are.


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## frank

I have a head. All I now need is a wall. See you perhaps further along some other post

Frank.


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## great68

Here you go









Good luck :thumbsup:


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