TN-S outside - but TT inside UPDATE -New Pictures

[CAWORK]

Second thread of the day!!

I have often been puzzled regarding the earthing of my parents cottage.

It is supplied by an overhead supply from a transformer serving three properties (including my parents)

It is a very long overhead line and my parents home alone is supplied by six poles, the other two properties are right in the vicinity of the transformer.

In 1990 the cable drop from the pole to the house was installed to replace three separate cables from the pole to the house (a short length of these redundant cables are still inside the house in the wall). The incoming cable appears to be a split-con cable and on the left hand side of the cutout where the earth block would be is a plastic cover (it 'wobbles' a bit....I know you shouldn't, and was wondering if this might have been a cover that slides off, but that is not possible now due to the consumer unit been in the way)

When the cable was replaced there was an old 3036 board and the old cutout together with the meter was directly below it. When the new cable was installed, the cutout was put in it's present position and the earth cable from the 3036 board was left connected to nothing! (not long enough to be connected to earth on new cutout)-house not earthed for a few years!.

We had a new consumer unit installed some time later (16th edition 100mA Time Delay RCD and 30mA RCD) and an earth cable was installed to a rod.

My main question is though, is the earthing house meant to be TT or TN-S?

A few other things I had thought about include the overhead line is very long with the earth been thin, so although you won't know the impedance for sure without testing, in the case of a 30A 3036 fuse protecting a ring circuit, I have wondered how long would it actually take to disconnect in an earth fault?

I'm posting (or trying) to post some pictures to help.

Many Thanks

 

[AdrianUK]

I'd be very surprised if the three conductors on the lines were live, neutral & earth. Never, ever, seen TN-S on a overhead. The cut-out doesn't have a PME label so it isn't TNC-S (rare with overheads but can be done with ABC cable). I suspect that when the feed was replaced it was intended that the customer supplied his own earthing arrangement but that never happened...most people seem to think that if the lights work then nothing can be wrong!

More likely to be split-phase with two 'live' conductor 180 deg apart & a neutral. It looks like there is a second fuse on the opposite side of the transformer pole? If there is then its highly likely to be split-phase. I suspect closer examination of the pole nearest the house will reveal that one of conductors isn't connected. Did the house ever have storage heating?

To answer your question... I think its supposed to be TT.

 

[CAWORK]

Hi, thanks for your reply.

If you go and look closer at the transformer, (couldn't get too close, area now private land lol) there is a link between the middle and bottom ( thinner) cable.

Also, if I look up to the pole supplying the cable drop with binoculars I can see three 'cables' red top black middle and what looks like bare copper to the bottom.

The house has never had storage heaters.

I was speaking to an electrician a while back who actually claims he has seen an overhead TN-S before, but quite rare.

Regards

 

[Screech20]

What area are you in? Can’t see from the DOD sign.
It’s a single phase tank. But I too have never seen tns on the overheads.

Your lead-in appears to be concentric, there’s no pme label or an earth coming out of the neutral block. So it should be a TT stystem.

If the EFLI is <0.35 then your DNO would probably ‘give you’ pme, but why would you?
TT is a much safer system, especially on the overhead network.

 

[CAWORK]

Hello,

I decided to add a few extra pictures (although I acknowledge the quality isn't great, like the previous photographs, since they were taken with my phone).

If you look at the image 'House Pole' and look carefully you can see a red cable (although fairly faded) at the top, a black cable, actually strands, in the middle and bare copper strands at the bottom (this is much clearer if I look up at the pole with binoculars)- possibly suggesting a 'split-concentric' cable?.

I have also included a closer view of the transformer. Looking from left to right, the three cables on the left go to another pole nearby, with a cable of the same appearance as that to my parents property in the image 'House Pole'.

The three cables on the right to the transformer, go to the pole in the first post (image 20200203) with one set of cables going to another nearby property (still served by three actual individual cables going from the chimney stack into the property) and the second set of cables going to the poles serving my parents property.

If you look carefully/zoom in to the image 'Other Side of Transformer' you can see a wire link between the middle cable and the thinner bottom cable.

When I had a look at the transformer pole, there is just the one fuse on it, in the grey enclosure, a cable from the transformer going into it, then a cable going out of it to the top cable, and from the other side of the transformer is another cable going to the middle cable (which has the wire link, shown in tho photo going to the bottom cable, from the bottom cable is a cable running, in some form of ducting/trunking, to the ground).

I have also include a clearer photo of the overhead line serving my parents property.

I am fairly sure, that when I was looking at a copy of BS 7671, many years back, a TN-S system was defined as having 'separate Neutral and Protective Conductors throughout, with the Neutral earthed at the source of energy only' and the earthing in the consumers installation being provided typically from the metallic sheath of the incoming distributors cable OR a separate conductor.

Many thanks, in anticipation for any further help given.

Regards.

 

[JohnW2]

I am fairly sure, that when I was looking at a copy of BS 7671, many years back, a TN-S system was defined as having 'separate Neutral and Protective Conductors throughout, with the Neutral earthed at the source of energy only' and the earthing in the consumers installation being provided typically from the metallic sheath of the incoming distributors cable OR a separate conductor.

