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Earth resistance of Neutral Earthing

Prasoon Raghavan
Good Evening,

Could anyone explain why a low earth resistance  of 20ohm is to be achieved for earthing the start point on the secondary side  of  a private distribution transformer with a TNS earthing system ? Rarely will any fault current pass through the ground to trip the protective device on the secondary. As I understand the purpose of earth is to provide a reference point for the neutral, in that case is 20 ohms required??
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    Where the earth is distributed from the transformer the earth is connected to the neutral at the star point of the transformer and to the earth electrode referencing the transformer to the general mass of earth.  This would be a TN-S earthing system.


    Where the the earth is not distributed by the supplier they may allow their neutral to be used as an earth return path. This would be a TN-C-S earthing system with the earth an neutral separated in the consumers installation.


    In both of the above installations earthing methods the transformer earth electrode is not in the earth fault path. The resistance between earth and neutral will be sub 1 ohm.


    If you want a comprehensive understanding of earthing and bonding I would highly recommend IET Guidance Note 8.

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  • 0
    Where the earth is distributed from the transformer the earth is connected to the neutral at the star point of the transformer and to the earth electrode referencing the transformer to the general mass of earth.  This would be a TN-S earthing system.


    Where the the earth is not distributed by the supplier they may allow their neutral to be used as an earth return path. This would be a TN-C-S earthing system with the earth an neutral separated in the consumers installation.


    In both of the above installations earthing methods the transformer earth electrode is not in the earth fault path. The resistance between earth and neutral will be sub 1 ohm.


    If you want a comprehensive understanding of earthing and bonding I would highly recommend IET Guidance Note 8.

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