On 18/06/2019 09:13, Martin Brown wrote:

Arising from the EV Charging in the UK thread I am having an argument
with a USian who insists that UK wiring is unsafe and uses a combined

Its a little rich for someone in the US to complain that *our*
electrical systems are unsafe... You only need look at the electrocution
stats where they have annual death rates of thousands a year compared
with the "tens" per year we would expect here.

protective earth and neutral to save money aka PEN and TN-C-S. This
seems to be permitted in BS 7671 although I have never seen it myself.

https://en.wikipedia.org/wiki/Earthi...of_TN_networks

Its typically used on all new installs, and is the preferred method of
installation.

It also means that the supplier can provide an earth on overhead
supplies where previously that was not usually possible.

All the installations I have ever seen are either rural TT or old build
TN-S where the sheath of the incoming armoured cable is a true earth.

So how common is TN-C-S with a single PEN in new build? What proportion
of homes in the UK have the potential for every metallic "earth" bonded
surface in them to be made live if the neutral return were to fail?
(It only requires one appliance to be on for this to happen)

Its important to understand the roll and significance of PME in this
setup. We tend to use the terms PME and TN-C-S interchangeably, although
technically I suppose you could have TN-C-S without PME.

PME requires that the PEN conductor is earthed at *multiple* locations
all along the delivery path. So on overhead wiring it would be common to
earth it every few poles. (its also how some overhead supply systems can
be upgraded to support TN-C-S even if they did not initially do so. For
example, my setup here is overhead and TT, however the network was
upgraded latter with PME, so I could switch to TN-C-S if I wanted.

(it is also probably one of the reasons that Aerial Bundled
Conductors/Cables ("ABCs") are favoured for new overhead LV distribution
in the street - it make it much harder to snag and break the neutral in
isolation)

So while there is a risk associated with TN-C-S should the PEN be
interrupted[1], its considered a worthwhile trade off for the very good
low impedance earth provided, the ability to supply an earth connection
in most circumstances now, and the reduction of potential difference
between neutral and local earth at the property.



[1] and that is why the wiring regs, have specific requirements for
mitigating the risks of TN-C-S supplies.





--
Cheers,

John.

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