Neutral Fault on a PME system

Can anyone advise on the following situation.
Background. My sister inherited a house from her father-in-law about 3 years ago and is know experiencing some strange faults with the plumbing and electric installations. First indications was when the hot water tank started to leek, when we looked at this we found that it had completely rotted. After we replaced the tank other leeks started to appear throughout the cold water and central heating pipe work, all on the ground floor close to external / sleeper walls. She called in several plumbers who fixed the leeks as they appeared only for another to appear a week later. All the plumbers indicated that this was due to the pipes not being lagged and the acid in the lime mortar causing corrosion, although we thought that this was strange because her father-in-law had never had a water leek in all the years he lived at the property. After much deliberation she bit the bullet and had a complete new boiler, heating and plumbing system installed with every bit of pipe work replaced and lagged at great expense. Now three months on the leeks have started again, the brand new copper pipworks look like it is 20 years old and full of pinhole leeks and corrosion. I decided to take a trip over to her house and do some more investigation on the electrical installation with the following results.
First check was to see if the earth bonding was correct. This was installed although not to the current size for a PME system. Next was a Earth Loop Impedance, this came up as 400 ohm on the socket that indicated that there was a major fault, this was checked again at the main earth block and again showed a Very High impedance. We called in the supplier to check out the system and he found that the earth strap in the head end was loose, he tightened this up and left without any further tests etc. The earth impedance is now .2 ohms. My Question / Worry now is that there may still be a fault on the installation as I cant see how a faulty / no earth on the incoming supply would put a high current through the earth bonding and associated pipe work to cause the damage to the copper pipe work.
Other tests carried out Incoming cable is a 2 core 32 mm I think PAT tested all appliances, all OK Incoming mains water is Plastic When a tap is left dripping it leave a blue stain. A new electronic metre was fitted a while back, this has doubled the elec bills from previous meter and may have something to do with the fault?????
I have fitted a new consumer unit with RCD on all ground sockets, shower etc without any tripping
Any advice on how to check for neutral faults / any other faults would be appreciated and an explanation on how the earth fault would cause these problems would be greatly appreciated.
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www.gas-elec.co.uk Welsh test paper?
Jim A
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"Jim Alexander" wrote
| www.gas-elec.co.uk Welsh test paper?
Despite the leeks, London North East test paper http://www.gas-elec.co.uk/contact/offices/?lid 
Owain
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On 28 Oct 2004 06:15:14 -0700, Keith wrote:
<snip>

Hmm, a bit of a poser. The blue stain from dripping taps suggests disolved copper salts in the water.
You don't say, is the electricity service into the house u/g or o/h? Is it in an urban or rural location? Do you know, are the distribution mains o/h or u/g? If it is rural, is it an isolated property, or are there any others nearby?
The meter change is a bit worrying if there's a significant change in unit consumption.
Do you happen to know anyone with a tongtest (clipon) ammeter? If so, it would be worth switching *everything* off at the sockets, so all you have is a 'bare' installation, and see if there's anything still going through the meter. The old disc-style meters were easier to check. coz the disc stopped if there's nothing flowing, but modern solid-state meters aren't quite so easy for a quick check.
--
wanderer at tesco dot net

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The supply is underground made up two core 32mm copper. I wouldn't say rural as there are neighbours both sides of the house.
I will try the ammpmeter check when next their
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i have just checked up on my previous reply and the supply is defiantly 2 core Overhead Supply.
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On 29 Oct 2004 05:33:07 -0700, Keith wrote:

Yep, but is it in a rural or an urban location, i.e village or town? Are the adjoining properties old, with fairly old electrical installations?
I presume you've checked polarity on the installation, given that the meter was changed recently? It's not unknown for those guys to get it reversed.
Certainly the things you describe are 'odd'. Do you have a reasonably high (say 1,000ohms/volt) impedance voltmeter, like an Avo or similar? If you use an electronic meter, these can give rise to all sorts of spurious readings.
Try sticking a seperate earth spike in the ground, well away from any buildings - say 15/20 yards - run a length of insulated cable from the spike back to the property and check what voltages you have from this separate spike to phase, neutral and earth on the installation. I'm just wondering whether you may be getting neutral inversion somewhere along the line. i.e. someone in an adjoining property has an earth fault that isn't clearing and if the resistance of that fault is lower than the main transformer earthing - most likely in a rural location with a pole mounted transformer - then that phase gets dragged down towards earth whilst all other phases and neutral rise. May not be, but at least it's worth eliminating the possibility, given the circumstances you describe.
--
wanderer at tesco dot net

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Think you mean it the other way round. ;-)
A DVM is usually about 10Mohm input impedance.
--
*Did you ever notice when you blow in a dog's face he gets mad at you? *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Fri, 29 Oct 2004 16:45:03 +0100, Dave Plowman (News) wrote:

Err, no, on a 500v range, that's only half a meg.

