Elsbeth's growing orb of protection

You cast a spell that instantly creates a small ball the size of a marble. It takes one full minute to reach its maximum size, but when it does any creature inside the orb will take no damage. But the shield can only take up 80 (100 damage if cast as a level 7 spell)  damage before it breaks. So if the party takes 80 damage as a group the spell ends .