Cable tension under stillness
That means steel sectional doors on extension or torsion springs, with lift cables running down each side to a drum or a pulley at the bottom bracket. Those cables are under real tension even when the door looks still, and a cable that's frayed or has jumped its drum can let a panel drop hard and fast.\n\nYou can look at a cable without touching it: check for rust bloom at the bottom fitting, strands that look splayed rather than tightly wound, or a cable that's visibly loose on one side while the other is taut.