TT on DB number 2

[Ross1]

Evening guys, I've got 2 basement rooms that are being supplied from their own cu which is fed from a 32A MCB in a split load rcd board in the house which is a TT installation. The cu in the basement is relying on the CPC in the 6mm supply cable to the basement cu as its main earthing conductor. The cu in basement has a lighting and socket circuit. With no rcd protection but is rcd protected back in the house cu.Do I need to install a rod for the basement cu and does it need rcd protection?Thanks

 

[telectrix]

if the 32A breaker feeding it is RCD protected, no need to fit another one. as for a second rod, you can fit 1 if you want. it may bring the overall Ze down.

 

[Ross1]

Thanks Telectrix, I've got to knock a deeper rod in for the main cu ( getting a reading of 225 Ohms ) So the 2,5mm CPC in the supply cable to basement cu is acceptable to act as the main earthing conductor for the basement cu?

 

[telectrix]

to determine that, you would have to know the Ze and PEFC. then apply the adiabatic equation. generally for a TT system, 2.5mm is adequate, unless you can bring your Ze down low. say you got your Ze down to 23ohms, that would give you a pefc of 10A, for which 2.5mm is adequate .