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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.
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  • 0
    However, it does need to be added to the 'robust details' type building advice, or it will not happen. JPEL 64 should be liaising with the likes of BRE and RICS, and the authors of the building regs if they want this to happen.

    Agreed. If we did conclude that foundation earthing was a good idea then surely a proper 'standards' approach would be to to get the requirement written into the standard for foundations (I'm sure there must be one (or more) - BS 8004 and BS EN 1992 series look like possible candidates) and then for BS 7671 just to say something like 'where such a facility has been provided, it should be main bonded as if it were any other extraneous-conductive-part'. (Mind you I thought that BS EN 61439 was the proper place for the fire perform of consumer units rather than BS 7671, so what do I know)


     
    So, what is the minimum length of footings required?

    It'll depend on the soil conditions. I saw some "beach homes" the other week down in the South East - which had built on what was basically reclaimed shingle beach. I'd bet you could have a 100m of footing on that and still not get 20Ω.


    Perhaps similar problems on normal soils for small independent buildings with only a few metres of outside wall - say a kiosk.


    Soil conditions will always be a large unknown variable - so being able to guarantee a result from a design would be difficult to impossible. Rather than insisting on a particular value it might be more practical to just accept whatever it turns out to be (other than for sole means of earthing for TT installations).


     
    Presumably this applies to an extension on a house, so if a 3 x3 metre extension has 9 linear metres of concrete foundations, how easily achievable is the 20 ohms requirement?

    It does say 'for new premises' - so hopefully we wouldn't cover additions or alterations to any existing premises.

       - Andy.
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  • 0
    However, it does need to be added to the 'robust details' type building advice, or it will not happen. JPEL 64 should be liaising with the likes of BRE and RICS, and the authors of the building regs if they want this to happen.

    Agreed. If we did conclude that foundation earthing was a good idea then surely a proper 'standards' approach would be to to get the requirement written into the standard for foundations (I'm sure there must be one (or more) - BS 8004 and BS EN 1992 series look like possible candidates) and then for BS 7671 just to say something like 'where such a facility has been provided, it should be main bonded as if it were any other extraneous-conductive-part'. (Mind you I thought that BS EN 61439 was the proper place for the fire perform of consumer units rather than BS 7671, so what do I know)


     
    So, what is the minimum length of footings required?

    It'll depend on the soil conditions. I saw some "beach homes" the other week down in the South East - which had built on what was basically reclaimed shingle beach. I'd bet you could have a 100m of footing on that and still not get 20Ω.


    Perhaps similar problems on normal soils for small independent buildings with only a few metres of outside wall - say a kiosk.


    Soil conditions will always be a large unknown variable - so being able to guarantee a result from a design would be difficult to impossible. Rather than insisting on a particular value it might be more practical to just accept whatever it turns out to be (other than for sole means of earthing for TT installations).


     
    Presumably this applies to an extension on a house, so if a 3 x3 metre extension has 9 linear metres of concrete foundations, how easily achievable is the 20 ohms requirement?

    It does say 'for new premises' - so hopefully we wouldn't cover additions or alterations to any existing premises.

       - Andy.
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