Isolating PME from TT addition

[JohnW2]

If that is what you are considering plus the rest of your argument then probably you should TT every installation regardless.

I tend to agree with that as a correctly installed and maintained TT installation remains safe in almost every fault situation where the fault is outside the property ( such as a failed Neutral in the local network )

Are you suggesting that we should not use TN (at least TN-C-S) 'earths' at all, and that all installation should only have a TT earth?

Merely adding a local earth electrode (essentially just an additional, fairly high impedance, extraneous-c-p) to a TN-C-S installation (using the TN-C-S 'earth') obviously would be no solution to the risks that concern you.

Kind Regards, John

 

[bernardgreen]

I don't think many electrical installations are carried out primarily with the thought of lost PENs.

I think it's a bit far-fetched to suggest that the (very rare) possibility of a 'lost PEN' should be the "primary" consideration when designing an electrical installation.

I did not suggest that the possibility of a 'lost PEN' should be the "primary" consideration, but it is something to be taken into account when designing the installation.

As I said the TT electrode needs some regular checks to ensure it's impedance is low enough to be effective.

The "Earth" derived from the Neutral requires some routine checks to ensure the continuity of the Neutral conductor is intact.

The balance between the effectiveness of the processes for checking these two requirements is part of the equation to decide TT or TN

 

[bernardgreen]

Merely adding a local earth electrode (essentially just an additional, fairly high impedance, extraneous-c-p) to a TN-C-S installation (using the TN-C-S 'earth') obviously would be no solution to the risks that concern you.

A local earth electrode with an acceptably low impedance that can be assured to remain low will provide adequate protection provided there is RCD protection.

 

[JohnW2]

I did not suggest that the possibility of a 'lost PEN' should be the "primary" consideration ...

It might not be what you intended but, per my previous quotes, that surely is what you 'suggested' (seems even stronger than that to me!). To remind you.....

I don't think many electrical installations are carried out primarily with the thought of lost PENs.

They should be !

As I said the TT electrode needs some regular checks to ensure it's impedance is low enough to be effective. The "Earth" derived from the Neutral requires some routine checks to ensure the continuity of the Neutral conductor is intact.

All very true, and ...

The balance between the effectiveness of the processes for checking these two requirements is part of the equation to decide TT or TN

Not really, since the same "process" can be used for either, so there is nothing to "balance". If one measures the Zs on a final circuit or, better, at the origin of the installation will give a good handle on the Ze, whether it be a TN ';earth' or a TT earth.

Kind Regards, John

 

[JohnW2]

A local earth electrode with an acceptably low impedance that can be assured to remain low will provide adequate protection provided there is RCD protection.

Did you read my statement which you quoted and to which you replied?

I was not talking about the adequacy of the TT earth in a situation in which it was the sole earth. As you correctly quoted, I was saying that adding a local (i.e. "TT") electrode to a an installation using a TN-C-S 'earth' would not necessarily give any appreciable protection against the effects of a 'lost PEN' in the TN-C-S supply.

Kind Regards, John

 

[flameport]

Broken PEN conductors isn't the only consideration.

The TNCS earth is rarely at the same potential as the actual Earth, and for items like hot tubs and swimming pools where wet people are standing on the Earth, that difference in potential between the Earth and items connected to the TNCS earth terminal can be felt as an unpleasant sensation.

However making a hot tub or other outdoor item TT isn't without problems either, and in many situations it's impossible.
Any TT electrode and anything else connected to the TT system must be located far away from anything connected to the TN system including underground cables, metallic pipes, outside lighting, water taps and so on.

 

[JohnW2]

Broken PEN conductors isn't the only consideration. ... The TNCS earth is rarely at the same potential as the actual Earth, and for items like hot tubs and swimming pools where wet people are standing on the Earth, that difference in potential between the Earth and items connected to the TNCS earth terminal can be felt as an unpleasant sensation.

Indeed but, although a conceptually similar consideration applies, much less of a matter/concern for outhouses. This thread might be a little confusing for some, since the OP referred to "hot tubs OR outbuildings", some people have responded in relation to one and some in relation to the other.

