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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 expect that there may be a range of opinions on this one!


    In any event, the premises must be sufficiently close to the transformer and the DNO must be willing.
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    Chris Pearson:

    I expect that there may be a range of opinions on this one!


    In any event, the premises must be sufficiently close to the transformer and the DNO must be willing.


    I have never had a refusal by U.K.P.N. to install a new P.M.E. earth terminal yet Chris. We have been treated to new A.B.C. over head cables out here in the sticks. And don't we need them with the strong winds coming off the North Sea!


    Z.


  • 0
    Zoomup:
    Chris Pearson:

    I expect that there may be a range of opinions on this one!


    In any event, the premises must be sufficiently close to the transformer and the DNO must be willing.


    I have never had a refusal by U.K.P.N. to install a new P.M.E. earth terminal yet Chris. We have been treated to new A.B.C. over head cables out here in the sticks. And don't we need them with the strong winds coming off the North Sea!




    Best PSSC that I have recorded at Daughter's house was 0.480 kA. So that's a best case of Ze = 0.48 Ω and no chance of PME there. (Nice shiny new ABC too.)


  • 0
    Chris Pearson:
    Zoomup:
    Chris Pearson:

    I expect that there may be a range of opinions on this one!


    In any event, the premises must be sufficiently close to the transformer and the DNO must be willing.


    I have never had a refusal by U.K.P.N. to install a new P.M.E. earth terminal yet Chris. We have been treated to new A.B.C. over head cables out here in the sticks. And don't we need them with the strong winds coming off the North Sea!




    Best PSSC that I have recorded at Daughter's house was 0.480 kA. So that's a best case of Ze = 0.48 Ω and no chance of PME there. (Nice shiny new ABC too.)




    Perhaps down the line they need to get the wire brush and metal polish out at pole tops. Does your daughter live off planet Chris?


    Z.


  • 0
    Zoomup:
    Chris Pearson:
    Zoomup:
    Chris Pearson:

    I expect that there may be a range of opinions on this one!


    In any event, the premises must be sufficiently close to the transformer and the DNO must be willing.


    I have never had a refusal by U.K.P.N. to install a new P.M.E. earth terminal yet Chris. We have been treated to new A.B.C. over head cables out here in the sticks. And don't we need them with the strong winds coming off the North Sea!




    Best PSSC that I have recorded at Daughter's house was 0.480 kA. So that's a best case of Ze = 0.48 Ω and no chance of PME there. (Nice shiny new ABC too.)




    Perhaps down the line they need to get the wire brush and metal polish out at pole tops. Does your daughter live off planet Chris?




    It's just that the distance from the transformer to the cutout is about 300 yards, so 25 mm² (copper equivalent) conductors will never get to 0.35 Ω or below.


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    Mind you over 300yds I'd expect them to have fitted something quite a bit heavier than 35mm wavecon. The neutral rise relative to terra-firma on full load (thinking about half of 0.48 ohms and perhaps 40A for an electric shower and a few lights ) must be quite noticeable! Or maybe the water pipes are taking quite  lot of neutral current, and the loop impedance of the main itself is really higher.

    There is a reason for the one metre one volt rule of thumb for planning networks - 230/400 V supplies become problematic in this way at the few hundred metres sort of range.

    Mike.
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    I never knew about the 1 meter 1 volt rule that makes  sense here at peak times we lose aprox 19 volts ie 231 at house assuming it starts as 250 at the Tx we are within those limits which is good to know most of the time the voltage is around 238 or 9  so all good
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    Same here zoom - I get every TT PME'd out of existence if possible but it isn't free! WPD charge £150.00 to insert a foot of 16mm earth into the neutral termianl on the cut-out.

    Mind you, the last one couldn't be done because the installation was end-of-line and the L-N loop was 0.45 ohms, so unless burying the last 15 metres or so to the house from the pole and ground-rodding it to death in the trench would solve the shortfall it was a no go. They wanted £1800 to bury the b***** though!
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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. Or is this a case where a neutral  is being bonded to earth  without the rest of the system being staked.
  • 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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