Austrian 230VAC.
I took a DMM and made a voltage measurement between phase and a 5 meter long curled 1.5mm² copper cable with isulation and got a voltage from 40VAC to 100VAC varieing how i curl the cable. The cable is not connected to anything on the other side.
What I am measuring here?
Is this a reflexion on the line? If thats the case, is it too fast for the FI circuit breaker to detect the flowing current? Ireflexion - Il = 0?
No, this is not reflection. Reflection only comes into play what the length of the wires are in the order of the wavelength of the AC frequency.
For 50 Hz that's \$\lambda\$ = \$c/f\$ = 3E8 m/s / 50 Hz = 6000 km. I'm guessing your wires aren't that long.
What is happening here is that you made a capacitor. Two conductors side-by-side have capacitance to each other. That will be a low value capacitance (less than 1 nF would be my guess) but enough to develop a voltage across your DMM as your DMM has a high input impedance. Usually that input impedance will be 10 M ohm.
As soon as you apply a load to that conductor, the voltage will drop as the capacitance has a high impedance. You cannot extract much power from it. The power is enough to show a reading on a DMM or make a neon-lamp voltage checker screwdriver light up (faintly) but that's it. Even if you touch it you might not even notice the AC current that is flowing as it will be small.
You get measurements like that in electrical installations frequently and as the above comments say, it's like a capacitor. And or induced voltage from adjacent wiring can cause this too, like a transformer of sorts. Some multimeters have a LoZ mode or Low impedance voltage measurement that will measure 0v in a case like this, which can be handy know.
