The following points highlight the three different metals used as fuse wire. The metals are: 1. Silver as Fuse-Wire 2. Copper as Fuse-Wire 3. Lead-Tin Alloy as Fuse-Wire.
Metal # 1. Silver as Fuse-Wire:
For fuse wires silver is quite satisfactory material. It is not subjected to oxidization as its oxide is unstable. The metal does not deteriorate when used in dry air. But when the air is moist and contains hydrogen sulphide, a layer of silver sulphide is formed at the surface of the element. This shields the metal from further attack. The only drawback in its use is that it is much costlier than other metals.
Metal # 2. Copper as Fuse-Wire:
Copper wire is used extensively as fuse-wire. It is generally used in distribution fuse boards and in main switch boards. Table no. 12 gives the fusing current of copper wires in air. Up to 0.4572 mm diameter the length of the fuse-wire should be 63.5 mm to 90.0 mm. For large diameter the length of wires must not be less than 100 mm.
Metal # 3. Lead-Tin Alloy as Fuse-Wire:
For the protection of final sub-circuits and small appliances where the current rating is low, fuse element made of lead-tin alloy is generally used. The tin content varies from two per cent and above. The main disadvantage of such a wire is that, as the alloy is soft, the molten metal is likely to scatter around when the element melts. Lead-tin alloy containing 37 per cent lead and 63 per cent tin is most preferred for fuse element. Such an alloy is known as eutectic alloy. It is quite homogeneous and has the highest brinnel hardness number of all the lead-tin alloy series. The maximum length of wire used should be 63.5 mm to 90.0 mm.
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Table no. 13 gives the approximate fusing currents of wires made of 75 per cent lead and 25 per cent tin. The value of current is true only when the fuse wire passes through the asbestos tube and does not come in contact with it. These wires are not used beyond 10 amperes range. Above 10 amperes copper wire is preferred as fuse-wire.
