Arc-chutes

Abstract

 An arc-chute for a D.C. circuit breaker in which arc runners extend up from the switch contacts to embrace the arc chute. This comprises a series of arc splitter plates insulated from each other, and open all around their edges which are coated with insulating ceramic to limit the spread of the arc. The open nature of the arc-chute provides very rapid decompression and cooling of the arc gases and hence a very much more compact chute and switch.

Description

[0001] This invention relates to arc chutes for circuit breakers. There is a well known problem inherent in circuit breakers, particularly high-current high-voltage D.C. circuit breakers, which concerns the arc which arises on opening of the breaker contacts. It is necessary to suppress this arc in as short a time as possible since the circuit is still partially closed while the arc exists.. It is common practice to guide the arc between extended arcing contacts which diverge so stretching the arc. The voltage drop in the arc'is thus increased and eventually the arc can no longer be sustained and dies. During the existence of the arc large volumes of very hot and partially ionised gases are produced which must be safely disposed of. It has been common practice to mount the extended arcing contacts, 'arc runners', in an insulated rectangular section box or 'chute' with the contacts at the bottom and the top open ended. The arc runners extend up the short walls of the chute, the arc extending between the arc runners along the major dimension of the chute. The hot gases rise up the chute, which acts in the manner of a chimney, and vent to atmosphere from the open top. The long walls of the chute, parallel to the arc, must be electrically insulated and refractory, and asbestos has been commonly employed for this purpose.

[0002] A typical high power D.C. circuit breaker of the above form might have an arc chute say 1½ metres long in the arc .direction and approaching a metre vertically. The hot gases emerging from the top end commonly require a free space of a metre or more above the chute to avoid doing any damage and at the same time impose no back pressure on the arc. An overall enclosure for such a circuit breaker therefore requires a large amount of space.

[0003] A modified version of such a circuit breaker is shown in Figure 1 of the accompanying drawings. The normal current-carrying contacts are often distinct from, although electrically connected to, the arcing contacts and are arranged to open before them so as to avoid being damaged. A single pair of composite contacts (1) are shown for simplicity in Figure 1. The fixed contact 3 has a current supply terminal 5 and is continuous with an arc runner 7 which extends upwards within the arc chute casing (not shown). The movable contact 9 is pivoted at a point 11 to a current supply terminal 13. An extention 15 of the contact 9, an arc horn, is arranged to lie adjacent the end of an arc runner 17 in the open condition, shown in broken lines, the arc runner 17 being electrically connected to the movable contact so that the arc may easily transfer from the arc horn 15 to the runner 17.

[0004] As mentioned above, the two walls of the arc chute parallel to the plane of the drawing must be made of some refractory material such as asbestos, in order to contain the arc and gases without suffering damage.

[0005] The modification referred to above consists of the use of 'splitter plates' 19. These are metal plates which extend across the small dimension of the chute in a direction transverse to the arc and also extend up the chute towards the open end. They are insulated from each other and from the arc runners and are sealed to the refractory wall at the front and back.

[0006] The object of the splitter plates is to break up the arc into a plurality of small component arcs shown in broken lines referenced 21. The voltage drop across an arc occurs mainly at the cathode. With multiple arcs therefore there are multiple cathodes and the volt drop is increased accordingly.

[0007] The lower ends of the splitter plates 19 are preferably slotted to accommodate the contacts, the slots providing a magnetic path around the arc current of such form as to drive the arc up into the plates.

[0008] The arrangement of Figure 1 does tend to suppress the arc rather better than the basic construction without splitter plates and the size can accordingly be reduced somewhat. However, the chimney effect still persists and a considerable amount of space outside the arc chute is still necessary in which to de_-pressurise and cool the hot gases. In addition, the use of refractory materials, particularly asbestos, in the very hot region within the chute is deprecated but unavoidable.

[0009] An object of the present invention is therefore to suppress the arc in a much smaller total volume and at the same time avoid the use of any refractory structural material in the arc zone.

