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

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!