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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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    I think the OP's question was Why 20Ω for RB? As I think is stipulated in BS 7430 (the code of practice for Earthing). Why not 2Ω or even 1000Ω?


    I've asked that question before (especially in the context of small domestic embedded generation that might run during power cuts - where achieving 20Ω might be quite challenging). As far as I can tell there's no maths behind 20Ω at all - it's just a long standing convention that probably started out in the supply industry for public systems and has been written into various standards for general use - and as a result all sorts of assumptions have been based on it (e.g. when designing a TT system derived from a TN system it's usually taken that the max Ze (excluding RA of course) is 21Ω - 20Ω for RB and max 1Ω for the line conductor.


    Clearly it's good to have RB as low as practical - especially if there's any chance of earth leakage to true earth - as even quite modest currents returning from the electrode can raise the potential of the entire system quite significantly.


       - Andy.
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  • 0
    I think the OP's question was Why 20Ω for RB? As I think is stipulated in BS 7430 (the code of practice for Earthing). Why not 2Ω or even 1000Ω?


    I've asked that question before (especially in the context of small domestic embedded generation that might run during power cuts - where achieving 20Ω might be quite challenging). As far as I can tell there's no maths behind 20Ω at all - it's just a long standing convention that probably started out in the supply industry for public systems and has been written into various standards for general use - and as a result all sorts of assumptions have been based on it (e.g. when designing a TT system derived from a TN system it's usually taken that the max Ze (excluding RA of course) is 21Ω - 20Ω for RB and max 1Ω for the line conductor.


    Clearly it's good to have RB as low as practical - especially if there's any chance of earth leakage to true earth - as even quite modest currents returning from the electrode can raise the potential of the entire system quite significantly.


       - Andy.
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