Nevermind. i found it! this is how its done if someone wonders

1. turn off 'View Hidden Geometry'
2. turn to 'Wireframe'
3. select ALL by dragging the selection box (left to right or right to left)
4. (see*) turn on 'View Hidden Geometry'
5. subtract all normally visible edges by selecting ALL again by dragging the selection box while [Shift] is held down.
   Now you only have hidden lines selected.

and how can i select all the lines from my building at once without selecting faces?
wireframe does not work

hi there!

i have a question... i exported my archicad drawing to a sketchup file..
but all my lines seem to change in hidden lines
i was wonering if there is something to change my hidden lines back to normal lines again?
maybe a plugin...