A fuse opens a circuit in the event of excessive what?

• A. Voltage
• B. Current
• C. Resistance
• D. Power

Answer: (B)

Fuses open when the current flowing through them becomes too large. The fuse contains a thin metal link that heats up as current passes through it (the heat is proportional to I^2R). When the current exceeds the fuse’s rated value, enough heat is generated to melt the link, breaking the circuit and stopping the current. This is why the protection is described in terms of current, not voltage, resistance, or power. Voltage alone isn’t the trigger unless it causes a surge in current; resistance affects current, and power is a related result of current and resistance, but the decisive factor for opening the fuse is the amount of current.