What happens when a fuse carries more current than its rating?

• A. The fuse cools and continues to run
• B. The fuse melts and breaks the circuit
• C. The fuse stores charge
• D. The fuse increases its rating automatically

Answer: (B)

A fuse protects a circuit by opening the path when the current gets too high. Inside the fuse is a thin metal link that heats up as current flows through it. Since heat produced is proportional to I^2R, if the current exceeds the fuse’s rating for long enough, the element gets hot enough to melt and break the circuit. That stopping of current prevents damage or fire. So the fuse melts and interrupts the circuit. It doesn’t just cool and keep running, it doesn’t store charge, and it won’t automatically increase its rating.