confusing two-way switch wiring

14 May 2012
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United Kingdom
The scenario:

The ceiling light at the top of the stairs is controlled by a two-way switch at the top and bottom of the stairs.

Strangely, power to the light is from the downstairs light circuit.
I can see why, the downstairs switch is one of a triple where the other 2 switches are controlling downstairs lights.

The problem:

The ceiling rose of the upstairs light has power from downstairs via the two switches but confusingly it also taps into the upstairs lighting circuit. The upstairs ciruit has been cut and then joined within the rose.
The two circuits are not connected, they just coexist in the same rose.

The only reason I can think for this is: that someone accessing the circuit to extend it might be expecting to find an access point in that ceiling rose. But if they are not careful (and of course they would be careful anyway :) ) they might get more than they bargained for!

The question:

Can you think of any other reason why the wiring would be like this?
Can you think of any good reason why I shouldn't just join the upper circuit in a choc-box and take it out of the rose?

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Where does the neutral for the upstairs light come from?
And the line is from downstairs.
That is called a "borrowed neutral" and will cause you problems if/when the consumer unit is updated - assuming how it is configured.

I would sugggest getting it re-engineered properly so the line and neutral for the upstairs light come from the same feed (the upstairs lighting circuit IMO.

It sounds like the two-way circuit is in place so it shouldn't be too big a job to do. See the WIKI for how to wire lighting circuits, if you are not sure.
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Cheers Taylortwocities.

If I rewire taking the line from upstairs, I assume it would be wise to separate the downstairs switch from the triple it's currently in?
No real need - you should document it at the CU so that anybody wanting to isolate the switch in the future knows he has to switch off both circuits.

Borrowed neutrals like that are not good (see // but they are quite common.

Ones where the landing light is on the downstairs circuit but takes it's neutral from upstairs arise when people take what was originally one lighting circuit for the whole house and split it into two by the simple means of finding the cable which runs from under the first floor up into the loft and connecting it to it's own way in the CU.

It's easy enough to sort out by running a 3C+E strapper between the two switches, but I'd advise, if at all possible, to keep it on the downstairs circuit and take the neutral from there too. That way if the upstairs circuit fails you'll still have light on the stairs and if the downstairs fails you can still get enough to navigate by from other rooms upstairs if you open doors.

And/or make the landing light a non-maintained emergency one.

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