earthing issues

[EddieB]

Hi all
just replaced a fuse board in a private flat in London. the meters are located in the basement along with the service fuses. There is no earth with the mains the conduit is acting as the main earth. The Ze reading at the DB is .14 the service type is TN-S. The sockets and lights are all in buried conduit and again using the conduit as the earth. I cannot get a good loop impedance reading on one of the rings, the boxes are rusting, I've drilled and taped into the boxes and put a fly-lead to the socket earth termnal but get an unsatisfactory 40 ohm reading. The other ring is coming in at 1 ohm. I've cross bonded all the outgoing conduits to the earth block but no improvment. Re-wiring the ring is a no go there is a fitted kitchen which has covered at least 1 existing box.I could I suppose transform the ring into a radial and sleeve the spare neutral and use it as an earth, not easy as it's all 3/4" an only one pipe so the cables are pretty solid.I've installed RCBO's If this was TT system then this would be ok. any suggestions please

 

[Spudnik]

Although not correct, providing there is 30m RCD protection on the RFC in question, then you have up to 1667 ohms to play with.

Not ideal at all though, especially if you are suspicious of the conduits breaking down.

Is there no way you could run a new cpc in?

If not, note everything on the cert.

 

[EddieB]

Cheers Jason
Yeah I'd love to get a cpc in but it's only 3/4" and there's 4 x 2.5's probably 7029's and they will not budge, there is the radial option but that's more cost for the customer and a sort of downgrade really. I will note it all down on the test certificate.