Water main going plastic: questions about electricity grounding

Hi all,

So, Thames Water has identified a leak in the pipe leading from the water main to our house, and is going to replace it with a plastic pipe.

They've advised that if our house is an oldish one (which it is) there's a chance that our electrics might be still grounded through the water supply. Obviously if they replace our connection with a plastic pipe, that's going to screw things up a bit.

So I've got an electrician coming over to check it out, but I want to make sure I really understand what's going on before making a monkey of myself.

(a) Should it be really obvious whether or not our electrics are grounded through the water pipe, or is it something that's going to involve a lot of digging around and detective work? (b) There are a couple of earth wires coming out of the distribution panel, one of which is connected to a water pipe. Is this evidence that we're grounded through the water supply, or is it just this 'bonding' thing I've heard about that stops us getting electric shocks when we touch the taps? (c) If we *are* grounded through the water supply, what do they replace it with? A ground rod in our cellar? (d) How much does that kind of installation usually cost? Big job?

Cheers for your help, Chris

Reply to
Chris Mear
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It should be pretty obvious. If you take a few pictures of your consumer unit and electrical supply gear and we could give you a good guess.

Your earth should definitely be connected to the water supply, whether it is being used as your earth or not. It is the absence or presence of other earthing methods that would be investigated.

They won't replace it with anything. It is not their problem. You may be able to replace a water pipe earth with an electrical supplier earth. It may be worth investigating if this option is available, either through a previously unused TN-C-S (PME) terminal, or via an upgrade.

If not, you will need to convert to a TT system.

The job involves (there are other options, but this will be most common):

  1. Replacing, upgrading or tweaking your consumer unit to be a split load unit, with a 100mA time delayed main RCD instead of a switch incomer.
  2. Bringing main equipotential bonding up to spec (insist they use conductor sizes appropriate to TN-C-S, so you can upgrade later).
  3. Burying an earth spike and connecting this up to your earth terminal.
  4. Test.

The cost depends on how difficult it is to do the above tasks. Your consumer unit might already be up to spec. Your main equipotential bonding might be adjacent to the consumer unit, or it might require runs of cables under expensive wooden or stone flooring.

Christian.

Reply to
Christian McArdle

Thanks for the excellent advice, Christian! All very helpful.

I'll take some snaps when I get home this evening.

Oh sure, I know the water board won't. I meant 'they' as in the electricians that I call out to deal with this ;)

Chris

Reply to
Chris Mear

If you are in a town location, it is quite likely that you have a TN-S system where the electrical ground is carried via the lead sheath of the incoming undergound cable.

This will be obvious from looking and a photo.

If it is the case then there is not a lot to do other than to make sure that there is a bonding conductor from this main earthing point to the incoming water and gas supplies. Under those circumstances it matters not what the water pipe is from there to the street. The main purpose is to make sure that copper water pipes in the house are bonded to ground.

Reply to
Andy Hall

Not at all, it's the water inside the pipe that does the earthing, not the pipe itself

I'd cancel the spark as he's going to get a grand out of you one way or another

I can't believe they are actually trying it on

Reply to
Phil L

Not at all, piped water resistance is high. Its the long metal pipe that provides the (good) earth.

no, disconnecting a water pipe earth from an old installatoin is a really dodgy thing to do. The pic should tell us whats going on.

NT

Reply to
meow2222

Eh? Clean water being conductive is an urban myth. It is a insulator. A bad one, but an insulator, nonetheless. Tests done by the IEE have shown that even 1m of plastic pipework is sufficient insulation such that it is considered counterproductive to supplementary bond bathroom fittings so supplied. There is no way it would have sufficiently low earth loop impedence to blow even a 30mA RCD.

I can't believe you are proposing that a house earthed only through plastic pipework is anything other than a deathtrap that needs to be fixed urgently.

Christian.

Reply to
Christian McArdle

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does look like something earthy is coming out of the big wire (in my professional opinion). The earth wire going off the right of the photo goes to the fusebox.

Thanks, Chris

Reply to
Chris Mear

Yes, not particularly pretty, but very clearly TN-S earthing there. The means of earthing is the lead sheath of the supply cable. The connection is by means of a BS 951 earthing clamp (with incorrectly fitted label, but that's hardly critical). Use of that type of clamp on lead is now a deprecated practice since they can work loose over time as the lead creeps. It would be worth checking to make sure that the clamp is still tight. If it is loose the DNO needs to be called out to tighten it, or preferably replace it with a sweated connection. Don't attempt to tighten it yourself as there is a finite risk of damaging the old supply cable, resulting in a very big bang.

Along with the meter tails, exactly as expected. And the two wires heading off upwards are presumably the main bonds to the water and gas services.

You have no need to worry about replacement of the water main affecting your earthing.

Reply to
Andy Wade

Fantastic. Thanks for the advice and explanation, much appreciated. Plus I now have a basic understanding of what's going on down there in general, which makes me feel much happier.

Cheers, Chris

Reply to
Chris Mear

Yes. I agree. TN-S with main equipotential bonding apparently installed. Nothing to worry about.

Christian.

Reply to
Christian McArdle

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