Structure Characteristics and Working Principle of High-Speed Fuses

A high-speed fuse consists of ceramic housing, conductive plates, fuse element, quartz sand, arc extinguishing agent and indicator. The fuse element is made of pure silver in the form of rectangular sheets with narrow necks perforated with round holes.

Pure silver features low resistivity, good ductility and excellent chemical stability, which enables the fuse element to be fabricated into thin sheets with narrow neck structures containing circular holes. When a short-circuit fault occurs, the current density at the narrow necks rises sharply, causing the necks to melt first. The generated arc is split into numerous segments by quartz sand. The segmented arcs carry lower current and occupy limited space, making them easy to be absorbed by the arc extinguishing agent. As quartz sand is insulating, it forms an insulating barrier immediately after arc extinction to isolate the circuit.

Inverse Time Current Protection Characteristic

High-speed fuses have an inverse time characteristic. Namely, the fusing time is longer under low overload current and shorter under high overload current. Within a specified range of overload current and duration, the fuse will not blow and can operate continuously. Fuses come with diverse fusing characteristic curves to suit different protected equipment.

Current-Limiting Characteristic

The fuse element of the high-speed fuse is a rectangular thin sheet with a series of narrow perforated necks, and the fuse interior is filled with quartz sand as the arc extinguishing medium. The narrow perforated necks have small cross-sectional area and low heat capacity. In the event of a short-circuit fault, the necks melt before the fault current reaches the prospective short-circuit current peak. The arc is divided into multiple segments by quartz sand. This restricts the rise of short-circuit current and accelerates arc extinction.

High Breaking Capacity

When a short-circuit fault takes place, the narrow perforated necks melt first, and the arc is split into small segments for rapid extinction. Thanks to the insulating property of quartz sand, the fuse turns into an insulator instantly after arc extinction to cut off the circuit. Therefore, high-speed fuses possess strong breaking capacity, up to 50 kA.

Low Impact Energy on Load Equipment

When a short-circuit fault occurs in the circuit, the impact energy endured by load equipment follows the formula:

W=I²RT

Where:

I — Short-circuit current

R — Circuit resistance

T — Time interval from short-circuit occurrence to circuit interruption

High-speed fuses feature short interruption time and outstanding current-limiting performance.