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EV install on a TT system - DNO PME?

Former Community Member
Former Community Member

Hi, new to this forum.

These might be obvious questions but I can't find an answer to it. 

Background.

A domestic house has a TT supply, has had a CU changed in 2019 but the new owners have no paperwork.

I can find no evidence of an earth rod anywhere and all the bonding cables go to the gas/water supplies. I did a quick Zs at the board = 2.9ohms. 

How do I know if the installation has been PME'd on the pole? (overhead wires)

Second part is - they want an EV charger installed that they brought with them from their old house. It will require TT, but as this property is TT it shouldn't need to have a separate rod.

If the installation is  a PME'd TT I would assume I would need to TT the EV?

Parents
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    Round here we get 0.238 ohm earth loop  but we definatley don't have PME the main earth is to the lead cable sheath

    Probably not a safe assumption these days. Just because you still have a TN-S style connection at the cut-out doesn't mean it's separate N and PE all the way back to the substation - since most DNOs have had a policy for many years of using a combined N and PE in any new or replacement cables and interconnecting N and PE at any connections with old cables. Any N-PE link after the substation and it's still TN-C-S, with the associated risks of a broken CNE/PEN even if the point of separation to -S is somewhere under a pavement or out on a pole rather than at the cut-out.

    We could almost do with an extra box on the certificates/reports for ‘worst of both TN worlds’ - to make it clear that we should be thinking of a Earth that could have the slightly raised Ze values of traditional TN-S together with the (sometimes) larger bonding and broken PEN risks of TN-C-S.

        - Andy.

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  • 0

    Round here we get 0.238 ohm earth loop  but we definatley don't have PME the main earth is to the lead cable sheath

    Probably not a safe assumption these days. Just because you still have a TN-S style connection at the cut-out doesn't mean it's separate N and PE all the way back to the substation - since most DNOs have had a policy for many years of using a combined N and PE in any new or replacement cables and interconnecting N and PE at any connections with old cables. Any N-PE link after the substation and it's still TN-C-S, with the associated risks of a broken CNE/PEN even if the point of separation to -S is somewhere under a pavement or out on a pole rather than at the cut-out.

    We could almost do with an extra box on the certificates/reports for ‘worst of both TN worlds’ - to make it clear that we should be thinking of a Earth that could have the slightly raised Ze values of traditional TN-S together with the (sometimes) larger bonding and broken PEN risks of TN-C-S.

        - Andy.

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