You can calculate the amount of energy which will be "let through" before the circuit is broken so long as you know how quickly the breaker will trip at the specific current level involved.
To use your example, if the C/B will open in 5 cycles at 1200A, then we can work out how much energy is dissipated:
Trip time = 5 / 60Hz = 0.08333 sec.
Power dissipated = 120V x 1200A = 144,000W.
One watt is equal to one joule per second, so:
Energy = 144,000 x 0.08333 = 12,000 joules.
