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We have a downstairs room with a single light and switch.

This is Lamp A and Switch A.

Switch A only has a single brown and blue wire into the switch.

We recently had some work done upstairs and the builder added a light switch and lamp. Lamp B and Switch B. He spured this off of the lighting wire in the eves of the house, and the wire goes behind the plasterboard, and into switch B, then onto lamp B with just the brown wire switched.

Now, lamp B works, and switch B works, however if it is on, then Lamp A downstairs is always on, and switch A does nothing.

If switch B is off, then switch A works as per normal.

Obviously, I'll get them back to sort but I really cannot figure out what has happened and how this is wired, in such that as soon as there's a circuit on switch B, Lamp A comes on and stays on and switch A does nothing!

How does this setup work?

  • If you paid a builder so it would be done right, this problem should be on him and his electrician.... – Jack Jan 25 at 22:44
  • Hi @jack yes he'll sort it out but it's just weird... I can't figure out how it must be connected! – RemarkLima Jan 25 at 23:07
  • The hot feed did not get wired the way it should have. It may just need a jumper off the hot wire to feed the other switch properly. Or perhaps the switch legs did not get tied in the way they needed..... – Jack Jan 26 at 0:15
  • When switch B is on, and you toggle switch A, does lamp A change in brightness? (You may have to wait until nightfall to test this.) – A. I. Breveleri Jan 26 at 6:48
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It looks like he got the switched load from lamp A tied into both lamps. The hot from switch A going to switch B and load switch B going to lamp B. Now when switch B is closed, both lamps will be on, wired in series, and switch A does nothing. If switch B is off, then A is normal.

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  • Except then, lamp B would light when switch A is on. This is a brain bender to be sure! – Harper - Reinstate Monica Jan 25 at 22:17
  • And is there any way to rectify this without pulling down some of the plaster board? Or do they have to get to the wiring they've spliced into? Just do not want to start decorating and find they rip a wall down! – RemarkLima Jan 25 at 23:12
  • It would depend on where he hooked into those wires. did he do anything in lamp A fixture? – JACK Jan 25 at 23:22
  • No, I do not think so, just cut the lighting wire and took a new line from there... No reason to touch the light fixture. – RemarkLima Jan 26 at 8:16

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