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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


Parents
  • 0
    And the answer is the lower touch voltage with a low impedance earth - during fault the earthed case of a hand held load rises to a voltage that is set by the relative resistances of the L and E paths forming a voltage divider. With a reduced CPC in T and E,  that voltage  may be about 2/3 of the 230V at the far end of a long circuit,  though  with a sub-main with an equal L and E cross -section it would be half the phase voltage. In reality somewhere in between is typical on a TN system.


    On a high resistance TT system ALL the exposed metalwork on every thing using the same CPC and also the surface of the ground near the rod itself rises to nearly the full phase voltage during a fault, so clearing that fault quickly is much more important

    If the Ze is really low then with larger fault currents we get some of our voltage division back..
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  • 0
    And the answer is the lower touch voltage with a low impedance earth - during fault the earthed case of a hand held load rises to a voltage that is set by the relative resistances of the L and E paths forming a voltage divider. With a reduced CPC in T and E,  that voltage  may be about 2/3 of the 230V at the far end of a long circuit,  though  with a sub-main with an equal L and E cross -section it would be half the phase voltage. In reality somewhere in between is typical on a TN system.


    On a high resistance TT system ALL the exposed metalwork on every thing using the same CPC and also the surface of the ground near the rod itself rises to nearly the full phase voltage during a fault, so clearing that fault quickly is much more important

    If the Ze is really low then with larger fault currents we get some of our voltage division back..
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