Any Electricians ?

[Mark Hennessy]

Thanks Mark, been reading up also, some of these internet forums are lethal!! According to IEE (I think) PME can be exported to a domestic outbuilding but if there are any conductive pipes within the building then they must be bonded and connected to the suppliers earth back at the house using 10 mm/2 (cross sectional square area) earthing cable. But apparently exporting PME to say caravan pitches and livestock buildings is a no no. Just to re iterate my caravan is not a old style aluminium caravan, it's basically a wooden chalet but constructed so as to satisfy the legal definition of a caravan just in case the planners have anything to say!

I'd seen that IEE document about the PME supply to the garage before, and upon re-reading it, it's surprising that no mention is made of the O/C Neutral problem. Even when it goes on to discus what to do in the event of metal pipes in the garage, it doesn't mention this as a reason to go down the TT route.

If I was in your shoes, I reckon that I'd want to convert the whole of the dwelling into TT. It's just a case of removing the connection between earth and neutral, and providing an earth spike - and making sure that all circuits in the property at RCD-protected. It's the sort of job that you ought to pay for, simply to get all the certification for when you come to sell the house. Oh, and that's the law

If your dwelling was converted to TT in the future, then it would make sense to extend the earth out to the caravan. So if you said that you are going to extend the PME earth now, fully aware of the risks, but with a medium-term view to converting the whole property to TT, that would be an outcome I'd be comfortable with

Incidentally, with your current PME supply, I'd be tempted to connect a mains lamp of some sort, and perhaps a buzzer, between neutral and and earth spike - this would hopefully give you decent warning of an O/C neutral condition.

The likelihood of an O/C neutral might seem fairly low, but the consequences of it are pretty dangerous. So, in "risk assessment" terms, where we would multiply the likelihood times the consequences, this would merit some action...

I have just been outside to check the SWA cable it's three core (3x 10mm square) So does that mean that I can use the spare 10mm conductor as an exported PME earth, I will not hold anyone to their answers, just trying to get a general consensus at the moment.

Yes, everything I've read says that this is fine. I said as much in last post, and nearly said it in my first. Just, as I'm sure you would, make sure it's suitably sleeved so it can be obviously identified at both ends by yourself and any future authority or whoever takes on the property after you...

Have spent the morning searching through various forums on this subject, it's alarming to read so many cavalier responses to questions on electrical safety.
I will try and find the IEE link to that PME article and post it, as I am sure many of us have outside sheds/ workshops etc that have or thinking of having electricity supply feeds.

Yes, I've probably spent more than a couple of hours on this as well. Of all the topics that come up, this is one of the more complicated ones, and it's good to discuss it.

All the best,

Mark

[pwdrive]

Hi Mark, yes so far so good but if the PME system is unsafe in the event of a neutral failure how come the electricity supplier installed it, therer must be many thousands installed in the UK especially in rurral properties?

Lawrence.

[Mark Hennessy]

The risk from PME supplies concerns an open-circuit on the Neutral, ahead of the RCD on your incoming mains. What happens is that everything in your house, including earth, goes high. The RCD doesn't trip out because it's lost power. So, you have a house that seems dead, yet by simply touching the metal case of any electrical device, you could get a nasty shock.

Interesting what you say there: it may explain a scenario I witnessed many years ago on a new-ish build house - presumably, therefore, with PME.

There you go Lawrence, it does happen

The upshot was that they had to dig a large trench in the road - about 20 metres long, back to a 'T' junction in the road - before they found the fault; it went on for about 3 days and nights. Being curious, one evening I spoke to the person supervising the work: "Real headache, this one. We've just got to keep digging back from the house until we find the trouble." Which they eventually did: an O/C neutral in a bitumen-filled junction box, so I was told.

Good on them for finding it!

My understanding is that when this happens, the supply companies often take the easy way out - and convert the property to TT, using a local earth spike. I wonder if this was before RCDs became commonplace?

Alternatively, perhaps it's safe to bet that these days, everything comes down to money

Peculiar thing was that several adjacent houses should have been also affected, but only this one house appeared to experience the fault. Perhaps the occupants of the other houses hadn't noticed, but I find that difficult to believe.

That is interesting, but I guess it depends on the exact detail of the wiring underground. Of course, it's common for houses to be on different phases, so perhaps only every 3rd house might have been affected. But you just never know - depending on when it was, perhaps there was much less electrical stuff in the houses, perhaps much more Class-II stuff? There is clearly potential division going on, so maybe other houses didn't have quite as many volts on their earths?

Luckily, we have TN-S, the Rolls Royce of electricity supplies. I suspect Alan does too... When they changed the termination on my incoming supply, the new one contained a 100A fuse - 3 times the previous one. When I expressed surprise the guy said that the incoming cable is good for several hundred amps - it apparently is many times bigger than modern cables. "They don't make 'em like they used to, mate!"...

Cheers,

Mark

[pwdrive]

Hi Mark, yes so far so good but if the PME system is unsafe in the event of a neutral failure how come the electricity supplier installed it, there must be many thousands installed in the UK especially in rural properties? shouldn't they have installed some cut out device in the event of such an occurence.

Interestingly we had a rarther alarming garden fire here last year, basically what happened was that the old aluminium uninsulated twin feed from the transformer started physically oscilllating causing the live and neutral to clash thus sending white hot lumps of aluminium showering down in several places in the garden, the grass was very dry after a prolonged dry spell, it instantly caught fire in no less than six places at the same time. It was caused by a horse rubbing it's bum on a stay wire on the pole the transformer was fixed to, the power company were immediately notified by myself. They came out and replaced the line with a twisted insulated cable. I asked if they were doing this to all the old uninsulated twin wire lines, they said no, only when they get a call out, negligence or what.
If that had happened at night the whole place could have caught fire unchecked, beggars belief really.

Lawrence.

[Ed_Dinning]

Hi Gents, the IET has a forum where questions such as these are discussed and answered. It on www.iet.org

Ed

[pwdrive]

I am finding this subject interesting, can't understand why the power distribution company can't install a safety cut out in the event of a O/C neutral on their feed on their side of the meter? Anyone know why.

Lawrence.

[Mark Hennessy]

Hi Mark, yes so far so good but if the PME system is unsafe in the event of a neutral failure how come the electricity supplier installed it, there must be many thousands installed in the UK especially in rural properties? shouldn't they have installed some cut out device in the event of such an occurence.

I think it's largely cost.

Interestingly we had a rarther alarming garden fire here last year

They came out and replaced the line with a twisted insulated cable. I asked if they were doing this to all the old uninsulated twin wire lines, they said no, only when they get a call out, negligence or what.
If that had happened at night the whole place could have caught fire unchecked, beggars belief really.

I think that largely backs up my previous sentence

Petrol stations aren't allowed to be fed with PME supplies. Nuff said

Mark

[pwdrive]

Thanks Mark, I think I have got thegeneral picture in all of this discussion.

Lawrence.

[Skywave]

At Wikipedia, I found this informative . . . .

http://en.wikipedia.org/wiki/Earthing_system

but I am puzzled about their drawing accompanying the IT system: it does not show a N connection from the source to the load; the star point of the transformer is not connected to anything at all . . . so why the earth connection at the consumer load? In effect, it is a 415-v floating 3-phase supply. Am I correct, have I misunderstood something or has Wiki made a mistake in their drawing?

Al.

[pwdrive]

No Al I don't think it's a mistake, see here:

http://www.schneider-electric.co.in/docu...ect178.pdf