Earthing Systems

[TheOriginalTonkaToy]

No, because in a TN-C system the consumer earth is actually supplied via the neutral conductor by the service provider. In a TT system no earth is provided and local arrangements have to be made.

Although I can see what you mean - the incoming cabling is identical up and to the service head and that's when the difference occurs.

Out of curiosity, has anyone here ever seen a TN-C in service?

 

[JohnW2]

No, because in a TN-C system the consumer earth is actually supplied via the neutral conductor by the service provider. In a TT system no earth is provided and local arrangements have to be made. Although I can see what you mean - the incoming cabling is identical up and to the service head and that's when the difference occurs.

Fair enough - but I don't think that I'm ever going to be convinced that the 'N' in TN-S makes any logical sense as a description of the 'earthing system'.

Out of curiosity, has anyone here ever seen a TN-C in service?

I don't think it's allowed in the UK, is it?

Kind Regards, John.

 

[Paul_C]

One problem, if one wishes to call it that, is that the various designations being considered here cannot be applied solely to the supply system or solely to the customer's installation, because the complete designation can be formed only by combining the appropriate letters which are determined by various aspects of both the supply/distribution network and the supplied installation.

So although it's common to hear talk of "a TN-C-S supply" and such like, that's not really correct, since it doesn't take into account the supplied installation as well.

You could have a TT arrangement where the house is earthed to its own local rod, then during a rewire the arrangement is changed to TN-C-S by connecting the installation's earthing lead to the supply neutral. The distribution network hasn't changed in any way, but the combination of supply and installation is now TN-C-S. That same pole might have a service drop to an adjacent house which is still on a local earth rod, and is thus still a TT system.

As far as that second letter of the designations goes, it refers merely to whether there is a solid metallic path back to the neutral point of the supply transformer (N) or whether the earth fault path is solely by way of the earth itself (T).

So for TN-S, although the distribution neutral conductor might not carry any actual current resulting from an earth fault, the separate earth conductor (or cable sheath) is still solidly connected to that neutral at the substation. The earth fault path is not, therefore, solely by way of the earth itself, so it's "N" and not "T."

 

[JohnW2]

One problem, if one wishes to call it that, is that the various designations being considered here cannot be applied solely to the supply system or solely to the customer's installation, because the complete designation can be formed only by combining the appropriate letters which are determined by various aspects of both the supply/distribution network and the supplied installation.

Indeed, as we've discussed at some length. For a start, regardless of what the DNO does and doesn't provide by way of an earthing arrangement, one is free to have a TT installation.

As far as that second letter of the designations goes, it refers merely to whether there is a solid metallic path back to the neutral point of the supply transformer (N) or whether the earth fault path is solely by way of the earth itself (T).
So for TN-S, although the distribution neutral conductor might not carry any actual current resulting from an earth fault, the separate earth conductor (or cable sheath) is still solidly connected to that neutral at the substation. The earth fault path is not, therefore, solely by way of the earth itself, so it's "N" and not "T."

I have to say that I still struggle to see the practical logic of that. What would you say if the separate earth conductor were connected directly to the transformer at the substation (rather than being connected via a very short bit of the neutral conductor)? Would you still try to justify the 'N' in TN-S, even though, in that situation, it would seem impossible for any earth fault current to flow through any of the neutral conductor?

Kind Regards, John.

 

[Paul_C]

What would you say if the separate earth conductor were connected directly to the transformer at the substation (rather than being connected via a very short bit of the neutral conductor)? Would you still try to justify the 'N' in TN-S, even though, in that situation, it would seem impossible for any earth fault current to flow through any of the neutral conductor?

So the distribution neutral conductor and the completely separate earth conductor (by way of cable sheath or separate overhead line) are both bolted directly to the transformer terminal. But what about that short length of neutral conductor inside the transformer casing which will carry both normal operating current and earth-fault current?

In either case, you'll get the same thing with TT.

Case 1. A conductor runs from the transformer terminal to the substation earth electrode system. The neutral which runs to the distribution system is tapped part way along that conductor. When an earth fault occurs, although the earth is providing the path from the installation electrode back to the substation earth, there will still be that short length of shared cable between where the neutral is connected and the transformer which will carry the fault current.

