Because of shortage of copper, it was decided to make the fuse in Example 7.4 out of iron (not a very good idea because of corrosion). If the same dimensions, including the width calculated in Example 7.4a are used and the current density at which the fuse melts is half the current density in copper, calculate:

(a) The current rating of the fuse

(b) The resistance of the fuse

Example 7.4

The fuse, in its various forms, is one of the most important, and often unappreciated, components in circuits and devices. In general terms, fuses rely on melting (or fusing) of a piece of conductor when the current in the fuse exceeds a given value. A fuse is made of copper in the shape shown in Figure 7.5. The two large sections are intended to clamp the fuse while the narrow section is the actual fuse. Copper can carry a safe current density of 108 A/m2. Above this current, copper will melt (because of heat generated by the current). The thickness of the copper sheet is t = 0.1 mm.

(a) Design the width of the fusing section so that it will break at currents above I = 20 A This design is typical of automotive fuses (although pure copper is almost never used because of oxidation problems).

(b) Calculate the total resistance of the fuse for the dimensions given in Figure 7.5 and the width found in (a).