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TNS Hybrid with PNB strap

SIGINT

Firstly I’d like to say, thank you for taking the time out of your day to reply!

Information about the property and installation:

2 core Live and neutral most likely 25mm2 because it’s protected by an 80A fuse PILC or PILS, looped supply, Semi detached 1968 build.

I’ve pretty much modernised everything in the property. Full Type A RCBO setup with CPN Cudis type 1+2 surge protection.

I’ve also installed an Isolator switch with an SPD too! Garo type 2 spec.

During my examinations and also bringing in my sparky. We’ve discovered that the system is no longer a proper TNS system. Essentially what’s happened from observation is that the lead sheath of the supply has lost its Earth. Most likely because of degradation and lifespan 40-50 years (BS6480) The system is now a hybrid of some sorts with a mechanical neutral bond to the sheath of the looped supply then wired to the MET terminal Earth block. There is also a tinned Earth CPC nave green that goes underneath somewhere I am assuming it’s an Earth rod…because of the tinned Earth copper configuration and the cable being dug deep down and no way to access it? The cutout also mimics 5-10 second buzzing/arching noises and then disappears?

I’ve contacted the DNO 4 times. First they said it’s “fine” 2nd attempt they just changed the meter tails that goes from the cutout to the meter. 3rd they sent two guys who only did a visual inspection, and I noticed both glared at each other? But didn’t comment? Like they were hiding something? (I could also be mistaken) this might sound picky or fussy or what not but…I don’t feel safe if I am being honest….I feel worried… they are refusing to replace the old PILC/PILS cable…

I don’t know what to do? I just brought this house? And I wish I did an EICR inspection before I brought it. The sparky lad said it would have most likely failed an EICR inspection because of the sheath bonding…

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  • I'm not seeing a obvious N-PE link in the pictures either - but as Mike say such things aren't uncommon - and most DNOs these say will say if asked "PME conditions apply" to just about any earthing facility they provide, whether it was originally TN-S or not.

       - Andy.

  • in reply to AJJewsbury
    AJJewsbury said:
    most DNOs these say will say if asked "PME conditions apply" to just about any earthing facility they provide, whether it was originally TN-S or not.

    As Figure 2.1 of the OSG clearly shows, if the N-PE link is in the service head, the earthing conductor will emerge from the latter; whereas if there is no N-PE link, it is connected to the supply cable.

    For SIGINT's benefit (as I have mentioned previously), my earthing at home looks very similar. However, Ze was excessive a few years ago. This is, apparently, normally due to deterioration in the connection between the street main and the supply cable. Rather than digging a hole in the highway and fixing the problem there, the DNO dug a hole in my lawn and linked N and PE there. So whilst the installation appears to be TN-S, it is in fact TN-C-S. (Or perhaps that should be TN-S-C-S?)

    ETA: JP beat me to it whilst I was typing.

  • in reply to Chris Pearson

    As Figure 2.1 of the OSG clearly shows, if the N-PE link is in the service head, the earthing conductor will emerge from the latter; whereas if there is no N-PE link, it is connected to the supply cable.

    I fear the real world is a little messier than the OSG might suggest. E.g. series 6 cut-outs (e.g. https://www.wt-henley.com/pdf/14688_series_6_house_service_cut_outs_2.pdf) had a removable link within the cut-out - so without opening things you're not supposed to open you can't tell if there's a link in there or not - hence it could be either combined N/E (CNE) or separate earth wire (SEW) as the DNOs say (or SEW to the premises but CNE to some common point outside, as is apparently now the fashion for multiple supplies to steel frame buildings).

       - Andy.

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  • in reply to Chris Pearson

    As Figure 2.1 of the OSG clearly shows, if the N-PE link is in the service head, the earthing conductor will emerge from the latter; whereas if there is no N-PE link, it is connected to the supply cable.

    I fear the real world is a little messier than the OSG might suggest. E.g. series 6 cut-outs (e.g. https://www.wt-henley.com/pdf/14688_series_6_house_service_cut_outs_2.pdf) had a removable link within the cut-out - so without opening things you're not supposed to open you can't tell if there's a link in there or not - hence it could be either combined N/E (CNE) or separate earth wire (SEW) as the DNOs say (or SEW to the premises but CNE to some common point outside, as is apparently now the fashion for multiple supplies to steel frame buildings).

       - Andy.

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  • in reply to AJJewsbury

    I see what you mean. So if it looks like TN-C-S, it could be TN-S; and if it looks like TN-S, there may still be a link nearby. Rolling eyes

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