TT Systems

[notrutsa]

Hi all,

I'm currently studying to become a qualified Domestic Electrical Installer and there is something I cannot find the answer to.

I have been given the task of designing a domestic circuit for a residence which uses a TT earthing system.

I have been told that the accepted Ze for this system would be 21 Ohms, however, I have not been able to find a way of calculating the Zs so that it falls within limit set by Appx 14 of BS7671; "Zs <= 0.8 x (Uo / Ia)" (0.8 x (Nominal Supply Voltage / Protective Device Trip Current)).

Can anyone help me with this?

Thanks.

 

[markysparky]

I think RCD is the key word for TT!!

Markysparky

 

[notrutsa]

Thanks for your reply.

I thought of that, but without the Ze to be able to calculate a Zs, it's not possible to then calculate the PFC or the cpc csa.

Andy

 

[Sintra]

Thanks for your reply.

I thought of that, but without the Ze to be able to calculate a Zs, it's not possible to then calculate the PFC or the cpc csa.

Andy

Why are you unable to measure the Ze and PFC?