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Foundation Earthing AMD 2

AJJewsbury
AMD 2 says:

 
542.1.2.202 For new premises constructed upon foundations, each building in which there is an installation using the protective measure of automatic disconnection of supply shall be provided with one of the following:


(i)         a concrete-embedded foundation earth electrode in accordance with Annex A542 or


(ii)         a soil-embedded ring earth electrode in accordance with Annex A542 or


(iii)         an equivalent earth electrode such as that afforded by metalwork of a steel framed building embedded in concrete foundations in contact with soil.


A value of resistance to Earth not exceeding 20 ohms shall be provided by the earth electrode, or collectively where the electrodes of two or more buildings are connected together.


The earth electrode shall be connected to the main earthing terminal of the installation by a main protective bonding conductor of that installation. For the purpose of this requirement, for an installation in a multiple premises building, the protective conductor of the service line or distribution circuit supplying that installation shall be deemed to be the main protective bonding conductor.


In dwellings, for outbuildings such as detached garages and sheds, an earth electrode in accordance with (i), (ii) or

(iii) need not be provided.

Which raises a few questions in my mind...

  1. In practice, how many electricians are involved with the design & construction of foundations (or how many groundwork engineers are likely to be familiar with the contents of BS 7671) - i.e. what are the chances of such a facility having been correctly installed by the time an electrician turns up on site? What's the electrician supposed to do if such a facility hasn't been constructed, or (worse) has, but doesn't meet the 20Ω requirement? I might suspect that demanding that the foundations of a near-complete brand new buildings are ripped up and re-done, or trenching for an extra electrode underneath all the newly installed services and landscaping isn't going to go down well (even if there is space). Will the electrician be unable to deliver a BS 7671 compliant installation? It's probably fine on large scale projects where there's a team of architects and engineers double checking every requirement before everything is built, but a typical small scale private domestic build, with a local builder who likes doing things in a tried and trusted manner, I foresee problems.

  • What the extra cost of all this likely to be? I gather that they go down this route in much of Southern Europe partly because seismic regulations often demand steel re-inforcment of concrete foundations (so the extra metal is there anyway), they need a local electrode as everything's TT and dry soil conditions mean a simple rod won't be sufficient. UK conditions tend to be different - a simple rod is usually fine for TT, with a damp climate that tends to corrode steel below ground unless very carefully constructed and no seismic requirements, foundations on everything other than very poor ground are usually just plain concrete - and even where reinforced rafts or ring beams are specified, they'll usually be tied with steel wire rather than welded - which A542 prohibits.

  • What's this 20Ω limit all about anyway? It's far too high to be useful in a broken PEN situation and probably overkill for TT. It aligns with BS 7430's requirement for earthing of sources (e.g. generators) but in that it seems that value was always a bit arbitrary anyway (if it's fine for a 1MVA transformer, do we really need the same for a 16A SSEG?)

  • Are there any potential harmful effects due to "exporting" fault voltages to the ground outside the building. Modern buildings with all plastic services probably contain hazardous voltages reasonably well within their walls (Class 1 outside lights and EV charging notwithstanding). With a foundation electrode is it possible that they very soil outside the building might become hazardous (e,g. during a broken PEN event) and contact with that at the same time with more remote soil (e.g. via metallic hand rails or fencing) could introduce a hazard that would otherwise not be there?

  • If, in many cases, using steel reinforcement isn't going to happen and so we'd end up adding a loop of several tens of metres of reasonably chunky copper wire or tape - from an overall point of view wouldn't we be better using a similar amount money and material to upgrade the DNO system to TN-S instead?



   - Andy.
  • 0
    As I understand it the 20Ω requirement for islanded generation is a somewhat arbitrary figure - probably something quite sensible for a large-ish public supply system, but is it really appropriate for a modest system supplying a single dwelling? Would there be a significant safety issue if it was say 40Ω or 100Ω or 200Ω or 1667Ω instead?


    Wouldn't it be worth challenging BS 7430's requirements before we attempt to move heaven & earth (well earth anyway) to try and comply with something that was probably never intended for this situation?


        - Andy.
  • 0
    Going back a step, does anyone have any thoughts on whether exporting the MET potential to the soil around/outside the building is a good thing or not?

      - Andy.
  • 0
    AJJewsbury:

    Going back a step, does anyone have any thoughts on whether exporting the MET potential to the soil around/outside the building is a good thing or not?