[0010] Accordingly, the present invention consists in an arc-chute adapted to be fitted across the contacts of a circuit breaker to receive and suppress an arc arising between the contacts on opening, the arc chute comprising a plurality of conductive splitter plates spaced apart, insulated from each other and mounted transverse to the path of the arc in operation to break the arc into a series of component arcs, wherein the plates are open to local atmosphere at their outer edges around at least the major part of their peripheries, escape of any component arc around the outer edge of an intermediate splitter plate being prevented by border portions of insulating material interleaved with the splitter plates.

[0011] The border portions preferably consist of refractory insulating coatings which may be vitreous or ceramic coatings on the outer parts of at least one face of at'least some of the splitter plates. Alternate splitter plates may have the insulating coatings on both sides, these insulating coatings extending both beyond and within the periphery of any adjacent uncoated splitter plates. Alternatively, the border portions may be peripheral plates of insulating material, which may form extensions of at least alternate splitter plates.

[0012] The splitter plates may conveniently be circular.

[0013] The splitter plates are preferably slotted in the vicinity cfthe arcing path, for reasons which will be explained subsequently. The term is to be understood to cover both notches and perforations.

[0014] According to another aspect, the invention consists in a D.C. circuit breaker incorporating an arc-chute such as aforesaid. Such a D.C. circuit breaker may include an enclosure for the arc-chute which is spaced from the splitter plates by an amount which is a fraction of the maximum dimension of those plates.

[0015] According to a further aspect of the invention, in a circuit-breaker incorporating an arc-chute as aforesaid, the arc chute comprises two stacks of splitter plates mounted side by side, the slots of each stack being substantially superimposed axially and being arranged to face the slots of the other stack and the circuit breaker contacts being mounted at a first end of the two stacks so that an arc between the contacts in operation extends between the said stacks by way of primary arcing horns mounted adjacent the first end of the two stacks, the circuit breaker incorporating a further arcing horn extending the length of the stacks and mounted between them so providing part of the path of an arc extending between the primary arcing horns, the further arcing horn having a shorting member at the other end of the stacks which provides a conductive path between the central portions of the'splitter plates at that other end, and the arrangement being such that an arc struck between the primary arcing horns in operation is split into two series arcs, by the further arcing horn, each such series arc being driven into a respective stack of splitter plates, and split into the multiplicity of series arcs linked by said shorting member.

[0016] Such a circuit-breaker may include a double-break series contact arrangement and two arc chutes each as aforesaid, the first end of one arc chute being adjacent the first end of the other, and the contact arrangement being disposed between tne two arc cnutes so that in operation two series arcs are formed by the double-break contacts and are transferred to the primary arcing horns of the respective arc chutes.

[0017] Several embodiments of an arc-chute for a D.C. circuit breaker in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a diagrammatic sectional side elevation of a D.C. circuit breaker of previously proposed form;

Figure 2 is a diagrammatic side elevation of an arc-chute in accordance with the invention;

Figure 3 is a perspective view of a similar arc-chute;

Figure 4 is a diagrammatic side elevation of a circuit-breaker employing an arc-chute which uses a twin arrangement of the module of Figure 3;

Figure 5 is a sectional view taken on V-V of Figure 4, and

Figure 6 is a diagrammatic side elevation of a circuit-breaker similar to that of Figures 4 and 5 but using a pair of arc-chutes each similar to that shown in Figure 4.

[0018] Figure 1 has already been described as showing the basic features of a conventional circuit breaker including splitter plates.

[0019] Referring now to Figures 2 and 3, the arc- . chute comprises a number of circular steel splitter plates 25 and 27 mounted in a stack by means of'sets of rods and spacers 23 of insulating material spaced around the periphery as shown in Figure 3. The rods are an easy fit in holes in the plates and standard spacers are assembled on the rods alternately with the plates.

[0020] There may be typically 20 to 30 splitter plates overall, these being spaced several millimetres apart and of similar or smaller thickness. As will be seen subsequently these splitter plates may be provided in modular groups. In this particular example there are two kinds of plates arranged alternately : larger plates 25 and smaller plates 27. The large plates 25 are coated with a ceramic insulating material 29 around their periphery and on both sides to the extent of about half their radius.

