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TN system for generator

lyledunn

This is the resistance (281 ohms) of the earth electrode connecting star point and frame of a stand alone TPN gen set. It was one of 15 used at a recent outdoor festival. I do appreciate the desire to keep the resistance within the norms usually applied, say around 20 ohms, but I don’t think there is anything in BS7671 that puts numbers on a TN system. I am not looking to debate the merits of such earthing or how this value could be reduced. I guess my question is more concerned about the value of earth resistance that the “T” in TN-S remains legitimate as far as 7671 is concerned. Is it solely related to some value that will ensure RCD protection will operate?

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    lyledunn said:
    This is the resistance (281 ohms) of the earth electrode connecting star point and frame of a stand alone TPN gen set. It was one of 15 used at a recent outdoor festival. I do appreciate the desire to keep the resistance within the norms usually applied, say around 20 ohms, but I don’t think there is anything in BS7671 that puts numbers on a TN system.

    Not that I'm suggesting we can apply a draft standard, but out of interest, the latest Draft for Public Comment for BS 7430 (see here, comments period closes on 10 September) steps away from 20 Ω in cases where generating sets are supplying an installation, and all circuits are protected by RCD, and recommends 200 Ω for TN system (Clause 6.1, Table 1). Where OCPDs are used, the recommendation of Clause 6.1 remains 20 Ω.

    200 Ω is of course provided as a recommendation as above this value it's generally accepted that the earth electrode might not be stable.

    20 Ω is a lot more interesting ... probably the true "source" is legislation and UK supply industry practice, particularly around PME ... interestingly, an earlier value used was 10 Ω.

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    lyledunn said:
    This is the resistance (281 ohms) of the earth electrode connecting star point and frame of a stand alone TPN gen set. It was one of 15 used at a recent outdoor festival. I do appreciate the desire to keep the resistance within the norms usually applied, say around 20 ohms, but I don’t think there is anything in BS7671 that puts numbers on a TN system.

    Not that I'm suggesting we can apply a draft standard, but out of interest, the latest Draft for Public Comment for BS 7430 (see here, comments period closes on 10 September) steps away from 20 Ω in cases where generating sets are supplying an installation, and all circuits are protected by RCD, and recommends 200 Ω for TN system (Clause 6.1, Table 1). Where OCPDs are used, the recommendation of Clause 6.1 remains 20 Ω.

    200 Ω is of course provided as a recommendation as above this value it's generally accepted that the earth electrode might not be stable.

    20 Ω is a lot more interesting ... probably the true "source" is legislation and UK supply industry practice, particularly around PME ... interestingly, an earlier value used was 10 Ω.

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  • 0 in reply to gkenyon
    gkenyon said:
    in cases where generating sets are supplying an installation, and all circuits are protected by RCD, and recommends 200 Ω for TN system

    The use of 200 Ω in these circumstances  is not a new concept, and has been used for a number of years in the IET Code of Practice for Electrical Energy Storage Systems, with broad industry buy-in.

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