Final answer:
Thermal magnetic circuit breakers protect against short-circuits by opening the circuit in reaction to the excessive current, preventing electrical fires and thermal hazards.
Step-by-step explanation:
Thermal magnetic circuit breakers are designed to protect against short-circuits by opening the circuit in response to increased current. When a short-circuit occurs, the current in the circuit increases dramatically. The circuit breaker detects this surge in current and responds accordingly. For example, many circuit breakers have a bimetallic strip that heats up due to the increased current. As the strip heats, it bends and triggers a mechanism that opens the circuit, breaking the electrical connection and stopping the flow of electricity. This mechanism acts to reduce the risk of thermal hazards and electrical fires, which can occur from excessive current. For larger currents and voltages, such as those in power-distribution systems, circuit breakers may employ insulating gases or jets of gas to extinguish any sparks that could allow the current to continue flowing.
In summary, the correct answer to the student's question is option b) They open the circuit in response to increased current. Circuit breakers do not increase current during short circuits (a), create a bypass for short circuits (c), nor do they only protect against overload without protecting against short-circuits (d).