Danger of PMR (TN-C-S) and TT earth when used close together.

Joined
27 Jan 2008
Messages
19,520
Reaction score
1,858
Location
Llanfair Caereinion, Nr Welshpool
Country
United Kingdom
When I had a caravan parked close to the house and plugged it in to charge battery I used the TN-C-S supply as you could touch the gas and electric meters as the same time as the caravan, so considered the two should be connected to the same earthing system.

When using the caravan fire regulations mean must have 3.5 meter gap at corner or 5 meters at sides, so having a caravan on TT and building on TN-C-S on a caravan site there is no real danger, and so on a caravan site always TT OK I know TN-S is allowed but in practice normally TT, but at home when parked close to house, then same earth as house.

However in last few years we have seen the electric vehicle, where more than 70 volt is used for the supply, so not mobility scooters, ebikes, fork lifts or milk floats, and it seems like the caravan, boat or petrol station TN-C-S is not permitted, however some of these supplies are actually on the building wall, local milk man has the supply to car within a meter of the gas and electric meters, so the van can be parked where you can touch both the meters and the vehicle at the same time, in fact it would be natural for one to touch the vehicle when standing up again after reading meter.

Clearly from the fires reported, where it took I seem to remember 5 days to put out the fire when Richard Hammond crashed an electric car, it would be daft to garage an electric vehicle while charging it in an integral garage, in fact really there should be the same gap as allowed for caravans, but often there simply is not the option. Even commercial the charge points tend to be close to buildings.

9623554.jpg

This shows my local one, very close to a steel roller shutter door, which will be earthed to building earth, and close to wood panelling of a building with no fire alarm system installed as waiting renovation. The building supply is two three phase TN-C-S supplies.

I would hope this is safe, and since no one sleeps in that building even if a fire one one should get killed, and in this case they do light fires in the local buildings although not that one, and it is common to see smoke pouring out of the building, which of course means any smoke would be ignored that's normal.

So how safe are these vehicle charging points, one electrical with different earth to main building and two fire wise.
 
Sponsored Links
Joined
28 Jul 2009
Messages
6,372
Reaction score
507
Location
Kent
Country
United Kingdom
When I had a caravan parked close to the house and plugged it in to charge battery I used the TN-C-S supply as you could touch the gas and electric meters as the same time as the caravan, so considered the two should be connected to the same earthing system.

When using the caravan fire regulations mean must have 3.5 meter gap at corner or 5 meters at sides, so having a caravan on TT and building on TN-C-S on a caravan site there is no real danger, and so on a caravan site always TT OK I know TN-S is allowed but in practice normally TT, but at home when parked close to house, then same earth as house.

However in last few years we have seen the electric vehicle, where more than 70 volt is used for the supply, so not mobility scooters, ebikes, fork lifts or milk floats, and it seems like the caravan, boat or petrol station TN-C-S is not permitted, however some of these supplies are actually on the building wall, local milk man has the supply to car within a meter of the gas and electric meters, so the van can be parked where you can touch both the meters and the vehicle at the same time, in fact it would be natural for one to touch the vehicle when standing up again after reading meter.

Clearly from the fires reported, where it took I seem to remember 5 days to put out the fire when Richard Hammond crashed an electric car, it would be daft to garage an electric vehicle while charging it in an integral garage, in fact really there should be the same gap as allowed for caravans, but often there simply is not the option. Even commercial the charge points tend to be close to buildings.

9623554.jpg

This shows my local one, very close to a steel roller shutter door, which will be earthed to building earth, and close to wood panelling of a building with no fire alarm system installed as waiting renovation. The building supply is two three phase TN-C-S supplies.

I would hope this is safe, and since no one sleeps in that building even if a fire one one should get killed, and in this case they do light fires in the local buildings although not that one, and it is common to see smoke pouring out of the building, which of course means any smoke would be ignored that's normal.

So how safe are these vehicle charging points, one electrical with different earth to main building and two fire wise.
This is actually a far more common problem than it should be and far too frequently the 2 get joined together during electrical works.
 
Joined
27 Jan 2008
Messages
19,520
Reaction score
1,858
Location
Llanfair Caereinion, Nr Welshpool
Country
United Kingdom
This is actually a far more common problem than it should be and far too frequently the 2 get joined together during electrical works.
This was my thoughts, I remember a good few years ago working on a gas terminal where I spent a few months knocking in earth rods, maybe I just had zero potential, but were the site earth connected to DNO earth there was a massive resistor so limiting the current flowing between the two earth systems. With a house even with an earth rod not likely to be that good so there is a limit I was putting in between 3 and 8 x 1.2 meter earth rods to get 8Ω and at 230 volts that's less than 30 amp so 6 mm² would be ample.

But there is no fuse in an earth wire, so with a radio ham who installed an earth pit, when the DNO lost the combined earth neutral the whole street tried to earth down his 4 mm² earth cable to his shack which melted. It was not as part of a cable, so easy to renew and more important see the damage, but it would be so easy for an earth to fail due to over current without being detected, had this a few times where welding has gone on.

