The material used for fuse elements must be of low melting point, low ohmic loss, high conductivity (or low resistivity), low cost and free from detraction. The material used for making fuse element has a low melting point such as tin, lead, or zinc. A low melting point is, however, available with a high specific resistance metal shown in the table below.
|Metal||Melting Point in Celsius||Specific Resistance|| Value of Fuse constant k
for d in mm
The material mainly used for fuse element are tin, lead, silver, copper, zinc, aluminum, and an alloy of lead and tin. An alloy of lead and tin is used for small current rating fuses. For current exceeding 15A this alloy is not used as the diameters of the wire will be larger and after fusing the metal released will be excessive.
Beyond 15A rating circuit, copper wire fuses are employed. Copper wire suffers from the drawbacks that it operates at a rather high temperature if a reasonably low fusing factor is desired. There is thus a tendency for the wire to overheat with the result that its cross-sectional area and fusing current are gradually reduced, and premature melting of the wire may occur.
Silver is used as a fuse element because it has the following advantages
- It does not get oxidized, and its oxide is unstable.
- The conductivity of silver does not deteriorate with oxidation.
- Owing to its high conductivity the mass of molten metal to be handled is minimized and thus operating speed is fast.
But silver is very costly as compared to other metals thereby copper or alloy of lead-tin is mostly used as a fuse wire. Zinc in strip form only is also used as a fuse element because it does not melt very quickly with a small overload.