A Mechanism for the Fuse Pre-Arcing Period [includes discussion]

The equation of heat conduction for a cylindrical fuse wire is considered, and approximate solutions are given for the case of a simple cartridge fuse to show the variation of the temperature distribution along the element for different fusing currents and different cartridge dimensions. The melting period is examined analytically, and it is shown how, above a certain current density, the element begins melting from the surface inwards instead of from the inside outwards. This mechanism depends on a variation of the melting temperature over the cross section caused by the "pinch" pressure. If the inside melts first, breakup is due to the internal pressure produced and is irregular; if the surface melts first, the breakup depends on the parameters of the molten metal. A general theory, from Lord Rayleigh, for the breakup of a liquid cylinder under surface forces is applied to the latter case, and criteria of size are established for the parameters so that the element will break up in a regular fashion to form a definite number of globules, and so that the element will break up irregularly at infrequent intervals.