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Max Ze values from DNO

Marc Lambert

Hi, long time no posting, busy busy I'm afraid.

Concerning supply Ze which is usually quoted as 0.35 max for TNCS, 0.8 for TNS.

On unmetered highway supplies UKPN says

Makes perfect sense as the fuse size (time and Ia)  dictates the max Ze. 

So where did the .35 and 0.8 come from, or go to in this case.

Kind regards

Marc

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  • The 0.35 and 0.8 figures have has more to do with the longest typical cable length used to feed a house in a street supply, and on a TNC-S feed, the resistance is partly related to the maximum voltage drop at full load as well, as the neutral and external 'CPC' resistance are the same.

    As such these figures are really for the house at the wrong end of the street, regardless of 100A service or 30.

    (and you might like to extrapolate that table you quote to a 100 A single phase supply like the largest you may get at a house, and see that it is in the same ballpark more or less, about half an ohm.)

    Mike.

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  • The 0.35 and 0.8 figures have has more to do with the longest typical cable length used to feed a house in a street supply, and on a TNC-S feed, the resistance is partly related to the maximum voltage drop at full load as well, as the neutral and external 'CPC' resistance are the same.

    As such these figures are really for the house at the wrong end of the street, regardless of 100A service or 30.

    (and you might like to extrapolate that table you quote to a 100 A single phase supply like the largest you may get at a house, and see that it is in the same ballpark more or less, about half an ohm.)

    Mike.

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