A 15 amp circuit breaker is tripping on a time curve until around 38 amps where it starts to react in it's instantaneous range.
As a rule of thumb you need 6 to 8 times the rated current to trip any circuit breaker and most fuses in less than 8 cycles (instantaneous trip)
A 15 amp breaker will carry 135% or its rating for around an hour or about 20 amps.
So how long did you apply a %100 overload?
Then there are loose set screws, loose bonding terminals, and other bad connections that can add impedance to the fault path which can create sustained faults in the many minutes range where good connections might trip a breaker faster. Imagine your fault path is the EMT and several set screws are not tightened or worse the pipe separates. 8 ohms is enough resistance to allow a 15 amp breaker to feed a fault as if it was a load and the breaker never trips. Can you start a fire with 1800 watts? Sorry I will stop now as it is becoming a lecture.
