Discuss TN-C-S or TT? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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phawk

Could anybody help me with this?

I'm starting a re-wire of an old property that has an overhead incoming supply. I've pulled the main fuse and there is a live & neutral supply, then the earth is taken from the neutral block (TN-C-S, right?). The problem I have is that I spoke to another electrician who seems to think that this shouldn't be the case if the supply is coming from overhead, as the system would need an earth rod, making it a TT system.

I've done a Ze which comes in at 0.20 ohms, and a PFC & PSCC which both come in at 1.17kA, which are well within the limits for a TN-C-S system.

So.......is the system earthed or not? Seems to me that it is but I want to make sure.

Also, the property has no gas (it has a stand-alone oil supply for the central heating). Where do I bond this?

Thanks a lot,

Paul
 

topquark

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Could well be TNC-S, check with the DNO if it is supposed to be available in that area.

Test to confirm it needs to be bonded then bond back to the MET the same as any extraneous incoming service.
 

stef

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If it is an overhead supply and TT then there must be a tranny somewhere down the line, on a mast or something. But your test results point more to a TNC-S.
 

telectrix

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it sounds very much like TNC-S. why would the guy think it needed a rod at the property? he needs to study his earthing diagrams. bond oil pipe where it enters the building ( assuming it's metal pipe.)
 

Guitarist

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Plenty of overhead supplies are TN-C-S mate, and pretty good ones too. Yours may well have been TT once upon a time, but for the last 20 years DNO's have been converting many on request.
Earth from neutral block and Ze of 0.2 ohms.... No-brainer really.
 
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Engineer54

PME was first introduced in the UK to provide a reliable earthing system, for the then many rural overhead supplies. You can check for yourself, if the overhead system supplying this property has been PME'D or not. Walk down the poles a few hundred metres, if PME'd you will see that at every 3rd or 4th pole, a N-E grounding cable that's going down to an earth rod, will be clearly present.

Overhead PME systems are in essence the purest form of PME you will find. Also if you have an earth rod on the property leave it in place, and don't disconnect from the installations MET. Look at it as just another PME earthing point!!!
 

Archy Styrigg

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PME was first introduced in the UK to provide a reliable earthing system, for the then many rural overhead supplies. You can check for yourself, if the overhead system supplying this property has been PME'D or not. Walk down the poles a few hundred metres, if PME'd you will see that at every 3rd or 4th pole, a N-E grounding cable that's going down to an earth rod, will be clearly present.

Overhead PME systems are in essence the purest form of PME you will find. Also if you have an earth rod on the property leave it in place, and don't disconnect from the installations MET. Look at it as just another PME earthing point!!!
I go over to the West of Ireland once or twice a year, very rural!
I've noticed that a lot of the O/H supplies appear to be quite new, so I take it they'll be mostly TNCS?
Might take my meter over this time...I need to get out more!
 

Risteard

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I go over to the West of Ireland once or twice a year, very rural!
I've noticed that a lot of the O/H supplies appear to be quite new, so I take it they'll be mostly TNCS?
Might take my meter over this time...I need to get out more!
Supplies in the south of Ireland are almost all TN-C-S although they must be "neutralised".
 

darkwood

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Ive seen plenty terraced shared overhead supplies converted to tncs .... just had to get DNO out to allow me to connect to the newly provided earth on a board upgrade that had a earth rod existing from the tt days.
 

Archy Styrigg

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I did a quick google and found this...

From Wikipedia (I know, not a definite source!) for ESB, Irish supply company:

Earthing/Grounding System

  • TN-C-S is preferred. This is referred to in Ireland as a "Neutralised" supply. Most homes and businesses are connected to such a system.
    Where this system is used, all metal pipe work must be bonded i.e. connected to the system's earth/ground creating an equipotential zone.
In such an installation earth straps and yellow & green earth wires can be seen connected to pipework under sinks, in heating systems etc.

  • TT system is used where TN-C-S earthing is not possible due to the design of the local distribution network, or the geology. This system is referred to as 'unneutralised' or 'not neutralised'.
Where a TT network connection is used, the entire supply must be protected by an RCD
Converting a TT supply to a TN-C supply "Neutralisation" may only be carried out by an ESB engineer with the approval of ESB Networks. Under no circumstances should a home owner, or an electrical contractor make any connection between the Earth/Ground and Neutral. To do so inappropriately could expose people to a risk of serious shock or electrocution.

