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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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    Odd. I presume an IR test that shows that there is no unintended NE- link is possible ? I think in your shoes I'd be wanting to verify that - a true L-E loop of 0.2 ohms is really more of a TNs or TNC sort of figure. Pure TT I'd expect many ohms, if not some tens of them, and then a need for RCDs at the very least at the building incomer.

    My money is on a metallic earth path, perhaps unintended.

    Mike.

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  • 0

    Odd. I presume an IR test that shows that there is no unintended NE- link is possible ? I think in your shoes I'd be wanting to verify that - a true L-E loop of 0.2 ohms is really more of a TNs or TNC sort of figure. Pure TT I'd expect many ohms, if not some tens of them, and then a need for RCDs at the very least at the building incomer.

    My money is on a metallic earth path, perhaps unintended.

    Mike.

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  • 0 in reply to mapj1

    Hi Mike,

    I am suspicious that there may be a damagfed insulated neutral support in a switchboard, either the submain or main. Will be testing all main and submain boards N-E insulation resistance tomorrow.

    If that does not provide answers will request authority retest their feeder cables N-E insulation resistance.

    Thanks for the relpies today, I had overlooked the basics when I was asking the original question, your replies made me wonder how the EFLI could possibly be lower than the earth resistance even if they were good results. As you say, unless there was a battleship buried it makes no sense Smiley

    Will let you guys know the results of the tests tomorrow and if problem has been identified, if not, will ask for more advice Slight smile

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