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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
    perspicacious:

    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............


    It is a money-spinner way in excess of AFDDs. Drill a hole, tap a thread, and screw it in. How hard can that be?


    Dr: that'll be 50 guineas.


    Patient: 'ow much?! It only took 5 minutes.


    Dr: 5 guineas for doing it and 45 for knowing how. ?


  • 0
    How about, I know we have put several hundred pounds worth of copper in your foundations as an earth electrode with a Ra of 18 ohms. But it is not good enough to use for your EV charger, so we need to put in another earth electrode or you will need to buy additional equipment for your EV charger to make it safer?


    Andy Betteridge
  • 0
    When reading this Daily Mail report you have to realise that the incident occurred in Perth in Australia, not Scotland and the supply company Western Power is not the UK network operator.


    So would a 20 ohm foundation earth stop such an injury?


    Andy Betteridge
  • 0
    Well, I think we know the answer really. A 20 ohm electrode may have helped a bit to reduce the exposed voltage - but it all depends on the load - anything more than a few kW ( 3kW would be ~ 20 ohms say 50/50 split of the voltage) and 20 ohms of electrode does not really pull down far enough to make things safe but it is an odds game - for quite a lot of houses the load is indeed quite a lot less than 3kW for most of the day when no one is cooking, showering or heating anything.

    What would have helped a lot would have been that non-existent, non-statutory, insulating section in the pipework to the tap. For that particular sort of accident, wiring the building as TT  would also have worked.  Equally, much as in the UK, there are parts of NZ where a 20 ohm earth connection would be hard to achieve.

  • 0
    It is, and I quote, "a protective bonding conductor" Not under the ESQCR regs it isnt. 


    And who is going to take responsibility for "20" ohms? Not the builder/developer.


    And if the "20" ohms fails, how is it going to be corrected?


    We will never know, Because the IET does not know, they will have no answer.


    Regards, UKPN


  • 0
    mapj1:

    Equally, much as in the UK, there are parts of NZ where a 20 ohm earth connection would be hard to achieve.

     



    Page 45.


    There should be an earth rod, but there is not a specific requirement for the resistance.


    Apparently it just needs to be what it is, so knock a rod in and that will do.

     


  • 0
    It seems to me that the whole of this idea is misguided. Would it not be better if anything outside is not bonded to the Earthing system at all and all pipes which lead outside etc, are plastic? The concept of the "Equipotential zone" seems to have disappeared and so the "Bonding" should also dissapear? Plastic plumbing, PME and much better Earthing of appliances have worked together to make PEN faults MORE likely to cause injury, and a 20 Ohm electrode will make no difference to that. RCDs provide some small protection because this kind of "touch fault" should still provide a currrent difference between phase and neutral, although an RCD which needs power may not be operative! I think we need a wholesale re-assment of our protection against shock at this point, it is the most dangerous problem, certainly ahead of fire. I suggest anyone who disagrees should do a resistance measurment of a few metres of water in a plastic pipe, in most areas you will be surprised. Note, do not use sea water!
  • Former Community Member
    0 Former Community Member
    Bonding will protect against faults external to the installation, however - and whilst "all plastic" services might be feasible in some buildings, in many facilities they will be metallic - eg medical gases, medium or high temperature heating mains, large cooling system mains, compressed air, LEV extract systems, cast iron drainage, sprinkler systems and so forth and so on.


    There is also a consideration to be made regarding lightning protection - although JPEL seem to have missed that one (possibly) in terms of Ohmic value


    Regards


    OMS

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