GB1598397A

GB1598397A - Vacuum circuit breaker

Info

Publication number: GB1598397A
Application number: GB2140978A
Authority: GB
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1977-05-27
Filing date: 1978-05-23
Publication date: 1981-09-16
Also published as: DE2822510A1; JPS53147270A; FR2392482A1; DE2822510B2; FR2392482B1

Description

(54) VACUUM CIRCUIT BREAKER (71) We, MITSUBISHI DENKI KABUSHIKI KAISHA, a Japanese Body Corporate of, 2-3, Marunouchi 2-Chome, Chiyoda Ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a vacuum circuit breaker having a relatively low chopping current characteristic and a relatively high current interruption capacity.

Conventional vacuum circuit breakers have a fixed contact assembly and a moving contact assembly. Complimentary main contact faces are provided on the assemblies and abut one another when the breaker is closed to carry the electrical current between the two assemblies. Each assembly also includes secondary electrode surrounding the main contact face and set back from it so as to leave a step between them. Spiral slots are disposed in the outer peripheral portion of the secondary electrode. When the breaker opens, an electric arc is struck between the separating main contact faces and is driven by electromagnetic forces produced by the arc current in an outward direction along the spiral slots to the outer periphery of the secondary electrode where the arc is eventually extinguished.

The transfer of the electric arc from the main contact face to the surrounding secondary electrode has the advantage that the wear on the main contact face caused by the arc is reduced thereby prolonging the working life of the assembly. However, if only moderate or low currents are interrupted by the breaker, the electric arc often does not transfer to the secondary electrodes but remains on the main contact faces until extinguished. This is believed to stem from the fact that there is insufficient drive obtained from moderate and low currents to transfer the arc over the step from the main contact face to the surrounding secondary electrode. Under these circumstances the main contact face which has a relatively small area is worn away rapidly compared with the secondary electrode with the result that the useful life of the assembly is reduced.

An object of this invention is to provide an improved vacuum circuit breaker and a contact assembly therefor.

According to the present invention there is provided a vacuum circuit breaker comprising a housing containing a pair of separable contact assemblies provided with complementary main contact faces which abut one another when the breaker is closed, at least one of the contact assemblies comprising a block of an electrically conductive material having relatively low chopping current characteristics and providing a flat annular contact face surrounding a well and meeting at its outer edge an inner edge of a frusto-conical surface of a secondary electrode which surrounds the contact face and makes an obtuse angle therewith to provide the assembly with a convex surface at its contact free-end portion, and radial slots extending outwardly of the convex surface and extending at their inner ends over the contact face and terminating at their outer ends on the periphery of the secondary electrode.

Preferably the breaker has its fixed and moving assemblies of the same shape. When the breaker opens, the electromagnetic drive causes the arc to travel outwardly to the edge of the main contact face from which it can transfer easily, in the absence of a step, onto the surface of the secondary electrode where it continues to be guided outwardly by the slots to the secondary electrode edge where arc extinction occurs.

Transfer of the arc from the main contact face to the secondary electrode occurs at low and moderate currents as well as at high currents.

Preferably the slots are equispaced and extend sprially outwards. In the preferred arrangement the main contact face is provided with a central well and the inner ends of the slots may extend to the edge of the well or terminate short of it. In the preferred embodiment of the invention the slots extend through the thickness of the secondary electrode so that it is effectively divided into lobes.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawing, in which: Figure I shows in sectional drawing a and b two contact assemblies of the prior art; Figure 2 is a vertical section through a vacuum circuit breaker; and Figure 3 is an end view of one contact assembly of Figure 2 taken on the line and in the direction indicated by the arrows III-III in that Figure.

Figure la shows a contact assembly comprising a main contact 100 fixed to the central portion of a secondary electrode 200 which is attached to the upper end of an electrode rod 300. The main electrode 100 is formed with a central well and provides an annular contact face which is displaced forwardly from the corresponding face of the secondary electrode so that there is a pronounced step between them. This step is enhanced by a peripheral gap between the cylindrical surface of the main contact and the surrounding secondary electrode. Spiral slots are formed in the secondary electrode.

Figure 1b shows a contact assembly having a main contact 100 embedded in the face of a secondary electrode 200 supported at one end of an electrode rod 300. Once again a clearly defined step is apparent between the face of the contact 100 and the corresponding face of the secondary electrode 200.

In both of the assemblies of Figure 1 the main contact 100 is formed from an electrically conductive material having low chopping current characteristics while the secondary electrode 200 is formed from an electrically conductive material having high chopping current characteristics and also a high current interrupting ability. For example, the secondary electrode 200 may be made of copper.

The slots provided in the secondary electrode 200 spiral radially outwards and serve to force an electric arc struck across the separating main contacts when the breaker opens, to translate smoothly radially outwardly of the secondary contact 200. This outward movement results from an electromagnetic force exerted on the arc and produced by the arc current. The operation of forcing the electric arc radially outwards of the secondary electrode is called the "arc driving" operation and facilitates the interruption of the current by the breaker.