Is it just an Irish way of referring to TNCS/PME? They do have their own ways! :)
Hopefully our man will come back and clarify.
 
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E

Engineer54

From Wikipedia (I know, not a definite source!) for ESB, Irish supply company:

Earthing/Grounding System


  • TN-C-S is preferred. This is referred to in Ireland as a "Neutralised" supply. Most homes and businesses are connected to such a system.
    Where this system is used, all metal pipe work must be bonded i.e. connected to the system's earth/ground creating an equipotential zone.

In such an installation earth straps and yellow & green earth wires can be seen connected to pipework under sinks, in heating systems etc.


  • TT system is used where TN-C-S earthing is not possible due to the design of the local distribution network, or the geology. This system is referred to as 'unneutralised' or 'not neutralised'.

Where a TT network connection is used, the entire supply must be protected by an RCD
Converting a TT supply to a TN-C supply "Neutralisation" may only be carried out by an ESB engineer with the approval of ESB Networks. Under no circumstances should a home owner, or an electrical contractor make any connection between the Earth/Ground and Neutral. To do so inappropriately could expose people to a risk of serious shock or electrocution.



Is it just an Irish way of referring to TNCS/PME? They do have their own ways! :)
Hopefully our man will come back and clarify.
Thanks for clearing that up ...lol!!!

Supplies in the south of Ireland are almost all TN-C-S although they must be "neutralised".
Though one has to wonder what a Southern Irish TNC-S system is called, if it isn't as they put it ''Neutralised''?? lol!!
 
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SPARTYKUS

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If you're used to working in cities for years then work somewhere that has alot of overhead Tncs (like Stroud, for example which is very hilly) it can throw you, experienced spark or not.
 
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Engineer54

If you're used to working in cities for years then work somewhere that has alot of overhead Tncs (like Stroud, for example which is very hilly) it can throw you, experienced spark or not.
In what way??
 

SPARTYKUS

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Good question. Just does. I didn't comment on said sample of electricians and their brain function.

Okay, I meant in the way that there are lots of things in the regulations and I am quite happy to admit I am rusty on things I do not spend time on (like Marinas).

But, having thought about it, should you now retort with something along the lines of "well basic knowledge of earthing systems should not throw any electrician", then I agree with you.

I only meant it can throw you for a moment or two. As a result of lack of familiarity.
 
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OP
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Jonny66

I've done a Ze which comes in at 0.20 ohms, and a PFC & PSCC which both come in at 1.17kA, which are well within the limits for a TN-C-S system.

So.......is the system earthed or not? Seems to me that it is but I want to make sure.


Paul
Thats what a Ze test shows (if done correctly) that you have an effective earth!!!!!!!!! tis the whole point!!!!!!
 
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sjm

Quite often the poles will have a large red PME sign on them, tends to be a bit of a giveaway really! lol
 

Risteard

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Though one has to wonder what a Southern Irish TNC-S system is called, if it isn't as they put it ''Neutralised''?? lol!!
Sorry for the delay in getting back to this thread, but my understanding is that all TN-C-S "neutralised" supplies in the south of Ireland must have an earth rod fitted as well. Unfortunately I don't have a copy of the Electrotechnical Council of Ireland's (ETCI) ET 101 ("National Wiring Rules") to verify.
 

Cbigfoot

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Depends where you took the ze from. If you took it with the earth connected to the neutral of the service head then you will get a low reading wherever you are and what ever supply you have as you are simply getting a reading through the phase and neutral conductors. This readin alone will not confirm a tncs supply.

Tncs supplies willnot have old cables on them. They require crmped connections, so the old cables with soldered connections would be removed. A pvc concentric service is usually a good sign that it has been upgraded.

I recently rewired a house that had a tncs service but had been left as a tt by the dno when the service was upgraded because there was no bonding.
 
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Engineer54

Sorry for the delay in getting back to this thread, but my understanding is that all TN-C-S "neutralised" supplies in the south of Ireland must have an earth rod fitted as well. Unfortunately I don't have a copy of the Electrotechnical Council of Ireland's (ETCI) ET 101 ("National Wiring Rules") to verify.
That's basically a very good standard to have, it's very much the same in most Western European countries too. Shame they don't insist on the same in the UK.
 

