Heater tripping off outside breaker
First the heater needs air flow prior to turning on the heat. If the blower is working then the contactor pulls in and energizes the heating coils. Since it is tripping the breaker at this point I would be looking for a broken nichrome wire in the heating element that is touching some metal or shorting to ground. Many times a visual inspection can find the problem a broken coil of wire or a hole in the metal where you can’t see the actual wire.