RCD impedance

[junksamiad]

Hi everyone,

I appreciate this is probably a really silly question.

I’ve come across the below test question and although I know how to find the answer, I don’t understand why we would need to know what the max permissible earth fault loop impedance is in relation to a 30mA RCD? I hope someone can offer some simple logic as to why we need to know this?

The question is:
Calculate max earth loop fault impedance permissible for a 30mA RCD used to protect a circuit on a TN-S supply

A. Nominal voltage / RCD trio rating = max earth fault loop impedance

Again, it’s just the theory of reason behind the question that I’m struggling with.

Thanks again as ever.

 

[Lucien Nunes]

If the EFLI is too high, in the event of a fault the leakage might be less than 30mA and the RCD might not trip. Same as with OCPD but a much higher EFLI is tolerable due to the low current involved. However, FWIW if the EFLI is that high it is likely to be unstable, e.g. if due to Ra of an earth electrode. Therefore, the practical application of this limit is unlikely.