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TT earthing arrangement

John Callaghan

I have found two earth electrodes at each , this is due to supply authority providing earth elecrode at their main panel and contractor supplying earth electrodes local to two seperate installations. I have two questions:

Is there an issue with using more than one earth electrode for each installation i.e. both the supply authority electrode connected to the installation via feeder cable SWA + supplimentary earth cable, and a local earth electrode connected to the main earth bar of each installation?

The project consistes of two buildings, one is a administration building and the other is a residential building with multiple demociles, both are approx 500m from the supply authority earth electrode. 

I note the measured Earth Fault Loop Impedance values are low enough to comply with BS7671.411.4.202, Table 41.3 and as such they comply with Note 1 of BS7671.411.5.2. while I understand the use of  Overcurrent Protection Devices without RCD protection for final circuits are considered acceptable, are there any issues I should take into account.

Note I am in the middle east and the ground is as dry as it gets there has been no rainfall or water on the ground since early April therefore the contribution of the general mass of earth to the fault path measured values are as high as they will ever be.

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    The Transformer earth electrode is approx10m from the main panel earth electrode providing an electrode to electrode resistance of approx 7 Ohms through the general mass of earth.

    I note the measured Earth Fault Loop Impedance values are low enough to comply with BS7671.411.4.202, Table 41.3

    I'm puzzled as how those those two statement can both be true (with the possible exception of B6 MCBs).

    How is shock protection provided for non-final circuits?

       - Andy.

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  • 0

    The Transformer earth electrode is approx10m from the main panel earth electrode providing an electrode to electrode resistance of approx 7 Ohms through the general mass of earth.

    I note the measured Earth Fault Loop Impedance values are low enough to comply with BS7671.411.4.202, Table 41.3

    I'm puzzled as how those those two statement can both be true (with the possible exception of B6 MCBs).

    How is shock protection provided for non-final circuits?

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Hi andy,

    This is why i am asking the question, the thinking is related to the disconnection time of 200mA identified in table 41.1 for voltages 230v<Uo<400v.

    With two earth electrodes in the system we have two resistances in parallel which greatly reduce the total earth resistance, hence the low measured EFLI values identified in my latest response to mapj1.

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