I think this has been covered once before, but one of the current IET publications shows on the schedule of test results Max Zs as 1666 when protected by RCD (possibly RCBO as well) even when having a TN-C-S connection. I wondered what was the current opinion on this.
Speaking personally if I got a result of even 10% of that, I'd be wanting to investigate. It may be safe, in the sense that any fault that would not trip the RCD would not expose a dangerous touch voltage, but testing for "this is safe to touch " is not really the same as "this is working as intended".
On TT supplies one might see a couple of hundred ohms of Zs, but I'd still expect to see the R1+R2 part of that to be sub ohm, and all the Zs in the soil path. Clearly after energisation, R1 + R2 is not as sensible to test as Zs, so there has to be some trust. But in this case, on TNS and TNCs I'd still expect Zs to be sub ohm in most cases, certainly less than ten ohms. A thousands ohms will operate the RCD, certainly, and that is what is being tested in that column - does the ADS work, but the awful connection still requires some investigation.
Speaking personally if I got a result of even 10% of that, I'd be wanting to investigate. It may be safe, in the sense that any fault that would not trip the RCD would not expose a dangerous touch voltage, but testing for "this is safe to touch " is not really the same as "this is working as intended".
On TT supplies one might see a couple of hundred ohms of Zs, but I'd still expect to see the R1+R2 part of that to be sub ohm, and all the Zs in the soil path. Clearly after energisation, R1 + R2 is not as sensible to test as Zs, so there has to be some trust. But in this case, on TNS and TNCs I'd still expect Zs to be sub ohm in most cases, certainly less than ten ohms. A thousands ohms will operate the RCD, certainly, and that is what is being tested in that column - does the ADS work, but the awful connection still requires some investigation.
