A fuse opens when there is what condition?

• A. Over-voltage
• B. Over-current
• C. Short circuit
• D. Ground fault

Answer: (B)

Excess current flowing through the fuse is the condition that makes it open. A fuse is designed to melt its internal link when the current exceeds what the fuse is rated to carry. That overheating from the higher current interrupts the circuit to prevent wiring and devices from overheating or catching fire.

Over-voltage isn’t what triggers a fuse—the problem is too much current, not too high voltage. A ground fault is detected by a ground-fault protection device, not by a fuse. A short circuit is a specific cause of excessive current, so it would typically lead to a fuse opening as well, but the fundamental concept is the fuse responding to over-current.