I was called to have a look at my in-laws house. They moved in a few months ago and the light at the top of the stairs has never been very bright. Also, there has always been a set of mystery switches on the stairs which they couldn’t figure out.
Upon further investigation I found the problem, and fixed it.
It turned out the slave wire to the top of the stairs from the bottom went via the back of an intermediate switch. In the intermediate switch the red and yellow wires were carrying the two lives, whilst the blue was a neutral up to the light.
What the people changing the switch had done is put both the live and the neural through the intermediate, and left the other live in a chocolate block connector. So all the intermediate switch was doing was switching live and neutral around.
Worse still, the neutral was then slaved onto the switched live of the downstairs fitting – so, both sides of the bulb were getting 230V. There was essentially no return feed! This showed because neutral had 230V present at it when it was on downstairs.
I rectified this, and sure enough one set of switches now turns on the upstairs light and one set now works the downstairs light. As a bonus the outside light also now works too – previously it only worked when the hall light was on.
Ironically, two electricians had already looked at this and suggested “major repair works”. Because two wires were in the wrong place.
My in laws now have a bright house and all are happy.