So there are dangers connecting earth systems together, and also dangers where not bonded together, with a PEN failure you can get 230 volt between to earth systems.

So we talk about a gradient, how true this is I don't know, but I was told 25 volt is enough to kill a cow, and I would say cows feet are a meter apart so around 10 meters between a TT earth and a TN-C-S earth is required to be safe with cattle around, and the old BS 7671 had pictures of how to earth live stock sheds.

So if the vehicle charging point has a TT earth, what measures are taken to stop any animal be it human, dog, cat, badger or any other from touching a metal surface connected to the TT and the TN-C-S earth? An earth fault through the ground I would assume any animal would feel discomfort before it reaches a dangerous point, however I did find my daughter at 4 year old licking a 9 volt battery, so may be not, but I would say if the fire regulations that apply to a caravan also apply to a car, then the 3.5 meters in the main for fire regulations should be ample.

But can't find any fire regulations for an electric car, seems you can if your daft enough charge it inside an integral garage, I watch a guy clearly having a problem walking putting his hand on each car as he passed them, OK when up a drive where passers by should not touch them although still postman and milkman etc. But if charged on the street what are the rules?

All the older electric vehicles the charger was static, and the transformer in the charger isolated the vehicle from the supply electrics there was a simple two pin connector so the fork lift or milk float was not earthed, same with mobility scooter or ebike. But the cars and vans have the charger in the actually vehicle, still no real reason which the vehicle needs earthing, I know with the narrow boat shore supply only supplied the class II battery charger so the earth was not imported, the step down transformer of the battery charger was the isolation between shore and boat, there is no reason why that should not happen with a car, however from what I read that is not the case.

So how are the two earth systems keep apart and safe?
 
Joined
28 Jan 2011
Messages
50,220
Reaction score
3,365
Location
Buckinghamshire
Country
United Kingdom
So there are dangers connecting earth systems together, and also dangers where not bonded together, with a PEN failure you can get 230 volt between to earth systems.

Whilst you are talking about TN-C-S 'earths' and (something you are calling) 'a TT earth', the situation would be identical if you were talking about a TN-C-S installation and an extraneous-c-p (which is 'a TT earth' under a different name).

In that latter case, it is deemed to be satisfactory/safe if the two are connected (bonded) via a conductor of adequate CSA, and not acceptable to not bond them together. Is there any difference in the situation you are discussing?

Kind Regards, John
 
Joined
27 Jan 2008
Messages
19,520
Reaction score
1,858
Location
Llanfair Caereinion, Nr Welshpool
Country
United Kingdom
Yes @JohnW2 you are correct, if I was to build a house using a reinforced concrete slab I could connect the re-bar in the slab to the incoming earth, and that earth could be rather good, I noted on the building of T5 air port there were copper strips connected to the re-bar before the concrete was poured, and these were connected to some thing, I assume incoming earth, the maximum current will be that provided by the step down transformer and at the size of the copper coming out of the slab it is unlikely that would melt, and with a row of houses all with the water pipes and gas pipes bonded, any earth currents will be shared between them all so non would have a large current, and it is unlikely to have enough of a difference for there to be any voltage differential between houses. And since the DNO decides what earth is used all adjacent houses will have the same voltage. Either whole row is TN-C-S or TT it is not mixed down the row of houses.

However with a TT charging post, it can be very close to a TN-C-S house and it is not bonded to the house earth, so there can be a voltage differential between the house and the car on charge.

So two independent problems, one excessive bonding conductor currents if one odd house has any earth rods bonded to system and two the voltage differential if the two earths not bonded together, personally I think the whole idea of TN-C-S is flawed, but with charging points the flaw is being highlighted, it was OK to put a shed at bottom of garden on TT and house on TN-C-S but there is then a voltage gradient, but the charging points are so close to the building there is no voltage gradient.
 
Joined
28 Jan 2011
Messages
50,220
Reaction score
3,365
Location
Buckinghamshire
Country
United Kingdom
However with a TT charging post, it can be very close to a TN-C-S house and it is not bonded to the house earth, so there can be a voltage differential between the house and the car on charge.
Yes, but anything and everything outside (but close to) 'a TN-C-S house' could be at a very different potential from the TN-C-S house's 'earth' (in the, very rare, event of a supply-side fault). For example, your photo showed a steel shutter which, as you said, could presumably be bonded to the building's TN-C-S 'earth' - so anyone touching that whilst standing on the adjacent ground would theoretically be at potential risk (just like the 'outside tap' issue).

In other words, I think the problem/risk always exists when anything (like that shutter) connected to a TN-C-S 'earth is touchable outside the (equipotential zone of the) building, whether there is a "TT charging post" in the vicinity or not. Maybe we should outlaw such things, which maybe we could call (from the point-of-view of the outside world) 'extraneous-c-ps' ??

