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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
    Had I known 10 years ago what I know now, I might well have put in a foundation earth electrode when I had my garage extended. With 60-odd m² of ground which is largely clay over a bed of flint, according to BS 7430 I might have been able to get below 1 Ω by using some form of mat.


    Could have been useful for an EVCP!


    FWIW, this proposal seems to be as much to do with prosumers as anything.
  • 0
    Today I explained this proposed requirement to a long standing customer who I have installed thirty complete installations for, he is about to start a substantial extension onto an existing bungalow and wants the DNO supply moved, a new consumer unit and other alterations.


    I was a bit stumped when he asked what it would achieve installing foundation earthing with a Ra of 20 ohms to a TNCS-PME supplied installation.


    Despite being well practiced at explaining the benefits of installing RCD protection and how they work along with other electrical matters, I really am at a loss trying to explain this proposal and cannot think of a justification.  


     Andy B.
  • 0
    We have been here before, same principle, but a different earth electrode arrangement.
  • 0
    What are we talking about, 25 metres of 25 mm X 3 mm copper tape at around £230?


    Andy B.
  • 0
    Sparkingchip:

    What are we talking about, 25 metres of 25 mm X 3 mm copper tape at around £230?


    Andy B.


    Depends on the size/shape of the building, but I'd expect a bit more than that for many new builds - even at say 75m² per floor and square it'd be closer to 35m - plus some to run up to the CU position (probably twice if the ring approach is adopted). For 100m² per floor it'd have to be well over 40m. Longer/thinner shapes will have a longer perimeters.


    Even at 25m, and 75mm² that's the equivalent of over 117m of 16mm² - you could "put right" (i.e. TN-S) an awful lot of the DNO's network with that much extra copper. Plus that's probably more copper than we'd currently put into the entire rest of the installation (equivalent to over 500m of 2.5mm and 500m of 1.0mm if my late night maths is right) - so what'll that do to the price of copper, not to mention the environmental impact of all that extra production.

       - Andy.


  • 0
    I had not thought about it being more copper buried in the foundations than there is in the rest of the installation ?
  • 0
    Is there an assumption that there will be rebar that can be used at little or no extra cost?


    Presumably it would have to be welded rather than positioned with potato sack ties?


    Andy B.


  • Former Community Member
    0 Former Community Member
    Wire ties are fine, given the multiplicity of connections.


    I once tested solid unbroken bar, welded bar, screw coupler bar and lapped, wire tied bar under design first and second (return) lightning stroke conditions (for an explosive facility)


    There was no credible difference from each of the test samples, beyond the 95th percentile of the postulated strike criteria (voltage, current, rise time etc)


    Regards


    OMS
  • Former Community Member
    0 Former Community Member



    Might be useful for some as a basic starting point - particularly, 7 - which just takes the foundation slab as a lumped parameter  - so all you need to estimate is the ground resistivity - probably along the lines of 40 Ohms/m as a reasonable starting point


    6, is the classic hollow square with perimeter electrodes interconnected with the slab edge (ie what is often described as a ditch tape arrangement


    Regards


    OMS
  • 0
    Wire ties are fine, given the multiplicity of connections.

    You might want to mention your experience to JPEL/64 - they seem to be of the opinion that "Connections made by a wrapped iron wire only are not suitable for protection purposes"...


    If it helps, A542 says:



    (Normative)



    A542.1   General



    A low resistance earth electrode is necessary for safety and to prevent significant voltage rises in the case of a fault. It also provides an earthing facility for prosumers’ electrical installations operating in island mode and assists in mitigating electromagnetic interference. Besides earthing, the foundation earth electrode provides a good basis for the main protective bonding of the installation.



    To provide a low resistance earth electrode it is necessary to have good contact with the soil, either directly or through concrete, and the soil must be damp and not liable to freezing or drying out. For concrete foundations without metal reinforcement concrete-embedded foundation earth electrodes must be coordinated with the type and dimensions of the foundation. One or more closed rings or rectangles mutually connected are preferred.