However making a hot tub or other outdoor item TT isn't without problems either, and in many situations it's impossible. Any TT electrode and anything else connected to the TT system must be located far away from anything connected to the TN system including underground cables, metallic pipes, outside lighting, water taps and so on.

I imagine that you have probably switched primarily to the 'broken PEN conductor' scenario again, but I'm not quite sure of the thinking behind what you say. ....

... the concern is obviously about potential differences between simultaneously touchable things. If (as one imagines will nearly always be the case) the TT electrode is fairly close to whatever it is 'earthing' (tub/outhouse/whatever) then, even if that TT electrode (and surrounding ground) is at a high potential relative to 'true earth' because it is also close to "something connected to the TN system" AND there is a 'broken PEN', then I can't see how any appreciable PD could exist between that TT 'earth' and anything else simultaneously touchable.

The only situation I can think of that might lead to the problem you seem to be alluding to would be if the TT electrode were close to "something connected to the TN system" but distant from whatever it was earthing (tub/outhouse/whatever) - but that would seem to be a rather bizarre, hence very improbable, scenario, wouldn't it?

What am I missing?

Kind Regards, John

 

[bernardgreen]

Not really, since the same "process" can be used for either, so there is nothing to "balance". If one measures the Zs on a final circuit or, better, at the origin of the installation will give a good handle on the Ze, whether it be a TN ';earth' or a TT earth.

That is a similar process for testing the earthing arrangement, it is NOT the same processes for maintaining the earth system's effectiveness.

Open circuit network Neutral, involve the DNO

TT ground rod becoming higher than acceptable impedance, use a bucket or two of water

 

[bernardgreen]

If you google for "power surge" metal theft you will get a lot of hits for incidents where a "power surge" caused damage to appliances. Thieves steal the Neutral conductor because it presents a much lower risk of them being electrocuted Loss of the Neutral due to criminal action.

"power surges" also happen when a local network Neutral conductor breaks for some non criminal reason.

 

[bernardgreen]

A posting on
https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=65452

by G J Ellis is well worth reading ( as is the rest of the thread )


OK, here's an opportunity to stop a few more of my colleagues perpetuating the misappropriation of standard electrical terminology. Just to get our terms correct here, to answer the original question: your earthing system is TN-C-S according to your description (consumer-side having a main earth bonded to the neutral at the interface with the supply-side -- that is Earth (Terra) and Neutral (N) are Combined (C) on the supply-side and Separated (S) on the consumer-side).

The visual confirmation of this is your last comment that the supply poles have Protective Multiple Earthing on at least every third pole. The existence of effective PME external to the installation (the DNO's equipment) is the only time a DNO will provide TN-C-S. If the PME fails or cannot be proven effective, the DNO will instruct that you are TT and you must install an earth electrode. (With TN-S and TN-C-S the means of earthing is provided by the DNO; with TT you must provide your own.) You state that you already have one, which is good as it overcomes the inherent risk associated with the potential loss of the neutral if the overhead cable gets damaged, which would leave the installation live but with no earth (a chance that DNO's state is small enough to be disregarded for a risk assessment, and yet seems to happen every year in the wild and is why most medical installations and the MoD prohibits its use, insisting only TN-S or TT are permitted).

To summarize my excessive waffle: No installation ever has PME! That belongs to the DNO, it is not an earthing system, it merely reinforces the earthing system. What the DNO provides you with makes you part of a TN-C-S earthing system.

 

[JohnW2]

That is a similar process for testing the earthing arrangement, it is NOT the same processes for maintaining the earth system's effectiveness. .... Open circuit network Neutral, involve the DNO ... TT ground rod becoming higher than acceptable impedance, use a bucket or two of water

All true - and I agree that, from the consumer's viewpoint, dealing with a high impedance TT electrode (if the consumer were awaree of it, which will almost never be the case) is, in some sense, 'easier' than dealing with a PEN fault (although, in some senses, even the later is 'easier'; for the consumer - since they pass the entire buck to the DNO.

