Protective arrangement for telecommunications line interface circuit

A circuit protection device designed to protect a pair of electrical circuits by fracturing at such time as an overcurrent condition has occurred in a first of said circuits, said device comprising a substantially flat substrate of non-conductive, frangible material upon which a resistive component and a fuse conductor reside in an adjacent relationship on a common surface, said substrate being operatively mounted in a substantially orthogonal relationship to a circuit board, with one edge of said substrate being comparatively close to such circuit board and another edge remote therefrom, said resistive component being disposed at a location on said substrate adjacent said one edge, and said fuse conductor being disposed essentially parallel to said resistive component but located closer to said other edge, said resistive component being part of said first circuit, and said fuse conductor being part of a second circuit, said resistive component, at the time of an overcurrent therethrough, becoming hot and causing a heating of the near surface of said frangible substrate, said frangible substrate, as a result of such heating, elongating and then fracturing, with the center of such fracturing occuring very close to the location of said fuse conductor, the fracturing of said frangible substrate thus bringing about a fracturing of said resistive component as well as a fracturing of said fuse conductor, effecting an interruption of said first circuit as well as a substantially simultaneous interruption of said second circuit.

A circuit protection device designed so as to project, by fracturing, a pair of electrical circuits, said device comprising a substantially flat substrate of non-conductive, frangible material having opposed parallel flat surfaces as well as first and second edges, said substrate being supported adjacent an associated electrical circuit board by means of a plurality of connection pins disposed in alignment along said first edge, with the second, opposite edge of said substrate remaining free, a resistive element tightly affixed on one flat surface of said substrate, extending widthwise across a substantial portion of said substrate and generally parallel to said first edge, said resistive element being electrically connected by one pair of said connection pins to a first associated electrical circuit, a fuse conductor also tightly affixed on said one flat surface, located generally adjacent said resistive element, but disposed nearer said second edge of said substrate, said fuse conductor being electrically connected to a second, associated electrical circuit by connection pins, said resistive element, when carrying an overcurrent, becoming heated and thus causing a heating of the near surface of said substrate, said near surface of said substrate then tending to elongate and to establish a considerable amount of mechanical stress in a central portion of said substrate, said first edge, being restrained by said aligned connection pins, remaining comparatively straight, whereas said second edge of said substrate, being unrestrained, tending to undertake a substantial curvature at the time a sizable overcurrent is flowing through said resistive element, contributing to the fracturing of said frangible substrate, of said resistive element, and of said fuse conductor, thus breaking and interrupting the electrical circuit of said fuse conductor, irrespective of the absence of an overcurrent flowing through said fuse conductor.

A circuit protection device designed so as to protect, by fracturing, more than a single electrical circuit, said device comprising a substantially flat substrate of non-conductive, frangible material of generally rectangular configuration, said substrate having opposed parallel flat surfaces, said flat surfaces terminating in one direction in first and second relatively straight, opposed edges, with said substrate being supported adjacent an associated electrical circuit by means of a plurality of connection pins disposed in alignment along said first edge, with the second, opposite edge remaining free, a resistive element tightly affixed on one flat surface of said substrate, extending widthwise across a substantial portion of said substrate and generally parallel to said first edge, along which said connection pins are located, said resistive element being electrically connected by a pair of said connection pins to a first portion of said associated electrical circuit, a fuse conductor also tightly affixed on said one flat surface of said substrate, located generally adjacent said resistive element, but disposed nearer said second edge, said fuse conductor being electrically connected to a second portion of the associated electrical circuit by a pair of said connection pins, said resistive element, when carrying an overcurrent, becoming heated and causing the near surface of said substrate to become heated and expand to a greater extent than the opposite side of said substrate, leading to the buildup of a considerable amount of stress in said substrate, said first edge, being restrained by said aligned connection pins, remaining comparatively straight, whereas said second edge, being unrestrained, tending to undertake a substantial curvature at the time a sizable overcurrent is flowing through said resistive element, contributing to the fracturing of said frangible substrate, of said resistive element, and of said fuse conductor, thus breaking and interrupting the electrical circuit of said fuse conductor, irrespective of the absence of an overcurrent flowing through said fuse conductor.

A telecommunications system comprising:

• a subscriber line comprising tip and ring conductors;
• a subscriber line interface circuit to which said subscriber line is connected for coupling the subscriber line to the remainder of the telecommunications system;
• surge voltage protective means coupled between the tip and ring conductors and earth;
• first and second current limiting resistances having nominally equal values inserted in the tip and ring conductors respectively;
• first and second feedback lines coupled between the tip and ring conductors respectively and the subscriber line interface circuit whereby to equalize the impedances of the first and second resistances in the subscriber line;
• a first electrical component comprised of said first resistance and a part of the associated feedback line, a second electrical component comprised of said second resistance and a part of the associated feedback line, wherein each of said first and second electrical components comprises;
• a frangible substrate, a resistance material formed on a surface of said substrate, a conductive strip of fusible material formed at a location adjacent said resistance on a surface of said substrate, whereby when in a surge condition the power rating of the component is exceeded and the temperature of the resistance material rises, a bending of the substrate is caused through a temperature differential whereby fracturing occurs of said substrate, said resistance material and said conductive strip, whereby to cause electrical and physical isolation of the subscriber line interface circuit from the subscriber line.

An electrical component for protection of a subscriber line interface circuit,

• the electrical component comprising a substrate, a resistance material having a predetermined power rating disposed on a lower part of one face of the substrate and a conductive strip disposed on an upper part of the substrate;
• first electrical terminal means carried at a lower edge of said substrate and electrically connected to said resistance material for electrical connection to a conductor of a subscriber line;
• second electrical means carried at a lower edge of said substrate and electrically connected to said conductive strip for electrical connection to a feedback line of a subscriber line interface circuit;
• wherein the substrate, resistance material and conductive strip are so disposed and arranged that upon fixing of said electrical terminal means in an external circuit and upon current flow through the resistance material exceeding the power rating thereof, the substrate bends in a first direction across a dimension of the resistance material and in a second dimension from the lower to the upper part of the substrate whereby to create a concentration of stress in the upper part of the substrate so that firstly the conductive strip and then the resistance material are physically fractured.