Earthing System

[studentspark]

I did actually phone the DNO, (with little hope) to see if they had any records, but of course they said they don't have such information

 

[JohnW2]

So you are suggesting that if the replacement cable happened, the DNO would leave the earth floating? I would have thought they would connect it to the Neutral. Anything else would be scandadous.

Exactly. As I wrote ...

The DNO could not leave a situation in which the sheath had become, or been made, discontinuous without making sure that all installations previously relying on that sheath as a TN-S earth were changed to deriving their earth from the neutral.

Maybe I'm misunderstanding, because I thought eric was suggesting that, even if it "looked like" (within the premises) TN-S (i.e. the earthing conductor were connected to cable sheath), it could 'have become TN-C-S' as a result of something that had been done external to the property. As we both have said, it doesn't become a TN-C-S installation unless/until the earthing conductor has been moved to the neutral.

Kind Regards, John

 

[JohnW2]

Ze of 0.09...

Apart from the fact that 'it would be nice to know', is there a reason wny you need to know whether it is TN-S or TN-C-S - given that it is clearly an 'adequate' earth?

Kind Regards, John

 

[studentspark]

Apart from the fact that 'it would be nice to know', is there a reason wny you need to know whether it is TN-S or TN-C-S - given that it is clearly an 'adequate' earth?

Kind Regards, John

MPB only 6mm and nice to fill cert in correctly

 

[DetlefSchmitz]

@JohnW2. I am having difficulties understanding what you are not understanding. Eric said that a TN-S connection could become a TN-C-S connection because of work in the street. Replacement of cable in the street, with the necessary connection of sheath to neutral, can have this effect.

 

[EFLImpudence]

Surely John is technically correct in that if the earthing conductor arrives in the premises separately then it is, strictly speaking, TN-S.

However, it has long been advised that apparent TN-S supplies should be teated as 'PME' because of the likelihood of the above mentioned (poor and money-saving) repairs in the supply cables by the DNOs.

 

[JohnW2]

@JohnW2. I am having difficulties understanding what you are not understanding.

I'm not being awkward, but I am also having difficulties in understanding why others do not seem to understand my difficulties.

Eric said that a TN-S connection could become a TN-C-S connection because of work in the street.

Work in the street can result in the sheath ceasing to be an effective earth, such that only usable two conductors (L and N) enter the premises, but whether it becomes a "TN-C-S connection" depends upon how it is used. The "S" of TN-C-S means "separated", so if the earth never 'separates' (from the 'C'), it can't really be TN-C-S.

My initial point remains - that if it "looks like TN-S" (in that the earthing conductor is connected to the cable sheath), then the installation either has a TN-S earth or no (effective) earth - regardless of what has (or has not) been done outside the property, it cannot possibly have a TN-C-S earth unless the installation's earthing conductor is connected to the incoming neutral (i.e. there is no 'S').

Replacement of cable in the street, with the necessary connection of sheath to neutral, can have this effect.

Could you perhaps clarify what you mean by "with the necessary connection of sheath to neutral"? Thanks.

Kind Regards, John

 

[JohnW2]

Surely John is technically correct in that if the earthing conductor arrives in the premises separately then it is, strictly speaking, TN-S.

Thanks.

However, it has long been advised that apparent TN-S supplies should be teated as 'PME' because of the likelihood of the above mentioned (poor and money-saving) repairs in the supply cables by the DNOs.

I'm not sure I fully understand that "advice". If one "treats a TN-S supply as PME", one would presumably feel that it was OK to connect the installation's earthing conductor to the incoming neutral - despite the fact that if were an 'intact and unmodified' TN-S supply, it would probably not have been PMEd.

As has been said, if a DNO address a faulty cable sheath by PMEing the supply (rather than repairing/replacing the cable), they are then duty bound to ensure that all affected premises' earthing conductors are shifted from the cable sheath to the neutral - whereupon (unless difficult to see because of the physical arrangement) it should no longer look like "apparent TN-S", should it? - not to mention the PME label that they would also 'install'.

Kind Regards, John

 

[ericmark]

TN-C-S means at some point the earth and neutral shared the same conductor, TN-S means at no point have they shared same conductor, TN-C-S is not quite the same as PME as some times there are not multiple earth rods, however as the user unless a petrol station, caravan park etc we don't need to know, either way it is TN, that's all we need to know.

