Form filling. Max Zs.

Certainly the maximum Zs for a 30mA RCD protected circuit is nominally 1667 Ohms, but it would be unwise to approach this figure, in the same way as TT earth electrode resistances. Really this column is only there to make sure that the Zs has been checked for the circuit, for whatever CPD is fitted. In this case it is an RCD and therefore the "magic" 1667 Ohms is the number to fill in, or some other one if the RCD has a different delta In value.

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.

You got me. Which IET publication is this? There is nothing in the model forms for max Zs, though of course some generic forms have this in the circuit schedules as a prompt for the Installer. I would have thought any figure would be to achieve ADS, in the first instance. If an RCD was required to achieve ADS then it would be the higher figure[ for the RCD ]. If the fuse/mcb could achieve it, then the  max Zs is for that, regardless if there is an RCD or not. Simply, for TN it would not be the RCD figure as it suggests something is wrong with the circuit.


Now I know some will disagree with the last sentence, as there seems to be some Schemes suggesting  that an RCD can be claimed for ADS on a TN circuit  that is a bit long and the cables a bit small, resulting in an overly large earth fault loop path, in order to save money on a properly sized set of conductors ?.


Now it could hinge really on what you are verifying for what purposes. So if it was for initial verification in a TN , it would be the fuse, but , perhaps if an in-service verification [EICR],  it could be the RCD if the original circuit is so rough that is all there is for ADS purposes ?.  RCDs are for fault protection and additional [ supplementary ] protection, now there is carelessness of the "user" , which is fine, but there does seem to be some mission creep where it seems to be for the carelessness of the installer also.

Thanks for the responses.

Current model forms have a column for max Zs and the publication is the current edition of the electricians guide to the building regulations.

Current model forms have a column for max Zs and the publication is the current edition of the electricians guide to the building regulations.

Squinting at BS7671 and GN3 forms again and yes, I missed the new box for max permitted Zs. However, in pages 126 and 127 of GN3 the "filled in " sample forms have that figure as per the circuit breaker not as the RCD. Same in Onsite Guide.


Do not have the latest building regs publication you refer to, so cannot comment.