Electrical contact construction

An electrical contact assembly includes a contact, a contact support integrally connected to a first surface of the contact, and an insert disposed proximate a second, arc-supporting surface of the contact for stabilizing an electrical arc. In one embodiment of the invention the insert comprises an electrically insulating material inset in the arc-supporting surface and forming a metal-insulator boundary therewith. In another embodiment of the invention, the insert comprises a material having a relatively higher electrical resistivity than the contact, and disposed inside the contact so as to leave an integral metal current path from the arc supporting surface of the contact to the contact support, substantially through the center of the contact.

This invention relates in general to electrical contact assemblies for use 
in circuit breakers and more specifically to such contact assemblies 
having inserts for stabilizing electrical arcs. 
BACKGROUND OF THE INVENTION 
In vacuum circuit interrupters and contactors, for example of the type 
taught in U.S. Pat. No. 4,109,123 to Lipperts (incorporated herein by 
reference), the interruption of current typically occurs at the end of the 
half cycle in which the contacts are separated. When this interruption 
occurs, the arc current becomes unstable just before the actual sinusoidal 
zero of current, and drops suddenly to zero. This abrupt drop can cause 
high voltage transients to be generated in the associated circuit 
inductance. These transients have a magnitude related to the instantaneous 
currrent prior to the sinusoidal zero at which instability occurred and 
can cause electrical breakdown in equipment used in the interrupted 
circuit. This problem of arc instability in vacuum interrupters has 
typically been addressed either through the use of surge suppressors, or 
through the use of a contact material for which the arc is stable to very 
small currents prior to the natural current zero. The first solution is 
very expensive. The latter solution compromises operation of the vacuum 
interrupter in that electrode materials having stable arcing 
characteristics, typically soft metals with low melting temperatures such 
as silver, are known to have poor dielectric capabilities. It would thus 
be desirable to provide a contact assembly for use in a vacuum interrupter 
which provides the interrupter with both stable arcing characteristics and 
a high dielectric strength. 
In typical air circuit breakers, for example of tthe type taught in U.S. 
Pat. No. 4,513,268 to Raymond K. Seymour et al. (incorporated herein by 
reference and assigned to the assignee of the present invention), contacts 
are constructed of an expensive silver-tungsten alloy. The silver is 
generally used to provide low contact resistance during switching, and the 
alloy is used as a sponge to hold molten silver during high level arcing 
short circuit conditions. In many applications, the size of the contacts 
must be very large to dissipate heat, and the cost of the contacts becomes 
prohibitive. However, it is not sufficient merely to cut back on the 
silver content of the contacts, as the silver provides the low contact 
resistance. Accordingly, it would be desirable to provide contacts for use 
in air circuit breakers which provide low contact resistance and are 
inexpensive to manufacture. It would be further desirable if such contacts 
acted to stabilize any arc drawn therebetween when the air circuit 
breakers opened. 
The Lipperts patent, cited hereinabove, shows a contact for use in a vacuum 
interrupter, the contact comprising an internal hollow in the form of a 
vertically disposed spiral and horizontally disposed cap overlying the 
spiral, this cap and spiral being filled with an insulating material. It 
would be desirable to provide a contact assembly for primary use in a 
vacuum interrupter which provides better arc stabilizing characteristics 
than that shown in Lipperts. It would be further desirable to provide a 
contact assembly for primary use in an air circuit breaker which uses 
inexpensive materials, provides stable arcing conditions, and which does 
not interfere with the main current flow through the closed contacts as 
does the cap shown in Lipperts. 
OBJECTS OF THE INVENTION 
Accordingly, a principle object of the present invention is to provide a 
new and improved electrical contact assembly for use in both air circuit 
breakers and vacuum interrupters. 
Another object of the present invention is to provide a contact assembly 
for use in vacuum interrupters, the contact assembly having both stable 
arcing characteristics and high dielectric strength. 
Yet another object of the present invention is to provide a contact 
assembly for use in air circuit breakers, the contact assembly being 
inexpensive to manufacture, and exhibiting both a low contact resistance 
and stable arcing characteristics. 
