Circuit breakers are not fuses. As the amp load goes above the breaker rating, the breaker does not trip right away. In fact, the time delay can be quite a while. A 15 amp breaker with 20amps going through it may take up to several minutes to trip. I used to have a design table that listed the delay vs.load but I can't put my hands on it right now. If I find it, I will post it.