It does - as I went on to say ('outside taps').
Quite so - but the only equivalent of that for an outbuilding would presumably be not to 'export' a TN-C-S earth to it, wouldn't it?
Kind Regards, John
Kitchen appliance service man advise us to upgrade 20A MCB to 32A! (Rant)
Kind Regards, John
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Maybe, but I got the impression that a lot of the reason related to lobbying about 'practicalities', not to mention the potential problems/dangers that could arise if all installations connected to a given transformer were not all connected to local earth electrodes more-or-less 'simultaneously'. As I implied, given that JPEL/64 saw fit to introduce this proposal into their draft, I would not be confident that it's not going to show it's face again!
I don't know about countries which are "standardised on TT supplies", but it is certainly my understanding that some which have TN-C-S do have a requirement for each installation to also have an local earth electrode.
I agree. In fact, I think it's only a very small proportion of people who are extremely concerned (at least, vociferous) about this.
What does rather surprise (and somewhat please!) me is that, in the climate of "if it's technologically possible, it must be made a requirement" (as in SPDs, AFDDs, and some might even say RCDs), is that we have not (yet!!) seen the emergence of devices (rapidly followed by a 'requirement' to have them!) to disconnect a TN-C-S supply in the event of a marked elevation in neutral potential (which would probably require at least a 'reference' earth electrode)
Kind Regards, John
I think eric went a fair bit further than merely saying that we should "be aware of these things" when he wrote ...
Kind Regards, John
No, don't call it 'export'. Pretend your house is bigger and reaches to the outbuilding.
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As I said, it's not as simple as that. It would only be a valid thing 'to pretend' if, say, there were a hole in your living room floor leaving exposed 'earth' below for you to stand on (whilst touching things within the 'equipotential zone'.
It's nothing to do with the size/'volume' of the equipotential zone (which can be as large as you like) - it's to do whether every part of the zone (in the case of an outbuilding, including it's immediate surroundings) really IS 'equipotential'.
Kind Regards, John
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The problem arises when the house earth is taken to a metal object for an extended time, it seems annually there is around 500 times when the PEN is lost, and 10% of those causes an electric shock, this is from a video on the methods used to keep electric car charging safe.
We have considered that 50 volt is the safe limit to touch AC supply to earth, but for car charging this has been raised to 70 volt, to detect this there are two basic methods, one is a reference earth rod, the second is the voltage of the supply. With a three phase supply it is very unlikely you will loose a PEN without the supply voltage on one phase going over 254 or under 207 volt, so with a three phase supply measuring the voltage is a good way to detect loss of the PEN.
What is however a problem is with the single phase supply, the 254 volt limit still works, but the 207 volt limit does not, tests show you can get over 70 volt to earth when the supply voltage is still over 207 volt, so the earthed frame of a car can have over 70 volt to earth without the system detecting a loss of the PEN and so present a real danger.
For cars being charged in a gated enclosure the time this is likely to happen means it is unlikely to cause a lethal shock, where the problem lies is where the car is being charged on the street or where people or other animals can easy touch the car. We have for many years banned the supply of boats, petrol stations, and caravans with a TN-C-S supply for good reasons, but the pressure to role out electric cars has resulted in the problem of TN-C-S being redressed in unacceptable ways, with salt with snow and ice clearance vehicles I have had a good belt from 24 volt, under normal conditions 24 volt is not even felt, but with wet gritters I have had a nasty belt with just 24 volt. Although on a dry summers day 70 volt may not be a problem, on a winters day when gritters have been salting the street 70 volt could be lethal. The gritters do not grit the drives, so off street not so much a problem, but with on street charging of electric cars it is just a matter of time before some one is killed. Unless better protection is employed.
We have considered that 50 volt is the safe limit to touch AC supply to earth, but for car charging this has been raised to 70 volt, to detect this there are two basic methods, one is a reference earth rod, the second is the voltage of the supply. With a three phase supply it is very unlikely you will loose a PEN without the supply voltage on one phase going over 254 or under 207 volt, so with a three phase supply measuring the voltage is a good way to detect loss of the PEN.
What is however a problem is with the single phase supply, the 254 volt limit still works, but the 207 volt limit does not, tests show you can get over 70 volt to earth when the supply voltage is still over 207 volt, so the earthed frame of a car can have over 70 volt to earth without the system detecting a loss of the PEN and so present a real danger.