[0021] The intervening smaller plates 27 have an edge which lies between the extremes of the ceramic coating on the adjacent coated plates, that is, the smaller plates overlap the coating to some extent and it is in this overlap region that the mounting rods 23 lie.

[0022] Slots 31 are formed in all of the plates in alignment, to accommodate the contact mechanism and the arc horns 33 which extend from the end slots and curve up to lie on a diameter of the plates, spaced from the end plates as shown in Figure 1. In addition, the slots 31 extend far enough in towards the centre to reach the uncoated central portions of the splitter plates.

[0023] The slotting of the plates assists the driving of the arc up into the plates as before. Various other known devices for 'blowing' the arc into the chute may also be used with advantage.

[0024] In operation, the contact mechanism is triggered, the contacts (not shown in Figures 2 and 3) open, and an arc is struck. This arc is extended across the arc runners 33 in similar manner to that described for-Figure 1 and is driven up into the splitter plates 25 and 27.

[0025] The arc will move towards the peripheral coated portion 29 but will be prevented from reaching the edge of the plates by the insulation coating which will in fact contain the component arcs within the uncoated central portion. Neither will the component arcs be able to reach the edge of the uncoated intermediate plates, since one end of the arc will be trapped at the inner edge of the insulation on the adjacent plates and the component arcs cannot adopt an oblique path between the plates.

[0026] The great difference between this construction and that of Figure 1 is that now the hot gases can expand in all directions from the arc and not merely in the 'upward' direction as in Figure 1. There is therefore no significant chimney effect, and the gases are de-pressurised and cooled very much quicker. In consequence, the arc-chute is small and very little space outside it is required for the venting gases..An enclosure can be fitted around the chute at a distance from the edges of the plates 25 which is a small fraction of the diameter of the plate. A local atmosphere is thus provided within the enclosure into which all of the arc gaps vent. No arc gap is closed at the edge of the plates forming it and thus no refractory sealing material is required.

[0027] The arc-chute shown in Figures 4 and 5 differs from that shown in Figures 2 and 3 in that it contains two stacks of steel splitter plates 19 mounted side by side, each stack being constructed as in Figure 3. The plates of each stack have notches or slots 11 in alignment. The two rows of notches so formed are arranged facing each other, as best seen in Figure 5, and thus lie in the plane of the stack axes. A copper arcing horn 7, in the form of a triangular flat plate, also lies in this plane, and is supported at its base by a copper shorting bar 8 so as to extend between the rows of notches with its apex lying adjacent a pair of arcing horns 4 and 5. The shorting bar 8 overlies the endmost plates L of the two stacks, extending-between the central areas of these two plates. The arcing horn 7 and shorting bar 8 form an electrically isolated assembly. As best seen in Figure 4, the edges of the triangular arcing horn 7 penetrate the rows of notches in the regions near the shorting bar 8.

[0028] The splitter plates are coated around their peripheries with ceramic insulating material (not shown) as in Figures 2 and 3.

[0029] As shown in Figure 5, two insulation pieces 10, of generally T-shaped cross-section, lie respectively above and below the arcing horn 7 and extending the length of the stacks. They are omitted from Figure 4 for the sake of clarity. The facing portions of the insulation pieces 10 are notched so as to mesh with the larger splitter plates 27.

[0030] The assembly of splitter plates is enclosed by, but spaced from,an arc-chute enclosure. The distance between the edges of the large plates and the enclosure is suitably a third, or less, of the maximum dimension of those plates. Sets of rods and spacers, precisely similar to those shown at 23 in Figures 2 and 3, are used to support the splitter plates, but are omitted from Figures 4 and 5 for the sake of clarity.

