Final answer:
Circuit breakers are installed in aircraft electrical systems primarily to protect the circuits from excessive currents and prevent thermal hazards.
Step-by-step explanation:
Circuit breakers are installed in aircraft electrical systems primarily to protect the circuits from excessive currents. They work by detecting and interrupting the flow of current when it exceeds a certain limit. Circuit breakers are designed to be resettable, unlike fuses which need to be replaced after they are blown. By interrupting excessive currents, circuit breakers prevent thermal hazards and potential damage to the electrical system.