Just responding to some quesions that came up. Germany doesnt use smurf tube on-wall, the picture is just a demo. The tube usually is plastered into brick/concrete walls or used inside drywalls. Then a NYM cable (our romex equivalent, but round with an insulated ground wire) or single wires are pulled in. It's also possible to directly plaster the NYM in or use it in drywall without a tube.
Also, most of our wall boxes are round. The one shown in the pic is a wet location on-wall box missing the plastic bushings for round NYM cable.
Those spring-loaded connectors are advertised as never coming loose (compared to the ones with screws we used back in time) because the spring compensates the copper's cold flow over the years. Wirenuts are seldom seen.
I have never seen a molten spring connector unless on internet pictures. Usually the cause then is that the wire was not inserted far enough into the spring, or the wire was damaged, corroded etc. The copper has to be straight and round. If the wire is deformed from being used in a screw connector before, you should cut off a bit.
The first ones were gray but most of them are clear now so you can see the proper wire insertion.
The ones Trumpy posted are used in Italy, but I believe they only have one screw there. We had the same ones back in time but open for wires on both sides.
