GB1453782A - Self-restoring current limitting device - Google Patents

Abstract

1453782 Conducting-liquid switches MITSUBISHI DENKI KK 25 Jan 1974 [25 Jan 1973 24 Aug 1973] 03666/74 Heading H1N A current limiting device of self-restoring type comprises a cylindrical metal shell 10 surrounding electrical insulation 14, 20 with a narrow passage 24 through it filled with current limiting material 48 responsive to a flow of excessive current through it to produce heat therein so as to be evaporated, and a terminal block 16 including pressure release means 38, 42 for decreasing the pressure arising on evaporation of current limiting material 48, terminal block 16 engaging shell 10 through electrical insulation 20. The pressure release means comprises a piston 38 with 0-ring 44 and chamber 40 containing a compressed gas 42. Needle valves 30, 34 are provided. A metal buffer member 12 is provided to absorb stress during moulding. Shell 10 may be of stainless steel, high tension brasses or hard copper alloys. Buffer 12 may be of a soft metal such as copper. Block 16 may be of copper or a chromium-containing copper alloy. Insulation 14 may be of a porcelain material such as alumina or beryllia. Insulation 20 may be a moulded mica produced by mixing a powder of natural or synthetic mica with a powder of low melting point glass and heating under pressure in a mould. Material 48 may be of sodium, potassium or a solid solution of sodium and potassium. Gas 42 is preferably inert. In a second embodiment, Fig. 3 (not shown), collar portion 18 of block 16 is longer for greater strength during moulding, and in a third embodiment, Fig. 4 (not shown), a stop 52 is provided in chamber 40 to prevent piston 38 hitting end portion 50 containing needle valve 30 and reducing the sealing effect of seal 46<SP>1</SP>. Three further embodiments, Figs. 5, 6, 7 (not shown), are as Figs. 2, 3, 4, except that a terminal block 32 is threaded on to the left hand end of shell 10 and contains a second pressure release piston and chamber, the block 32 not being electrically insulated from shell 10. Also the length of passage 24 is greater for higher voltage and contains a restriction 26. Three further embodiments, Figs. 8, 9, 10 (not shown), are as Figs. 2, 3, 4, but have a second terminal block 16 at the left hand end of shell 10 as that at the right hand end and containing the same pressure release piston and chamber and insulated from shell 10 so that shell 10 is completely insulated and can be used for mounting. Also passage 24 is longer for a higher voltage and contains a restriction 26. Reference has been directed by the Comptroller to Specification 1,214,711.