However, in the case of a TN-S system with no extraneous-conductive-parts, in the event of a PE break, a consumer's earth electrode connected to MET will help divert at least 83 % of the available touch-current (assuming body resistance of 1 kΩ, and earth electrode resistance at the 200 Ω maximum recommended for stability).. So, let's say we had a potentially lethal touch-current of 30 mA, this is reduced to a much less problematic 5 mA (at least in dry conditions).

Agreed - however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.

   - Andy.

AJJewsbury said:

however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.

Andy, you can't compare apples and oranges.

My reply regards controlling touch-currents in certain cases of TN-S systems with broken PE in the distribution network, is in no way relevant, applicable, or comparable, to the case of trying to control touch-voltage PME with broken PEN conductors!

I think the assumed value of 200 Ω in the example calculation gave that away!

As above, to control PME touch voltage, you really need low values of resistance - typically sub 1Ω, which is seen as not realistic in a number of areas in the country (unless you have a large amount of steelwork from a large steel-frame building to use as your electrode).

However, it's also the case that 5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes) could well help ... as discussed above.

5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes)

We have to be careful when the local connection to terra-firma on the fault side gets too good, as now it starts to matter about the "electrode" at the substation side as well.
The traditional assumption is that the transformer star point is well earthed,  and the fault is not. If the reverse is true, then it is all the folk on the non- fault side that get shocked  as the whole transformer is in effect lifted off true earth by a voltage set by the total current  coming back via terra-firma, and the combined electrode resistances on that side.
In effect the  planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )
Mike.

mapj1 said:

In effect the  planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )

Agreed ... but that raises more questions - like what voltage do you choose for protecting yourself, which we've not delved into yet either?

• 50 V ("traditional" BS 7671, and the value of PE potential rise that was often quoted that BS 7671 limits you to ... but it never did)?
• 70 V (e.g. Reg 722.411.4.1)?
• 100 V (typically used when assessing the distribution network for this and similar situations)/

mapj1 said:

In effect the  planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )

Agreed ... but that raises more questions - like what voltage do you choose for protecting yourself, which we've not delved into yet either?

• 50 V ("traditional" BS 7671, and the value of PE potential rise that was often quoted that BS 7671 limits you to ... but it never did)?
• 70 V (e.g. Reg 722.411.4.1)?
• 100 V (typically used when assessing the distribution network for this and similar situations)/