Patent GB-703299-A (GB)
Title: Improvements in electric current-limiting cartridge fuses
Inventor: EDSALL WILLIAM SHERMAN
IPC: H01H85/055
Filed: 1951-11-23
Abstract: 703,299. Fusible cut-outs. CHASE-SHAWMUT CO. Nov. 23, 1951, No. 27562/51. Class 38 (1). A current-limiting, low-voltage, high-capacity cartridge fuse in which the arc energy dissipated on circuit interruption is minimized and the ability to absorb the energy is improved thereby giving a fuse of reduced size, is characterized in that the fusible link has substantially along its entire length equidistantly-spaced, reduced cross-section portions dimensioned to cause the formation of a plurality of seriallyrelated arcs at current intensities far below the available short-circuit current of the circuit into which the fuse is connected, in that the length of the non-reduced portions of the link is a multiple of the length of the reduced portions, in that the number of the reduced portions is so related to the arc-extinguishing capacity of a pulverulent filler surrounding the link as to cause in less than 1/100 second after fault inception permanent interruption of any shortcircuit current of the circuit, and in that the whole length of the link is not vaporized even under the most onerous interrupting conditions. A fuse, intended for an A.C. or D.C. circuit of 600 volts comprises a fibre casing 3 enclosing a fusible link 4 surrounded by a quartz sand arcextinguishing filler 6. The link 4, which is spot welded to terminal caps 1, 2, has a plurality of reduced cross-section portions 4a. On shortcircuit the portions 4a fuse instantly thereby limiting the "let-through" current to, e.g. 10 per cent. of the available short-circuit current, and hence limiting the arc energy. The non-reduced link portions 4b, which are initially cool, together with the filler 6 then rapidly absorb the arc energy. The fuse link portions which remain substantially unaffected by the circuit interruption are denoted in the drawing by cross-hatching. On small overloads the temperature of the reduced portions 4a rises slowly so that the whole link assumes an even temperature before arc initiation and hence the arc energy necessary for eventually producing the gap to effect interruption, is reduced. The Specification discusses in detail the theory underlying the fuse operation and shows how the dimensions of the fusible link may be determined by calculation and experiment. In a modification, Fig. 5 (not shown), the reduced link portions are formed by rectangular-shaped perforations. A fuse for a small current rating, Fig. 6 (not shown), has the link formed with three circular perforations. According to another modification, Fig. 7 (not. shown), rise in temperature at the link centre is delayed by a rivet-like insert positioned in the central perforation of the link, and made of a metal which, on fusing, forms with the link a low-melting point alloy. To prevent "aging," contact between the insert and link is normally prevented by a layer of fluxing agent, e.g. rosin. Charring of the centre portion of the casing 3 on circuit interruption is prevented by employing a filler 6 of poor heat conductivity, e.g. gypsum, in the centre part of the fuse, Fig. 9 (not shown), the gypsum also evolving gas under pressure to improve the fuse dielectric strength, and providing an electrically-insulating barrier between the semi-conducting hot fused quartz portions at either end. In a further construction for high current-carrying capacity, Figs. 10, 11 (not shown) a number of cartridge fuses are arranged in concentric circles within an insulating shell and separated by quartz sand, the terminal caps 1, 2, of the fuses, being respectively connected to terminal elements at the ends of the shell whereby the fuses are connected in parallel with one another. Venting of the gases within the casings 3 is obtained by loosely mounting the caps 1, 2 on the casings. This construction is more fully described in Specification 703,303.