Patent GB-702582-A (GB)
Title: Improvements in current-limiting electric cartridge fuses
IPC: H01H85/055
Filed: 1952-01-22
Abstract: 702,582. Fusible cut-outs. CHASESHAWMUT CO. Jan. 22, 1952 [Jan. 30, 1951], No. 1775/52. Class 38 (1)  A current-limiting cartridge fuse in which the fusible link comprises a sheet metal strip surrounded by a pulverulent filler, is characterised in that the strip has at least two portions of reduced cross-section, one portion having a larger cross-sectional area than the .other, the larger area portion being adapted to determine the minimum fusing current of the fuse, and the smaller area portion determining the maximum let-through fuse current. The fuse, Fig. 1, comprises an insulating casing 1 accommodating a fusible link 2 of silver strip spot welded to end caps 6a, 6b and surrounded in part, by a quartz sand filler 3, the right end of the link 2 being surrounded by a gas evolving filler 3a, e.g. chalk. The two fillers 3, 3a are separated by a partition 4. The link 2 has a number of holes 2a forming pairs of reduced cross-section portions 2c and a pair of juxtaposed notches 7 forming a reduced cross-section neck portion 26, the cross-sectional area of a pair of portions 2c being four times that of the portion 2b. On small protracted overloads approximating to the minimum fusing current the temperature of the neck portion 2b rises slowly owing to the cooling effect of the cap 6a and the adjacent link portions 2b<SP>1</SP>, whereas the temperature of the portions 2c at the link centre rises more rapidly as the intermediate link portions 2c<SP>1</SP> are relatively ineffective for cooling being heated at each end by the portions 2c. Hence fusion and arc initiation occurs at the reduced portions 2c at the link centre and is followed rapidly by fusion of the remaining portions 2c to produce a number of arcs in series. The neck portion 2b may not reach the fusing temperature. On fault interruption of short-circuit current proportions, however, the temperature rise in the link either at the reduced portions 2c or.at the neck portion 2b is too great for heat dissipation to have any cooling effect, and the link 2 therefore fuses initially at that point having the smallest cross-sectional area, i.e. the neck 2b, the remainder of the link then fusing instantly. The Construction of the fuse link permits the minimum fusing current of the fuse to be varied independently of the letthrough current; e.g. the minimum fusing current may be increased without a consequent increase in the let-through current. In a modification, a tin rivet 5 is inserted in one of the holes 2a surrounded by the gas-evolving filler 3a. On a low protracted overload the rivet 5 forms a low melting point alloy with the silver of the link 2 and arc initiation occurs at this point. A fuse for high current-carrying capacity, Fig. 4; comprises 'a number of'fuses 1 as in Fig. 1 connected in parallel between copper blocks 8a 8b having recesses 10a, 10b tightly gripping the fuse caps 6a, 6b. The blocks 8a. 8b have terminal blades 9a, 96 and the several fuses 1 are separated by quartz sand 11. The Specification contains graphs, Figs. 6-12 (not shown) illustrating the circuit interrupting performance and temperature rise characteristics, of the fuse. Actual dimensions are given of a fusible link having a minimum fusing current of 20 amps.