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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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    Very glad there was a clear problem to be found, kind of makes it worth it.  I presume now the Zs is more like a slack handful of ohms, and RCDs are needed in more places after all ;-)

    Mike.

  • 0 in reply to mapj1

    Hi Mike,

    Yes a reliefe indeed.

    We also isolated the each local buliding electrode from all other return paths and measured 28 Ohms across the disconnected incoming feeder armour (with supplimentary) and the isolated building electrode, which I thought was low but this time the numbers chrunch, with the main panel electorde approx 7 Ohms, the measured EFLI at the building incomers was measured at approx 6.5 Ohms.

    Another problem found during the electrode isolation process above was that the lightning network was connected directily to the building electrode, via underground copper tape. This was removed and will remain so.

    Again thanks for your advice.   

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  • 0 in reply to mapj1

    Hi Mike,

    Yes a reliefe indeed.

    We also isolated the each local buliding electrode from all other return paths and measured 28 Ohms across the disconnected incoming feeder armour (with supplimentary) and the isolated building electrode, which I thought was low but this time the numbers chrunch, with the main panel electorde approx 7 Ohms, the measured EFLI at the building incomers was measured at approx 6.5 Ohms.

    Another problem found during the electrode isolation process above was that the lightning network was connected directily to the building electrode, via underground copper tape. This was removed and will remain so.

    Again thanks for your advice.   

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