No one is suggesting that both live and neutral are disconnected. In fact with a borrowed neutral this would be dangerous.
If neutrals were truly "permanently bonded together" then it would be OK.
However in practice they're bonded half-a-house away, down cable that could be unreliable. There will also be a path to live through the lightbulbs. So if this other path is to a live that isn't isolated (i.e. borrowed neutral) and is switched on at the same time, and the neutral loop impedance climbs to a fraction of that of a cold lightbulb (and remember, there's a fault somewhere, which is why we're fiddling) then that can be enough to put a harmful voltage onto a supposedly "dead" circuit.
It's like driving behind one of those Fords with a poorly earthed rear light cluster, so that it indicates every time it brakes.
Also, if working on something where you don't _know_ there isn't a borrowed neutral, isolate all the lighting circuits, don't just pull fuses until the light turns off.
I fitted my loft lights off the ceiling rose for the landing light(powered from the downstairs lighting circuit) so that any work in the loft on upstairs lights would be lit.
When I came to move an upstairs light, as soon as I disconnected the neutrals
--flash-- darkness, a shared neutral, and a jumbled loft to cross. It could have been a lot worse.
I now am certain I haven't any shared neutrals.
I should have looked at this kind of thing before buying my house - the electrician who sold it to me had rewired the place. Power circuits (on 30A rewireable fuses) were 1 cooker point in kitchen, with single 13A socket integral, and one ring covering cellar, ground, first and second floors, with fused spur to garage! Lighting was a little more sensible, though the switches do seem to have been put where it's convenient to wire rather than convenient to use - the only swirch for the hall light is 4m away from the front door.
If you are working on a circuit that is protected by a RCD (especially if the RCD protects other circuits as well), then you will frequently find that a neutral to earth short will trip the RCD (or even a neutral to "any large lump of metal" connection for that matter).
You also need to keep in mind that contrary to expectation, neutral is also a "live" wire! See notes here:
Depends on where the neutral is borrowed from. Say you had a split load CU, and the borrowed neutral was from other "other" side of the split. You could disconnect line and neutral of one side with the isolator/RCD for that section, and that would leave its neutral bus bar floating or pulled up to mains potential via the borrowed neutral if present.
I thought they were, and in particular that certain types of circuit breaker would disconnect both live and neutral. Was that a misunderstanding on my part, do such devices exist, and if so, why do they disconnect both poles?
In an analogous situation to that described by the diagram to which you refer below, a tripped or disconnected breaker on ciruit 1 would cause
*both* live and neutral of circuit 1 to become live when the lamp in circuit 2 is switched on, and that's before you even open it up.OK, I see what you mean. In this case the two blues would no longer be connected to each other.
I have some 2 pole RCBOs (Hager) for outside circuits. It is not a
*general* requirement, but I feel it isn't a bad idea. One situation where it would be a requirement is if that circuit fed a remote TT installation from a non TT source, eg a workshop. TT must have DP RCD protection. Normally you'd use a DP Type S 100ma RCD followe by a 30mA RCD for sockets or anything else that required 30mA RCD protection.But having a 100mA RCD for one 30mA RCD circuit (like a shed supply) would be a bit unnecessary though perfectly correct.
They would be if they were connected downstream of the CU, unless I'm not following you?
Cheers
Tim
The diagram shows two circuits emanating from the CU. Circuit 1 feeds live and nuetral to lamp 1. Circuit 2 feeds live to lamp 2, but lamp 2 takes its neutral from lamp 1.
The idea is that if you switch off circuit 1 (but not necessarily its nuetral), in order to do some work on lamp 1, like remove it which involves disconnecting both neutrals from it (both the one coming from the CU and the one going to lamp 2), that if you then switch on lamp 2 it will make its neutral (which is now open-circuit) live. Lamp 2 will of course not light because it has lost its neutral feed.
When you then come to re-fit a replacement for lamp 1, you will encounter an unexpected live neutral coming form lamp 2.
If the two blues were still connected to each other, they would not become live, unless the MCB switch at the CU was 2-pole.
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