Conventional contact assemblies such as those shown in Figure 1 have the disadvantage that it is difficult to drive the electric arc off the main contact face when low or moderate currents are to be interrupted. It appears that this disadvantage stems from the presence of the step hitherto thought to be necessary between the main contact face at the adjacent surface of the secondary electrode. Because of the reluctance of the arc produced by a low current to jump the step, there is a tendency for moderate and low currents to be interrupted at the main contact face. If, therefore, the breaker is used mainly to interrupt low or moderate currents, the main contact face, which has a relatively small area, becomes worn relatively rapidly so that the working life of the breaker is shorter than is intended and the unitary assembly of the main contact and the secondary electrode has to be disgarded when the secondary electrode 200 is scarcely worn.

The contact assemblies shown in Figures 2 and 3 have a longer working life, when fitted onto a vaccum breaker, than is the case with the previously discussed electrodes.

Referring to Figure 2 the vacuum breaker has a fixed electrode rod 10 extending through and sealed into an electrically insulating cover which closes one end of an electrically insulating housing in a vacuumtight manner. A circular fixed contact assembly 12 of unitary construction is electrically and mechanically connected to the rod 10. Opposite the fixed contact assembly 12 is an identical circular moving contact assembly 14 mounted at the end of an axially displaceable electrode rod 16 which extends through and is sealed into a closed end of a bellows unit which is fixed to a second electrically insulating cover closing in vacuum-tight manner the other end of the housing.

The assemblies 12 and 14 are each made from an electrically conducting material having low chopping current characteristics, for example an alloy of copper and a substance such as bismuth, tellurium, antimony, graphite, carbide or another element of the iron group.

The identical contact assemblies 12 and 14 each have a central well 18 whose diameter is larger than that of the electrode rod 16 supporting the assembly. The well is coaxially arranged in the centre of an annular flat contact face 20 which is coaxial with and parallel to the contact face of the other assembly. Each flat face is surrounded by a frusto-conical surface of an annular secondary electrode 22. The frusto-conical sur face of the secondary electrode meets the outer edge of the contact face 20 at a circular corner edge 20a. The frusto-conical surface and the contact face make an obtuse angle with one another so that they provide, together, a convex surface to the assembly.

As shown in Figure 3, four spiral slots 24 which are equiangularly positioned extend radially outwardly through the outer portion of the main contact face 20 and the secondary electrode 22 so that it is effectively divided into four lobes. The slots 24 extend completely through the secondary electrode and terminate at their outer ends on its peripheral edge while their inner ends lie on a circle 24a which lies approximately in the centre of the annular contact face 20 and is concentric therewith.

As is dearly apparent from Figure 2, there is an absence of any step between the frusto-conical surface of the secondary electrode and the flat contact face of the main contact which is integrally made from the same material as the secondary electrode.

The contact breaker operates as follows: During opening movement of the contact assemblies, a current flows between the contact faces 20 by way of the current path shown by the broken line labelled L. An electric arc which is struck between the contact faces 20 as they separate, is rapidly electromagnetically driven outwards to the circular corner 20a. In the absence of any step between the contact face 20 and the frusto-conical surface of the secondary electrode, the arc is able to move readily outwards over the frusto-conical surface of the secondary electrode under the guiding action of the spiral slots 24. This outward movement of the arc occurred even if the magnitude of the current is low or only moderate.

As the design of contact assembly shown in Figures 2 and 3 allows a low or moderate current arc to translate readily from the flat contact face 20 to the secondary electrode, the arc wear of the contact face 20, which has a small area, is reduced to give a corresponding increase in the useful life of the contact assembly.

As the secondary electrode presents a frusto-conical face, rather than a planar one, to receive the arc transferred from the main contact, the area of the arc face of the secondary electrode is increased with respect to prior art arrangements without there being any increase in the radial dimension of the contact assembly. Also, as the slots 24 extend close to the central well 18 of the contact face 20, an electric arc occurring on the flat face 20 is substantially uniformly dispersed outwards and driven onto the surface of the secondary electrode so that the surface over which the ends of the electric arc travel is extended while simultaneously the length of the arc is increased.

These factors contribute to increasing the current interruption capacity of the breaker with a respect to a breaking having the contacts shown in Figure 1.

As the main contact and the secondary electrode comprise a unitary structure made from the same suitable electrically conducting material having a low chopping current characteristic, the resulting vacuum circuit breaker has a low chopping current characteristic over its entire current range rather than only over the higher current end of the range as was the case with the assemblies shown in Figure 1.

Various modifications of the arrangement shown in Figures 2 and 3 are of course possible. For example, the portion of the slot 24 located on the flat contact face 20 may be of concave cross-section sufficient to prevent jumping of the electric arc but not extending through the axial thickness of the main contact part of the assembly. The inner ends of the slots 24 may extend as far as the well 18. There may be more or less than four such slots if desired.