However, each time I respond to you, you seem to be coming back with 'moved goalposts' (or 'additional goalposts' I was responding to you:

The balance between the effectiveness of the processes for checking these two requirements is part of the equation to decide TT or TN

... but, after I pointed out that the same 'checking process' was equally applicable to either TN or TT (so nothing to 'balance'), you have now added "the process for maintaining"

In passing, you might have added to your argument the fact that, in reality, the "bucket or two of water" (if the consumer realised it might be required, which they almost certainly won't). A TT installation relies on RCDs for fault protection. If everything in the installation is protected with 30 mA RCDs, then the impedance only has to be below about 7,666 Ω for that to work, and if there are some non-RCD circuits, such that fault protection relies on an up-front 100 mA RCD, then it needs to be below about 2,300 Ω. Unless the earth electrode becomes totally disconnected (which, again, is something of which consumers would almost never be aware) is is very unlikely that the impedance of the ear rod would get anything like that high.

I should perhaps add that, as you and others here should know, I have no 'axe to grind', in that I am no fan, let alone 'champion', of TN-C-S. In my own house, although I have been offered a TN-C-S 'earth', I have declined that offer, being much happier sticking with my TT.

Kind Regards, John

 

[JohnW2]

If you google for "power surge" metal theft you will get a lot of hits for incidents where a "power surge" caused damage to appliances. Thieves steal the Neutral conductor because it presents a much lower risk of them being electrocuted Loss of the Neutral due to criminal action.

Yes, one often hears/reads that, but it rather surprises me.

My understanding is that most of these 'metal thefts' occur at substations/transformers. As far as I can see, resultant problems of the sort we are discussing would only arise if the perpetrators stole the conductor between transformer and the distribution network, but left in place the conductor conductor connecting the transformer to earth - but since the latter is presumably also copper (and quite possible more accessible) I don't quite understand why they don't steal that as well. If they did, then there would simply be a 'power cut' with no risk of any other problems ("power surges").

Kind Regards, John

 

[bernardgreen]

The critical cut is the separation of the network Neutral from the star point in the transformer. The network Neutral conductor is then either

(1) the star point of the local distribution of 3 phases to the local area and as such its potential can fluctuate depending on the ratio of the loadings on the 3 phases

(2) the open circuit return from a single property which has it's own single phase transformer and hence will be pulled to the Live potential when there is a load on the supply,

( Option (2) is unlikely to be the result of metal theft but could happen if overhead cables are damaged )

 

[bernardgreen]

in reality, the "bucket or two of water" (if the consumer realised it might be required, which they almost certainly won't)

It is because the average customer cannot be relied upon to maintain a Ground electrode that the DNO will offer an Earth with the supply,

In the early days of domestic power supply the domestic "Earth" was derived from the metal water pipe in the street. Often the metallic water supply network was connected to the star point at the substation.

When water supply pipes became plastic the DNO had to think about alternative means to earth the average customer's installation.

 

[JohnW2]

It is because the average customer cannot be relied upon to maintain a Ground electrode that the DNO will offer an Earth with the supply,

I would be more inclined to say that DNOs (usually/often) offer an earth because DNOs (usually/often) offer an earth.

I would imagine that the proportion of domestic earth electrodes that have ever been 'maintained' is so tiny as to be negligible. However, as I said, given that we now have RCDs, and given that we don't live in a desert, I doubt that that any (connected) domestic earth electrode ever develops such a high impedance as to not 'serve the purpose', even without any 'maintenance'.

When water supply pipes became plastic the DNO had to think about alternative means to earth the average customer's installation.

I would have thought that TN-S was around for decades before anyone even knew what a plastic water pipe was.

TN-C-S is the cheapest method for a DNO to provide an earth 'ab initio', and 'conversion to TN-C-S' is clearly their cheapest way of fulfilling their obligation to continuing providing and earth (once they have started providing one) in the case of a 'failed TN-S earth'.

Kind Regards,
John