The fuse wires are made of such materials which have low resistance viz, silver, gold, lead tin alloy, tinned copper wire etc. The silver is a quite satisfactory material to be used for fusing elements, because it is not subjected to oxidation and the oxides are also even unstable. There is no deterioration of material when used in dry air and remain bright. In case of moist air and contains hydrogen sulphite, the surface is attacked and a layer is formed which protects from further attacks. But even then it is not conveniently used because of the cost, similar is the case with gold. But in very precious cases the fusing elements are also made of silver and gold.
Fig. 1: Fuse wire
Lead tin alloy
The lead tin fuse wire is mostly used for the low currents. In this alloy the tin is 2% to 3% which is used for low current. Generally the alloy used for normal currents contains lead 37% and tin 63%, such alloys are known as outectic alloys. These wires are generally used in open type and semi-open type fuses for low currents. It is a homogeneous material and the heat developed is dissipated uniformly and has less tendency to spread over. But mechanically it is weak. These are generally used up to 5A for heavy currents it is not used since the diameter of the wire used will be large and after fusing, metal release will be comparatively excess. It is known as standard alloy.
Tinned copper wire
It is a general purpose fuse wire, which can be used for any value of currents. It is made of copper, generally tinned. The rating of current depends upon the size of the fuse wire, more thickness large current, less thickness low current. These are used for open type and semi-enclosed type fuse, up to 100 A.
What are the factors which limits the rating of a fuse?
There are the following factors which effects the rating of fuse:
- The type of material used.
- The cross-sectional area of the wire to be used for this purpose.
- The methods of placement of wire, for example the wire is open, enclosed or semi-enclosed.
- The surrounding atmospheric conditions.
- The length of the fusing element as the heat developed is proportional to the length i.e. the resistance.