Tn-C-S Or Tn-S

[revor]

I am somewhat confused by the earth arrangement to my property. I have a 3 phase supply to the outbuildings and a single phase to the house. I have had a new underground supply to the house (45M 16mm) replacing an overhead supply.( It cost me 850 notes and I did the trenching and laying the trunking and back filling).

At the pole where the supply comes onto the land there are the 3 phase cables a neutral and earth. There is a transformer on a pole some 80 M away on the roadside.

Where the house supply connects at the pole there are the 3 connections phase N and E. Where the supply terminates in the house I cannot make out the arrangement but the equipment installed by the board looks exactly like the arrangement in Fig 2.1 in the on site guide except that the earth coming out of the board's termination is not quite like that in the figure but my conclusion is that it nearer to a TN-C-S than a TN-S. The other point directing me in this direction is that a few years ago the house was rewired and on the EICC it states it is a TN-C-S arrangement. So why would the supply company do it in this way when there is a supply earth at the pole?. Cost of cable?

 

[apprentice87]

photos????

john..

 

[Andy™]

if it comes from overhead then it cant be TNS since there will not be a separate earth

 

[Sidewinder]

Not quite true Andy, I've seen at jobs I've gone to where we have had overhead TN-S 3 wire coming in.

Going to a site meeting with WPD, engineer possibly, and one of their line teams, where I am going to be asking for the same again TBH!

 

[Steve3948]

I am somewhat confused by the earth arrangement to my property. I have a 3 phase supply to the outbuildings and a single phase to the house. I have had a new underground supply to the house (45M 16mm) replacing an overhead supply.( It cost me 850 notes and I did the trenching and laying the trunking and back filling).

At the pole where the supply comes onto the land there are the 3 phase cables a neutral and earth. There is a transformer on a pole some 80 M away on the roadside.

Where the house supply connects at the pole there are the 3 connections phase N and E. Where the supply terminates in the house I cannot make out the arrangement but the equipment installed by the board looks exactly like the arrangement in Fig 2.1 in the on site guide except that the earth coming out of the board's termination is not quite like that in the figure but my conclusion is that it nearer to a TN-C-S than a TN-S. The other point directing me in this direction is that a few years ago the house was rewired and on the EICC it states it is a TN-C-S arrangement. So why would the supply company do it in this way when there is a supply earth at the pole?. Cost of cable?

It may be that the DNO are not happy with the earthing arrangement for various reasons, and cannot be relied on.

Should it be TT and it's been connected otherwise.

 

[Andy™]

Not quite true Andy, I've seen at jobs I've gone to where we have had overhead TN-S 3 wire coming in.

Going to a site meeting with WPD, engineer possibly, and one of their line teams, where I am going to be asking for the same again TBH!

didnt think they were allowed a separate E on overhead pole, only line(s) & neutral

 

[revor]

Thanks for discussion so far, have taken some photos. There are 5 lines overhead and one is an earth that is for sure. I have had to blow up an increase brightness on the connections on top of  the pole so hope you can make out the detail. The cable coming to the house makes a loop at the top and as it comes down you  can see 3 terminations.

 

[ProDave]

I would have an educated guess (as the connections are not that clear) that you have a very old and very unusual setup of mains electricity and telephone sharing the same pole and same type of insulators.

Look at the connections to the bottom 2 wires, VERY thin by comparison to the top 3.

I'd still put my money on TNCS, that looks like a concentric (i.e 2 core) incomer, and no other earth connection visible.

 

[Sidewinder]

I believe the pole is TPNE, as TN-S with PME.

The presentation at the cut out is TN-C-S, no PME sticker though.

 

[Andy™]

of the 5 wires on the pole, is the 2nd from bottom thinner than the others?

also looks like T&E connected between bottom and 2nd bottom cable too.

looks like a normal 5 wire pole with a switch wire for the street lights. so no separate earth, just L1 L2 L3 SL & N

as for the cutout, cant really say for sure. could be either, but usually if it was TNCS then the earth block would be on the side on the cutout

 

[revor]

Gosh did not think was going to be so difficult to ascertain what we have. Will try and take a better pic if lighting better tomorrow. Been watching DVD aimed at C&G 2391 and a mention in there is that the distributor should inform us of the declared values for prospective fault current and Ze for the particular earthing arrangement. Should they have provided us with this info when they connected up the supply? We got no documents on completion.

 

[Andy™]

an loop test L-N & L-E, or a continuity N-E would easily identify if its TNCS or TNS. or a phone call to DNO

 

[Sidewinder]

The presentation in the property MUST be TN-C-S as there is no separate E connection before the cutout, that is all we are concerned with.

Further up the DNO network it will be TN-C-S PME anyway, though that is not out concern.

You will always have N-E continuity, it is just the resistance that will vary, depending upon the distance to the first N-E link in the DNO network, if that is in the cutout, then the N-E resistance will be zero.

What does the "warning" sticker above the cutout actually say?

 

[apprentice87]

Hi Paul,

I think it says "interference with this installation is illegal" etc etc,,

john...

 

[steptoe]

AFAIAC it is presented as TN-C-S

that is what we are asked, not what it actually is, or might be, or we think it is,

we state what it is presented to us as,

 

[Sidewinder]

Exactly my point Steps, though you said it better like!

 

[steptoe]

Exactly my point Steps, though you said it better like!

:slap

 

[revor]

First of all many thanks for all who have helped with this query. Been ages getting back as had difficulty in getting to speak to someone at SP or just even pick up the phone. However have now had the info we need and that is a TNCS (PME) supply, PFC 16KA, and fault loop impedance 0.35.

Merry Xmas

 

[ProDave]

Those figures don't add up.

A loop impedance of 0.35 gives a PSSC of just 657 Amps

Were those figures by measurement? or just somebody's guess?

 

[kerching]

It is some monkey on the phone reading from the " shut the customer up quickly " screen script.

They have just quoted you the worst case scenario out of the book.

But what do i know, wife will be along shortly to correct me

Just sleighing