[0031] In use, the current path between terminals 1 and 2 is broken as a moving contact 3 is rotated by an activator (not shown) away from arc horn 4 towards arc horn 5, thereby transferring an arc from position A to position B. The arc is then "blown" in towards the arcing horn 7, to a position C for example, by any suitable conventional means, such as a gas blast or electrical blow-out coil. Magnetic fields induced by the arc in and around the splitter plates then force the arc out to positions such as D and E, until eventually the arc is forced to pass from the arc horn 4, through the adjacent stack of splitter plates 19 to the shorting bar 8, and back through the other stack of splitter plates to the arc horn- 5, as shown at F, for example. In this position the arc is rapidly extinguished.

[0032] The motoring forces driving the arc into the stacks of splitter plates are shown by the arrows 6 in Figure 5, and result from the distorted magnetic field (shown as a triangular arrowed path) surrounding the arc in the vicinity of the notches 11 in the steel splitter plates.

[0033] Two or more arc chutes of the type described above may be connected in series as shown in Figure 6, in order to increase the operating voltage. The separate moving contacts are replaced by a common moving contact assembly 20, which breaks the current path between terminals 1 and 2 on moving in the direction shown by the arrow. Arcs initially formed between contacts 20 and 21, and 20 and 22, are transferred to the gaps between each pair of arcing horns 4, 5. These arcs then split up into sets of series arcs as shown in Figure 4. Each arc-chute may be supported in any convenient orientation, since chimney effects are negligible. The pair of arc chutes are enclosed in a common enclosure 9.

[0034] Each arc-chute may be in unit or modular form, each module comprising a standard number of splitter plates stacked together by means of sets of insulating rods and spacers. An arc-chute suitable for any given line voltage may then be constructed, a particular circuit breaker requiring a standard number of such modules stacked end to end. The sets of rods and spacers (which are precisely similar to those shown at 23 in Figures 2 and 3) are omitted from the modules 19 for the. sake of clarity. All the parts common to such multiple-module circuit breakers may be made in standard sizes, other than the arc horns ? and the enclosures 9.

[0035] It will be apparent that the shape of the splitter plates does not have to be circular. They could equally be square or of any other shape.

[0036] It will be noted that in the constructions described above, the only structural material, that of the rods 23, which is at all close to the arc zone, is in fact never in contact with the arc since the rods are positioned, as shown in Figure 3 particularly, in the insulated area. The gases have cooled sufficiently at this point and therefore no refractory material is required for the rods 23.

[0037] Various other methods of controlling limiting the travel of the arc components are possible. The plates may all be the same and all coated. They may all be the same and coated on one side only. The insulation may be a solid annulus forming an extension of the plate. It may be an annulus of insulating material suspended between plates at their periphery. It may be shaped to guide the arc into a pre-determined location.

[0038] It will be seen that the invention provides a ready means of reconditioning existing conventional circuit breakers, to reduce their size and remove the undesirable asbestos constituents, merely by replacing the 'chimney chute' of the conventional switch by the omni-directional chute of the invention. It will be appreciated that, while the best results are obtained by leaving the plate spacings open to the 'local atmosphere' all around their-peripheries, a major advantage will still be gained if some part of the periphery has to be obstructed for some reason. The advantages of the invention will be gained if a major part of the periphery is 'open'.

[0039] Because of the omni-directional nature of the gas path expansion it will be apparent that the chute has no inherent 'proper' orientation. The contact mechanism is at the bottom in a conventional chimney chute but there is no such limitation on the operation of the chute of the present invention. This is seen clearly in a double-break circuit-breaker employing the invention in which the series-contacts are conveniently arranged back to back, each having its own chute. If one is arranged with its contact at the bottom the other must necessarily be arranged with the contact at the top and the chute below it. It is found that in such circumstances the two chutes work equally well. Such a construction would be highly impractical, if not impossible, using chimney chutes.

[0040] In an alternative construction to the slotted plate form described above, each plate may be pierced by a hole drilled through its axis and the arc horns 33 may be replaced by two facing tubular or cylindrical arc horns which axially penetrate the plates through the holes and meet at the centre of the stack of plates. In use the arc initially formed between the arc horns will subsequently expand radially through the plates and extinguish itself.

