Current-limiting fuses are designed to operate within what time after sensing a short-circuit current in a 60 Hz system?

Current-limiting fuses are designed to operate within what time after sensing a short-circuit current in a 60 Hz system?

• A. 1
• B. 1/10
• C. 1/60
• D. 1/120

Answer: (D)

Current-limiting fuses are designed to interrupt a short-circuit very quickly to limit the energy released in the circuit. In a 60 Hz system, one full cycle lasts about 16.7 ms, so half a cycle is approximately 8.3 ms (1/120 of a second). The fuse is designed to clear within that half-cycle, or faster, to keep damage and let-through energy to a minimum. The other time options are too slow to achieve this rapid energy limitation—waiting a full cycle or more would allow more heating and potential damage, while 1/10 of a second is far longer than a half-cycle. So the best characteristic is clearing within about 1/120 of a second.

Current-limiting fuses are designed to interrupt a short-circuit very quickly to limit the energy released in the circuit. In a 60 Hz system, one full cycle lasts about 16.7 ms, so half a cycle is approximately 8.3 ms (1/120 of a second). The fuse is designed to clear within that half-cycle, or faster, to keep damage and let-through energy to a minimum. The other time options are too slow to achieve this rapid energy limitation—waiting a full cycle or more would allow more heating and potential damage, while 1/10 of a second is far longer than a half-cycle. So the best characteristic is clearing within about 1/120 of a second.