question for ericmark

[sparkybird]

Hi there

Didn't want to hijack the post about the high Zs readings by student spark, but at the end of your post you said

"Remember the DNO does not have to provide an earth but they do have to give same system to all property supplied from same transformer. Hard luck if a petrol station is near by."

Can you elaborate on this a bit? I'm always have a battle with EDF about earthing and have put in maybe a dozen rods over the years - but in LONDON. This means that there are now different earthing systems on the same transformers....

I presume that this is a BAD thing? I guess that as PME and TNS are both TN systems, I suppose they are the same?? But TT ain't!

Thanks in advance

SB

 

[RF Lighting]

I've never heard of this and it certainly isn't the case as far as YEDL are concerned.

 

[PrenticeBoyofDerry]

The DNO doesn't have to supply an earth system but generally they do, other than TT systems, as far as all transformers being of the same system, I too never heard of this but would be interested to find out more.

 

[ericmark]

Maybe old teaching now but TN-C-S is where the earth and neutral share the same conductor for part of it's run so if anywhere on output from transformer they are combined whole becomes TN-C-S by definition so can't be a mixture of TN-S and TN-C-S.

Problem with TN-C-S is the water pipes, gas pipes and drains are bonded to the DNO's earth and this is also the case with a TT earth system so either direct or worse through the ground the earths will be connected together.

With the latter electrolysis can cause erosion of the pipes and you can also get a voltage gradient between the two earth systems.

I was always taught because of this all earths from same transformer should use same system.

Of course where there is a big separation between properties then there is not so much of a problem and a garden shed 50 meters down the garden would likely present a problem with PME as there will be non of the boards earth rods near the shed and it may be permissible to in that case use a TT from a TN-C-S supply.

In spite of the ESQCR saying caravans should be TT if parked next to a house which is TN-C-S with any metal which could be touched at same time as caravan then one would be rather silly connecting the caravan to a TT supply.

I am sure some where there is a laid down separation distance and a complex set of rules but in a row of houses built close to each other the earth system should be the same.

This is of course why we should ask the DNO what earth to use. It is not up to us we have to do as they say.

 

[1john]

Maybe five of more years ago the DNO made a TNS supply into TNCS on one property I was rewiring due to the supply cable being boxed in. Also all new supplies are PME weather supplied from a typically TNS supply or a TT supply. Also TT supplies are regularly upgraded to PME. Just wondering how this fits in with ericmark's knowledge, not being awkward or having a go, just curious.

 

[ban-all-sheds]

All supplies are TN at the origin - that's what (nominally) ties neutral to 0v.

 

[Click-Sure]

I'm most active in the old EMEB area and see TNS, TNCS and TT all on the same transformer on a very regular basis.

 

[Lectrician]

WPD will only provide a single earthing system per transformer. The only exception to this is on older supplies using PILC cables where the earth is failing - they tend to PME these supplies.

 

[sparkybird]

Thanks ericmark and everyone else - glad I did hi-jack the original post, makes for interesting reading.

As mentioned I'm in EDF land (London) and they regularly tell my clients that 'you'll have to get a rod' if there is no/poor TN earth. In fact even where there was evidence that there had previously been a good TNS earth (cert readings) they still insisted on TT. Strikes me as a bit short sighted as surely this is indicative of cable sheath deterioration??

Anyway, if someone can find the chapter and verse on terraced houses having to have the same earthing system I'd be interested and would take it up with EDF...... as trying to install a rod in a 1st floor flat with no garden is a bit of a challenge!

Interesting how different DNO's seem to have different rules, eh???

SB

 

[Adam_151]

I certainly wouldn't worry about not using the PME and providing your own TT (unless there is an issue such as metalwork from the two systems in close proximetry, or shared metallic services)

Generally when you ask for a supply for a building site, farm or petrol station, etc you'll get a supply thats installed in the same way as a PME supply, but they just wont make the terminal on the side of the cutout accessible.

Oh and even if you have what looks like TNS, theres a good chance that it isn't... if the cable has failed in the street in the past a section of waveform will have been jointed in with the neutral conductor and the sheath jointed onto the armouring/sheath of the waveform at each end.... AFAIK the guidences for petrol stations insist on TT or TNS only where it can be guarented TNS and guarented to stay TNS (i.e large shopping site with dedicated transformer on site)

 

[JohnW2]

I certainly wouldn't worry about not using the PME and providing your own TT....

I rather suspect that, four years down the road, sparkybird is probably not worrying about any of this any more

Kind Regards, John

 

[ericmark]

Oddly the issue has raised it's head again with electric cars. I was rather over egging it by saying same transformer but clearly where there are extraneous-conductive-parts close to each other connected to a different earthing system and there is a possibly that a person could touch both at the same time or an animal could receive a shock due to it's leg base and the voltage gradient then it is possible building in close proximity to each other will need to use the same earthing method.

If one looks at my caravan and father-in-laws motor caravan they are within one meter of the gas and electric meter boxes so there would be a real danger if the caravan and motor caravan used a TT system with the house on a TN-C-S the meter readers under fault conditions could get a shock.

Where electric cars are being charged at home it is it seems permitted to use TN-C-S earthing for the charge point. Ones house is not a caravan / camping parks or similar location. There for there is nothing to say it needs a TT supply for the caravan.

Wiring Matters Autumn 2005 talked about earthing for outbuildings and we are looking at a similar situation it's all down to distance between installations.

We also of course have a problem with TT where the earth rods are a distance apart under fault conditions the two earths could be a different potential but once the RCD trips in most cases they will be close enough not to worry as all connection to supply is removed. But with TN-C-S even after the RCD trips the earthed parts are still connected to the neutral.

It is very much down to a risk assessment and must be worked out on an installation by installation basis there really can't be a hard and fast rule.

In the main with commercial buildings fire regulations mean there is little chance of a earth on a caravan causing a problem due to different earth on the building as the caravan can't be parked that close.

 

[Adam_151]

Bloody hell, why did this come to the top?

Its not the first time its happened either, I reply to a thread that comes up and find its years old!

 

[JohnW2]

Bloody hell, why did this come to the top? ... Its not the first time its happened either, I reply to a thread that comes up and find its years old!

We've all done it

Did you perhaps find the topic at the top of the list of "Similar Topics" at the bottom of the page (which seems to specialise in ancient threads!)?

Kind Regards, John

 

[Spark123]

All supplies are TN at the origin - that's what (nominally) ties neutral to 0v.

It is just the initial "T" which means the system is connected to mother earth.
"N" means there is a direct connection to the neutral of the supply from the installation, which is connected to earth at the transformer etc.