GB1354035A - Self-recovering current limiter - Google Patents

Abstract

1354035 Conducting-liquid switches MITSUBISHI DENKI KK 21 Dec 1971 [25 Dec 1970] 59356/71 Heading H1N A self-recovering current limiter comprises electrodes 1, 4, an insulator 2 situated between electrodes 1, 4, and having a passage 2a for containing a current limiting material, an outer tube 3 surrounding insulator 2 and insulating material 5 packed between insulator 2 and outer tube 3 at a temperature above the softening point of insulating material 5, material 5 having a coefficient of linear expansion higher than that of insulator 2 at temperatures below the softening point and being the same or lower than that of outer tube 3. Electrode 1 is held down by a nut 8 acting through insulating washer 7. The current limiting material is potassium or sodium or a mixture of the two. Electrodes 1, 4 are of copper or a copper alloy such as chromium-copper. Insulating tube 2 is preferably beryllia porcelain or alumina porcelain. Outer tube 3 is of stainless steel or brass. Insulating material 5 is a mixture of powdered glassy material and a synthetic mica such as fluoro-phlogopite mica, with the glassy material comprising 30-70% by volume of the insulating material 5 and the glassy material comprising a major part of potassium oxide, sodium oxide, lithium oxide, barium oxide or boric anhydride and a minor part of lead oxide and silica. A method of moulding the mixture is described, giving temperatures and pressures. The object is to prestress insulating tube 2 so that it will withstand the pressure generated in the tube 2 when the current-limiting material vaporizes to limit the current. In a second embodiment, Figs. 2, 3 (not shown), washer 7 and nut 8 are omitted and electrode 1 is held in the outer tube by flanges 1a on electrode 1 co-operating with internal flanges on the outer tube.