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Hello! Seeking some expert guidance. Main protective bonding of metal structural parts.

Former Community Member
Former Community Member
Good evening everybody. I hope you are all well.


It's been a while since I've prostrated myself on here to beg your guidance and assistance but I have a few scenarios that are hurting my head that I hope you can help with.


I have managed to acquire two installations (both single unit detached dwellings) where I have main protective bonding conundrums that are taking me round in circles. I have discussed with the NICEIC and have had various responses and would be interested to know your thoughts.


Installation 1 - TNS earthing arrangement (at least visibly). Metal frame structure (aluminium conservatory essentially). Frame has structural supports in ground contact that will also be paved over so will be at least partially buried in the ground. The frame is measuring circa 200 ohms (continuity) from supplier earth already so very much less than 23 kiloohms and very much extraneous. Attached to the frame are the bifold doors and window frames that form the rear of the extension. Part of the frame also forms a lean to that is outside of the equipotential zone of the building and one of the supporting pillars is located in this area. The frame is proximate to kitchen sockets that are intended to be metal and it is almost certain that class 1 equipment will be in use and be simultaneously accessible to the frame. As far as BS7671 goes I am of a mind that as this is introducing earth potential into the building then main protective bonding is required. I have checked continuity around the frame from a convenient bolt to the structure and it all seems good and should be feasible to achieve. However I am slightly concerned at connecting so much metalwork into the installation earthing arrangement that is just as accessible from outside the equipotential zone (garden/doors and lean to) as within it (exporting faults etc.). I am not sure if the dreaded PEN conductor loss situation applies here - although with car chargers we have to consider TNS systems to be TNCS so I am not even convinced of this. Similarly I'm not really seeing the advantage in converting the installation to TT prior to connecting a bond as this comes with its own risks (failed RCD's etc./non-maintained earth rods) and there will still be the risk of exporting a fault. I would welcome your thoughts and guidance. Is the risk to people outside of the equipotential zone just one that has to be tolerated?


Installation 2 - Similar but TNCS. Client has built his own metal doors that span the whole rear of his kitchen extension and bolted them to the ground/brickwork (presumably straight through the damp proof course). Again measuring in the 100's of ohms continuity back to the MET. Class 1 equipment/sockets will be proximate. Erring on converting this one to TT but would welcome your thoughts as to whether this is necessary or better practice.


I hope this has all made sense. Any assistance or advice you could provide to better inform my decisions would be much appreciated!


  • 0
    This will be one of those questions where whatever happens you can imagine a fault where treating it the other way would have been better.


    The doors are clearly accessible from outside (and inside..) and the exam question, is to balance the risk of taking the outside earth voltage indoors, or the inside CPC 'not quite earth' voltage outside. Regs wise you should bond the doors frame to the  CPC, and the decision is then TT or suppliers earth. However, in practice there must be loads of conservatories, greenhouses metal sheds, etc that are not connected to anything at all, that have been and will be like that for decades and give no real problem.

    Most of the time it will not matter, as the CPC and terra-firma voltages will be more or less the same anyway, we are looking at rare fault events. Unless the street cables in your area are the aluminium jacketed ones 'of an age' and the risk of lost PEN is much higher than normal. Consider all the PME supplied street lights and bus shelters, and the total lack of dead bodies piled beneath them each morning, tells us the real risk is quite low.

    If the area is paved so anyone outside touching the door is not well connected to ground, and/or also if TT would be difficult to segregate, or an electrode is likely to be poorly maintained, then the company earth is probably the way to go.

    On the other hand If folk will be standing on bare earth, and there is an obvious place for an electrode, then the balance swings the other way towards TT.


    I appreciate this is not an answer, but it is a list of things to consider.

    And whichever you do, realise that it is most unlikely to be a problem in practice, so do not feel stressed about it.

    Mike.


    Mike.
  • 0
    I can certainly see the difficulty.


    It seems to me that the risk is having a reasonably firm grip of something from which one could not let go in the event of a lost PEN connexion as opposed to merely brushing against it. So will window and door handles be in continuity with the structure? Clearly windows could only be opened from inside, but the door will be used inside and out. People will presumably be more likely to be inside the conservatory than in the immediate vicinity outside it.


    So if I have to come off the fence, I would bring the conservatory into the equipotential zone by bonding it.
  • 0
    The BS7671 view is that extraneous conductive parts should be bonded, see Reg 411.3.1.2. If this were a risk to persons outside in any serious way, we would find shocked persons everywhere, but we do not. The connection of the structure to the local "ground" will probably raise the ground potential significantly, so the shock potential will be quite low, at least within touching distance, say 1m. The danger from the kettle to the structure is much greater, so bonding is a good idea.
  • Former Community Member
    0 Former Community Member
    Gentlemen. I am grateful for your input. As there does not seem to be a 100% solution I think, inline with the general consensus, I will make the bonding connection as per BS7671 requirements. It's been a bit of a headspinner so I am thankful as ever to be able to bounce this off you all. Many thanks.

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