Neutral to earth fault affecting both RCDs

[Roadhousezamora]

Zs was 0.4, on the circuit I was installing the extractor on.

With regards to leads, no I haven't replaced them yet, I will look at picking up a set if I can't seem to find a solution on the system itself, I haven't had them that long but it's a good suggestion for me to look into, thank you

 

[Aaron b]

I was reading someones explanation and what I took from it was that the amount of current lost from the neutral was less than 30mA and therefore not triggering the RCD... Didn't fully understand how that happens though.

Ah that makes sense for why that fault didn't cause the RCD to trip, I was thinking more of a direct short from a screw or the like but you're right it could be a lot lower than 1M and not trip I suppose. Assuming in most cases a 30mA RCD's only operate above 20mA..... 230V/0.02A=11.5kohm. Any chance something could be wet?

Hopefully checking your leads and using a long lead to check the earths should identify the problem with the RCD test.

 

[Roadhousezamora]

Ah that makes sense for why that fault didn't cause the RCD to trip, I was thinking more of a direct short from a screw or the like but you're right it could be a lot lower than 1M and not trip I suppose. Assuming in most cases a 30mA RCD's only operate above 20mA..... 230V/0.02A=11.5kohm. Any chance something could be wet?

Hopefully checking your leads and using a long lead to check the earths should identify the problem with the RCD test.

Yes possibly, there is an external wall that the faulty cable is buried on if there's water from the outside I suppose it could be wet but I would probably expect to see that on the plaster on the I side. Will get the damp meter on it next time, thanks

 

[Lucien Nunes]

To clarify with certainty:

An N-E fault downstream of an RCD can cause it to fail a test. because some of the leakage current generated by the tester can 'get back into' the neutral via the fault and go back through the RCD, hence reducing the imbalance. Equally, the N-E fault can defeat the RCD's own test button, because that might simply send some current from the L-in terminal to the N-out terminal, only for it then to disappear down into the CPC via the fault without returning through the RCD to create the imbalance.

But this only happens on circuits protected by the RCD in question. An N-E fault on a circuit protected by a different RCD won't affect the first one. That is logical: N & E are always connected together upstream of the RCD anyhow, whether at the origin (TN-C-S) or the substation (TN-S) so it doesn't matter if they are also connected at an upstream fault.

What can happen that 'crosses over' from circuits on one RCD to another, is the effect of a heavy load current showing up an N-E fault and causing a trip. In that case, it doesn't matter where the load is, whether on the RCD that trips or another. Anything that creates voltage drop in the supply neutral can raise the N-E voltage and drive current through the fault. The fault will however always be on the one that trips.

 

[Roadhousezamora]

As a quick update, the RCD on the non fault side was indeed faulty itself, the RCD in its place is functioning. It would seem as stated above there was no link between the neutral to earth Fault on the other side with this faulty RCD.