Current in TN-C-S main earth conductor

[JohnW2]

I wonder if nothing to do with power being used on the premises, I remember a case where the owner of a house was a radio ham, and he had earth rods for his hobby, connected to the DNO earth, and a fault with the PEN caused excessive current to run, in his case until the earth wire joining the two earths melted.

With the OP's initial figures, it would seem that about 33A of the 50A Line current was returning through the DNO's PEN, so I would doubt that there could have been much of a "fault with the PEN".

I haven't personally heard of PEN faults other than 'complete' (i.e. broken/open circuit) ones, but I suppose it's not impossible - in which case if the TT rods were connected to the installation's 'earth' (i.e. joined to the installation's neutral) that might conceivably offer an explanation - but I'm still a bit doubtful about getting 17A down TT rods (unless, I suppose, there were a lot of them.

Charlie: How many TT rods are we talking about, and are they effectively connected to the installation's (TN-C-S) 'earth' (via SWA armour or anything else)?

Kind Regards, John

 

[JohnW2]

It needs to be TN or TT you can't mix the two.

Does that mean that one should not bond extraneous-c-ps (of which a TT rod would be but one example) in a TN-C-S installation?

Kind Regards, John

 

[SUNRAY]

I wonder if nothing to do with power being used on the premises, I remember a case where the owner of a house was a radio ham, and he had earth rods for his hobby, connected to the DNO earth, and a fault with the PEN caused excessive current to run, in his case until the earth wire joining the two earths melted.

It needs to be TN or TT you can't mix the two.

Oh yes, we know about mixing them

A real moan tonight...

It is one of those 'several people have had a look so can you go in and sort it out' jobs, I popped in to aquaint myself with it with the expectation I'd get the call when required. This is an outbuilding [happens to be an ex telephone exchange] currently used as changing room and pump house...

www.diynot.com

 

[SUNRAY]

Does that mean that one should not bond extraneous-c-ps (of which a TT rod would be but one example) in a TN-C-S installation?

Kind Regards, John

We go to all sorts of lengths to make sure they remain isolated from each other. Well some of us do, others appear to be completely oblivious to the problems and hazards.

However it has been said that a draft of latest edition of regs proposed fitting an earth rod at each installation and bonding to DNO earth.

 

[JohnW2]

Oh yes, we know about mixing them ...

We do - but, as I've just implied in my last post, "mixing them" is essentially unavoidable in a high proportion of TN-C-S installations!

(unless, of course, you "don't believe" in bonding extraneous-c-ps in TN-C-S installations )

Kind Regards, John

 

[SUNRAY]

We do - but, as I've just implied in my last post, "mixing them" is essentially unavoidable in a high proportion of TN-C-S installations!

(unless, of course, you "don't believe" in bonding extraneous-c-ps in TN-C-S installations )

Kind Regards, John

Indeed. But then it isn't TT.

 

[JohnW2]

We go to all sorts of lengths to make sure they remain isolated from each other. Well some of us do, others appear to be completely oblivious to the problems and hazards.

You may go to all sorts of lengths to prevent extraneous-c-ps (including connects to TT rods) entering a building which has a TN-C-S installation, but if such entry does occur, you have no choice (if you want to comply with BS7671,and avoid C2s on an EICR) to connect them together (rather than 'isolating' them from one another).

However it has been said that a draft of latest edition of regs proposed fitting an earth rod at each installation and bonding to DNO earth.

"It has been said", because it was true ...

542.1.2.202
For new premises constructed upon foundations, each building in which there is an installation using the protective measure of automatic disconnection of supply shall be provided with one of the following:
(i) a concrete-embedded foundation earth electrode in accordance with Annex A542or
(ii) a soil-embedded ring earth electrode in accordance with Annex A542or
(iii) an equivalent earth electrode such as that afforded by metalwork of a steel framed building embedded in concrete foundations in contact with soil.
A value of resistance to Earth not exceeding 20 ohms shall be provided by the earth electrode, or collectively where the electrodes of two or more buildings are connected together.
The earth electrode shall be connected to the main earthing terminal of the installation by a main protective bonding conductor of that installation.

I suspect/imagine that it is only a matter of time before it reappears as 'a requirement'!

Kind Regards, John

 

[JohnW2]

Indeed. But then it isn't TT.

What "isn't TT"? We're talking about connecting a TT rod (which is nothing more than one possible extraneous-c-p) to the 'earth' of a TN-C-S installation (which, as you said, was nearly required by Amd2 of BS7671:2018).

As eric is pointed out, if one does it then, if the TT electrode provides a sufficiently low impedance to earth and if the cable connecting it to the installation's earth does not have a large enough CSA, then in the case of a TN-C-S installation which 'loses its PEN', there is a risk of that cable melting. All we can do to avoid that is to make sure that the cable has an adequate CSA, taking into account the resistance of the rod to earth.

Kind Regards, John

 

[JohnD]

I suspect/imagine that it is only a matter of time before it reappears as 'a requirement'!

Years ago, buildings used to have metal water pipes and gas pipes, doing the same thing.

Effectively not just an earth spike, but an earth mesh covering the entire town and much of the country

 

[JohnW2]

Years ago, buildings used to have metal water pipes and gas pipes, doing the same thing.

