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?