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Main bonding in a split property

Alan B

I am currently doing an EICR in a large thatched cottage that has been split on to a main house and small rental.

They share one supply but have separate DB's and are on a single TT earth,

They also have a single 30mA RCD protecting both properties, not great for selectivity but at least there is RCD protection.

Main bonding is in place to the oil feed for both properties.

The properties share a single water supply that's bonded when it enters the main property but all the pipework in the second property is giving me a reading of 0.54 ohms. My working assumption is some plastic pipe somewhere along with supplementary bonding to a lighting or power circuit. I believe the pipes just run through the fabric of the building rather than going in the ground but the outer walls of the property are damp, quite a bit of corrosion in back boxes etc, and the pipework is partially buried in the walls in places.

In my mind at least if the walls are damp they are probably also conductive to some extent and providing a path to earth,  I would love to be able to run an insulation test on the pipes but so far I cant find the supplementary bonding to disconnect it.

Ze 47 ohms, Zs at DB 22 ohms.

The MET is more or less diagonally opposite where I would need to make a bonding connection, therefore getting a bond in place is far from trivial.

Would you agree that I probably have no choice and need to find a way to get bonding in place

Is it worthwhile me spending more time trying to locate and isolate the supplementary bonding or is it very likely that an insulation test between the pipes and MET will come back below 22K because of the damp walls.

If I was to make a crimp connection to an existing 10mm earth bonding cable outside and protect it with amalgamating tape would this be acceptable or would I need to give it better protection?

Thanks

Parents
  • 0

    your Ze Zs suggest that the plumbing and the oil tanks manage another 50 ohms or so to put in parallel with the electrode. If there is only one supply fuse and one TT earth, then  the fact the building is split is a pain, but electrically makes it nicer, as it is one installation.

    When you say 0, 54 ohms, relative to what -  the CPC or the plumbing in the other half of the property? Some of the resistance could also be the wiring to the second DB. Unless the water is practically brine, that is not the sort of figure from a plastic pipe break  - expect something  more like kilo-ohms per cubic cm of drinking water, so  many hundreds of ohms to thousands per cm of plastic pipe, so even one single plastic elbow  could add the best part of a couple of k ohms - it will vary a bit with local water hardness, and could be as low as hundreds,  but not down to single figure ohms.

    Also with RCD protection, and the other earth impedances involved, there is no good technical reason to worry about 0.5 ohms - if a live wire hits the plumbing or the water heater splits, the RCD will safely remove the supply and the earth system cannot pass enough current to make the voltage across that half ohm resistance much more than half a dozen volts.

    I'd not be rushing round nailing G and Y to the brickwork to get all round outside the building without some convincing it was really needed.

    Mike

Reply
  • 0

    your Ze Zs suggest that the plumbing and the oil tanks manage another 50 ohms or so to put in parallel with the electrode. If there is only one supply fuse and one TT earth, then  the fact the building is split is a pain, but electrically makes it nicer, as it is one installation.

    When you say 0, 54 ohms, relative to what -  the CPC or the plumbing in the other half of the property? Some of the resistance could also be the wiring to the second DB. Unless the water is practically brine, that is not the sort of figure from a plastic pipe break  - expect something  more like kilo-ohms per cubic cm of drinking water, so  many hundreds of ohms to thousands per cm of plastic pipe, so even one single plastic elbow  could add the best part of a couple of k ohms - it will vary a bit with local water hardness, and could be as low as hundreds,  but not down to single figure ohms.

    Also with RCD protection, and the other earth impedances involved, there is no good technical reason to worry about 0.5 ohms - if a live wire hits the plumbing or the water heater splits, the RCD will safely remove the supply and the earth system cannot pass enough current to make the voltage across that half ohm resistance much more than half a dozen volts.

    I'd not be rushing round nailing G and Y to the brickwork to get all round outside the building without some convincing it was really needed.

    Mike

Children
  • 0 in reply to mapj1

    Hi Mike

    Thanks for the response, which is more or less in line with one of my chains of thought on the issue.

    The 0.5 ohms is relative to the met in the main house. I only have 0.02 ohms between the MET in the main house and earth bar for DB in the rental property, therefore would be a similar resistance between this DB and the pipes.

    There is a very short run (10cm) in the ground between pvc incoming water main and house plumbing, oil pipe is insulated.

    My assumption is that some of the 50 ohms is coming from the house plumbing being in significant contact with the walls but it could be the oil pipes or there are also 3 swa cable runs out to the garage.

    Alan

  • 0 in reply to Alan B

    "They also have a single 30mA RCD protecting both properties, not great for selectivity but at least there is RCD protection."

    With the TT earthing arrangement the R.C.D. is essential not just desirable, as you probably know.  Have you advised against keeping this very vulnerable  unsatisfactory arrangement. Paying guests where the mains fails will become very angry and expect a free stay or other financial compensation. You may be well advised to advise the owners in writing to cover yourself.

    Z.

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