A further object of the present invention is to provide electrical contact 
assemblies for use in both air circuit breakers and vacuum interrupters 
which are relatively inexpensive to manufacture. 
SUMMARY OF THE INVENTION 
New and improved electrical contact assemblies are provided for use in 
circuit breakers, these electrical contact assemblies using inserts of 
relatively inexpensive material both to reduce cost and stabilize 
electrical arcs. Contact assemblies constructed in accordance with the 
present invention include a metal contact having first and second 
surfaces. A metal contact support is connected integrally with the first 
side of the contact. In a first embodiment of the invention for primary 
use in vacuum interrupters, an electrically insulating insert is inset in 
the arc conducting second surface of the contact, thereby forming a 
metal-insulator boundary on this surface. This metal-insulator boundary 
acts to stabilize the arc on the arc conducting surface, permitting the 
contact to be constructed from a high dielectric material, such as copper, 
which would otherwise cause the arc instability described hereinabove. 
In another embodiment of the invention for primary use in air circuit 
breakers, an insert comprising a material having a higher electrical 
resistivity than the metal contact is disposed inside the contact so as to 
leave an integral metal current path between the second contact surface 
and the contact support substantially through the center of the contact. 
This insert permits typically expensive contact material such as a silver 
alloy to be replaced with a cheaper material such as a ceramic. The insert 
acts further to stabilize an arc drawn on a contact surface, while not 
interfering with the main current path through the contact.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 1 and 2, an electrical contact assembly 20 is shown 
comprising a disc shaped metal contact 22. A concentrically disposed metal 
contact support in the form of a rod 24 is connected integrally with 
contact 22 at a surface 26 of the contact. This integral contact between 
contact 22 and rod 24 is formed either by a good electrical connection 
such as welding or soldering, or by forming the two components as a single 
piece of metal. A generally disc shaped, electrically insulating insert 28 
is inset concentrically in an arc supporting surface 30 of contact 22 and 
forms an exposed insulator-metal boundary 32 therewith. Insert 28 includes 
a lip portion 28a for secure mounting in electrical contact 22. 
For aplication in a vacuum interrupter, contact 22 preferably comprises a 
metal having a high dielectric strength, for example copper. For 
application in an air circuit breaker, contact 22 preferably comprises a 
softer metal with a low melting temperature, for example a silver alloy 
such as silver-tungsten, silver-molybdenum, silver-nickel or 
silver-graphite. In either application, insert 28 preferably comprises an 
electrical insulator, for example a ceramic and rod 24 preferably 
comprises a conductive metal, for example copper. 
In operation, when electrical contact assembly 10 is used in an air circuit 
breaker or a vacuum interrupter, any electrical arc generated on surface 
30 of contact 22 will tend to stabilize at the exposed insulator-metal 
boundary 32 between contact surface 30 and insert 28. When used in a 
vacuum interrupter and constructed of the appropriate materials described 
above, contact assembly 20 will provide excellent dielectric strength and 
stable arcing characteristics. When used in an air circuit breaker and 
constructed of the appropriate materials described hereinabove, above, 
contact assembly 20 accommodates the substitution of a low cost ceramic 
for the expensive silver alloy while providing stable arcing 
characteristics. An environment in which electrical contact assembly 20 
experiences continuous arcing may cause a metal film (not shown) to be 
deposited oer the exposed surface of insert 28. This film will have a 
further stabilizing effect on subsequent arcs, allowing these arcs to burn 
on a metallic layer without attendant heat losses to the bulk of insert 
28. 
FIGS. 3 and 4 show a second embodiment of the invention including a contact 
assembly 38 comprising a contact 40, contact support 42 and insert 44. 
Electrical contact assembly 38 is substantially identical in structure to 
electrical contact assembly 20 (FIGS. 1 and 2), with the exception of the 
shape of insert 44, and the inclusion of an optional cylindrical 
passageway 47 extending coaxially through contact support 42 and contact 
40. Insert 44 is in the shape of a doughnut, and disposed concentrically 
with contact 40 and contact rod 42. Insert 44 includes two lip portions 
44a for stabilizing its position in contact 40, and two metal-insulator 
boundaries 46 on a surface 48 of contact 40. Passageway 47 terminates in 
an aperture 50 disposed concentrically on surface 48 of contact 40. 
