Neutral Fault on a PME system

[Keith]

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.

[Jim Alexander]

Welsh test paper?

Jim A

[Wanderer]

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.

[Jim Ingram]

lagged

thought

water

heating

copper

socket

called

without

cause

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

[Malc]

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

[Keith]

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

[Keith]

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

[Andy Hall]

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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[Christian McArdle]

When you replace the pipework again, you might want to consider using PEX pipe, rather than copper...

Christian.

[Keith]

i have just checked up on my previous reply and the supply is defiantly 2 core Overhead Supply.

[Wanderer]

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.

[Owain]

"Jim Alexander" wrote | "Keith" wrote |

Welsh test paper?

Despite the leeks, London North East test paper

[Dave Plowman (News)]

Think you mean it the other way round. ;-)

A DVM is usually about 10Mohm input impedance.

[Wanderer]

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.

[N. Thornton]

A couple of maybes, but take qualified advice before touching anything like this.

1. switch off, disconnect suppliers earth, measure V and i between supply earth and a local earth rod
2. convert to a TT system with local earth rod. Fairly simple to do, and eliminates any possible supply earth problems.

However, I'd be inclined to look elsewhere, at the water. You may well have significant copped coming out the drinking taps, which sounds toxic, and youre losing copper rapidly on the CH piping. Now assuming your water doesnt run through the CH and then out the kitchen tap, the likely conclusion is that the water is fairly quickly dissolving the copper. So whats in the water? I would test the pH. Garden centres will sell you pH tests for a few pounds.

Also I would not drink or cook with the water until its been fully analysed. Tesco water is only 18p/ 2 litres.

NT

[Dave Plowman (News)]

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.

[Pete C]

Hi,

From the sort of corrosion described, it sounds like something connected to the mains is leaking excess current to earth, and part of all of this current is returning through the pipework of your heating system.

If your pipework is metal all the way to the ground and earthed water heaters etc are connected to it, it could be that it creates a lower impedance earth than that provided by the earth rod. Earth leakage currents will then return though the pipes instead of mains earth.

One way to stop this would be to put a section of plastic pipe where any water pipes go into the ground, and bond the pipework to the earth rod instead.

Once this is done it will be easy to connect an AC ammeter in line with the mains earth and/or the bonded earth from the pipework to get an idea of how much leakage current there is.

It could be that neutral is shorted to earth somewhere, or a low power device like a valve is connected between live and earth.

cheers, Pete.

[Capitol]

Copper oxide was used for rectifiers early in the last century. It may be that accidental rectification from stray currents has occurred. The forward volt drop of a cu rectifier I believe was about 0.3V.

Regards Capitol

[Keith]

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.

[Andy Hall]

AC though?

.andy

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