You don't understand ? So I don't...
To make it simple :
from the main box (in the garage), I have some breaker with a black cable going to the wire, a second white one from a common metal bar on the box, and generally nothing else. The black and the white are going together, that's my wire... travelling along the house.
In my desk switch (this example), I had this in the box (where the switch is located) : one black cable coming (I suppose) from the garage, connected to one side of the switch, and going (from the other side of the switch) to the lamp.
The white cable is coming (I suppose) from the box, connected through a round plug to a white wire going to the lamp.
That's it !
I disconnected eveything and connected the ws13, like explained in my previous message.
So for me, no third cable or whatever...
BTW, If I can't explain the case (...), the system is functionning (the black box mystery), and that's ok for me
Gilbert