Measuring Zs with RCBO

JuniorSparky · Apr 17, 2015

Hi everyone,

I've read that when carrying out earth fault loop impedance test we have to disconnect the functional earth of the RCBO (in case there's one) from the earth bar to avoid misleading result. Is that correct and is there anything else we have to disconnect?
I know about carrying out IR with RCBO and it even says on the device what to disconnect, but there's nothing about disconnecting the flying earth to avoid misleading results!

PS: What is your practice with the functional lead? I thought it has to connect to the corresponding circuit earth, but I've heard some people just connect all the FE from the RCBOs together in a connector block and then just one earth to the earth bar!

Thanks

davesparks · Apr 17, 2015

It is better to connect the functional earth to the appropriate terminal on the earth bar which corresponds to that circuit.

Richard Burns · Apr 17, 2015

If you are measuring Zs for a final circuit then the wiring should be connected as it would be in use as this gives you the real measured value.
When calculating for the worst case then you are avoiding parallel paths on your R1+R2 and may disconnect a functional earth.

Two schools of thought on FE connections to bus bars, I would go with use the same terminal as the cpc.
However there is the thought that if the cpc gets disconnected then so will the functional earth, therefore it may be better to put them in adjacent terminals, this makes a right mess of a CU, but may be safer!