Higher permitted zs on a rcbo protected circuit

Assuming 30 mA residual current rating of the RCBO's, it sounds very strange to have Z_s > 1667 Ω

What is Z_e ?

Is volt-drop on the circuits OK?

It's perfectly compliant BS 7671 wise to select a residual current device for ADS even on a TN system - so as long as Zs is below 1667Ω I don't see an issue.

I might double check that the C type provides adequate (thermal) protection to the conductors (both live and c.p.c.) under fault conditions though.

   - Andy.

AJJewsbury said:

It's perfectly compliant BS 7671 wise to select a residual current device for ADS even on a TN system - so as long as Zs is below 1667Ω I don't see an issue.

And volt-drop is met.

Of course, though, we'd be worried if Zs approached that value in a real installation.

Happier, if (R₁+R₂) were as expected (for the assumed length of the circuit), then, ignoring parallel paths, quite roughly, Z_s should be of the order of Z_e+(R₁+R₂) give or take a little, and particularly if Z_e is very low, some instrument error.

And volt-drop is met.

Absolutely - my comment was only meant to be taken in the context of the value of Zs itself. I didn't mean it imply that there was nothing else to consider - be it voltage drop, protection of the conductors from faults, possibility of poor joints or a break in the ring that might lead to an increased impedances, thermal insulation around the cables or many other things.

   - Andy.

The wording of 411.4.204, "Where an RCD is used to satisfy the requirements .....Table 41.5 may be applied..." suggests that it should be a choice of the designer of the circuit. It doesn't say "If an RCD is present then table 41.5 should be applied". In which case, ideally the designer should indicate in the certification that this choice has been made ie max Zs permitted by BS7671=1667

An inspector doing an EICR is surely not in the position to choose whether to use 41.5, because they are not the designer. They can take a view on whether it is safe since BS7671 allows a designer to use 41.5. 

As to whether a code is appropriate, don't know, i don't do EICRs.

OlympusMons said:

An inspector doing an EICR is surely not in the position to choose whether to use 41.5, because they are not the designer. They can take a view on whether it is safe since BS7671 allows a designer to use 41.5. 

I think I agree with the statement "They can cake a view on whether it is safe".

The inspector can only decide whether there is (or is not) now adequate means of protection against electric shock.

This is very important, because the periodic verification is done against the current standard, not the one in place at the time the installation was originally designed.

So, in this case, clearly Z_s for ADS by OCPD is not met, but is the installation safe? Well, the inspector can apply the current version of BS 7671 to determine (and of course this permits the RCD element in an RCBO to provide ADS).