US 3252038 A
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May 17, 1966 Filed Jan. 28, 1963 J. W. CALVESBERT ETAL SPARK GAP DEVICES 2 Sheets-Shea?I l May 17, 1966 .1. w. cALvl-:sBERT ETAL 3,252,038
SPARK GAP DEVICES Filed Jan. 28. 1963 2 Sheets-Sliema?l 2 United States Patent C) 3,252,038 SPARK GAP DEVICES James William Calvesbert and Alfred Alexander Robinson, Stafford, England, assignors to The English Electric Company Limited, Strand, London, England, a British company Filed Jan. 28, 1963, Ser. No. 254,331 Claims priority, application Great Britain, Feb. 2, 1962, 4,002/62 4 Claims. (Cl. 313-243) This invention relates to protective spark gap devices.
Protective spark gaps made with metal electrodes are standard equipment on many types of electrical apparatus and are usually satisfactory where the gap breakdown voltage is high and the arc drop and the deterioration of the electrode surfaces are relatively unimportant.
There are many medium and low voltage applications where the fault current may be as high as 20 ka. for which metal electrodes are unsatisfactory because the iirst operation at such currents may significantly damage -the electrode surfaces.
Graphite surfaces are much less eroded by high current arcs than are metal surfaces so that the initial gap setting and hence the breakdown voltage may be maintained for many discharges. There is the problem, however, of different relative expansions which may give rise to mechanical damage or variable contact resistance.
An object of the present invention is to provide a spark gap device capable of employing graphite electrodes.
According to the invention, a protective spark gap device including two non-metallic electrodes separated by a spark gap has a said non-metallic electrode urged into electrical contact with an electrode holder of a different material at a-substantially constant pressure irrespective of different relative expansions in the device.
According to a preferred feature of the invention, the non-metallic electrode is urged into electrical contact with said electrode holder by a taper plug of the same material as the electrode holder, the taper plug being caused to positively engage a correspondingly tapered hole in said electrode by the action of tensioning means acting simultaneously on the taper plug and on said electrode, the arrangement being such that the apex of taper of the plug coincides approximately with the centre of action of the tensioning means on the electrode holder.
According to another preferred feature of the invention, the tensioning means comprises a bolt attached to the electrode holder and having ahead portion engaging the taper plug.
Further preferred features of the invention will appear from the following description with reference tothe accompanying drawings. FIG. 1 is a vertical section of one embodiment of protective spark gap device according to the invention for the protection of series capacitors used for reactive -compensation in a power system, FIG. 2 is a sectional plan on the line A-A in FIG. 1, and
FIG. 3 is a vertical section of another embodiment of spark gap device according to the invention.
Referring now to FIGS. 1 and 2, the spark gap device includes two electrodes 10 of graphite providing an annular spark gap 11. Three saw cuts 12 are formed radially at the outer end of each electrode 10 and end faces 13 and side faces 14 are in contact with a copper electrode holder 15. A conical hole 16 is formed centrally in each electrode 10 to provide a seating for a taper plug 17 formed of copper. The taper plug 17 itself has a central hole 18 which is enlarged at the inner end thereof as indicated at 19 to accommodate the head 20 of a bolt 21. The bolt 21 is secured to a tapped hole 22 in the electrode holder 15 in alignment with the hole 18. It is arranged that the apex of the -conical taper of the plug 17 coincides approximately with the centre of attachment 3,252,038 Patented May 17, 1966 of the bolt 21 in the hole 22, that is, the mid-point of the portion of the bolt 21 in threaded attachment with the hole 22.
Each electrode holder 15 abuts a terminal plate 23, the two plates 23 being arranged to provide diagonallyopposed line and earth connections as indicated at 24 and 25 respectively.
Three insulators 26 are interposed between each terminal plate 23 and an end-plate 27, the insulators being secured to the end-plate 27 by screws 28 and to the terminal plate 23 by studs 29. The end-plates 27 are rigidly coupled together by circumferentially disposed rods 30 which are threaded at the upper and lower ends thereof. Each rod 30 is encompassed by an insulating sheath 31 which rests on a nut 32 co-operating with the threaded lower end of the rod 30 and is separated by shims 33 from a nut 34 co-operating with the upper threaded end of the rod 30, a further nut 35 being provided for securing the assembly together. It will be clear that adjustment of the spark gap 11 to cover a wide range of voltages is easily produced by addition or removal of the shims 33.
When the bolt 21 is tightened, the taper plug 17 urges the graphite electrode 10 into engagement with the electrode holder 15. The effect of the very different expansions of copper and graphite is minimised by arranging for the apex of the taper of the plug 17 to approximately coincide with the centre of attachment of the bolt 21 (as described above) and by forming the taper plug 17 and the electrode holder 15 of the same material. This arrangement allows sliding contact between the taper plug 17 and the graphite electrode 10 without release of the Contact pressure exerted by the pre-stress in the bolt 21, over a wide temperature range.
The embodiment of the device described above is suitable for the protection of electrical apparatus in the 2 to 20 kv. voltage range for fault'currents up to 20 ka.
Referring now to FIG. 3, in which parts similar to those inFIGS. l and 2 bear the same reference numerals, the terminal plates 23 are attached to the end plates 27 by set lscrews 50. The terminal plates 23 are separated from each other `by four ceramic insulating pillars 51 which are circumferentially spaced at equal angular intervals. The amount of the separation is adjustable by utilizing accurately machined metal shims 52. Screws 53 secure the end plates 27 to the pillars 51. It will be noted that in this arrangement of spark gap device, the electrode holders 15 are insulated from each other by the insulating pillars 51 instead of by the insulators 26 shown in FIGS. 1 and 2. The disposition of the parts of this embodiment of the device is in other respects similar to that of the previous embodiment, and the device is suitable for the protection of electrical apparatus in the 2 to 40 kv. voltage range for fault currents up to 20 ka.
What we claim as our invention and desire to secure by Letters Patent is:
1. A protective :spark gap device having two nonr'netallic electrodes, wherein at least one of said electrodes is urged into contact with an electrode holder of a different material at a substantially constant pressure irrespective of different relative expansions in the device, by a taper plug of the same material as the electrode holder, the taper plug being caused to positively engage a correspondingly tapered hole in -said electrode by the action of tensioning means acting simultaneously on the taper plug and on said electrode, the arrangement being such that the apex of taper of the plug coincides approximately with the centre of actionof the tensioning means on the electrode holder.
2. A protective spark gap device according to claim 1 wherein the tensioning means comprises a bolt attached to the electrode holder vand having a head portion engaging the taper plug.
3. A protective spark gap device according to claim 1 wherein said plug has a conical taper, the axis of the core coinciding with the central axis`of the electrode holder.
4. A protective spark gap device according to claim 1 wherein the electrode holders associated with the electrodes are each in electrical contact with a separate terminal plate which is separated from an end plate by at l least one insulator, the end `plates being maintained a predetermined distance apart by a plurality of rods interconnecting the end plates.
References Cited by the Examiner UNITED- STATES PATENTS 0 GEORGE N. WESTBY, Primary Emminer.
R. JUDD, Assistant Examiner.