The invention includes a contact assembly as well as a breaker incorporating two such assemblies.

WHAT WE CLAIM IS: 1. A vacuum circuit breaker comprising a housing containing a pair of separable contact assemblies provided with complementary main contact faces which abut one another when the breaker is closed, at least one of the contact assemblies comprising a block of an electrically conductive material having relatively low chopping current characteristics and providing a flat annular contact face surrounding a well and meeting at its outer edge an inner edge of a frusto-conical surface of a secondary electrode which surrounds the contact face and makes an obtuse angle therewith to provide the assembly with a convex surface at its contact free-end portion, and radial slots extending outwardly of the convex surface and extending at their inner ends over the contact face and terminating at their outer ends on the periphery of the secondary electrode.

2. A breaker as claimed in claim 1, in which the slots extend spirally outwards.

3. A breaker as claimed in any one of the preceding claims, in which the contact face is formed at its centre with a cylindrical well lying on the axis of an electrode rod which extends away from the assembly on the side opposite the well and has an external diameter smaller than that of the well.

4. A breaker as claimed in any one of the preceding claims, in which the slots extend through the axial thickness of the secondary electrode.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. face of the secondary electrode meets the outer edge of the contact face 20 at a circular corner edge 20a. The frusto-conical surface and the contact face make an obtuse angle with one another so that they provide, together, a convex surface to the assembly. As shown in Figure 3, four spiral slots 24 which are equiangularly positioned extend radially outwardly through the outer portion of the main contact face 20 and the secondary electrode 22 so that it is effectively divided into four lobes. The slots 24 extend completely through the secondary electrode and terminate at their outer ends on its peripheral edge while their inner ends lie on a circle 24a which lies approximately in the centre of the annular contact face 20 and is concentric therewith. As is dearly apparent from Figure 2, there is an absence of any step between the frusto-conical surface of the secondary electrode and the flat contact face of the main contact which is integrally made from the same material as the secondary electrode. The contact breaker operates as follows: During opening movement of the contact assemblies, a current flows between the contact faces 20 by way of the current path shown by the broken line labelled L. An electric arc which is struck between the contact faces 20 as they separate, is rapidly electromagnetically driven outwards to the circular corner 20a. In the absence of any step between the contact face 20 and the frusto-conical surface of the secondary electrode, the arc is able to move readily outwards over the frusto-conical surface of the secondary electrode under the guiding action of the spiral slots 24. This outward movement of the arc occurred even if the magnitude of the current is low or only moderate. As the design of contact assembly shown in Figures 2 and 3 allows a low or moderate current arc to translate readily from the flat contact face 20 to the secondary electrode, the arc wear of the contact face 20, which has a small area, is reduced to give a corresponding increase in the useful life of the contact assembly. As the secondary electrode presents a frusto-conical face, rather than a planar one, to receive the arc transferred from the main contact, the area of the arc face of the secondary electrode is increased with respect to prior art arrangements without there being any increase in the radial dimension of the contact assembly. Also, as the slots 24 extend close to the central well 18 of the contact face 20, an electric arc occurring on the flat face 20 is substantially uniformly dispersed outwards and driven onto the surface of the secondary electrode so that the surface over which the ends of the electric arc travel is extended while simultaneously the length of the arc is increased. These factors contribute to increasing the current interruption capacity of the breaker with a respect to a breaking having the contacts shown in Figure 1. As the main contact and the secondary electrode comprise a unitary structure made from the same suitable electrically conducting material having a low chopping current characteristic, the resulting vacuum circuit breaker has a low chopping current characteristic over its entire current range rather than only over the higher current end of the range as was the case with the assemblies shown in Figure 1. Various modifications of the arrangement shown in Figures 2 and 3 are of course possible. For example, the portion of the slot 24 located on the flat contact face 20 may be of concave cross-section sufficient to prevent jumping of the electric arc but not extending through the axial thickness of the main contact part of the assembly. The inner ends of the slots 24 may extend as far as the well 18. There may be more or less than four such slots if desired. The invention includes a contact assembly as well as a breaker incorporating two such assemblies. WHAT WE CLAIM IS:

1. A vacuum circuit breaker comprising a housing containing a pair of separable contact assemblies provided with complementary main contact faces which abut one another when the breaker is closed, at least one of the contact assemblies comprising a block of an electrically conductive material having relatively low chopping current characteristics and providing a flat annular contact face surrounding a well and meeting at its outer edge an inner edge of a frusto-conical surface of a secondary electrode which surrounds the contact face and makes an obtuse angle therewith to provide the assembly with a convex surface at its contact free-end portion, and radial slots extending outwardly of the convex surface and extending at their inner ends over the contact face and terminating at their outer ends on the periphery of the secondary electrode.

5. A breaker as claimed in any one of

the preceding claims, in which the portions of the slots lying in the contact face are of concave cross-section.

6. A breaker as claimed in claim 1, arranged and adapted to operate substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawing.