High-voltage switches with arc preventing or extinguishing devic – Arc preventing or extinguishing devices – Vacuum
Reexamination Certificate
1998-02-18
2001-12-04
Donovan, Lincoln (Department: 2832)
High-voltage switches with arc preventing or extinguishing devic
Arc preventing or extinguishing devices
Vacuum
C218S123000, C218S127000
Reexamination Certificate
active
06326573
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a vacuum switching device, and in particular a vacuum interrupter.
Vacuum interrupters are commonly used in electrical equipment for interrupting an AC supply in the event of a fault, e.g. a short-circuit on a power line. A typical vacuum interrupter is shown in very general terms in FIG.
1
. The interrupter comprises an insulator
10
, normally made of a ceramic or glass material, housing two electrically conductive contacts
11
,
12
. Contacts
11
,
12
are taken out of the interrupter unit by means of respective stems
13
,
14
, the stems terminating in end-portions
15
,
16
, normally referred to as “end-stubs”, for connection to further electrical equipment (not shown). The end-stubs
15
,
16
may have external or internal threads for effecting the connections. The interrupter by means of its contacts serves selectively to establish or remove electrical continuity between the further electrical equipment and the AC supply.
Also included in the interrupter is a bellows unit
17
and a shield
18
. The bellows unit
17
allows axial movement of the stem
14
to make and break, as afore-mentioned, electrical contact between the contacts
11
and
12
, contact
11
and stem
13
being fixed relative to the insulator
10
.
The shield
18
is an electrically conductive component which serves two main purposes: to prevent an arc, which is drawn when the contacts are separated, from striking the insulator and to impede the deposition of metal vapour, which is given off from the contacts when the arc is present, on the insulator.
The arc that is drawn when the contacts are separated during the presence of a normal or a high fault-current, for example, allows the current flowing prior to the interruption to continue by the medium of metal vapour given off from the contact faces. Nominally the arc would extinguish when the current passing through the arc passed through its next zero-crossing, however a phenomenon known as “current chopping” causes the arc to cease (“chop”) before that zero-crossing point by virtue of the reduction of the energy in the arc. When chopping occurs, a high voltage can be caused to appear across the contacts which is passed on to equipment (e.g. a motor load) connected to the interrupter, and if the voltage is high enough damage can be done to that equipment. There is therefore a desire to keep such voltage to as low a level as possible, which in turn means minimizing the current at which chopping occurs.
The value of the chopping current depends on the nature of the contact material and it has been found that, although an element such as copper or silver by itself gives rise to a high level of chopping current, if such a high-conductivity material is combined with an arc-resistant material such as tungsten, tungsten carbide or chromium, the chopping current can be brought down to very low levels, e.g. of the order of 4 A or less.
Use of a combination of materials for the contact instead of just the high-conductivity material considerably increases the costs of the interrupter and in an attempt to minimize such costs it is common practice to make only part of each contact of a combination of materials the remaining part being of the high-conductivity element only. The combined-material part is that part from which the arc is struck, the remaining part of each contact serving to sink heat from the arc-exposed part and to physically and electrically connect that part to the contact stem. It is clear that, since this remaining part plays no role in arc generation, it is not required to supply a vapour which has the afore-mentioned low chopping current quality, and can therefore be made exclusively of inexpensive high-conductivity material.
It is an aim of the present invention to provide a vacuum switching device which permits increased savings in contact-material outlay.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided a vacuum switching device comprising first and second contacts for making or breaking an electrical circuit, wherein said contacts each comprise a high-conductivity material and, prior to the drawing of an arc from said contacts, said first contact alone additionally comprises an arc-resistant and/or anti-weld material whereas, following the drawing of an arc from said contacts, said second contact has formed thereon a thin layer of said arc-resistant and/or anti-weld material. The first contact may be either the moving or fixed contact, as convenient.
The advantage of this construction is that, since only one contact comprises an expensive combination of materials in contrast with both contacts in prior-art arrangements, considerable savings can be made in materials outlay for the vacuum switching device, yet without significantly compromising the performance of the device. This is because in use, following the drawing of an arc from the contacts, the second contact develops its own thin layer of the arc-resistant and/or anti-weld material found in the first contact.
The high-conductivity material may be, for example, Ag or Cu, or a mixture thereof, the arc-resistant material may be selected from the group consisting of Cr and W and their carbides, or mixtures thereof, and the anti-weld material, where present, may be selected from the group consisting of Bi, Pb, Te and Sb, or mixtures thereof.
In accordance with a further aspect of the invention, there is provided a vacuum switching device having a pair of contacts for making or breaking an electrical circuit, wherein said contacts at facing ends thereof each comprise a layer composed of a high-conductivity material and an arc-resistant and/or anti-weld material, the layer of one of said contacts being a bulk layer as present in a manufactured state of said contact, whereas the layer of the other of said contacts is a vacuum-deposited layer produced by arcing between said contacts. The bulk layer may be substantially thicker than the vacuum-deposited layer.
In a yet further aspect of the invention, a method of manufacturing a vacuum switching device having first and second contact members and an insulating housing comprises the manufacture of said first contact member from a high-conductivity material and an arc-resistant and/or anti-weld material and the manufacture of said second contact member from said high-conductivity material, but excluding said arc-resistant and/or anti-weld material, and the enclosure of said contact members in said insulating housing.
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Donovan Lincoln
GEC Alsthom Limited
Kirschstein et al.
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