What is the grey wire dangling from my ceiling?

Question

We recently moved into a new apartment in Berlin. We are the first occupants and thus there are no existing lighting fixtures. In the living room there are 4 light switches (1 & 2 on the left of the room, 3 & 4 on the right) and 5 cables dangling from the ceiling:

• Green/Yellow: Earth
• Blue: Neutral
• Brown: Live when switches 1 or 3 are on
• Black: Live when switches 2 or 4 are on
• Grey: Gets a "small" current when switches 2/4 are on.

By small current I mean - the only tool I have is a simple test light. When the Brown/Black wires are active the test light glows very brightly, but when the Grey wire is active the test light is very dim.

I get that there are two sets of three way switches here, but what is the Grey wire for then?

According to https://en.wikipedia.org/wiki/Electrical_wiring#Colour_code Black, Grey, Brown are colour codes for 3 phase, but I don't know what that means as far as installing a ceiling light goes here.

Any help much appreciated!

Answer 1

You need a multimeter, in voltage measuring mode.

And please post pictures of the overhead and two wall junction boxes.

Without further information, most likely Brown is hot for fan, Black is hot for lighting, and Grey is wired through some dimmer you haven't yet discovered, or neglected to mention you had.

Answer 2

Ceiling fan wiring? Here is a discussion with the same color wires. https://www.askmehelpdesk.com/advice/t-348809.html