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 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.
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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)
The best description of it I have found so far is here:
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So it seems some UK households do have this configuration.
Have we not had this conversation quite recently? Interestingly enough, our council put out a press release this morning about enabling people to get charging points that were good enough at home and of course on the street operated by mobile tech, apparently. All well and good but surely they must also realise that all of this puts up demand for power in a climate of lack of investment in Nuclear, the only current viable way to generate it without either greenhouse gasses being emitted or the high cost and intermittent nature of renewables. Brian
AFAIK PME has been the /default/ for new, ordinary connections for quite some time. (By 'ordinary' I mean not a caravan, boat, railway etc.) And a fair few older houses which started out as TN-S have been switched to PME.
I doubt they realise at all. The general populus has been lulled into a very deep sense of security due to the reliabilty of the Britain's mains electricity. It's always there, it happens by "magic". They haven't clue how much effort goes on in the background to keep the lights on or the limitations of the whole system from generation to local distribution. Even less idea of how much energy transport consumes.
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.
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.
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.
Overhead supplies have often had PME added later. This place (overhead supply) was fully rewired in the mid 80's I believe, and was TT then. However at some time in the 10 to 15 years after that, the network must have been upgraded to PME since the cutout now sports a yellow PME label (well spotted Adam!)
I suspect the last time the general public were affected was whn a Duisrtibution board in Battersea failed. That took out most of West London
- including BBC TV Centre. The reserved feed to that building was "fed directly from Battersea" - from the power board that failed. The emergebncy generator hadn't yet been commissioned so BBC2' first night didn't happen.
I am also sure that I am not sure. New builds in Epworth that I have just worked on (one street). Half are TN-C-S with a 100A supply and half are TN-S with a 80A supply.
However I would say that 80% of newbuilds I have worked on in the last
20 years are supplied with a single core concentric cable.
What would be the reason for not using PME with overhead supplies in the first place? Were some ot the neutral wires too long to have a low enough resistance? Or was it just that most overhead supplies were put in before PME was fashionable? Our overhead supply is PME but it was redone in the 80s.
PME is used by supplier networks, others have already mentioned the 'multiple' bit. Combined N&E is not used within the house wiring itself - in America it has been in the past.
I would guess that the neutral was designed to be just that, and not a PEN conductor, and it was only earthed only the once at the sub station in the same way that a TN-S supply would be. Hence there would be a much greater risk if the neutral was cut anywhere along the route.
As to why, perhaps because it is more expensive to PME an overhead supply, or simply they did see the need at the time, and considered that leaving it TT was ok.
No. I had to administer first aid to a US installation engineer being trained at our facility in the UK who used that approach on UK mains. It threw him across the room knocking the wind out of him entirely. After the incident a warning was added to training materials never to do this!
Warm sweet tea aided his recovery but he was lucky not to be more seriously hurt. One of my friends at university got hold of the wrong end of a Jesus lead with serious very burns to his hand resulting.
Thanks. You would think it made more sense to do one or the other.
So the incoming armoured cable has a live core and outer PEN sheath?
I expect it is against the wiring code but is there anything to prevent the householder having a stake in the ground bonded to the household protective earth in addition? It won't make a blind bit of difference most of the time but if the neutral return ever fails then at least you don't suddenly find every household appliance is live to the touch.
It is just after all another parallel multiple earth local to the premises and although it cannot sink anything like the current that the PEN can it does have the advantage of keeping the water pipes from becoming live in the unlikely event of a fault.
Round here single phase or neutral return fail faults are not at all uncommon if a tree branch hits a line as it falls. Usually the people on the phase at the top of four wires bear the brunt of such cuts. The first wire snapping slows the branch down enough that the second or third one down survives.
Interestingly the new fully insulated steel cored combined aluminium cable can survive a whole tree falling on it sometimes although it bent all the poles and ripped several mains supplies off houses along one side of the road. You get some interesting mains faults in rural areas.
Last big one was when the milk tanker skidded on ice on the coldest day of the year and took out a pole wrecking all mains and phones.
The Sizewell/Longannet incident of May 2008 is likely to have affected the power to far more people than either of those. The automatics dropped customers off supply across London, Cheshire, Merseyside and East Anglia.
I expect the whole country felt the effects of it as well. We didn't lose power but had a big dip and unstable supply for a few seconds.
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