Claims

1. An arc-chute adapted to be fitted across the contacts (20, 21, 22) of a circuit breaker to receive and suppress an arc arising between the contacts on opening, the arc chute comprising a plurality of conductive splitter plates (25, 27) spaced apart, insulated from each other and mounted transverse to the pith of the arc in operation to break the arc into a series of component arcs (21) characterised in that; the plates (25,27) are open to the local atmosphere at their outer edges around at least the major part of their peripheries, and escape of any component arc (21) around the outer edge of an intermediate splitter plate (25) is prevented by border portions (29) of insulating material interleaved with the splitter plates (25, 27).

2. An arc-chute according to Claim 1 in which the splitter plates (25, 27) are substantially parallel, are slotted, and are constructed of magnetic material, the positions and orientations of the slots (31, 11) being such that in use, the arc passes through or near at least some of the slots (31, 11) and the magnetic field associated with the arc drives the arc (21) into the central regions of the splitter plates.

3. A circuit-breaker incorporating an arc-chute according to Claim 2, the arc chute comprising two stacks (19) of splitter plates mounted side by side, the slots (11) of each stack being substantially superimposed axially and being arranged to face the slots (11) of the other stack and the circuit breaker contacts (3) being mounted at a . first end of the two stacks so that an arc between the contacts (3) in operation extends between the said stacks by way of primary arcing horns (4, 5) mounted adjacent said first end of the two stacks, the circuit breaker incorpor- ting a further arcing horn (7) extending the length of the stacks and mounted between them so providing part of the path of an arc (B, C, D, E, F) extending between the primary arcing horns, the further arcing horn having a shorting member (8) at the other end of the stacks which provides a conductive path between the central portions of the splitter plates at that other end, the arrangement being such that an arc struck between the primary arcing horns in operation is split into two series arcs (C), by the further arcing horn, each such series arc being driven into a respective stack of splitter plates (19) and split into the multiplicity of series arcs (D, E, F) linked by said shorting member (8).

4. A circuit-breaker including a double-break series contact arrangement and two arc chutes each as specified in Claim 3, the said first end of one arc chute being adjacent the first end of the other, and the contact arrangement being disposed between the two arc chutes so that in operation two series arcs are formed by the double-break contacts (20, 21, 22) and are transferred to the primary arcing horns (4, 5) of the respective arc chutes.

5. A circuit-breaker according to Claim 4 in which all the splitter plates (25, 27) are approximately parallel.

6. An arc-chute according to Claim 1 or Claim 2 wherein said border portions (29) consist of insulating coatings on the outer parts of at least some (25) of said splitter plates (25, 27).

7. An arc-chute according to Claim 6, wherein said insulating coatings (29) are ceramic coatings.

8. An arc-chute according to Claim 6 or Claim 7, wherein alternate ones (25) of said splitter plates have said insulating coatings on both sides, these insulating coatings extending both beyond and within the periphery of the adjacent uncoated splitter plates (27).

9. An arc-chute according to Claim 1 or Claim 2, wherein said border portions (29) are peripheral plates of insulating material.

10. An arc-chute according to Claim 9, wherein said peripheral plates (29) form extensions of at least alternate splitter plates (25).

11. An arc-chute according to any of Claims 1, 2, 6, 7, 8, 9 and 10, wherein the splitter plates (25, 27) are substantially circular.

12. An arc-chute according to any of Claims 1, 2, 6, 7, 8, 9, 10 and 11, incorporating a plurality of modules (19) stacked end to end, wherein each module (19) comprises a standard number of splitter plates (25, 27) stacked together by means of sets of insulating rods and spacers (23).

13. A D.C. circuit breaker incorporating an arc-chute according to any of Claims 1, 2, 6, 7, 8, 9, 10, 11 and 12.

14. A D.C. circuit breaker according to Claim 13 and including an enclosure (9) for the arc-chute which is spaced from the splitter plates (25, 27) by an amount which is a fraction of the maximum dimension of those plates.

Drawing