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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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  • SIGINT said:
    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.

    I am not sure how you reach that conclusion. If the two cables which enter the bottom of the service head are the loop, their sheaths would have to be connected to maintain your neighbour's earth. The earthing conductor is the twists of copper from the supply to the MET. Although they may appear to be rather weedy, I would expect that there are 4 x 4 mm² bare copper, which seem to have been painted.

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  • SIGINT said:
    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.

    I am not sure how you reach that conclusion. If the two cables which enter the bottom of the service head are the loop, their sheaths would have to be connected to maintain your neighbour's earth. The earthing conductor is the twists of copper from the supply to the MET. Although they may appear to be rather weedy, I would expect that there are 4 x 4 mm² bare copper, which seem to have been painted.

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