we had a lucky escape with roller shutter drive chain snapping the door drops like a stone scary !!!
we fitted a new chain
we then were so rattled that it could have killed somebody or seriously damaged a vehicle we made a safety device
we took the door to the top and secured it we made two round pins and drilled some holes through the door
guide so pins could be inserted through we welded some flat steel to end of pins
so they could be inserted from floor safely
they were inserted every time the door went up from then on
if the chain snaps door can only move a few milimeters
cost a couple of quid time involved an hour
peace of mind pricelees
best regards