UK RCD issue baffling me

brianmoooore · 2024-10-31T23:53:27+0000

ChiliTom said:
So am I right in saying when I use a socket for a kettle the current is going from the mains through the LH RCD to the MCB to the kettle and the return current is then going down the neutral back through the LH RCD (all ok at this point) up to the neutral bar where the RH RCD neutral is also connected, but because there is a N to E fault in a circuit that side the current is instead flowing down the RH RCD neutral to the earth somewhere which then trips the RCD as there was a imbalance (due to no live current being drawn that side)

Yes, that's the general idea, but there might also be a balanced current flowing through the RH RCD as well, if the RH circuits are in use. May be 30A flowing through the live of the RH RCD, but if there's 30.02A flowing through the neutral of the same RCD, even for just a few milliseconds, then off it goes.

ChiliTom · 2024-10-31T23:56:23+0000

brianmoooore said:
Yes, that's the general idea, but there might also be a balanced current flowing through the RH RCD as well, if the RH circuits are in use. May be 30A flowing through the live of the RH RCD, but if there's 30.02A flowing through the neutral of the same RCD, even for just a few milliseconds, then off it goes.

But instead of the lighting current returning form the circuit down its neutral the sockets neutral current is going the other way trying to get to earth through the failt

ChiliTom · 2024-11-01T06:04:50+0000

brianmoooore said:
As I explained earlier, there is a parallel path for the neutral current to return to the service head. One that results in a small proportion of the neutral current from the socket circuits, but not the live current, flowing through the RH RCD, thus unbalancing it.
The proportion of diverted current is the ratio of the resistance of the neutral tail compared to the resistance of the RH circuit neutral, fault resistance, RH circuit cpc, and main cpc, which can be a very small proportion, BUT 20mA is a very small current, when compared to the many tens of amps that can flow as an inrush current on the socket circuits.

Would this be the case for the other lighting circuits tho because they all work fine and don't trip the RCD. Surely any current would try to use the parallel path to earth from the faulty neutral. How come the other lighting circuits are ok and work okay