The problem is the line not neutral, the name is confusing, it's call borrowed neutral, but is borrowed.
So years ago electricians would put two switches together and take one line feed for both switches, and in the main this was OK, however when we started using down lighters we regularly found five amp was not enough, and although regulations allow up to 16 amp for lighting, the ceiling roses are typically rated 5 or 6 amp, so if there were any ceiling roses they could not use over a 6 amp supply, also the tungsten bulb should have a built in fuse, so if when the filament ruptures there is ionisation, the fuse will rupture so only that bulb fails, but many imports don't have this fuse in the bulb, so when the bulb blows, the high current has been know to melt the solder on the bulb contacts and weld the bulb into the holder, so it makes sense to limit the lighting supply to 6 amp.
So to draw more than 6 amp the supplies were split, some times it was split to start with, but often this split was done latter, when split the electrician should clearly check that there is no borrowing of a line which will technically result in borrowed neutral, however pre use of RCD protection everything would still work, so often missed.
As to what to do when found, returning to a single MCB for all lights is the easy way, your not making any worse, you just returning it to how it should be, however putting the two MCB's on the same RCD may work, but is clearly wrong, and once you realise there is a borrowed neutral to put both MCB's on one RCD leaves no excuse, if an accident then the person doing it could be excused, no body is perfect, but doing it when you know there is a borrowed neutral is criminal.
What we now consider is should a power circuit trip, we don't want this to affect lights, so it is considered safer to have two RCD's split so the sockets in a room should they trip the RCD will not take out lights in that room.
However clearly a main fuse will take out both, as will a power cut, so really down to a risk assessment, some houses do have stair wells in centre of house with very little light from outside, my old house had windows above the internal doors to light landing, which really was not good enough, I fitted an emergency light, this house there is a window at top of stairs, however I have a rechargeable light plugged in which will auto light with a power cut.
This house sockets split front to back, lights split up and down, so only way with two RCD's is all lights on one RCD and all sockets on other RCD, this means loose one socket due to RCD trip and you loose all, so the all RCBO consumer unit (CU) is clearly the way to go. Which is easy enough as TN supply.
MK do a kinetic switch, that is the easy way around the two way switch problem, but the standard wiring for two way switching the line is taken to second switch,
View attachment 55400you can see from diagram earths missing for clarity and loops around the cables you need three wires between the two switches but one of the three is permanent line. So this was "borrowed" from another switch, so twin and earth could be used between the two switches. It is a bad idea anyway as it can cause mains hum on some equipment if you send a line in one direction only.
The two way switch shown between them has 4 options with a truth table, two are off so not interested in them, of the other two if different lines are used, from different RCD's one will trip RCD the other will not, so it is easy not to realise there is a problem.
In my own house at front door a block of 4 switches, upstairs landing lights, hall lights, and two sets of outside lights, and an error had been made, wrong line selected, both lines were in the switch, just some one selected wrong one.
If you think of it as borrowed line instead of borrowed neutral does it make more sense? Remember both Line and Neutral are considered Live, I try to remember to call it Phase or Line, but some times I forget.
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View attachment 55401Does this help, the right hand light line feeds the switch second from left which will work the left hand light, it will work, but the line can feed between the two lamps with both left hand switches down, and with just second from left down light is powered from right hand fuse instead of left hand fuse. As can be seen it is line that is borrowed, but it is called borrowed neutral.
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Just watched John Ward no wonder you can't work it out, he shows line to com, but that is not the way most of us wire it, we connect all terminals to the like terminal in other switch, so L2 to L2 and L1 to L1 and com to com. The com only connects switch to switch, so if L1 is line in than L2 is line out to light.