Enquiry as already done is correct method, and if the DNO can't tell you, then nothing more that can be done, since TN-C-S is worst scenario if forced to enter either/or in software I would enter TN-C-S.

 

[skenk]

If the earth loops/fault currents are identical L-N and L-E it suggests PME

 

[davelx]

It looks like TN however only the DNO can say if TN-S or TN-C-S as only they know is some where along the supply root is the earth and neutral have been combined or not.

A big clue as it if TN or TT is the ELI, although a TT supply can have a very low ELI in domestic unlikely to be under the 0.35Ω considered as limit for a TN-C-S or 0.8Ω considered limit for TN-S however with a three phase supply to commercial that does not really work. In one job I spent 6 months knocking in earth rods, non over 8Ω and main earth pit under 1Ω and the incoming earth connected through a large resistor to limit earth current.

So only real way to know is ask the DNO.

You can't rely on Ze, especially in built-up areas - I'm definitely on TN-S and my Ze (on the sheath with it isolated from the building CPCs and bonding) is also 0.09Ω. Round here no-one is more than 500m from an 11Kv substation - I have one 150m away.

(This is a good thing when it comes to using a type C MCB for an EV charging point, as specified by some of the home charge-point suppliers)

 

[JohnW2]

TN-C-S means at some point the earth and neutral shared the same conductor, TN-S means at no point have they shared same conductor, ...

As I've said, TN-C-S means that a Combined N and E subsequently splits into Separate N and E. If one does not have any sheath (as with my overhead supply) or does not have an ineffective sheath - i.e only two usable conductors enter the premises - then whether or not it is TN-C-S depends on how we use it. If one 'taps off' the neutral to derive one installation's earth, then (and only then) it becomes TN-C-S.

Given that I am using a TT earth in my house, it would, in my opinion, be less than helpful (I would say 'silly'), as well as being technically incorrect, to say that my 2-conductor supply is a "TN-C-S" supply - even though (since it is PMEd) I could use it as TN-C-S if I wanted (by adding the "S").

TN-C-S is not quite the same as PME as some times there are not multiple earth rods ....

I would call that an understatement - TN-C-S and PME refer to two totally different things - the important thing being that one is not allowed to use a 2-conductor supply as TN-C-S unless it has been PMEd.

Mind you, I was a bit disillusioned when the late lamented westie told us that the 'M' of PME often merely meant that they had added second earth rod at the end of distribution cable - I had previously thought that it really did mean 'multiple' (i.e. several). Of course, the new reg that got pulled from BS7671:2018 'at the last moment' may well show itself in the future - in which case we will move gradually towards the situation in which the supply neutral will be connected to an earth rod at every installation supplied.

Enquiry as already done is correct method, and if the DNO can't tell you, then nothing more that can be done..

In practical terms, that's probably the case. However, even if they don't have records (which I can believe), anyone who opened up the cutout could determine whether the supply is being used as TN-C-S (and that's what matters). As above (and as per my house, as an example), the fact that it could be used as TN-C-S is irrelevant if it is not being used as such (just as in my TT installation).

Kind Regards, John

 

[JohnW2]

If the earth loops/fault currents are identical L-N and L-E it suggests PME

It would, indeed, be an amazing co-incidence if PEFC and PSCC were identical with TN-S. However, I wonder if they really were both exactly 2,500A - it sounds as if that could be a matter of meter precision (or an 'upper limit' of the meter).

Kind Regards, John

 

[EFLImpudence]

https://www.enwl.co.uk/globalassets...ervice-connections-and-application-of-pme.pdf

 

[JohnW2]

https://www.enwl.co.uk/globalassets...ervice-connections-and-application-of-pme.pdf ...

Thanks.

It's interesting to see that they have used only the terms "CNE", "SNE" and "PME", which makes sense, since those are characteristics of the network., rather than TN-S and TN-C-S, which also relate to how the supply is used (particularly in the case of the latter). As the installations on that diagram illustrate (and as I have been saying), whether or not a supply is TN-C-S depends upon how it is used. That's particularly true of the TT installation in the middle, since the supply to it is identical to the supply to the installation described as 'PME'.

Kind Regards, John