What does a fuse do in a circuit?

A fuse is a safety device — a thin wire designed to MELT (blow) when current exceeds its rated value, opening the circuit and protecting downstream components from damage. Once blown, the fuse must be REPLACED — single-use device. Types: (1) GLASS TUBE fuses — small, used in electronics, vehicles; (2) BLADE fuses — automotive (typical colors indicate amperage: tan=5A, brown=7.5A, red=10A, blue=15A, yellow=20A, clear=25A, green=30A); (3) AGC/MDL fuses — household; (4) High-voltage / industrial fuses. Ratings: marked with maximum continuous current (e.g., 5A, 10A, 20A); voltage rating (e.g., 250V); response speed (fast-acting vs slow-blow / time-delay — slow-blow tolerates brief startup surges from motors). CIRCUIT BREAKERS are like reusable fuses — they trip when current is too high, but reset by flipping the switch. Used in residential/commercial breaker panels instead of fuses. Modern circuit breakers: thermal (heating element bends), magnetic (electromagnet opens contacts), or combination; GFCI (ground-fault circuit interrupter) detects current imbalance between hot and neutral wires — used in bathrooms, kitchens, outdoors to prevent shocks; AFCI (arc-fault circuit interrupter) detects arcing — used in bedrooms to prevent fires. Replacing fuses: ALWAYS replace with the SAME rating — using a higher-rated fuse defeats the protection and can cause fire or damage; lower-rated fuse will blow repeatedly. Resettable polyfuses (PPTC) in some electronics — increase resistance when hot, recover when cool; reusable but slower response. Surge protectors are different — they protect against voltage spikes, not excess current. NEVER bypass a fuse or breaker — it's there for a reason. Common ASVAB EI safety items address fuse function and proper replacement.