High current flow through a thin fuse wire will raise its temperature causing it to melt and break the circuit before the current excess reaches a level sufficient to damage other, more substantial, parts of the system. Melting temperature depends on the material used (tinned copper in rewireable fuses melts at 1080ºC, the silver in cartridge fuses at 960ºC). The wire is sized so that the normal current is carried without overheating, but due to the resistance of the relatively small wire, that excess current will produce heat sufficient to melt it. Current rating gives the normal current that may be carried: minimum fusing current is the smallest current that will cause melting.
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A fuse will melt much quicker with very large fault current than when the value of fault current is only just above the minimum fusing current. Time/current characteristics are found by testing six or more of the same type of fuse at different currents and plot ting the results. The bottom current for the test is not more than 1.05 X minimum fusing current, and the top current is one that will melt the wire in not more than 0.5 second. The other test currents are equally spaced between these. Fuses are rated for particular ac. and/or d.c. voltages
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Cartridge Fuses
High Rupture Capacity (HRC) fuses have silver wire enclosed in a quartz powder filled ceramic tube with metal end caps (Fig. 9.1). Arcing when this type of fuse blows is buried in the powder, fusion of which in the arc path helps to extinguish it.
HRC fuses can be used for very high fault levels: deterioration is negligible; they have accurate time/current characteristics and reliability for discrimination; they are safer if accidentally inserted on a fault: there is no issue of smoke or flame; cartridges are sized to ensure that the correct value fuse is fitted.
Semi-enclosed Fuses
The rewireable fuse has an insulated carrier for safe handling and containment of the wire in an asbestos lined tube
The wire is easily replaced after operation, but the des is open to abuse as too heavy a wire can be used which could mask a fault and also cause severe arcing if it did operate. Another fault is that of premature failure if the wire is made thinner by oxidation or con tact with air, or by being stretched when fitted (a problem with wire made of lead, tin or an alloy of the two).
Fuses in Service
Fuses may be used as the only protection in a steady load circuit, such as for lighting. An ac. motor with its high starting current and varying load has fuses in each of the supply conductors, but fitted as a backup for the other forms of protection and to break the circuit in the event of a short-circuit current greater than that which the ordinary contact breakers are designed to interrupt without damage. Very accurate time/current characteristics are needed for fuses used in conjunction with other safety devices, to ensure that the overload trip is allowed time to operate for moderate over- current but that the fuse blows first if there is very high short-circuit current.
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