Kind Regards, John
 
Joined
28 Jul 2009
Messages
6,372
Reaction score
507
Location
Kent
Country
United Kingdom
I should have a photo of a situation where the 2 earths were linked by really poor engineering practice via the earth wire to a socket. The first that was known of the missing DNO earth being the socket fuse blowing where insulation had melted.
 
Joined
18 Nov 2008
Messages
1,540
Reaction score
199
Location
Surrey
Country
United Kingdom
Much of the New Charging systems allow to Directly connect to PME now and have a cut off device if the Earth is lost from what I've read.
 
Sponsored Links
Joined
28 Jan 2011
Messages
50,220
Reaction score
3,365
Location
Buckinghamshire
Country
United Kingdom
Much of the New Charging systems allow to Directly connect to PME now and have a cut off device if the Earth is lost from what I've read.
Do you mean that it would 'cut out' in the event of the TN-C-S supply's neutral (i.e. CNE) being 'lost'? If so, that would presumably require a local TT electrode to enable that situation to be detected?

Kind Regards, John
 
Joined
28 Jan 2011
Messages
50,220
Reaction score
3,365
Location
Buckinghamshire
Country
United Kingdom
Interesting. I suppose it's fair enough up to a point, in that they are assuming that if there is a supply neutral fault that results in a high potential between N and (true) earth, than the L-N voltage will then decrease.

However, what they describe is (relay) switching which includes switching of the earth which, in general, is not allowed in an electrical installation.

Kind Regards, John
 
Joined
2 Mar 2020
Messages
4,316
Reaction score
106
Country
United Kingdom
They are concerned by the battery being a power source - good idea as well as short circuit currents from that can be spectacular. That seems to the general reasoning behind the regs.
 
Joined
28 Jan 2011
Messages
50,220
Reaction score
3,365
Location
Buckinghamshire
Country
United Kingdom
They are concerned by the battery being a power source - good idea as well as short circuit currents from that can be spectacular. That seems to the general reasoning behind the regs.
Possibly, but I'm not really sure what that has got to do with the earthing arrangements - and one certainly cannot get 'spectacular currents' to flow into a TT electrode.
 
Joined
10 Mar 2007
Messages
10,374
Reaction score
2,124
Location
Poole, Dorset
Country
United Kingdom
To properly detect an open PEN conductor where the voltage between the Earth and the circuit protective conductor is above 70V, then an earth electrode is required, the voltage being monitored between that and the CPC. Disconnection of all conductors including the CPC is required.

However that requires that the electrode is installed a significant distance away from any other items connected to the installation earth terminal, including any cabling underground, metallic gas, water or other pipes, and any other extraneous conductive parts. Distances do vary but several metres would be the absolute minimum, perhaps up to 10m in some cases.
This is not likely to be practical or even possible for many installations.
Any single phase device which does not have a connection to an earth electrode cannot determine this voltage and therefore can't comply with 722.411.4.1(iii)
There are some three phase devices which can do this with no earth electrode, by determining the relative voltages between all 3 phases and the neutral, but rather obviously those can only be installed where three phase is available - which rules out the vast majority of domestic locations.

Another method described in 722.411.4.1(iv) which is now included in BS7671 Amendment 1 (which can be downloaded free here: https://electrical.theiet.org/bs-7671/updates-to-18th-edition/ ) is to monitor the voltage between L&N and disconnect if that voltage is outside the normal range (207-253).
However that only detects the fault in some situations - it's entirely possible for an open PEN to cause both L&N to rise relative to Earth, so that the voltage between them is still within limits but the voltage between the CPC (N) and Earth is greater than 70V.

Some charge points also include a current sense transformer on the CPC, so that if current flows there, the supply including the CPC is disconnected.
However that relies on a current path from the CPC to Earth, which in most cases would be through the unfortunate person touching the vehicle. It is also using the equivalent of an RCD for the only means of protection, which is not permitted (415.1.2).

Just installing everything as TT isn't a particularly valid option either because once again, that electrode must be installed far away from anything else.
 
Joined
28 Jan 2011
Messages
50,220
Reaction score
3,365
Location
Buckinghamshire
Country
United Kingdom
To properly detect an open PEN conductor where the voltage between the Earth and the circuit protective conductor is above 70V, then an earth electrode is required, the voltage being monitored between that and the CPC.
That's what I said in my initial post but when I read the description that was posted, I was sort-of persuaded that (in a single phase installation) detection of a substantially reduced L-N voltage would probably achieve the same, since I can't see how/why L voltage would rise (relative to true earth) in the event of an 'open PEN'.
Disconnection of all conductors including the CPC is required.
That's clearly 'necessary' in the situation we're discussing but, as I implied, how can one square that with the regulatory prohibition of having a switching device in a CPC?

I hope everyone is managing to keep well.

Kind Regards, John
 
Sponsored Links
Top