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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.
  • Former Community Member
    0 Former Community Member
    Hello Andy


    I think we are slightly at cross purpose here - I completely agree with JPEL 64, as they are talking about the connection between the installation and the means of earthing - and they probably wouldn't listen to anything I've got to say anyway ?


    I wasn't suggesting that (for example) we strip back the end of a bit of G/Y singles and wire tie the stripped end to the re bar and shovel in the concrete - more that the continuity of the rebar can be assured if wire tied (ie typically 40 x the bar diameter as the lap length and wire tied, plus any link bars etc are normally tied


    If you take a browse through say the Furse catalogue you should find a range of products that include phosphor bronze tubes flared out to give an M10 connection one end, and one or two flyleads that are mechanically clamped to the re bars, on the other.  If you then put the button end next to the concrete formwork on the inside (ideally drill the form and put an M10 through to secure to the form), when it's struck you are left with an M10 connection onto which you can add a tape or a crimp for a circular conductor.


    I can't help think this approach is going to cause chaos in the jobbing electrical sector, however - it's fine on the big projects where it's all neatly designed out, planned, inspected, continuity tested  with witness overview, etc - I can't see any of that happening on the average small construction site however - the foundations may well be in and the place built as a shell before the building contractor even thinks about appointing the electrical subbie


    Best regards


    OMS
  • Former Community Member
    0 Former Community Member
    As a post script, in my world we would probably be evaluating the whole of the reinforcement and concrete arrangement in terms of multiplicity of paths to act as dividers and cover thickness of concrete to ensure that the rapid temperature rise due to the passage of current wouldn't result in spalling (particularly internally) where those spalled fragments could cause mechanical insult to a component


    Regards


    OMS
  • 0
    Potato sack ties don’t comply with the regs, now there’s a surprise ?
  • 0
    For the uninitiated Potato bag ties


    And the Tool for tying them
  • 0
    I think we are slightly at cross purpose here

    Interesting point - I'd read it as talking about connections within the rebar system (where I understand that twisted wire is a common approach and welding a high spec alernative) rather than specifically the connection to the bonding (earthing?) conductor. I can see now that it is ambiguous at the very least. It wouldn't have occurred to me to try to wire twist copper to steel, or ideed weld copper to steel ... but then I'm not as ingeneous as some builders it seems (judging by some of the bodges I've uncovered and would not have dreamt of doing myself).


       - Andy.
  • 0
    I had a year working for a major house building company as a site agent and had to try to organise and supervise the ground workers installing methane gas barriers under houses, because the houses are adjacent to a former landfill site that had been used for household and other waste; and the firms buyers decided they could get the ground workers to do it for half the cost of the specialist contractor.


    That’s one of life’s experiences it’s probably best not to dwell on.
  • 0
    I too interpreted the DPC as referring to the connections between the re-bar elements.

    It is within a para that starts talking about the reinforcement and ends talking about reinforcement.

    [It also says wire wrapping might be suitable for EMC - does this really refer to wire-wrapping the conductor to the reinforcement].


     Connections to the reinforcement are covered in A542.4?


    So ambiguous at best.
  • 0
    Whilst I understand the gist of this discussion, I still say the regulation is in the wrong place! This is so clearly Building Regulations material that further discussion will be wasted. Tell the DPC it is in the wrong place! The problem is that the whole of the "Standards industry" (and it is just that, trying to exactly regulate everything) is incapable of overall thought. Because the BRs are not part of the BSI, the regulations which they would like are spread all over the place, making understanding by any normal person impossible. Consultants (Sorry Graham) are then hired to navigate the mess. This is not the way to go. Who in their right mind would look in the wiring regulations to find out about foundation construction? Will the BRs specifically reference BS7671 in the foundation section, or even allow BS7671 to specify the details of foundations when someone here decides a more conductive kind of concrete must be used in some soil types without consideration of the structural effect? I think not. All that BS7671 needs is a connection to electrically conductive reinforcement via a piece of conductor of some size which will not corrode away. BS7671 should say that requirement and reference the BRs for details of how it is to be done.  BS7671 also needs to detail how this problem is solved if such a connection is not present, which it presently does not, and simply references BS 7030 for Earthing. OMS has made a number of good points, That the current must be limited, the safety of the foundation is paramount. Causing reinforcement corrosion in steel pile reinforcement could endanger a complete highrise building. Electrochemical corrosion is very likely in certain conditions such as seawater. Is all this going to be in BS7671 too?


    While I am at it, can we remove all reference to energy saving in BS7671, that is in the wrong place as well! Energy measures are BR material which contains a whole lot more. It would be much better if all considerations were together in one place.


  • Former Community Member
    0 Former Community Member
    I'm pretty sure JPEL 64 didn't intend to suggest that wire tied rebar isn't acceptable, clearly it is, and it would cut across a large number of codes that deal with the specification and application of reinforcing cages in concrete


    I'm pretty sure they are actually talking about the means of connection to the rebar.


    That said, you would have to wonder what's going wrong in the committees check and review process that allows such ambiguous (sloppy) wording into a DPC.


    Regards


    OMS




  • 0
    I would like to make a comment on the wiring together of reinforcement rods etc. The reason that the joints do not matter much is because the rods are in a basically conductive matrix, it is not an insulator. Welding reinforcement elements is a huge can of worms, because the stronger kinds of rods are made of carbon steel which is both heat treated and then cold worked to maximise strength. Heating this to weld it would destroy its properties and make it little better than mild steel. Pieces are wired together using soft steel ties (either ready prepared ones or just plain wire in large rolls) with overlaps of pieces sufficient for the concrete to transfer the expected forces. Whilst similar wires may be used for sacks, this is a deliberate slur on a perfectly satisfactory product. Many things have many diverse uses. The sand in concrete is very important although may also be a child's plaything and an adult bed! I agree with OMS above.

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