Case 2. The earthing conductor and distribution neutral are both bolted directly to the transformer terminal. Again, the fault-current going to earth by way of the electrode at the faulty installation has to return to the transformer by way of the substation earthing system, the conductor which runs from there to the transformer terminal, and then along the shared short piece of conductor inside the transformer.

That's why I think it's probably easier to regard that second letter as indicating only whether there's a solid metallic path for the fault current or whether at any point it relies solely upon the earth itself.

 

[JohnW2]

What would you say if the separate earth conductor were connected directly to the transformer at the substation (rather than being connected via a very short bit of the neutral conductor)? Would you still try to justify the 'N' in TN-S, even though, in that situation, it would seem impossible for any earth fault current to flow through any of the neutral conductor?

.... But what about that short length of neutral conductor inside the transformer casing which will carry both normal operating current and earth-fault current?

Prize quibbling

That's why I think it's probably easier to regard that second letter as indicating only whether there's a solid metallic path for the fault current or whether at any point it relies solely upon the earth itself.

Agreed, but I'd still say that to use 'N' for that second letter would be misleading - maybe 'M' (for 'metallic path')?

Kind Regards, John.

 

[Paul_C]

Prize quibbling

Couldn't the same be said of a couple of feet of conductor outside the transformer?

That's really the same debate as a few weeks ago about TN-S vs. TN-C-S and whether it can never truly be "S" if there's a short length of cable at any point which serves both functions. I think there has to be an element which comes into play of discounting very short interconnections of that nature.

Agreed, but I'd still say that to use 'N' for that second letter would be misleading - maybe 'M' (for 'metallic path')?

That probably would have been better, especially given the other point I've made before that if it's a simple 2-wire 1-phase 240V secondary the earthed conductor is not really a neutral conductor anyway, in the true meaning of the term.

 

[JohnW2]

Prize quibbling

Couldn't the same be said of a couple of feet of conductor outside the transformer?

Indeed, and that's been my very point - that 'couple of feet of conductor' is, as I understand it, the usual situation - and I find it unconvincing that this is a good reason for an 'N' in TN-S!

Agreed, but I'd still say that to use 'N' for that second letter would be misleading - maybe 'M' (for 'metallic path')?

That probably would have been better...

Good to see that we are agreed about something!

Kind Regards, John.

 

[Paul_C]

Well, as I think I've also mentioned before, I've never really been keen on this notation anyway. Some designations are really too broad to describe fully the arrangement in use, yet they seem to have become very widely adopted in just that role.

This designation system wasn't adopted widely in the U.K. until the 1980's, and we could easily do without it as we did for decades before then.

 

[Spark123]

Out of curiosity, has anyone here ever seen a TN-C in service?

You may see TN-C used on things such as electrified tramways and railways, it is illegal to use TN-C without permission from some part of the government, it is in the ESQCR.

 

[TonyS]

You may find this interesting, http://www.eon-uk.com/distribution/...l/E2 Earthing Manual Earth Guidance Notes.pdf

 

[JohnW2]

You may find this interesting, http://www.eon-uk.com/distribution/...thing Manual Earth Guidance Notes.pdf[/QUOTE]
Thanks. Yes, although most of it a bit tangential to the discussions here, it looks as if it might make an interesting read. My knowledge about anything other than LV parts of the network is very limited and there's at least one HV concept in that document that I need to educate myself about - namely 'earthing transformers'!

Kind Regards, John.

 

[ricicle]

Fill yer boots John...

http://www.qualitypower.co.in/earthing-transformers.php

 

[westie101]

namely 'earthing transformers

They are fun and usually only used on the higher voltages

 

[JohnW2]

Fill yer boots John...http://www.qualitypower.co.in/earthing-transformers.php

Thanks - fascinating. I didn't even know such things existed, but I guess that's inevitably true of lots of things in the HV (and higher) world as well. This is obviously of no practical importance to me - but, nevertheless, interesting and 'good for my education'

Kind Regards, John.