Just preparing a room at home to redecorate. Removed the ceiling light fitting and there's an extra cable …
As expected, there are 2 x loop cables and a switch cable. Then a single, double insulated wire. Black inner insulation and it was connected to the rest of the black/neutral wires. Not connected to any other wires, it's carrying 240 V with respect to earth.
Disconnected from light and light works OK. Haven't yet found anything else not working.
Any ideas what it might be?