     


    NOTE: The methods in this annex can also be used for an extension to existing premises.



    To enable a first approximation of the earth electrode resistance, a calculation may be made, using average soil resistivity values for the soil type. Calculations made from these values only give an approximate result of an earth electrode resistance. See BS 7430 for further guidance on such calculations and approximate soil resistivities.



    In practice, horizontally buried conductors are laid down in two different ways:



    -            concrete-embedded foundation earth electrode, and/or



    -            soil-embedded ring earth electrode.



    A542.2   Concrete-embedded foundation earth electrode


    The construction of a concrete-embedded foundation earth electrode during erection is an economical solution to obtain a good earth electrode because:



    -            it does not necessitate additional excavation works



    -            it is erected at a depth which is normally free from adverse influences resulting from seasonal weather conditions



    -            it usually provides good contact with the soil through the concrete



    -            it extends over most of the foundation surface and results in the minimum earth electrode resistance which can be obtained with this surface, and



    -            from the beginning of the building erection it can be used as an earth electrode for the electrical installation of the construction site.



    Concrete used for the foundations has a certain conductivity and generally a large contact area with the soil. Therefore, bare metal electrodes completely embedded in concrete can be used for earthing purposes unless the concrete is isolated from the soil by the use of thermally insulating material or other similar measure. To protect them from corrosion, bare metal electrodes embedded in concrete shall be at least 50 mm from the outer surfaces of the concrete at all points.



    Metal reinforcement of the foundations may be used as an electrode provided it is soundly connected. For welding, clamping or where suitable mechanical connections are utilised (see Regulation 542.2.8 ), the permission of the person responsible for the structural design and analysis of the construction is required. Connections made by a wrapped iron wire only are not suitable for protection purposes but may be sufficient for electromagnetic compatibility purposes for information technology equipment. Pre-stressed reinforcement must not be used as an electrode.



    The electrodes shall not go over joints between different parts of larger foundations. At such places, suitable malleable connectors shall be installed outside the concrete to provide the necessary electrical connections. Connections between the malleable connectors shall not be galvanised steel. The use of copper or copper alloy is permitted.



    After preparing the electrodes and/or the connected reinforcement, but before the concrete is poured, a survey, continuity test between connecting components and documented record of the results and arrangement shall be made.



    A542.3 Soil-embedded ring earth electrode


    If the building foundation is to be protected against loss of heat energy by thermal insulation using non-conductive materials, or if the foundation is to use protective measures against water ingress, e.g. using plastic sheets, earthing using the foundation concrete is not viable. In such cases a soil-embedded ring earth electrode may be installed buried in the soil outside the foundations to an equivalent depth to that which an electrode would be in the foundations.



    The constituent parts of earth electrodes may include buried elements of:



    -            steel (hot-dip galvanized)



    -            steel (copper-sheathed)



    -            steel (electro-deposited copper coated)



    -            stainless steel



    -            bare copper, or



    -            copper alloy.



    Other metals and alloys are not to be used.



     


    Joints between dissimilar metals shall not be made. Minimum thickness and diameters of the parts must consider the usual risks of chemical and mechanical deterioration. However, these dimensions may not be sufficient in situations where significant risks of corrosion are present. Such risks may be encountered in soils where stray currents circulate, for instance the return DC currents of electric traction installations or in the proximity of installations for cathodic protection.



    Trenches shall not be backfilled with stones, cinders or similar materials, but with earth liable to retain moisture.



    After preparing the electrodes, a survey, a continuity test between connecting components and a documented record of the arrangement shall be made before any backfilling.



    A542.4   Connections


    The foundation earth electrode shall have at least one terminal lug for connection to the electrical installation



    – either rising through the concrete for a concrete-embedded type or ducted through the foundations for a soil- embedded type – to a suitable connection point (e.g. to the main earthing terminal) or ending at a special connection clamp embedded in concrete of a wall at its surface. At the point of connection, the terminal lug must be accessible for maintenance and resistance measurement purposes.




    - Andy.

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