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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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  • 0

    I am concerned that the Ze value appears so low.You say 0.22 Ohms at the main panel, but unless there is a major Earthing system this is not right. Typical values in the UK even for substation Earths are 10-20 Ohms. What regulations are you operating under, is it BS7671? If so the TT installation requires that all final circuits are RCD protected to provide disconnection in <200ms (411.5.3 etc). to provide fast enough ADS in the event of an Earth fault and a nominal touch voltage of 50V (see regulation). Also see Table 41.1. The question then comes does the Main Panel Earth fault relay meet the above for submain protection as well as all the exposed conductive parts of the installation?

    There are a number of changes in this area of the regulations 18th ed Amdt 2.

    If it were me I would very carefully examine what has actually been installed, because I suspect a neutral Earth short, maybe deliberate, has occurred. The 7 Ohms you mention might be possible, but this would require all of the TT parts of the Regs to be followed.

    It would be useful to know the whole supply rating being supplied, I suspect it is quite significant. I suggest each dwelling is supplied with 30mA RCD protection, as a final safety feature.

    Kind regards

    David CEng.

  • 0 in reply to David Stone

    Hi David,

    Submain protection was not a concern as 411.3.2.4 allowed 1 second max for disconnection of distribution circuits and I have required ELR's for all submain circuits. I have not seen amendment 2 yet does this affect 411.3.2.4?

    I agree, I think there is a neutral earth fault somewhere, I have asked for switchgear flash test results as I suspect there may be a damaged insulation of a neutral support in the panel, will test this out tomorrow, as previously advised there are no N-E links in any of the switchboards.

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  • 0 in reply to David Stone

    Hi David,

    Submain protection was not a concern as 411.3.2.4 allowed 1 second max for disconnection of distribution circuits and I have required ELR's for all submain circuits. I have not seen amendment 2 yet does this affect 411.3.2.4?

    I agree, I think there is a neutral earth fault somewhere, I have asked for switchgear flash test results as I suspect there may be a damaged insulation of a neutral support in the panel, will test this out tomorrow, as previously advised there are no N-E links in any of the switchboards.

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