Confused by unusual switch wiring

[sam]

My wiring was done in the 90s in black and red. I was considering re-wiring a switch so I could have two switches controlling the light and I found the following wiring which confused me. There are two sheathed cables coming into the switch. Cable A is twin+earth; cable B is three core+earth as per the attached diagram. Earths (green in diagram) are connected; black and blue are connected; reds are connected and the switch connects red to yellow. The ceiling rose has just a single red, black & earth cable. Is this a normal set up or does anyone have any suggestions as to what's going on? I must have some hidden junction boxes. It's chased into a wall where there's no obvious reason to have the ring going down to the switch and both cables appear to go upwards from the switch. A nearby switch doesn't have any yellow and there isn't space for a junction box in the wall.

 

 

[ProDave]

Without some testing with a volt meter it is hard to be certain.

 

One guess is the 2 core is L and N feed.  The 3 core is then L and N out and switched L out on the yellow. The fact there is no yellow at the ceiling rose means another junction box somewhere.

 

It is quite common to "loop at the switch" like this. The unusual thing is the 3 core containing L and switched L This would typically feed a bathroom fan giving permanent L and switched L for the fan, with just the switched L then going on to the light.

 

Is this an upstairs room? if so look in the loft.

 

Don't discount stupidity / ignorance and another hidden junction box above the ceiling.