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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.
  • 0
    It is a 3 ph & N supply

    Actual TT loop value on site is around 14 ohms so nice and low. I would have thought that it wouldn't take too much to drop 0.1 of an ohm to get it to meet  the PME requirement.
  • 0
    This subject popped into my head again just now.


    Current situation: supply to single premises branches off close to the transformer, perhaps where the LV side of the transformer is earthed. (I have not looked closely.)


    Assuming that the PSSC, and thence the PEFC could be brought below 0.35 Ω, where and how do you plant the extra earths which will make it PME?
  • 0
    Chris Pearson:

    This subject popped into my head again just now.


    Current situation: supply to single premises branches off close to the transformer, perhaps where the LV side of the transformer is earthed. (I have not looked closely.)


    Assuming that the PSSC, and thence the PEFC could be brought below 0.35 Ω, where and how do you plant the extra earths which will make it PME?


    If your consumer's supply "branches off" then presumably there's a "main" continuing off to serve other consumers. If so I think the minimum requirement for PME is to have an additional electrode connected to the CNE at the point where the last consumer branches off from the main.

      - Andy.


  • 0
    Update to original post.


    The metering people are attending in April, and about a week later U.K.P.N. is attending to install the new P.M.E. earth terminal.


    The house has got two nice earth rods at present. Big galvanised water pipes. One was installed at the front of the house where the earthing conductor runs from the meter position through an air brick to the front garden, very handy. The second is located right at the back of the property and is wired in 10.00mm2 green and yellow, which at first I suspected was a bonding cable.


    Then there is a redundant rod in the front  garden near to the first rod, These have been exposed due to trenching to prevent "rising damp".


    I might join the two front rods together and retain them.


    Z.
  • 0
    Zoomy, I cannot see any problem with keeping two perfectly decent earths, but those cleverer in here might disagree.


    Andy, without driving up to have a butcher's (not essential travel!) I am pretty sure that it is not like that. So transformer strung between a couple of poles. LV to a pole nearby, presumably where it is earthed. Then the cables spread out into what was once a single estate. I suppose that an earth (the existing rod - 80 Ω) could be used, but we are still stuck with the PSSC of 0.480 kA. The issue in hamlets (if you can call it that) is that some premises are just going to be too far from the transformer to get a DNO's earth.

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