That certainly happens but it's hard to think of a way that it could happen the other way around - so, even if the 40Ω N-E measurement were at low voltage (e.g. with a multimeter), it's very unlikley that the true N-E resistance is not at least as low as 40Ω, perhaps even lower, isn't it?
You are correct. Have you discovered another thing that no one realised?
I don't think I deserve any credit for this one. I'm pretty sure that I first came to realise it (a long time ago) when someone pointed it out to me - maybe even here
That obviously "depends" on the circumstances - in particular the impedances of the 'normal return path' of the circuit without the fault and that of the path via the affected RCD ('backwards') and the N-E fault.
Sure, the device feeding the other circuit is irrelevant. Indeed, even if, hypothetically, there were no OPD (e.g. circuit fed directly from CU Main Switch), the same would still happen.
Yes, but I don't remember anyone saying such a fault could be anywhere.
Yes, again, but it is not common for two or more RCDs to trip - the one on the circuit with the fault and perhaps all the others.
OK. As I said, I think someone one pointed it out to me - but I'm happy to take the credit
It did, and his text said the same. That's why I jumped in with my comment that it could even be something on a 'different busbar' (different RCD)
That won't happen with a situation I described. The 'unaffected' RCD won't trip, because the L and N currents through it will be balanced - it's only the RCD protecting the circuit with the N-E fault.
No, the point I was making was that the neutral current due to a load on the circuit on the 'other ('unaffected') RCD' splits upstream of that RCD, with the majority going through the normal return path to the tranny and a small amount going backwards through the 'affected' RCD, hence via N-E fault to earth - and since there is zero current through the L side of that 'affected' RCD (but the fault current through the N side) ,it will trip.
As above, I hope?
I'm not so sure about that. The total current (normal blue path plus dotted green 'sneek path') through the N side of the lower RCD will surely be the same as the L side of the RCD (hence no trip), won't it?
There is no dotted green sneak path through the lower RCD.
The only sneak path is the black line along the top neutral to the fault.
Indeed - so the top one will trip. However, as I said, I see no reason why the L and N currents through the lower RCD should differ, since they are simply the currents going into and out of the load, so that one shouldn't trip.
That depends upon what you mean by the "sneak path".
I was using your wording.
Yes but the Neutral current from the load is not increased because there is a leak elsewhere - so no sneak current at the bottom RCD; just the load current.
Agreed, but that small proportion is part of the load return current to the joint which is then missing from the back to tranny return path upstream of the joint.
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