Yes, if they were bonded or, at least, incidentally in continuity with the electrical installation's earth. However, those were probably the days when TN-C-S was not around (much or at all), and when "incidental continuity with the electrical installation's 'earth" was probably much less common than it is today

Kind Regards, John

 

[SUNRAY]

Years ago??
My metal gas pipe still exists and is bonded.
The house I grew up in still has metal gas and water supplies.

I thought you were going to say that in the days of open overhead supplies and DNO didn't supply the earth, it was common practice to use the gas and/or water pipe as the only earth connexion on the electrical installation.

 

[JohnW2]

Years ago?? ... My metal gas pipe still exists and is bonded.

So does mine (short length of underground metal LPG pipe) - as does my underground metal water supply pipe - and both are bonded.

The house I grew up in still has metal gas and water supplies.

The same may well be true of the house in which I grew up - it certainly was the case when I left it about 50 years ago,and the situation may well not have changed since then

I thought you were going to say that in the days of open overhead supplies and DNO didn't supply the earth, it was common practice to use the gas and/or water pipe as the only earth connexion on the electrical installation.

That was the point I thought JohnD was making, and to which I responded "Years ago" those 'networks of underground metal pipes' were the only means of earthing of many electrical installations. I'm not sure when that ceased to be 'allowed', but it was a good while ago!

Kind Regards, John

 

[ericmark]

However it has been said that a draft of latest edition of regs proposed fitting an earth rod at each installation and bonding to DNO earth.

That is OK if every supply from that transformer has earth rods, this is what the M in PME stands for, the DNO should fit the earth rods or other earthing device, so any current is shared, if every house in the street has water pipes earthed, then the load is shared, and if the earth rod is 60Ω then max current is 230/60 = 3.8 amp which is not really a problem as long as the earth rod can't be touched.

However where I fitted earth rods in a gas terminus each rod was 8Ω or less, so 29 amp per rod if 230 volt, and there were many rods, so at the DNO input there was a box the size of a small van with a resistor in it to limit the current in the earth.

Earth current is a problem and I know canal boats often have diodes in the earth connection so no current will flow until more than two volt difference, you don't want to connect an aluminium hull to a steel hull, or soon there would be no aluminium hull.

There are special earthing arrangements where animals are concerned, and I would need to do some reading up, but likely best option is all TT. Using TN-S is OK but TN-C-S is a bit risky. What is unknown is the amount of metal in the ground, and what the current is doing to that metal.

Lucky the guy is an electrical engineer, as otherwise this would go over his head, it is a complex problem.

 

[JohnW2]

That is OK if every supply from that transformer has earth rods, this is what the M in PME stands for, the DNO should fit the earth rods or other earthing device, so any current is shared, if every house in the street has water pipes earthed, then the load is shared, and if the earth rod is 60Ω then max current is 230/60 = 3.8 amp which is not really a problem as long as the earth rod can't be touched. ...... However where I fitted earth rods in a gas terminus each rod was 8Ω or less, so 29 amp per rod if 230 volt

I don't really understand some of the concerns that have been voiced (and which you seem to be implying). As you say, adding TT electrodes to a TN-C-S supply at the location of individual installations is merely increasing the 'M' of PME and, as you also allude to, although it is gradually becoming less prevalent (due to the increasing appearance of plastic supply pipes), it's still the case that a substantial proportion of properties have metal water/gas supply pipes bonded to the TN-C-S - and they probably have far lower impedances than any standard domestic TT rod.

The perceived problem seems to relate to the situation if there is a 'lost PME' in a situation in which only a small proportion of affected properties have connected TT electrodes. However, as you have illustrated with your numbers, the current that could flow into anything approaching a standard domestic TT electrode would be nowhere near high enough to melt any credible earthing or bonding conductor - even if the electrode has a resistance/impedance as low as the 8Ω you mention (far lower than a standard domestic rod in most locations). For what it's worth, the drafted regulation which didn't quite make it into Amd2 of BS7671:2018 called for a maximum of 20Ω.

I therefore do not really understand what the perceived problem is. Perhaps you can educate me?

However where I fitted earth rods in a gas terminus each rod was 8Ω or less, so 29 amp per rod if 230 volt, and there were many rods, so at the DNO input there was a box the size of a small van with a resistor in it to limit the current in the earth.

You often talk about that, and I think I must be missing something, since it sounds daft. Why install so many (and so low impedance) earth rods that the net impedance to earth is considered so low as to be 'unsafe' (in terms of possible current through them) and then install a massive/expensive 'resistor' in series with them to increase their resistance/impedance in order to limit the current?? i.e. why not just install fewer and/or higher impedance rods in the first place?

Kind Regards, John

 

[bernardgreen]

would be nowhere near high enough to melt any credible earthing or bonding conductor

credible being 6mm or 10mm.

That size of conductor can carry a lot of current before it melts.

( probably specified as such to cater for the situation where the ground electrode is actually a metallic water main with near zero impedance to the TN junction at the substation. )

i.e. why not just install fewer and/or higher impedance rods in the first place?

The reason may be the need to sink to ground potential any extraneous current that is imported to the site by the gas pipe lines from other gas processing plants.