Guitarist

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Well, I have just had my in-laws' cottage (overhead supply) changed from TT to TNC-S by UKPN and we now have a Ze of 0.33 ohms. I am going to leave the earth electrode connected also, which is a stable 70 ohms (measured 22 ohms with oil and water bonding connected).
 
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E

Engineer54

Depends where you took the ze from. If you took it with the earth connected to the neutral of the service head then you will get a low reading wherever you are and what ever supply you have as you are simply getting a reading through the phase and neutral conductors. This readin alone will not confirm a tncs supply.

Tncs supplies willnot have old cables on them. They require crmped connections, so the old cables with soldered connections would be removed. A pvc concentric service is usually a good sign that it has been upgraded.

I recently rewired a house that had a tncs service but had been left as a tt by the dno when the service was upgraded because there was no bonding.
Well that's not completely true at all. So what do you think happens when a TN-S supply is converted to TNC-S/PME??
They will only use crimped connections when a joint is replaced, they certainly aren't going to replace every joint on the distribution supply run before converting a households service head. It maybe several years before a converted distribution cable is fully PME'd, and is more likely than not, supplying both TN-S and TNC-S during that period.

By the way, there is nothing wrong at all, with soldered joint connections, they have been around long, long before crimped connections, they are as sound as any crimp connection too. There are very few circumstances where soldered terminations are not advised or permitted...
 
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Edd

Well that's not completely true at all. So what do you think happens when a TN-S supply is converted to TNC-S/PME??
They will only use crimped connections when a joint is replaced, they certainly aren't going to replace every joint on the distribution supply run before converting a households service head. It maybe several years before a converted distribution cable is fully PME'd, and is more likely than not, supplying both TN-S and TNC-S during that period.

By the way, there is nothing wrong at all, with soldered joint connections, they have been around long, long before crimped connections, they are as sound as any crimp connection too. There are very few circumstances where soldered terminations are not advised or permitted...

Funny thing was I was also told that Only crimped distributions systems would be converted to PME. From EEB (as you know), was quite a few years ago though!!
Also every 3rd or 4th pole as you said in a previous post to be staked. If they are working on an area and having to fit all these stakes at the poles, It would make sence to sort the joints out at the same time. Saying that I have seen many soldered overhead PME systems, so maybe they have changed their rules or its been converted DIY.

Edit,, I think it was the old U bolt type joints they were more on about
 
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OP
E

Engineer54

Funny thing was I was also told that Only crimped distributions systems would be converted to PME. From EEB (as you know), was quite a few years ago though!!
Also every 3rd or 4th pole as you said in a previous post to be staked. If they are working on an area and having to fit all these stakes at the poles, It would make sence to sort the joints out at the same time. Saying that I have seen many soldered overhead PME systems, so maybe they have changed their rules or its been converted DIY.

Edit,, I think it was the old U bolt type joints they were more on about
Can't say i've seen a soldered joint on an overhead system. All the joint's connections i've seen on such installations have been via multiple clamp connectors. On some of the very old overheads it was by means of splicing (a lost art form) and clamps.

Sorry if it wasn't too clear, but I was talking about underground cables (PILC and the like) that are being converted from TN-S to TNC-S/PME Where every joint will most certainly be soldered connections unless it's been repaired/replaced in recent years. Now converting every joint at the same time in those circumstances, would be both time consuming, very costly and be an intolerable inconvenience for those customers connected to that distribution line...
 

Geordie Spark

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Well that's not completely true at all. So what do you think happens when a TN-S supply is converted to TNC-S/PME??
They will only use crimped connections when a joint is replaced, they certainly aren't going to replace every joint on the distribution supply run before converting a households service head. It maybe several years before a converted distribution cable is fully PME'd, and is more likely than not, supplying both TN-S and TNC-S during that period.

By the way, there is nothing wrong at all, with soldered joint connections, they have been around long, long before crimped connections, they are as sound as any crimp connection too. There are very few circumstances where soldered terminations are not advised or permitted...



Thank you !!! :):):):)
 
OP
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Edd

My typo above,, I did try edit it... One too many beers I think! Yep Beer and the Electricians forum makes for intresting reading :)
 
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