    Like down a gas pipe?


  • 0
    Like down a gas pipe?

    Indeed - but problem that should be reducing with time as new gas pipes are MDPE. My thought was that many reasonably modern or fully refurbished homes will be pretty well insulated from the soil around them (water/gas/oil in plastic pipes, various electrical service cables with plastic sheaths), plastic plumbing to outside taps and so on  - which might be thought to have benefits from a shock protection point of view - especially for people/animals just outside (esp. barefoot childeren etc.) Might we be actually going backwards safetywise by exporting fault voltages to the outside?

       - Andy.
  • 0
    Andy


    How would your all plastic home perform when we are banned from using fossil fuels and we go all electric and we heat our homes and water with our Class 1 heat pump outside and your smart meter rations your supply and you go to island mode relying on your PV and battery storage?
  • 0
    How would your all plastic home perform when we are banned from using fossil fuels and we go all electric and we heat our homes and water with our Class 1 heat pump outside and your smart meter rations your supply and you go to island mode relying on your PV and battery storage?

    I'd imaging the heat pump unit would be placed out of reach anyway to avoid tampering - most commercial ones around here are either at a height or in a cage if at ground level. No need for island mode and own means of earthing if I'm only supplementing the grid supply from battery - the grid can remain connected and the grid's L-N link and Earth connection utilised. I suspect real island mode systems will be comparatively rare as they'd only be needed in case of complete grid supply failure.


      - Andy.
  • 0
    I was exploring the bungalow and garden one of my customers has bought in a Herefordshire hamlet on Tuesday afternoon.


    No gas, plastic water main, plastic sewer pipe to a shared private sewage treatment plant and a fibre optic cable laid to the boundary left coiled up in a small plastic inspection pit ready for connection with the promise of 70 mbs internet through a bit of plastic to replace the copper overhead telephone line.


    New build homes aren’t going to have shared metallic pipes and cables are they? It’s only the older homes around towns where people are trying to upgrade homes there are such issues.


    Edit- This bungalow has a DNO pole in the garden that supplies several other homes with a DNO earth under it, which is two metres from the bungalow, so the bungalow is within the zone on influence of this DNO earth, in rural locations there could easily be a DNO earth run out of the meter box. So that potentially puts the DNO earth immediately adjacent to the consumers foundation earth, which I guess isn’t a problem, but the DNO earth will be a short rod or tape versus many metres of copper tape on the consumers side, so whose earthing what?


     Andy B.
  • 0
    John Peckham:

    Andy


    How would your all plastic home perform when we are banned from using fossil fuels and we go all electric and we heat our homes and water with our Class 1 heat pump outside and your smart meter rations your supply and you go to island mode relying on your PV and battery storage?




    Well, you will have to ensure the hot tub on the patio heated up overnight ready for you to walk bare foot into the garden for your early morning soak.


    So whilst installing this foundation earth electrode it will be a good idea to install an earth mat under the patio and hot tub as well as another earth mat under the driveway where cars or vans may be parked to be charged. In fact the earth mats could be extended under the heat pump as well.


    A copper perimeter tape in the foundations with mats under the front and back gardens should do the trick.


    Andy Betteridge 


  • 0
    A foundation earth with a Ra of 20 ohms is not going to meet the requirements of 722.411.4.1(ii) to hold touch voltage down in installations with EV charging outdoors down to 70 volts or less if there is a lost neutral scenario, is it?


    This proposal to make foundation earthing compulsory in new build properties is not a cure all panacea to earthing issues.


    Andy Betteridge
  • Former Community Member
    0 Former Community Member
    I cannot see any reference in the above to the means of earthing such as rebar in concrete being required to have "Two reliable connections to the grid, protected against corrosion and preferably at opposite ends, should be accessible to facilitate testing and inspection," as GN5 once required (it may still do, my copy not to hand).


    To me, having extensive requirements without this "good engineering practice", doesn't seem sensible.


    As to structural steel buried in concrete foundations, the 3 bay agricultural barn with 12 such steels I tested last month gave a value of 37 Ohms. It was on quite stony ground not far from Crumlin for OMS's geography.


    I recall testing years ago a golf driving range "practice shot canopy" of some 50 m length with many buried steels giving under 5 Ohms.


    Drilling into a hard structure is topical for me having had a dental implant two days ago..... The extraction from my wallet was the most painful part though............


    Regards


    BOD

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