Breakers are designed and tested to protect the branch circuit conductors under all conditions. This is one of the reasons that the UL test for circuit breakers (and fuses mounted in holders) includes 4' of conductor. This way we know the conductor is protected up to the AIC rating of the device and to the thermal rating of the termination. If there is enough impedance to limit the fault current below that of the short circuit portion of the protective device then its thermal portion will protect the conductor.