Contact 40 further includes an optional lip portion 40a disposed about its 
other circumference for forming an edge 40b to further stabilize an 
electrical arc. Contact assembly 38 is preferably constructed of those 
materials described with respect to FIGS. 1 and 2 above. 
The operation of electrical contact assembly 38 is substantially identical 
to that of contact 20 (FIGS. 1 and 2). Electrical contact assembly 38, 
however, provides two metal-insulator boundaries 46 at which an electrical 
arc can stabilize. The metal edges situated on surface 48 at aperture 50 
and at edge 40b provide further areas at which an electrical arc can 
stabilize. Passageway 47 functions to better disperse an arc in a manner 
well known to those skilled in the art. 
FIGS. 5 and 6 show a third embodiment of the invention including an 
electrical contact assembly 60 comprising a contact 62, a support 64 and 
an insert 66. Contact assembly 60 is substantially identical in 
construction to those embodiments of the invention described above, with 
the exception that insert 66 is in the shape of a spiral originating 
proximate the center of contact 62 and terminating proximate the 
circumferential edge thereof. Contact assembly 60 is preferably 
constructed of those materials described hereinabove. In operation, this 
spiral shape of insert 66 stabilizes an arc by giving it two 
metal-insulator boundaries about which to migrate from the center to the 
edge of contact 62. These two metal-insulator boundaries are indicated at 
68. 
FIGS. 7 and 8 show a fourth embodiment of the invention including an 
electrical contact assembly 70 comprising a contact 72, a contact support 
74, an insert 76 and an arc runner 78. Contact 72 is generally rectangular 
in shape, and includes a first surface 80 disposed in integral contact 
with rectangular, horizontally disposed (from the view of FIG. 8) contact 
support 74. Insert 78 is generally rectangular in shape, and inset into an 
arc supporting second surface 82 of contact 72 for forming a 
metal-insulator boundary 84 therewith. Arc runner 78 preferably comprises 
a layer of thin copper having a rectangular aperture 86 centrally disposed 
therein, the arc runner being situated proximate surface 82 of contact 72. 
The remaining elements of contact assembly 70 preferably comprise those 
materials described with respect to FIGS. 1 and 2 above. A generally 
opposing, identical contact assembly is shown in phantom at 88 (FIG. 8), 
representing a typical arrangement of such contact assemblies in air 
circuit breakers. 
The operation of electrical contact assembly 70 is substantially identical 
to those embodiments of the invention described hereinabove. In addition 
to the stabilizing effect metal-insulator boundary 84 has on an arc drawn 
at surface 82, arc runner 78 further functions, in a manner well known in 
the art, to permit that arc to be driven away from the surface and along 
aperture 86. The rectangular shape of contact 72, as well as the inclusion 
of arc runner 78, is typical of the construction of contact assemblies in 
air circuit breakers. However, it will be appreciated by those skilled in 
the art that the shape of contact 72 is not so limited and can comprise, 
for example, a disc shape similar to contact 22 (FIGS. 1 and 2). 
Referring now to FIGS. 9 and 10, a fifth embodiment of the invention is 
shown wherein an electrical contact assembly 90 comprises a disc shaped 
metal contact 92 having an optional ridge 94 disposed concentrically on a 
surface 96 thereof. A metal contact support in the form of a 
concentrically disposed rod 98 is connected integrally with contact 92 at 
a second surface 100 thereof. A doughnut shaped insert 102, comprising a 
material characterized by a higher electrical resistance than the material 
of electrical contact 92 and rod 98, and capable of withstanding high 
temperatures in the range of those normally encountered in air circuit 
breakers and vacuum interrupters, is disposed concentrically inside of 
contact 92 so as to leave an integral metal path for electrical current 
between surface 96 and contact rod 98, substantially through the center of 
contact 90. This current path is indicated as dashed-line 104. Contact 92 
and rod 98 preferably comprise those metals described with respect to 
FIGS. 1 and 2 hereinabove. Insert 102 preferably comprises a ceramic or a 
high-temperature metal having an electrical resistivity relatively higher 
than that of contact 92. For example, when contact assembly 90 is used in 
an air circuit breaker and contact 92 is constructed of a silver alloy, 
insert 102 preferably comprises tungsten or molybdenum, or a ceramic such 
as alumina. 