For cars being charged in a gated enclosure the time this is likely to happen means it is unlikely to cause a lethal shock, where the problem lies is where the car is being charged on the street or where people or other animals can easy touch the car. We have for many years banned the supply of boats, petrol stations, and caravans with a TN-C-S supply for good reasons, but the pressure to role out electric cars has resulted in the problem of TN-C-S being redressed in unacceptable ways, with salt with snow and ice clearance vehicles I have had a good belt from 24 volt, under normal conditions 24 volt is not even felt, but with wet gritters I have had a nasty belt with just 24 volt. Although on a dry summers day 70 volt may not be a problem, on a winters day when gritters have been salting the street 70 volt could be lethal. The gritters do not grit the drives, so off street not so much a problem, but with on street charging of electric cars it is just a matter of time before some one is killed. Unless better protection is employed.
No it wouldn't.
Are you saying all outbuildings have a hole in the floor?
There you go.
So - it could be.
Yet people would/do not consider the immediate surroundings of (part of) the house when it is at the same place/distance.
No, but it's far more likely that an outbuilding is such that one can stand on 'earth' whilst touching things inside it. That's true of all my outbuildings (through the door) and, in the case of my greenhouse (which has no floor, but does have an electricity supply) I stand 'on earth' inside it. In contrast, even if there were any exposed- or bonded-conductive parts touchable from outside, there is no 'earth' for me to stand on outside of any of the external doors of my house.
Well, as discussed, they sometimes do in relation to outside taps but, with that exception, it is fairly unusual to have anything earthed to stand on or touch whilst standing outside of a door and touching something inside the house.
However, if I had a TN-C-S installation (which I don't), and was sufficiently concerned about extremely small risks (which I am not), then if there were earthed things to stand on or touch immediately outside of any of my exterior doors, I would not want any exposed- or bonded-conductive parts just inside the door which were touchable whilst on I was in contact with earth outside of the house.
Kind Regards, John
I don't know how correct that figure is, but it sounds credible. I presume that relates to the whole UK, and that the faults usually only persist for a short period of time. What do you think the annual probability is that you would be within 5 miles of, let alone close enough to touch, one of the resultant 'live' conductors during the relstively brief periods during which it was 'live'? Do you worry about being struck by lightning?
I find that a little hard to believe, but I have no facts on which to base a disagreement.
Those figures are, indeed, correct, but I'm not sure where they come from. 'Skin resistance' is not uncommonly 1kΩ or less (particularly in the case of 'wet hands'), which would mean currents through a person of at least 50mA and 70mA respectively with those 'touch voltages'.
Kind Regards, John
What does rather surprise (and somewhat please!) me is that, in the climate of "if it's technologically possible, it must be made a requirement" (as in SPDs, AFDDs, and some might even say RCDs), is that we have not (yet!!) seen the emergence of devices (rapidly followed by a 'requirement' to have them!) to disconnect a TN-C-S supply in the event of a marked elevation in neutral potential (which would probably require at least a 'reference' earth electrode)
I believe that is now a requirement for EV charging, is it not? Certainly my employer requires it, hence Pod Point, Zappi etc now providing chargers with built in broken PEN detection and three pole disconnection.
Indeed - and that's what the recent discussions here have been about. I suppose I should have been clearer in saying that I was talking about a possible 'general' requirement for such 'protection', not just for A-V charging.
Should such a 'general requirement' ever come, I wonder if those who (like me) have 'declined' an offer of TN-C-S will be able to claim an exemption from the requirement?
Kind Regards, John
My wife keeps an eye on a house for someone. I often go with her when we are out that way. One day there was no lecky - unfortunately I didn't have any tools with me. But I looked back along the overhead supply, and two poles along I could see a cable hanging loose - I'm fairly certain it was the CNE. We called the DNO and they were out to fix it the next day.
There's a whole section in BS7671 about EV charging. It's very explicit that using the DNO's PME earth is not allowed on it's own. (From memory) you basically have 3 options :
- Ensure that the combination of loads will hold the CNE potential to within 70V of local earth - only practical on 3 phase supplies with lots of balanced loads.
- Add a local earth electrode to keep the CNE potential down - has practical problems in terms of proximity to other services.
- Use equipment which will disconnect both the supply and each connections to the charger. I think this is the only place where switching the CPC is allowed.
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