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TT to TN-C-S

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I advised a customer to have a new P.M.E. earth terminal installed by U.K.P.N. This was done. When U.K.P.N. attended and tested, the men found a weakness/imperfection/fault at the top of the nearest pole. A repair was carried out.


In my area many premises have old T.T. earthing. As a mater of course I would recommend that all T.T. premises be converted to TN-C-S where permissible, to improve the safety of the installation. 


Today I visited a house under renovation. The earthing conductor to an earth rod had been disconnected in the garden. The whole house was very dangerous due to an old and unearthed installation. The old Voltage operated earth leakage circuit breakers were unreliable. I disconnected many circuits.


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Z.
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    I may be missing the point a bit but if a supply is being made PME then the DNO would have to put earth steaks in at various poles isn't it every 4th pole? That would get rid of the hi loop impedance problem.

    I doubt a few extra stakes would make much difference at all. The reistance of the soil around each electrode (usually referred to as the electrode's resistance) will make the path via the ground such high resistance that it would make very little difference compared to the existing metallic path.


    For example, say the existing N is say 0.24Ω (one half of the 0.48Ω loop) - with the existing electrode at the transformer possibly approaching 20Ω to Earth. Try adding in one additional electrode at the consumer's end - say 100Ω. Conductors in parallel so 1/R = 1/R1 + 1/R2 etc. So the original resistance is 0.24Ω, the new parallel path 120Ω - which gives 0.2395Ω for the CNE, so 0.4795Ω for the loop (adding back in 0.24Ω for the L side) - an improvement of just 0.005Ω. Certainly adding in a few more electrodes will bring it down a bit further, but it's in very very small steps.


       - Andy.
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  • 0
    I may be missing the point a bit but if a supply is being made PME then the DNO would have to put earth steaks in at various poles isn't it every 4th pole? That would get rid of the hi loop impedance problem.

    I doubt a few extra stakes would make much difference at all. The reistance of the soil around each electrode (usually referred to as the electrode's resistance) will make the path via the ground such high resistance that it would make very little difference compared to the existing metallic path.


    For example, say the existing N is say 0.24Ω (one half of the 0.48Ω loop) - with the existing electrode at the transformer possibly approaching 20Ω to Earth. Try adding in one additional electrode at the consumer's end - say 100Ω. Conductors in parallel so 1/R = 1/R1 + 1/R2 etc. So the original resistance is 0.24Ω, the new parallel path 120Ω - which gives 0.2395Ω for the CNE, so 0.4795Ω for the loop (adding back in 0.24Ω for the L side) - an improvement of just 0.005Ω. Certainly adding in a few more electrodes will bring it down a bit further, but it's in very very small steps.


       - Andy.
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