BS7671:2018 DPC - Local Earth Electrode with TN systems

[JohnW2]

I like the comment in another forum - The need for this is the fault of the DNOs and it should be up to them to correct it.

I think that's probably a bit unfair. Things break and/or develop faults. If we did not have to accept that, we would be able to do without a lot of the 'precautionary' measures we take - whether in terms of electrical installations or anything else.

Kind Regards, John

 

[EFLImpudence]

Yes, but that is OUR property for which WE have to take precautionary measures.

Had the DNOs replaced damaged TNS supplies, this would not be an issue.

 

[EFLImpudence]

"to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth."

How does an electrode do that?

 

[JohnW2]

Yes, but that is OUR property for which WE have to take precautionary measures. Had the DNOs replaced damaged TNS supplies, this would not be an issue.

Supply cables can, and do, develop faults or become damaged at any time, for a wide variety of possible reasons. No matter how much maintenance work DNOs did/do, there will always be a risk of a problem developing in some cable at some point in time - so (if one is a 'precaution type of person') one still needs to take precautions.

Kind Regards, John

 

[JohnW2]

"to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth." How does an electrode do that?

Yes, I wondered about that, too. Provided the property is properly configured as an equipotential zone, there should be no significant 'touch voltages' as a result of the loss of an earth connection (and maybe simultaneous loss of neutral), per se - but I suppose there are always those 'outside taps' etc. to consider!

I suspect what they are thinking of is the possibility of an earth being lost (possibly unbeknown to the house owner for a potentially long time) with an 'L-E' (L-CPC) fault subsequently developing somewhere. With no earth, no protective device would operate as a result of that fault but, with a 'supplementary earth electrode', one would hope that an RCD would operate - and, with the 18th, we will (as expected) be approaching the point at which everything will be required to be RCD protected.

Kind Regards, John

 

[ban-all-sheds]

Unless, I'm misunderstanding, this "low resistance electrode" is, at least in domestic properties, likely to have an impedance of at least a few dozen ohms, isn't it - in which case that would very much be the limiting factor, limiting the current to just a few amps even if full phase voltage were applied to it??

In contrast, extraneous-c-ps, already required by regs to be bonded to the MET, might, in some cases, have very low impedances to earth.

In one of my boxes of round tuits I've got 10m (IIRC) of 25x3mm copper tape, to be buried and used as the earth for a generator if I decide that such a facility is a Good Idea.

So would that be a "low resistance electrode", or an extraneous-conductive-part? Does it make a difference if the generator is installed to provide a TN-S supply vs a TN-C-S?

 

[ban-all-sheds]

Supply cables can, and do, develop faults or become damaged at any time, for a wide variety of possible reasons. No matter how much maintenance work DNOs did/do, there will always be a risk of a problem developing in some cable at some point in time - so (if one is a 'precaution type of person') one still needs to take precautions.

True, but more diligence on the part of DNOs to repair flaky TN-S cables might well reduce the risks from said flakiness to a level so low that no additional mitigation is required.

And if any of these proposals in the 18th are driven by risks inherent in TN-C-S supplies, if the DNOs had always had a practice of adding an earth rod at every property they supplied with TN-C-S then maybe they too wouldn't be needed.

 

[EFLImpudence]

Yes, I wondered about that, too.

That's good. I thought I was missing something.

I suspect what they are thinking of is the possibility of an earth being lost (possibly unbeknown to the house owner for a potentially long time) with an 'L-E' (L-CPC) fault subsequently developing somewhere. With no earth, no protective device would operate as a result of that fault but, with a 'supplementary earth electrode', one would hope that an RCD would operate - and, with the 18th, we will (as expected) be approaching the point at which everything will be required to be RCD protected.

Ah. I thought it was to counteract a lost supply neutral in TNCS supplies.

The regulation would seem to be pretty universal - no mention of new builds or if RCDs fitted or to be fitted.

Mr.Customer may be presented with "I can't do so and so unless you have an electrode and new CU".

 

[JohnW2]

Ah. I thought it was to counteract a lost supply neutral in TNCS supplies.

As I said, if one has a properly constituted equipotential zone, loss of neutral (and/or earth) will not result in any 'touch voltages' arising within that equipotential zone.

The regulation would seem to be pretty universal - no mention of new builds or if RCDs fitted or to be fitted. Mr.Customer may be presented with "I can't do so and so unless you have an electrode and new CU".

I don't doubt that some will be presented with that, just as they are currently presented with "I can't do so and so unless you have an electrode and new CU" - but there's no accounting for cowboys! However, as we know, BS7671 regulations are virtually never retrospective. If a customer actually wanted their installation to be brought up to "18th ed standards", they would presumably need the electrode and, if they didn't already have them, a 'non-combustible CU' and RCDs protecting almost everything (virtually all sockets, plus buried cables, luminaires and bathrooms) - to name but a few issues

One question is whether (although the primary TT will not be 'distributor supplied') this proposed reg will require TT installations to have a second electrode (to facilitate RCD operation if the primary one 'fails').

Kind Regards, John

 

[JohnW2]

True, but more diligence on the part of DNOs to repair flaky TN-S cables might well reduce the risks from said flakiness to a level so low that no additional mitigation is required.

That obviously depends upon how 'risk averse' one is. I doubt, for example, that any level of dilogent maintenance would convince Bernard that nothiong was ever going to go wrong.

And if any of these proposals in the 18th are driven by risks inherent in TN-C-S supplies, if the DNOs had always had a practice of adding an earth rod at every property they supplied with TN-C-S then maybe they too wouldn't be needed.

Maybe they will (with new TN-C-S installations), if/when this new reg comes into force.

Kind Regards, John

 

[flameport]

properly constituted equipotential zone,

Many properties do not and cannot have that, due to the prevalence of plastic piping for services both inside the building and services connected to it.

Most new supplies of gas and water to properties are plastic, as are sewers and other drainage.
Most new build property has plastic pipes for the internal water and heating.

 

[ban-all-sheds]

I doubt, for example, that any level of dilogent maintenance would convince Bernard that nothiong was ever going to go wrong.

Nor me.

But I am open to the suggestion that better maintenance would reduce risks to a level low enough to not warrant other measures being put in place to reduce them still further.

I'm also open to the suspicion that JPEL/64 have become evidence-free rule makers.

 

[JohnW2]

So would that be a "low resistance electrode", or an extraneous-conductive-part?

As we've discussed before, although the intended purposes may be different, I don't think there is much practical difference, particularly in the sense that either would have to be adequately connected to the MET.

I suppose the main logistical difference is that if something is regarded (and 'labelled' etc.) as an 'earth electrode', it is likely to remain in place, whereas most extraneous-c-ps might disappear overnight if some workmen crept in quietly with a lorry loaded with plastic pipe.

Kind Regards, John

 

[JohnW2]

[of an equipotential zone]Many properties do not and cannot have that, due to the prevalence of plastic piping for services both inside the building and services connected to it. Most new supplies of gas and water to properties are plastic, as are sewers and other drainage. Most new build property has plastic pipes for the internal water and heating.

You've rather lost me there, since you seem to be describing the ideal conditions for an equipotential zone - the primary requirement of which (in present context) being that no conductor with a path to true earth should enter the building. If that is the case, then there will be no touchable pds within the building, regardless of what potential (relative to true earth) the incoming neutral and/or earth may have.

Kind Regards, John

 

[ban-all-sheds]

Even if that tape were to disappear one night, I doubt that the disappearers would replace it with plastic pipe.

And, with the disclaimer that I still haven't looked at the DPC yet, might there now be a practical difference with the requirement to disconnect earthing of alternative supplies?