On a sink in a toilet. But it energised the drainage of all the sinks of the building! It was sheer luck that the guy who called me was from where the fault came.
After analysing the problem, it became clear how the fault came to be: The appartment above were doing renovation work smashing the floor open without caring about any wires since they thought that all wiring will be renovated. But they don't think about the wiring of the lighting circuit from the appartment underneath that is also passing through their floor.
