BS 7430 resistance to earth 1

[Pgee630]

Hi,

May I seek some advice regarding BS 7430 requirement on resistance to earth.
I see in Annex A regarding legislation (the Electricity Supply Regulations), it states resistance to earth has to < 1 ohm. I think it's referring to TN-S system.

But how about TT system?

And how about the HV (or MV) side, if a HV or MV substation is built on-site?

I also can't find a required value for the telecom earth resistance, is it covered in another BS standard?

Thanks
P

 

[7anoter4]

No such a limit for earth resistance it is now required. See BS 7430 A2 Legislation.
. In BS 7671 the maximum earth fault loop impedance is limited for different earth fault system protection.
See Regulation 411.3 Requirement for fault protection
In Table 41.5 it is a note that the maximum recommended earth resistance has to be less than
200 ohm.

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[Baltoche]

Hi,

I've always been against the maximum earth resistance value (except for EMC). In terms of safety, and especially for MV systems, the maximum value is dictated by step and touch potentials permissible limits.

For TT systems the earth resistance does not really matter as long as residual circuit devices are used.
By the way, is there any table giving the maximum permissible touch voltage in function of time ?

Rgds

 

[7anoter4]

See BS EN 50522:2010 Annex B
Table B.1-Permissible body current IB depending on fault duration tf

 

[7anoter4]

Sorry. Better see Table B3 -Calculated values of the permissible touch voltage UTP as a function of the fault duration tf