A fuse blows immediately after replacement. The cause could be:

• A. A loose fuse holder
• B. A short to ground between the fuse and the load
• C. An incorrect fuse rating
• D. An overvoltage condition on the system

Answer: (B)

Fuses open when current exceeds their rating, so an immediate blow right after replacing a fuse usually means there’s a fault creating a very high current path as soon as power is applied. A short to ground between the fuse and the load provides a direct low-impedance path from supply to ground, causing a large surge current that instantly exceeds the fuse rating and makes it open immediately to protect the rest of the circuit.

Other possibilities don’t fit as well. A loose fuse holder tends to cause intermittent contact or arcing rather than an immediate high-current event. An incorrect fuse rating that’s too low could cause nuisance blowing, but it’s not guaranteed to blow immediately on power-up unless there’s already a heavy load; and a rating that’s too high might not protect at all. An overvoltage condition can lead to damage or triggering protection in other ways, but the fuse’s action is driven by current, and a short to ground is the most direct, common cause of an instant fuse blow.