Can anyone clarify what sort of earthing we have.

[aesmith]

Hi,

I'm trying to confirm what sort of earth we have, and the information seems to be conflicting. We're supplied by overhead 11kV, two lines on our poles separating from a 3 phase line 1/2 mile away. These feed our transformer which serves only us. The Hydro Board guy who came round to trace cable routes for us described the service cable as being a central live conductor surrounded by strands some of which are insulated N, and some uninsulated Earth, which would be separated into N and E connections at our end. He also described how the earth is referenced to an earthing cable from the foot of the transformer pole. So far that sounds as if we're fed by separate N and E conductors. It also sounds to me as if the supply can't be PME as there's only one E to N connection, at the foot of our pole. Am I on the right lines?

The conflicting information is the electrical certificate from a re-wire, which has the supply classed as TN-C-S.

Is it possible to tell by looking at the connection? It seems to me that they can't both be correct. Let me know if you need to see a different bit of the supply.

Untitled

• aesmith
• 9 Dec 2013
• 1

Thanks, Tony S

 

[PrenticeBoyofDerry]

Looks and sounds like TN-C-S to me.

 

[aesmith]

Cheers. The reason I wondered was that the Hydro engineer's description indicated that separate conductors carry N and E from the transformer, which I thought would have been classed as TN-S. However if that still counts as TN-C-S (because E and N are combined at the transformer?) then that's one uncertainty out of the way.

What do you reckon on the PME question?

Tony S

 

[westie101]

TN-S without a doubt as the N & E are separate at the customer's end, be it that or TN-C-S the N & E are ALWAYS combined at the transformer weather it supplies 1 customer or a hundred!

 

[PrenticeBoyofDerry]

Sound like a concentric supply cable.

 

[westie101]

Two types of concentric!
Plain where the N & E are combined
Split where the neutral has each strand insulated and a bare copper earth

In either case they would use the same cut-out, the only difference is that for a PME supply there would be an internal link between the N & e terminals to provide the customer earth. This link is not visible unless the cut-out is partially dismantled.

I recently had the same discussion where an electrical contractor was convinced he had a PME supply with a high ELI, until I explained about the different types of concentric.

Back to the OP if the DNO says it is TN-S it will be!

 

[aesmith]

Back to the OP if the DNO says it is TN-S it will be!

Cheers, I'll see if I can ask them specifically. The info from the engineer was really just me picking his brains a little while he was out doing the cable tracing for me.

Sorry to labour the point, but either way am I correct that it is not PME, since there's no opportunity for multiple N to E connections? The conductors on the 11kV must be two from a three phase line, so I can't see how either could be referenced to N or to E. Even if it was TN-C-S with the service cable combining N and E, there's still only one point of connection to true earth.

The reason for wanting to confirm is that our outbuildings have a lot of metalwork and I'm reviewing whether that's going to be a problem.

Tony S

 

[JohnW2]

The reason for wanting to confirm is that our outbuildings have a lot of metalwork and I'm reviewing whether that's going to be a problem.

As I understand it, it's not really the 'PME' which is a potential issue with outbuildings but, rather, TN-C-S. As westie has said, if you have a split-concentric supply cable (and the single earthing point) such as you have described, it must be TN-S.

Kind Regards, John

 

[aesmith]

As I understand it, it's not really the 'PME' which is a potential issue with outbuildings but, rather, TN-C-S.

I must admit I am struggling to understand. I thought I had concluded that the issue arises from PME in the unlikely event of a break in the supplier's Neutral, in a PME supply with that fault "my" earth could be carrying someone else's neutral current and thus be raised well above true earth. Would the same issue arise if I'm the only connection?

 

[westie101]

The conductors on the 11kV must be two from a three phase line, so I can't see how either could be referenced to N or to E.

The way an 11kV line is referenced to earth bears absolutely no connection in anyway to what the LV earthing provision is.
For it to be PME or TN-C-S (which are electrically the same thing) there would have to be another earth electrode, possibly at the cut-out (but not necessarily)

 

[westie101]

Would the same issue arise if I'm the only connection?

In the situation you describe, as applies to your supply, the loss of the neutral anywhere between the cut-out and the transformer would raise all the earthed & bonded metal work to 230V (nominal)

 

[JohnW2]

I must admit I am struggling to understand. I thought I had concluded that the issue arises from PME in the unlikely event of a break in the supplier's Neutral, in a PME supply with that fault "my" earth could be carrying someone else's neutral current and thus be raised well above true earth.

What you say is correct, except that it is because of the TN-C-S (i.e. the combined neutral and earth), not the 'PME' (which usually goes with TN-C-S), per se.

Would the same issue arise if I'm the only connection?

Well, the precise issue as you describe obviously couldn't arise if there was no 'someone else' to raise the potential of your neutral. However, if you were the only connection, loss of the supply neutral would result in the potential of your installation's neutral (and hence also your installations's 'earth'/CPCs etc.) trying to rise to close to line potential (via connected loads).

Kind Regards, John

 

[aesmith]

Thanks everyone, I need to refresh my memory on the references that I found.

I am sure there was some special concern for PME (as opposed to a single TN-C-S supply) which was to do with the installation potentially carrying N current from other customers loads. I can't remember if this was in normal operation, or only in the case of a remote fault. If I remember correctly the multiple earth connections (the M in PME) are to mitigate the risk from a break in the combined conductor by providing an alternative path via earth points either side of the break.

Tony S

 

[Lectrician]

Sound like a concentric supply cable.

Sounds like SPLIT concentric to me, and TN-S.

Split Concentric

Straight Concentric

 

[JohnW2]

I am sure there was some special concern for PME (as opposed to a single TN-C-S supply) which was to do with the installation potentially carrying N current from other customers loads. I can't remember if this was in normal operation, or only in the case of a remote fault.

As I said, it's the TN-C-S which is the issue, not the multiple earthing. Your confusion probably results from the fact that, for virtually everyone, TN-C-S and PME go hand-in-hand, so they use 'PME' to refer to a TN-C-S supply.

If I remember correctly the multiple earth connections (the M in PME) are to mitigate the risk from a break in the combined conductor by providing an alternative path via earth points either side of the break.

Exactly. The multiple earthing (i.e. the 'PME') cannot, in itself, cause any hazards - it is there to reduce the potential very small hazard (under supply-side fault conditions) of TN-C-S. As far as I am aware, at least in the UK, one is, for that reason, not allowed to have TN-C-S without PME. Your situation, of a single customer being supplied by a transformer may be a sort-of exception (I'm sure that others will be able to talk more authoritatively about that) - but I would then be more comfortable with a second earth connected to (or very close to) my installation - just in case someone 'snipped' the neutral supply cable!

Kind Regards, John