Maximum Zs on circuit protected by RCD - 1667?

[Lainan01]

Does a RCBO protecting a single circuit mean it can have a maximum Zs of 1667 Ohms since it's being used as fault protection?

 

[pc1966]

Technically yes - but if you see that sort of impedance you have a whole lot of other questions to be asked as very, VERY likely there is something seriously wrong.

If you have a TN earthing system then your CPC is unlikely to be less than 37% of the phase conductors' size (thinking 4.0/1.5 T&E here as worst-case) so if it were genuine no-fault Zs of 1.6k that implies about 1k for the line and your 5% drop is going to be a load of 11mA so basically a 2.5W bulb maximum (ignoring lamps should be 3% drop for now). Is that realistic?

Also your cable, even if 1mm as smallest / highest resistance in use for fixed wiring, would be over 36km long. Again is that realistic?

The answers are no. So in the TN case anything beyond an ohm or so points to a fault, and for that passing the RCD's limit is not acceptable - it has to be fixed.

For TT earthing again it is simply pointing to a fault. The regs say no more than 200 ohms Ra (Table 41.5 Note 2) for a reason - anything more points to a faulty rod (probably corroded badly or broken connection and bearly the wrie in contact with Earth) or one in very dry / rocky ground which is not fit for purpose - it needs to be deeper to find moisture, etc.

 

[newfutile]

In any case it wont be for fault protection unless its TT earthing.
over 200 ohms Ra is considered unstable .
Your better off conducting R1+R2 and comparing these to the maximum ZS (roughly) , this will also ensure that it disconnects during a line-neutral short circuit.

 

[Pretty Mouth]

Short answer: yes the max Zs for a circuit protected by 30mA RCD is 1667 ohms, and this value is what you would write on the cert. As above though, an actual measured value approaching even a fraction of this indicates other issues.

Also: You can use RCDs for fault protection on TN-S and TN-C-S installations. The regs say so, and there is a list of standard circuits in chapter 7 of the OSG that rely on RCDs for earth fault protection. There's nothing I'm aware of that suggests that overcurrent protective devices are preferred for earth fault protection in TN systems.