I'm definitely not quite sure here, but the way I understand this snippet it suggests that once the screws are snapped off (for whatever reason) they are no longer able to penetrate/perforate the Isolierkoerper, since they break off 'formschluessig'.
If that does not quite work in your context, here are some other suggestions from Siemens Woerterbuch Antriebstechnik & Fachwoerterbuch Industrielle Elektrotechnik:
1)formschlüssig
form-locking
form-closed
positive
non-slipping
non-slip
form-fit
keyed
mit Federkeil (auf Welle) befestigt
geared
über Zahnräder verbunden
no-slip
rigidly engaged
rigid
starr
2)abreißen
break
break off
snap
zerspringen, zerreißen, entzweigehen
come to a dead stop
ein Prozeß, eine Beziehung usw.
tear off
auseinanderreißen (aktiv
tear down
niederreißen
pull off
auseinanderreißen (aktiv
pull down
Gebäude
strip
Werksanlage, Maschine
dismantle
Werksanlage, Maschine
3)Abreißkontakt
arcing contact
4)formschlüssige Bauart
form-fit design
5)Abreißdiode f
snap-off diode

good luck with that one!
Chris