There are some conditions that the neutral earthing resistor  does not guard against - and the grounding to terra firma of any one phase  like that barn in the incident described above

Thinking about it, I reckon it could help even then - provided the chosen resistance was much larger than the source's electrode resistance - which shouldn't be too hard - given that DNOs work to max 20Ω anyway (often <1Ω).

e.g. with a resistance >200Ω  and Rb < 20Ω you shouldn't see PE dragged more than 1/10th of Uo above true Earth, no matter how low the resistance of the fault to Earth.

   - Andy.

I must admit I find the faults described a bit confusing  I can understand that if the neutral is broken then the current goes back via customers earth we had that at a house on the estate I lived on  everything metallic  was at a decidedly painful voltage above earth the DNO came out and sorted a damaged neutral in the meter box  the weird thing was the estate was only converted to PME maybe a year before. What's needed now I think is everytime an EV charger is installed is for the DNO to install a long length of copper pipe in a trench in the street bonded to that customers CNE terminal in the house they can afford it  what do you think?

Kelly Marie Angel said:

What's needed now I think is everytime an EV charger is installed is for the DNO to install a long length of copper pipe in a trench in the street bonded to that customers CNE terminal in the house they can afford it  what do you think?

Come to think of it, when I put in the duct for my service cable, it wouldn't have been difficult to put in a 20 ft length of 15 mm copper pipe, or indeed some form of copper mat. Even better would have been to put something under the 500 sq ft of garage floor, or at the very least to have made a connection to the rebar.

When we wired my daughter-in-law's house, there was a 60 ft trench, let alone their garage floor, which were available.

Hindsight is a wonderful thing, but I think that Kelly Marie Angel makes a very good point.

I must admit I find the faults described a bit confusing

Apologies- that's probably down to me going off at tangents (again). To try an clarify, a broken N causing shocks only happens when we have a common neutral and PE conductor (i.e. TN-C or TN-C-S system (e.g. PME)) - so maybe going back to the old TN-S system might be an improvement. The other tangent was recognising that TN-S still isn't entirely without problems, so a further "improvement" might be earthing the neural through a resistance (IT style) but still having a common earthing conductor throughout (TN-S style) - not something that BS 7671 entirely  recognises at present, but has been talked about for a number of years now.

    - Andy.

AJJewsbury said:

so maybe going back to the old TN-S system might be an improvement.

Or perhaps another way of thinking about it.

I think the issue is, that a break in a Protective Conductor is really what causes the electric shock issue, and due to protective conductor currents, it's a problem in any TN system (and potentially, any TT system, although in that case, the break would be within the consumer's installation, and not using aluminium foil). It's not really a break in a Neutral that causes the potential electric shock (although, from a fire protection perspective, a broken Neutral in any three-phase system is potentially a huge issue).

AJJewsbury said:

so maybe going back to the old TN-S system might be an improvement.

Or perhaps another way of thinking about it.

I think the issue is, that a break in a Protective Conductor is really what causes the electric shock issue, and due to protective conductor currents, it's a problem in any TN system (and potentially, any TT system, although in that case, the break would be within the consumer's installation, and not using aluminium foil). It's not really a break in a Neutral that causes the potential electric shock (although, from a fire protection perspective, a broken Neutral in any three-phase system is potentially a huge issue).

I think the issue is, that a break in a Protective Conductor is really what causes the electric shock issue, and due to protective conductor currents, it's a problem in any TN system (and potentially, any TT system,

Often the protective-conductor current from a single domestic installation is small (witness the number of installations that survive perfectly well with a single 30mA RCD as the incomer) - not always granted -  but the available current is relatively limited. A TN-S system with additional electrodes on PE scatted about the distribution system (either in each installation or at strategic points in the network PME style) would have a very good chance of keeping touch voltages well down. The trouble with open PEN faults is that they simultaneously open the PE and make available a very substantial current, so that additional electrodes are very unlikely to make a significant difference. 10mA or even 100mA on a 200Ω electrode is a much more comfortable proposition than several 10s of A on a 200Ω electrode.

    - Andy.

We can't assume, for a broken TN-S PE conductor, that only "leakage" from one installation might be involved ... it may well involve a number of installations.

We also can't assume that there will be lots of earthing of the PE (unless BS 7671 requires consumers to each have a supplementary earth electrode ... only a recommendation now). In fact, we are moving to plastic external service pipes.

If the earthing is put along the TN-S cable, then it would be even more costly, and of course, only the electrodes on the consumer's side of the broken PE will be effective ... which really is one of the issues with PME broken PENs in the first place?

We can't assume, for a broken TN-S PE conductor, that only "leakage" from one installation might be involved ... it may well involve a number of installations.

Very true - equally and open-PEN event can affect multiple premises (perhaps something 722.411..4.1(ii) might want to consider). Still even as the number of installations affected increases the  relative scale of the currents involved remain similar - maybe 10,000x higher for a broken PEN than for a broken PE. There may be some cancellation of PEN currents in the 3-phase distribution system - but likely the same would apply to TN-S PE leakage currents.

There are known problems with PME. If we were to look for something better, perhaps TN-S with additional electrodes (ideally one per installation) might be a candidate. (Presuming we don't have the appetite for something more radical that is.)

If the earthing is put along the TN-S cable, then it would be even more costly

Maybe 4-core + armour cables with a semiconducting outer sheath? (I think semiconducting 'insulation' is already used in some audio cables)

    - Andy.

I would be worried that after a few decades underground, a 4-core wire armoured cable could have deteriorated into a 4-core rust armoured cable.  Wouldn't 5 core be better, using the armour only for mechanical protection?

Simon Barker said:

using the armour only for mechanical protection

The armour would need to be earthed.

Simon Barker said:

Wouldn't 5 core be better

It would be more difficult to break the PE link back to the transformer. However, it would do nothing for providing regular contact with the ground along the length of the run (which is one advantage that hessian-sheathed armoured cables had, when the hessian gets wet, you've got good contact with the ground along the length of the cable, and additional earthing ... until the armour is lost to corrosion of course).