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Tell me please how long it takes the breaker to trip after you reset it, when the thermostat is set for heating and up to 90F.
Next turn the thermostat down to 50F, and set the fan switch for 'fan ON' and tell me if the fan runs or not.
If the fan runs, and the breaker trips instantly when set for heating, then there is a dead short in the wiring to the heating elements or in the heating elements themselves.
Tell me what test instruments you have please, and a bit about your experience working with electricity, we can go from there without any time limit, please hold your rating until we arrive at a diagnosis.