A fuse is a device for protecting an electrical system against the effects of overcurrents (excess currents), by melting one or more fuse elements, thus opening the circuit. Very fast-acting fuses are widely used for the protection of diodes, thyristors, and other power semiconductors in AC and DC power electronics applications, and provide excellent protection against the potentially damaging effects of short-circuit currents. Current-limiting fuses achieve this protection by limiting both the amount of energy produced during an overcurrent and also the peak current which is allowed to flow. The term ‘semiconductor fuse’ means a very fast-acting fuse specifically intended for the protection of power semiconductors.
Figure 1. Construction of a typical smiconductor fuse
Figure 1 shows the construction of a typical semiconductor fuse. The fuse elements are usually made of pure silver strips, with regions of reduced cross-sectional area (often called notches). Theremay be several strips in parallel, depending on the ampere rating of the fuse. They are enclosed within an insulating tube or ceramic body, which is filled with pure quartz sand. At each end there are terminals with a variety of designs to permit installation in fuse-holders or connection to busbars.
Figure 2. Fuse element interrupting a short cicuit current
Fuses such as that shown in Figure 1, with the elements surrounded by sand, are called currentlimiting, high breaking capacity fuses. Semiconductor fuses are sometimes referred to as rectifier or ultra-fast fuses. They are available with voltage ratings up to 12.5kV, and with rated currents up to 10000A.
During normal circuit operation the fuse elements carry the required currents without melting. The resistance is as low as possible to minimize the power loss. However, when a short-circuit or fault occurs, the elements melt very quickly at the notches (regions of reduced cross-sectional area), and a number of small electric arcs are produced in the notch zones, which causes the fault current to be rapidly reduced to zero, and the arcs extinguish. The total time taken by a fuse to clear a fault is the sum of the melting time (pre-arcing time) and the arcing time.
Figure 2. Fuse interrupting a short circuit current
Special fuse designs are needed to provide protection of power electronic components. The p-n semiconductor junction can be very easily damaged, and so a very fast-acting fuse is required. For example, a typical thyristor may fail when subjected to a 10ms (one half-cycle in a 50Hz system) pulse of only 10 times its nominal r.m.s. current rating. A low-voltage fuse designed for general industrial applications may require 15-30 times its ampere rating to melt within 10ms, which is not fast enough to provide protection, whereas a very fast-acting semiconductor fuse typically requires only about 5-10 times its ampere rating, thus protecting the thyristor.
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