Current Limiting Fuses

Current limiting fuses are used in systems where high fault levels can result in excessive fault currents. The fuses function as normal fuses; however, they are designed to limit the fault current to low levels when they operate.

During normal operation, the fuse has a low resistance. However, when the fault occurs and the fuse ruptures, the heat created by the arcing inside the fuse causes the compacted quartz sand to create a high resistance environment. This quenches the arc and ensures that a very rapid fall in current.

The fault is thus cleared within the first half-cycle of the fault within 10 ms. Thus current-limiting fuses also protect systems from voltage sag in the event of a fault in one part of the system.

The current limiting fuses contains elements made of copper or silver. The elements are designed to have constrictions at a number of places which will heat up in the event of a fault. This enables quick operation. The arcing is also made to occur in a pre-determined number. The arcing which occurs in many streams enables easy quenching instead of one single arc. The quenching medium is usually compacted quartz sand.
Current limiting fuses also reduce hazards of arc-flashing, since they are extremely fast acting and also able to restrict the currents.

Arc flashing occurs when different conductors accidentally come into contact. The resulting arc can cause flashovers which generate tremendous amounts of heat causing danger to personnel nearby.

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