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Hidden main bonding strap connector

Discussion in 'Electrics UK' started by riveralt, 12 Sep 2009.

  1. riveralt

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    I would welcome your views on this one. Please stick with me as I try to adequately describe the situation.

    I've just fitted a new Consumer unit in the garage as a replacement for my old semi rewirable one. I have fitted a 10mm earth cable from the Main Earth Terminal to the Consumer unit earth block. A separate 10mm earth cable runs from the copper gas pipe (within 600mm from entry) to the Consumer unit's earth block.

    The plastic water service feed pipe is about three metres from the Consumer unit. The plastic pipe becomes a 15mm copper pipe before passing through the water meter. Aside from the earth clamp and cable to the meter there is no sign of the water main bonding strap. The water pipes pass through the wall into the house at this stage.

    Back at the Consumer Unit a third 10mm earth cable runs from the CU earth block to the Central heating boiler water pipes where it runs unsheathed but unbroken through an earth strap and off across the garage toward the incoming water pipe. The earth cable then disappears into a wall 'seemingly' to meet up with the water pipe inside the house. Lets call that the 'hidden cable'

    I have done R2 continuity checks with the two visible earth cables and got acceptable very low ohm readings. I obviously cannot do the same with the third hidden cable but I decided to do a wander lead test between the CU end of the hidden cable and the copper pipe near to the water meter and found a very low ohn reading 0.005ohms.

    Despite this reading I do not think it is acceptable for the bonding strap to be hidden from view. I therefore intend to fit a new bonding strap and 10mm cable to the incoming copper water pipe and run it back to the CU.

    My key question is whether it is permissible to run the water earth cable from the water pipe back through the Gas boiler water pipes (which is clearly supplementary bonding) onto the CU.
     
  2. ColJack

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    what size are your tails?
    main earth should be 16mm²..
     
  3. skenk

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    How long is the run from House-Garage CU?
    How is power fed to the garage CU?

    I have never seen a water meter, where is the clamp and where does the cable go? There should be an 'earth strap' somewhere on the pipe presumably within 60cm of the meter on 'your' side (assuming this is copper). Is this plastic feed pipe the only means of supply of water to your house?

    I would say connect this cable to the MET (Main Earth Terminal) rather than the CU earth bar. If the other end connects up 'inside the house' where is that, that you cannot access? If your water supply is located 3m from the CU and bonded there, and it is the only supply to your property, then this hidden cable is a supplementary bonding cable (which probably should not be connect back the service point but . . . whatever). The cable between CU and earth bar should be 16mm sq.

    Can't see a problem leaving it connected, especially in view of your incoming supply being plastic although I would move it to MET so all the non-CPCs arrive at the same place. Where does your main earth come from? Your property should not be relying on Gas and/or Water supplies for its earth.

    A few pictures of the supply point and surrounding area would be good.
     
  4. riveralt

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    Thanks for your replies.

    To Clarify: The consumer unit is situated in the integral garage and is the sole unit for the house. The earthing system is TN-C-S . The Meter Tails are 16mm2 and the main protective bonding conductor is 10mm2 which seems to meet the regs. The energy suppliers main fuse is 60amps.

    The meter is situated on the outside wall of the garage in a plastic cupboard which also houses the MET. A single 10mm2 earth cable runs from the MET to the Consumer unit earthing terminal. The CU is situated on the inside of the same garage wall.

    The sole water pipe enters the garage approx 3 meters from the CU. I know that the plastic incoming water pipe is irrelevant because I want to bond the water metalwork for inside the house back to the CU. The water meter bonding is supplementary and is solely for the water meter - vital because I keep prodding it to stop spinning.

    The hidden bonding strip appears to be buried with the pipes inside the wall of the room adjacent to the garage and is not accessible without making big holes in the wall next door - which will not please someone who must be obeyed.

    As I said, I am happy to run a new main bonding conductor from the water pipe back to the CU but is it within the regs to pick up the exposed water pipes of the garage boiler enroute back to the CU since I can't be sure they are properly earthed.

    No photos because no camera other than Tesco's cardboard range.
     
  5. ColJack

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    up to ( but not including ) 16mm, the CPC must be the same size..
    between 16mm and up to ( but not including ) 35mm the CPC must be 16mm
    35mm and over it's half size..

    so even with 16mm tails, you need 16mm main earth..
     
  6. riveralt

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    While I bow to your (Coljack) greater knowledge, I took my information regarding the size of the main protective bonding conductors for PME systems from the red book - reg 544.1 and Table 54.8.

    I am obviously looking in the wrong place - could you point to the right one please.
     
  7. ColJack

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    easy mistake to make..
    those are main bonding conductors.. IE the ones from the MET to the water, gas, structural steelwork etc...

    the reg you want is 543.1.4 and table 54.7.. or you can calculate it using the adiabatic equation from 543.1.3..

    as it is an earth for a circuit, it's just a CPC..
     
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  8. riveralt

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    Thanks for that, I seem to remember getting that equation as a question on 2382. Just gone out (in the dark) and checked the cable sizes.

    I think there is a bit of a mix up. I am talking about the main bonding conductors running from the MET to the CU and then to the gas and water pipes, they are 10mm2.

    I've been 'bubbled' so I cannot get too close to the MET but it looks like a 16mm coming into the MET and a 10mm going from the MET to the CU.

    Still could do with advice regarding running a 10mm2 earth cable from the water inlet pipe back to the CU picking up the boiler water pipes on the way back. The boiler has copper pipes in the garage but changes to plastic as soon as it enters the house. The boiler water pipes (exposed metalwork) would not otherwise be connected to the earthing system.

    So is it okay to effectively use a single cable from the water earth bond route through an earth clamp attached to the boiler pipes and then onto the CU earthing bar.
     
  9. flameport

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    Boiler pipes do not need bonding, since they won't be extraneous conductive parts (not too likely that heating pipes will be going outside or underground!)

    Your bonding conductors should not be going from the main earth terminal to the CU and then to the pipes. They should be connected to the pipes and the MET only.

    The MET could be a bar inside the CU, or a separate block mounted outside of the CU, not both.

    The setup for TNCS should be:
    Wire from the supplier cutout to an earth block (your MET) - size would be whatever the DNO chose to put there (usually 16mm)
    16mm from the earth block to the CU
    10mm from the earth block to the gas/water and any other metallic services.
     
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  10. riveralt

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    Many thanks flameport
     

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