NEIEP Magnetism And Electromagnetism (355)

The key idea is overcurrent protection that sacrifices a part of itself to stop the current. A fuse uses a thin metal element that carries the circuit current. When the current exceeds its rating, the element heats up due to I^2R losses and melts, opening the circuit. This makes fuses single-use devices that must be replaced after they operate. Circuit breakers perform protection by tripping a switch—often via a bimetal thermal mechanism or a magnetic trip—so they interrupt the circuit without melting anything. They are resettable after the fault is cleared. Relays are control devices that open or close contacts in response to a signal, not primarily protective devices that melt to interrupt current. Transformers, on the other hand, are energy-transfer devices and do not function as protective elements that open circuits by melting.