Earth Loop Impedance

[bigfra67]

I have several circuits on an installation that are failing their Earth loop impedance readings, they are on an ABB Type C RCBO is there a table in which would tell me if an RCBO has more tolerance than a normal Circuit breaker?

I know i could change them to Type B MCB's but they are on RCBO's for a reason?

[Ohmslaw]

Hi.

You'll probably find that they are on RCBO's for that very reason, the Zs being too high for a standard MCB to disconnect in the required time. Is it a TT earthing system???

With it being on an RCBO (assuming 30mA) the max Zs permissable in order for the RCD part of the breaker to operate before the touch voltage exceeds 50V (safe touch voltage) is 1667 ohms.

I would hazard a guess that your Zs is nowhere near this high??? or at least it shouldn't be.

Are you able to make a comparison be measuring some other circuits or taking a Ze reading just to make sure there is nothing seriously wrong.

[Central Scrutinizer]

I have several circuits on an installation that are failing their Earth loop impedance readings, they are on an ABB Type C RCBO is there a table in which would tell me if an RCBO has more tolerance than a normal Circuit breaker?

I know i could change them to Type B MCB's but they are on RCBO's for a reason?

rcbo's have the same Zs values as 60898's, take a look at the table (think its on page 49 of the regs, something like table 41.3)

but as Mr Ohmslaw says, the RCD element of the RCBO takes over the duty of providing earth fault protection, so the Zs reading, if less than 1667 ohms, is irrelevent

as long as the R1+R2 is ok and the Zs is being caused by the Ze then i wouldnt worry