A fuse in a circuit is rated 3 A. If 4 A flows, what happens to the fuse and why?

• A. The fuse remains intact and passes 4 A
• B. The fuse will heat and melt, breaking the circuit
• C. The fuse will reset automatically
• D. The fuse will explode violently

Answer: (B)

Fuses are designed to carry current up to their rated value and protect the circuit if the current goes higher. When the current exceeds the rating, the fuse element heats up because power dissipated as heat is proportional to I^2R. With 4 A flowing through a fuse rated at 3 A, that extra current causes enough heating to melt the fuse element and open the circuit, stopping the current to protect the rest of the components. It doesn’t pass the extra current, it doesn’t reset automatically (in a standard fuse), and while a fuse may make a brief popping sound, it doesn’t typically explode violently.