Indeed, the 'S' of TN-S merely means that the 'protective conductor' enters the supplied premises separately from the neutral. Whilst, in practice, that 'separate conductor' is almost invariably the cable sheath, there is, as you imply, no reason why it cannot be a separate conductor, whether part of a multi-core cable or totally separate - not that I have ever seen that done. If a sheath providing a TN-S earth fails, DNOs usually react by changing it to TN-C-S (not by re-establishing the failed 'S')

I currently have, and have previously had overhead supplies, and have seen countless others in the rural areas I live in and frequent, but I have certainly never knowingly seen one which appears to provide 'a TN-S earth' - installations with overhead supplies have, traditionally, invariably been TTd, but they also can (and, I think, increasingly are) be TN-C-S (I have been offered that - and declined the offer!).

Kind Regards, John

 

[ericmark]

I was told it is up to the DNO what they supply, and what your earthing system is, the idea is where two houses are close together the items earthed outside on one house should be same voltage as neighbours so you can't touch two earthed items of a different voltage, clearly with the house in question there is no chance of this, however it is still up to the DNO to tell you what earthing system you should use, so answer is simple, ring them and ask them.

I would suspect TT however it does seem there were three cables, so maybe some time in past it was TN, however I have seen where old cables enter the house in Mid Wales which seems odd at first glance, but I know reason was when I was a lad it had a 110 volt DC supply from generator in the village a mile away. So you have no idea if there was a third wire when supplied with 230 volt AC or if the third wire is some thing from very distance past.

 

[CAWORK]

Afternoon All, Thanks again for your replies.

In this near surrounding area, there are a fair few of these transformers around with the wire link between the middle and bottom cable - if I can get a chance to photo another one, I will.

The cottage has always been in the family and apparently the overhead line was installed in the early 1960s (the first time the cottage, and probably other two properties, had electricity installed.

Just to say as I previously did though, when the black cable was installed in 1990 with the cutout moved to it's current position, a piece of earth cable was left unconnected from the old 3036 board, which if I remember rightly, had a small 'pinch' mark in it, suggesting it had been previously connected to something, this bare wire end was where the old cutout was sited.

I will try and find out more information when I can

Regards

 

[JohnW2]

I was told it is up to the DNO what they supply, and what your earthing system is, the idea is where two houses are close together the items earthed outside on one house should be same voltage as neighbours so you can't touch two earthed items of a different voltage, clearly with the house in question there is no chance of this, however it is still up to the DNO to tell you what earthing system you should use, so answer is simple, ring them and ask them.

That's a laudable enough intent but they obviously cannot force anyone to us a TN earth (of any sort) which they supply. I have TT (although, as mentioned), I could have TN-C-S) but at least one of my immediate neighbours has TN-C-S - but there's no problem such as you describe, since the properties are far too far apart.

My most immediate neighbours are buildings which are actually attached to mine (prior to 1950 was part of 'my' house). They share my overhead supply (which certainly doesn't have any 'earth'), and traditionally were certainly bot TTd, like me. However, either of them could have been changed to TN-C-S (or may be changed in the future), in which case the concern you mention could arise.

Kind Regards, John

 

[JohnW2]

Just to say as I previously did though, when the black cable was installed in 1990 with the cutout moved to it's current position, a piece of earth cable was left unconnected from the old 3036 board, which if I remember rightly, had a small 'pinch' mark in it, suggesting it had been previously connected to something, this bare wire end was where the old cutout was sited. ... I will try and find out more information when I can ...

Thanks for the further information.

I may have missed it somewhere, but the one (clearly important) thing I haven't noticed you explaining is the question of what (if anything) your installation's 'earthing system' (i.e. earth bar in CU and/or an external 'MET') is actually connected to (or appears to be connected to)?

Kind Regards, John

 

[CAWORK]

I may have missed it somewhere, but the one (clearly important) thing I haven't noticed you explaining is the question of what (if anything) your installation's 'earthing system' (i.e. earth bar in CU and/or an external 'MET') is actually connected to (or appears to be connected to)?

There is an Earth Block (4 way) now near the Consumer Unit, with 3 connections to it : a conductor going to an electrode rod in the garden, Equipotential Bonding to the water and the Earth from the CU (connected to the Earth Bar within the CU) connected to it.

This was all installed when the CU was installed.

Previously there was no Equipotential Bonding or separate Earth Block, just the old 3036 board with the final circuit CPC's connected within and a (probably 6 mm) piece of Earth cable (with the 'pinched' mentioned previously) end coming out of the bottom of it, connected to the Earth Bar in the old board.

As I have already mentioned, the old cutout removed in 1990, was directly in that position, incoming supply prior to the black cable installed was three actual conductors (see photo 'Old Redundant Cables' in original post).

Thanks for your reply

Regards.

 

[JohnW2]

There is an Earth Block (4 way) now near the Consumer Unit, with 3 connections to it : a conductor going to an electrode rod in the garden, Equipotential Bonding to the water and the Earth from the CU (connected to the Earth Bar within the CU) connected to it.

That sounds fine, then (provided the earth rod in the garden is satisfactory). That's obviously a standard TT setup and, as I've said, there is no reason why one has to use a TN (TN-S or TN_C_S) 'earth' even if the DNO provides one - one is always free to use TT if one wishes (which is what I do, by choice).

Kind Regards, John