The Calculation of the Complete Time/Current Characteristics of Cartridge Fuses With Single Wire Element [includes discussion]

A review is given of previous attempts to solve the thermal equations for a filled fuse with single wire element lying along the axis of the cartridge. A new solution is then developed, which is based upon analysis applied by Gr??nbergl to high-voltage cables. This solution is used to calculate prearcing times, and is checked experimentally for four copper wires of different diameters and for one silver wire. For currents above about 1.4 times minimum fusing current, the error in the calculated prearcing time at a given current for all these wires does not exceed 25 per cent (%), and is generally much less, throughout the whole range. A comparison with other methods of predicting time/current characteristics shows the present solution to be superior. Minimum fusing currents for silver and copper wires can be calculated within about 5% of the true value. An allowance for the effect of oxidation of copper is given for this purpose. The prearcing time below which heat loss from the element is negligible can be calculated accurately, and is shown to be a function of element cross section. The effect of varying filler parameters on time/current characteristics is shown, and the thermal conductivity of quartz powder is discussed.