Patent GB-1513932-A (GB)
Title: Protector for electric circuit
IPC: H01H85/10
Filed: 1975-09-19
Abstract: 1513932 Electric fuses McGRAW-EDISON CO 19 Sept 1975 [1 Oct 1974] 38631/75 Heading H2G An electric fuse comprises a first and a second electrical conducting path which are in electrical parallel relation, each path having a controlling weak spot and a dependent weak spot wherein the controlling weak spots are longitudinally displaced and the controlling weak spot of each path is adjacent the dependent weak spot of the other. Each controlling weak spot forms a first arc when it fuses at a predetermined overcurrent. Each dependent weak spot responds to the respective adjacent first arc to form a second arc. Thus the electric fuse acts, whilst the current therethrough is less than the predetermined overcurrent, to provide two weak spots in electrical parallel relationship, and responds to a predetermined overcurrent to form two longitudinally spaced arcs, each comprising a first and second arc, which become seriesarranged arcs. The first and second conducting paths may be defined in part by a single slot 56 (Fig. 1) or slit 230 (Fig. 15). As shown in Fig. 1, the controlling weak spots 62, 64 and dependent weak spots 66, 68 may be defined by the ends of the slot and substantially triangular notches, or by a slot and rhombic notches (Fig. 5, not shown), by a slot with straight ends, and triangular notches (Fig. 10, not shown), by a slot having triangular offsets, and triangular notches (Fig. 11, not shown) and by a slot and frusto triangular notches (Fig. 19, not shown). In these embodiments, the slot is longitudinal but, alternatively, the slot may be at an angle to the longitudinal axis. For such an inclined slot either the controlling or dependent weak spots may be defined by the slot and a side wall of the fusing element of the fuse (Figs. 13 and 14, not shown) notches defining with the slot the remaining weak spots. The controlling and dependent weak spots may alternatively be defined by rectangular and semi-circular notches respectively each co-operating with a respective end portion of the inclined slot (Fig. 12, not shown).  The first and second paths may alternatively be defined by a slit, as shown in Fig. 15, the ends of which may terminate in delimiting holes. The portions defining the first and second paths may be bowed in opposite directions out of the plane of the remainder of the fusing element and as before the weak spots are defined by the slit cooperating with rectangular notches (Fig. 4, not shown), triangular notches (Figs. 8 and 9, not shown), circular notches (Fig. 20, not shown) or by transverse slits 236, 240, 244, 248 (Fig. 15). In this embodiment the controlling and dependent weak spots 252, 254 and 256, 258 respectively are defined by bending down portions 238, 250 and bending up portions 242, 246. For both the slit and slot embodiments the weak spots may be alternatively defined by apertures (Figs. 6 and 7, not shown). In each case a plurality of controlling and dependent weak spots are defined at each end of the slot or slit. In alternative embodiments, metal plates may be provided on both sides of the slot to constitute a further thermal mass (Figs. 24, 25, not shown) or this portion may be of increased width and subsequently folded. In yet a further embodiment a layer of alloying material, e.g. tin, is situated between notches in addition to those defining the weak spots (Fig. 26, not shown). Thus when a current in slight excess of the rated current passes for sufficient time, fusing occurs in the alloying layer rather than at the weak spots. In a still further embodiment, no slit or slot is provided, the weak spots being defined by opposed notches in parallel plates spaced by a barrier of insulating material having apertures corresponding to the notch positions (Figs. 21-23, not shown), or by air or arc extinguishing filler. Alternatively, the controlling weak spots may be defined by a notch and the separation of the plates, whilst the dependent weak spot comprises the entire width of the adjacent plate. The fusing element may, in a further embodiment, comprise two wires which may be spaced and parallel (Figs. 35 and 36, not shown) or touching at their ends only (Figs. 33, 34, not shown). In each case the controlling weak spots are defined by wire of reduced diameter, the dependent weak spots being the adjacent portion of the other wire. Further embodiments of fusing element are described wherein a plurality of slots are employed. The slots may lie on the axis of the element (Figs. 31 and 32, not shown), lie on opposite sides of the central longitudinal axis, Figs. 28 and 38, not shown, or lie on and on both sides thereof (Figs. 30 and 37, not shown). In each case notches, which may be of differing sizes, define with the slots the various weak spots. The fusing elements are, as described, incorporated singly or in spaced pairs within a casing of ceramic material, e.g. alumina, porcelain and steatite, or may be cooled by moving air. A filler of quartz sand is also employed. As shown in Fig. 29, two elements 408, 410 are joined via a connector 422 and a heat absorber 398, the connector 422 being soldered to a tab 418 of element 408. If a current in slight excess of the rated current is maintained for a sufficient time, the absorber 398 warms sufficiently to melt the solder, a spring 420 breaking the contact between the connector 422 and tab 418. One of the elements may be replaced by a conventional fusing element (Fig. 41, not shown). The fusing element is of silver, silver copper alloy, copper, or of copper zinc alloy with high copper content. The cross sectional area of the dependent weak points are greater than that of the controlling weak points but are less than eight times as large.