When you're looking for the voltages associated with 'genuine' extraneous earth leakage currents, a DVM is a PITA.
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Not disagreeing with you on that point - merely that you suggest you need a meter with a reasonably high input impedance. You don't - it's the opposite way around. And meters don't come much lower than 1000 ohms per volt.
--
*I brake for no apparent reason.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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We are talking village so I presume that this comes under the rural heading, there are transformers mounted on poles in certain locations
What you say sounds good but not being an electrician and living quite a long way away means its hard to check this out. I have approached several electricians and the supplier but they all come across as either not interested or just have not got a clue what I am asking.
First of all you mention polarity. Do you mean that the incoming Phase and neutral may be reversed? This would also mean that the PME earth is connected to the Phase, what is the best way to check this so it can be eliminated
Neutral Inversion! Again not fully understanding what this means and how it can occur but please explain exactly how to eliminate this.
My sister has just spent another 500 on replacing the copper pipe so must try and get to the cause.
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On 29 Oct 2004 13:20:47 -0700, snipped-for-privacy@gas-elec.co.uk (Keith) wrote:

Have you talked to the water supplier and found out about the supply.?
So far we know that there has been an electrical problem (may still be) and obviously that should be corrected, but there doesn't seem to be any clear evidence relating the electrical installation to the demise of the pipes.
Does the water come from the mains or from a well?
Do the neighbours have the same issue with their pipes?
.andy
To email, substitute .nospam with .gl
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On 29 Oct 2004 13:20:47 -0700, Keith wrote:
<snip>

<snip>
At the moment, you are coming back with more and more questions and no factual information on which to make a judgement. I did note also the email addy you were using, and find myself wondering why the very firm you represent cannot do this work for you.
--
wanderer at tesco dot net

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Wanderer wrote:

But the supply system here does appear to be PME, from the references to a 2-wire overhead line and a 0.2 ohm Zs value. That being the case there should be a number of other earth electrodes in parallel with the main transformer earthing, making it pretty hard, I'd have thought, for an consumer's earth fault to drag one phase down very far. Moreover such an earth fault would have to be to a local means of earthing (in a TT installation) and not to the CNE[1] conductor.
A broken supply neutral might be another possibility, but I think we can rule that out as there's been no mention of supply voltage problems.
*** *** ***
On further reflection though, perhaps there's something in what you say: IF something is causing the potential of the CNE conductor (and therefore all the protective conductors in the house in question) to be well above earth, AND the water supply is from cast iron mains (clearly well earthed) THEN the said potential difference will appear across the section of plastic water mains feeding the property. This could cause electrolysis in the water and perhaps some of the strange effects described.
One thing that would certainly do this is if the overhead supply has been connected with reversed polarity, so that the consumer's PME terminal is actually connected to the phase! Hmm, yes, your suggestion of making voltage measurements to a local independent test earth electrode is a good one.
*** *** ***
I would also like to point out that the OP, Keith, is the NE London Regional office contact[2] for a company whose business is gas and electricity safety inspection and testing.
[1] Combined neutral and earth. [2] http://www.gas-elec.co.uk/contact/offices/?lid 
--
Andy

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Yes, such a test should be done, but I doubt the earthing is all live: if it is the OP would have been bitten numerous times by now. Equipotential bonding would not stop that happening, it would merely reduce the number of opportunities for it. There are real life earths inside houses as well as supply provided ones.
NT
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On Sat, 30 Oct 2004 19:58:25 +0100, Andy Wade wrote:

Must get my eyes tested, missed the decimal point in front of the 2 when I first read the OP! Mind you, all that proves is that the phase to neutral loop on the mains is sound, coz that's all the earth loop impedance tester is checking.

Not at all. The notional maximum value for additional electrodes is (or was - I've been out of the industry for a few years now) 40 ohms maximum, but they were rarely tested anyway. If the main earth for the transformer happened to have been broken with ploughing (and it happens).....
Not exactly a common occurence, but I came across two or three instances during my time, although the usual warning is reports of shocks in the property, or someone reporting that they're reading 300-400 volts from phase to a water pipe or similar.

Unfortunately we don't live in an ideal world where every electrical installation complies with the current edition of the regs - after all, the problem doesn't necessarily have to be in the property in question. There are countless voltage operated trips still in use - something of a contradiction in terms - and there will be many installations where the local water distribution system offers a better connection to the general mass of earth than the local REC's system. We also have the incompetent DIYer who carries out work without understanding the theory behind what he's doing, but that's altogether a different story. <snip> > I would also like to point out that the OP, Keith, is the NE London

<snip>
Oh yes, I had noticed that.
--
wanderer at tesco dot net

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ago
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Too many bits of relevent information for a typical DIY request -reads like an exam question. I suggest you ask your colleagues at gas-elec.co.uk or look up electrolysis on google for yourself.
Jim
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Snipped

It certainly sounds like some sort of electrolysis cos the blue stain must surely be the copper leached from the pipes. I thought that it would have to be a dc electric voltage to cause this not ac. So if there is something in contact with the copper piping that generates a dc potential..... It may be your new earthing will have stopped the problem.
The other possibility that occurs to me is dissimilar metals. I don't know the chemistry (A levels were a long time ago) but two metals have different potentials or something when immersed in an electrolyte (water) and this causes one to be corroded away. This is why boats have sacrificial anodes on them [1.23]. Have you installed something in contact with the copper pipes recently?
[1.23] Metal ones that is. -- Malc
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Where would a DC potential come from ??? and how could I check for this. It would be easy enough to remove the incomming PME earth from the bonding to replicate the same fault
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On 29 Oct 2004 02:19:29 -0700, snipped-for-privacy@gas-elec.co.uk (Keith) wrote:

This is normal electrolytic action - junction between dissimilar metals in solution.
Are there any direct connections from steel to copper?
These should be done via brass fittings to minimise release of copper. Plastic pipe in certain places could be a help as well to electrolytically separate sections of the installation
Did you say where the house was? If the water is soft and acidic, this can be part of the story as well. It would be worth asking the water supplier about this.
Clearly you had an electrical problem, but I'm not entirely sure that it is responsible for the pipe corrosion.
.andy
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