Circuit breaker trip timing
Ideally, a circuit breaker should never trip while carrying its rated current.

Looking further down the graph to where it should trip, we find that a 30 Amp breaker will trip in about 10 seconds if carrying 60 Amps (200% of rating).  A short pulse of 200% overload (say, 5 seconds) will not cause a trip, but a continuous 200% overload will, after about 10 seconds.

The spec sheet states that that is a thermal breaker - something inside has to heat up before it will trip.  There are also magnetic breakers which will operate much faster than the thermal types.