The operation of electrical contact assembly 90 is substantially identical 
to that of the previous embodiments of the invention described 
hereinabove. Because insert 102 is disposed totally inside of contact 92, 
its stabilizing effect on an arc at surface 96 is due to the use of a 
higher resistivity material for insert 102 and will be subtler than those 
contacts having the exposed, metal-insulator boundary described above. 
This stabilizing effect is in the form of a magnetic field on surface 96 
which causes an arc to stabilize along a circle above insert 102. Ridge 94 
enhances this stabilizing affect by providing a metal edge for further 
stabilizing an electrical arc in this same general area. 
Because contact assembly 90 has a buried insert 102 and thus a subtler arc 
stabilizing affect than those contact assemblies with the exposed 
metal-insulator boundaries described above, it has particular application 
in air circuit breakers where unstable arcing is less of a problem. When 
used in air circuit breakers and constructed of those materials described 
above, insert 102 has the specific advantage of replacing the bulk of the 
expensive silver alloy used to construct contact 92 and contact support 98 
with a less expensive metal or ceramic material. Further, the positioning 
of insert 102 is such as to leave an integral metal current path 104 
substantially through the center of contact 92, thereby interfering as 
little as possible with the flow of current through the air breaker. 
Referring now to FIGS. 11 and 12, a sixth embodiment of the invention 
includes an electrical contact assembly 120 comprising a contact 122, a 
contact support 124 and an insert 126. The construction of contact 
assembly 120 is substantially identical to that of contact assembly 70 
(FIGS. 7 and 8) above, with the exception of the shape of insert 126 and 
the inclusion of the insert totally within contact 122. In this embodiment 
of the invention, insert 126 is generally rectangular and includes a 
hollow disposed generally coaxially with the contact about an axis 128. 
Because insert 126 is buried in contact 122, contact assembly 120 is 
particularly suited for use in air circuit breakers and preferably 
comprises those materials described hereinabove with respect to FIGS. 9 
and 10. The operation of contact assembly 120 is substantially identical 
to that of contact assembly 90 (FIGS. 9 and 10), insert 126 producing a 
stabilizing affect at an arc supporting surface 130 of contact 122 while 
replacing contact metal with insert material. Contact assembly 120 further 
leaves an integral metal current path 132 between arc supporting surface 
130 and contact support 124. It will be appreciated by those skilled in 
the art that a contact runner, similar to contact runner 78 (FIGS. 7 and 
8), may be optionally included with electrical contact assembly 120. It 
will be further appreciated that contact 122 is not limited to the 
rectangular shape illustrated, but can comprise for example, the disc 
shape of contact 92 (FIGS. 9 and 10). 
There is thus provided multiple embodiments of improved electrical contact 
assemblies for use both in air circuit breakers and vacuum interrupters, 
these electrical contact assemblies providing improved stabilization of 
electrical arcs. When used in air circuit breakers, these electrical 
contact assemblies provide the additional benefit of substituting 
inexpensive materials for normally expensive silver alloys. When used in 
vacuum interrupters, these electrical contact assemblies provide the 
further advantage of high dielectric strength. It will be appreciated by 
those skilled in the art that those embodiments of the invention which 
include an exposed metal-insulator boundary are intended primarily for use 
in vacuum interrupters where arc stabilization is the most important 
consideration. Those embodiments of the invention wherein the insert is 
buried and an integral current path is left substantially through the 
center of the contact are primarily intended for use in air circuit 
breakers where material expense and high current flow are primary 
considerations. However, either type of contact assembly can be used in 
either type of breaker with the attendant advantages described 
hereinabove. 
While preferred embodiments of the invention have been illustrated and 
described, it will be clear that the invention is not so limited. Numerous 
modifications, changes, variations, substitutions and equivalents will 
occur to those skilled in the art without departing from the spirit and 
scope of the present invention. Accordingly, it is intended that the 
invention herein be limited only by the scope of the appended claims.