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ELECTRICAL ENGINEERING
Construction of HRC fuse The HRC fuse consists of a ceramic body usually of steatite, pure silver element, clean
silica quartz, asbestos washers, porcelain plugs , brass endcaps and copper tags (see Fig. 11. 7).
The brass end-caps and copper tags are electro-tinned. The metal end-caps are screwed to the
ceramic body by means of special forged screws to withstand the pressure developed under short Fuse element
Contact blade
Fig. 1 1 . 7 H RC fuse link. circuit condition. The contacts are welded to the end-caps. The assembly also includes solder of various types, cement and indicator devices. Deterioration of the fuse must involve a change in one or more of these meterials or a change in their structure. Normally the fuse element has two or more sections joined by means of a tin joint. The fuse wire is not in the form of a long cylindri cal wire as after it melts, it will form a string of droplets and will result into an arc between the droplets. Afterwards these droplets will also evaporate and a long arc will be struck. The purpose of the tin j oint is to limit the temperature of fuse under small overload conditions. The melting point of silver is 960°C while that of pure tin is 230°C. As the circuit is overloaded the melting of tin prevents the silver element from attaining high temperature. The shape of the fuse element depends upon the time-current characteristic required.
Fuse operation When an HRC fuse operates, the element absorbs energy from the circuit and heats until it melts. The heat produced during operation is given by resistance and
i
J i2R dt where R is the instantaneous
the instantaneous current during the operating time. The fuse element melts
before the fault current reaches its peak value. As the element melts , it vaporizes and disperses. This action is then followed by a period of arcing during which chemical reaction between the silver vapour and the quartz powder takes place, which further results into building up a high resistance and reduces the current to zero. Thus the arc is quenched. Generally, the filling pow der used is quartz-sand as it can absorb heat at a very high rate and it does not evolve appreciable amount of gas.
Cut-off Current When an HRC fuse interrupts a heavy fault it exhibits an ability to limit the short circuit current. This ability is referred to as a 'cut off as shown in Fig. 11.8 and has the effect ofreducing the magnetic and thermal stresses both in the system and within the fuse itself under fault
conditions. Cut-off is in fact one of the main reasons why HRC fuse is so successful as a protective device and it is at times preferred over the circuit breaker of low ratings . Due to this property of