Under fault conditions, current-limiting fuses can limit the let-through current and clear the fault in 1/2 cycle or less. The combination of decreased clearing times and reduced arcing current can greatly reduce incident energy when arcing current is within the current-limiting range. True or False?

• A. True
• B. False
• C. Not applicable
• D. Unknown

Answer: (A)

Current-limiting fuses protect by two powerful actions during a fault: they interrupt very quickly and they constrain the arc current to a lower level. When a fault happens, the fuse can open in about half a cycle, so the duration of the fault current—and the arc that feeds it—is shortened dramatically. At the same time, the arc current is held within the fuse’s current-limiting range, which means the energy produced by the arc is much smaller than it would be if the current were allowed to soar or persist.

Incident energy is basically proportional to I^2 times the time the fault current flows. Reducing both the peak current (by current-limiting) and the clearing time (by fast interruption) leads to a substantial drop in energy reaching equipment. So, when arc current stays within that current-limiting range and the fuse clears quickly, incident energy is greatly reduced. That’s why the statement is true.