Yep. The instantaneous magnetic trip would be what should have worked here. Most likely, as was said, the circuit impedance limited the instantaneous fault current to a level below the mag trips on that breaker. I think (strictly from memory) that CH breakers have mag trips set at a range between 11-17X the breaker rating. So on a 20A breaker, the current would have had to jump to over 220A before the breaker would trip instantaneously.
Originally Posted by templdl
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)