Circuit breakers and fuses will protect against both overloaded circuits as well as overcurrent in a circuit. It’s important to know the difference between both.
An overload in a circuit is when there’s an increase of current (amperage) above the rating for the circuit. If the circuit is allowed to draw the excess current, the wire insulation could become damaged or even catch fire from the added heat. An overload in a circuit usually happens over time and not instantly.
A circuit with #14 wire is rated for 15A with a 15A breaker required. If you use a 20A breaker instead, you could run into problems if amperage in the circuit goes above 15A as the wire is not designed to handle that much. It will overheat and could become damaged.
A seized blower motor attempting to start and run could add to an overload condition. but probably not create an overload condition as most motors in the HVAC industry have built-in thermal overload protection to shut the motor down when it begins to overheat.
Too many pieces of equipment on a circuit could create an overload condition.
Overcurrent situations usually happen almost instantly. Overcurrent could be a short circuit or a sudden rush of current upon a motor startup in a circuit not designed to handle it.
Upon motor startup, a motor will draw a higher amount of amperage than it will during normal operation. This is called the LRA, lock rotor amperage. It’s the amperage a motor will draw when the supply voltage is applied, and the rotor is locked (not turning). This lasts for a second or less before the motor gets up to speed.
Overcurrent protection happens much quicker than overload protection.