Patent GB-908240-A (GB)
Title: Improvements in spark gap devices
Inventor: HICKS BRUCE CLIFFORD
IPC: H01T1/06
Filed: 1959-09-15
Abstract: 908,240. Protective arrangements. E.M.P. ELECTRIC Ltd. Dec. 13, 1960 [Sept. 15, 1959], No. 31407/59. Class 38 (4). A spark-gap unit for a lighting arrester comprises opposed pairs of electrodes in a shallow chamber formed in ceramic discs having flat permanent magnets of oppositely polarized faces located on opposite sides of the electrode chamber. Two arc electrodes 24 are housed in a spark-gap chamber 12 formed by a recess in one face of each of the ceramic discs 8 which are in facial contact. The other face of each disc is recessed to provide half a circular chamber 13 having deep non-recessed portions at diametrically opposite points. Each full chamber formed by stacking the plates 8 accommodates a permanent magnet 14 of circular shape but with deep cut-outs and preferably made from ferrite material. Each magnet 14 is oppositely polarized on its two faces so that the adjacent faces to each spark-gap chamber 12 produces a powerful are-quenching magnetic field. The whole area of the arc chamber 12 is covered by the magnet 14 which is surrounded by a magnetic shield formed from two metal pressings, one of which has a web extending across the cut-out portion to form contact blades 16, each of which is provided with pressed-out springy tongue members adapted to provide electrical contacts with a series of grading resistors 27 housed in a tunnel formed by a circular aperture 26 in the peninsular of each ceramic disc 8. A second tunnel may be formed by a further aperture to house a further grading resistor or capacitive grading element. Further spring- tongue portions 29 engage copper connecting members 25 used to connect the pairs of electrodes 24 so that the spark-gaps formed are in a series arrangement. Circular polepieces 30 made from a high permability material and/or a high dielectric ceramic wafer are accommodated in the spark-gap chamber 12 in order to concentrate the magnetic field between the electrodes 24 thus pre-stressing the arc gap and ensuring that an arc formed between the electrodes will move outwards at the earliest possible moment. Additional shields of high permability material, elliptical in shape and with their major axis in the plane containing the connectors 25 between adjacent electrodes, may be accommodated in the discs 8 beneath the magnets 14 and their shields 15. Connecting members 25 may also be surrounded by insulating magnetic shields. Each ceramic disc 8 embodies a raised rim portion 17 extending over half its peripheral span whilst the other corresponding half contains a rebate 18 which is so arranged that when the discs are assembled the rim portion of one disc fits into the rebate of the adjoining disc, this providing registration for the discs, and ensuring a perfect seal around the entire chamber 12. An arc initiating between the electrodes 24 is drawn outwards into the chamber 12 by the concentrated magnetic field and is thus rapidly cooled and quenched. The shields 15 around the magnets 14 protect them from heavy current surges, aid the cooling of the grading resistors and provide a predetermined gap capacity so that the " spark-over " characteristics of the gap assembly may be easily controlled.