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table 41.1 max. disconn. times and the extra notes

psychicwarrior
good day folks


"Where in TT systems the disconnection is achieved by an overcurrent protective device and the protective equipotential bonding is connected with all extraneous-conductive-parts within the installation in accordance with Regulation 411.3.1.2, the maximum disconnection times applicable to TN systems may be used."


Does this [also] imply that where protective equipotential bonding is not needed, then TN times can be used ?


And what reasons might there be for no similar relaxation of effectively 411.3.2.4 to 411.3.2.3  (1s to 5s  for other circuits not covered in 411.3.2.2 and in the light of the above extracted note to 411.3.2.2 Reg ) ?


Regards

Habs


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  • 0
    OK so 5 ohm rod for a really small circuit like one lighting circuit in a barn perhaps. To reliably give TN-s like rise of earth potential/ shock duration against  a 50 or 63 A breaker needs loop impedances perhaps a 10th of that - which is not all that likely to be met at the supply transformer end if it is a modest rural one ( a single pole pig could be could be on an LV electrode as high as 20 ohms, but probably no more than half that), let alone the resistance  any electrodes at the load end.

    Note that if you do have a really low electrode resistance at the load end, better than at the substation, then while fault current is flowing, all other users see the neutral voltage move off- earth , and the faulty live to earth voltage is depressed, while (if present) the other 2 phase to ground voltages rise.
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  • 0
    OK so 5 ohm rod for a really small circuit like one lighting circuit in a barn perhaps. To reliably give TN-s like rise of earth potential/ shock duration against  a 50 or 63 A breaker needs loop impedances perhaps a 10th of that - which is not all that likely to be met at the supply transformer end if it is a modest rural one ( a single pole pig could be could be on an LV electrode as high as 20 ohms, but probably no more than half that), let alone the resistance  any electrodes at the load end.

    Note that if you do have a really low electrode resistance at the load end, better than at the substation, then while fault current is flowing, all other users see the neutral voltage move off- earth , and the faulty live to earth voltage is depressed, while (if present) the other 2 phase to ground voltages rise.
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