Note: Descriptions are shown in the official language in which they were submitted.
CA 02508989 2005-06-01
HIGH CURRENT SWITCH AND METHOD OF OPERATION
FIELD OF THE INVENTION
The present invention relates generally to high current switches used in
electric power
distribution systems and, more particularly, to an electrically insulated,
deadfront, single
operation, medium voltage, high current closing device.
BACKGROUND OF THE INVENTION
An urban utility experiences approximately 1,500 failures on its network
feeders each
year. Each feeder outage duration is directly proportional to the risk of
customer interruption
and the stress experienced by other feeders and transformers in the network.
The defective
component must remain out of service during repair and/or replacement. This
means that the
whole feeder remains out of service or a live end cap must be installed to
separate the main
feeder from the spur containing the defect. If a live end cap is installed,
the feeder must be de-
energized a second time to reconnect the required spur. This second outage is
usually scheduled
as soon as possible to restore the system to normal full capability. However,
the perceived risk
of scheduling the entire feeder out of service to pick-up a small spur is very
large, especially
during the summer or other high load periods.
Encapsulated switch assemblies with sub-atmospheric or vacuum type circuit
interrupters
for electric power circuits and systems are well known in the art, such as is
shown in U.S. Pat.
Nos. 4,568,804; 3,955,167; 3,471,669; 3,812,314; and 2,870,298. In some prior
art switch
assemblies and circuit breakers, a pair of coacting contacts, one fixed and
the other movable, are
provided for controlling and interrupting current flow. The contacts are
provided in a controlled
atmosphere contact assembly which may include a relatively fragile glass or
ceramic housing,
commonly referred to as a "bottle" for housing the contacts. A metal bellows
may be provided
on one end of the bottle, and the movable contact is linked to the inside of
the bellows. An
operating rod attached to the outside of the bellows can be moved so as to
move the movable
contact inside the bottle. The interior of the bottle is maintained under a
controlled atmosphere,
such as air or another gas under a low subatmospheric pressure, to protect the
contacts from
damage caused by arcing when the contacts are opened and closed. The glass or
ceramic wall of
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the bottle provides a permeation-resistant enclosure which maintains the
controlled atmosphere
for the life of the device.
More recently, elastomer-insulated switch housings using a controlled
atmosphere
contact assembly have been introduced for underground power distribution
systems and other,
similar applications. Switches for use in such applications must meet several
demanding
requirements. Those parts of the switch assembly connected to line voltage
during use, including
the contact assembly and operating rod, must be encased in a solid insulating
housing having
dielectric strength sufficient to withstand the maximum voltage which may be
imposed on the
system, which may be tens of thousands of volts for a distribution-level
system. For safety, the
insulating housing should be covered with a conductive layer that can be
grounded. The switch
should be operable from outside of the dielectric housing, without opening the
housing and
should be capable of withstanding many years of exposure to temperature
extremes, water and
environmental contaminants.
Elastomers such as EPDM (ethylene propylene diene monomer) combine high
dielectric
strength with excellent resistance to the effects of ozone and corona
discharge. These elastomers
can also provide good physical properties such as abrasion resistance, and can
be molded at
reasonable cost. Additionally, these elastomers can be compounded with
conductive additives
and molded to provide an electrically conductive grounding layer integral with
the dielectric
housing. For these and other reasons, elastomers molded and vulcanized under
heat and
pressure, such as EPDM, have been almost universally adopted as materials of
construction for
the housings used in many underground electrical distribution systems.
An important feature in such switch assemblies and circuit breakers is the
ability to
visually determine the switched condition of the contacts. This is obviously
important for safety
reasons in that power must be disconnected before accessing or repairing a
switch branch. U.S.
Patent No. 4,568,804 discloses a high voltage vacuum type circuit interrupter
having a one-piece
ceramic insulating housing connected to a two-part metallic base. The base
encloses a solenoid
operated toggle mechanism that controls and operates movement of a switch
contact to open and
close the switch. The base further includes a sight glass or lens secured to
the bottom of the
base, through which a switch position indicator is visually discernible.
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One drawback with the circuit interrupter disclosed in the '804 patent is its
size and
complexity in manufacture. Another drawback relates to the fact that the
position indicator is
located at the toggle mechanism away from the switch contacts. In other words,
while the
position indicator of the '804 patent may show the condition of the toggle
mechanism, there is no
provision for visually confirming whether the switch contacts are indeed in
contact or separated.
As mentioned above, another concern with such switch assemblies is flashover
or arcing
of the electric current between switch contacts. Aside from safety concerns,
such arcing causes
damage to the contacts and the surrounding housing. While efforts to reduce
arcing by enclosing
the contacts in an evacuated chamber or by insulating the contacts with an arc
quenching gas or
oil have proven somewhat successful, arcing still occasionally occurs in the
field. Additionally,
vacuum chambers typically require a housing made from ceramic. Air insulation
chambers are
generally very large. Chambers filled with SF6 arc quenching gas must be
hermetically sealed
and maintained to ensure no leakage and insulating oils have been found to
fail catastrophically
resulting in injury to people and damage to equipment.
Yet another problem with high current switches described above is related to
electromagnetic fields which generate undesirable bending forces. In
particular, the feeder
contact is arranged generally at a 90 angle to the switches current carrying
contact pin. These
electromagnetic forces are produced on the current carrying members causing a
cantilever
bending movement at the connection interface.
Accordingly, it is desirable to provide a simply constructed, electrically
insulated, switch
assembly having direct visible verification of open or closed contacts. It is
further desirable to
provide such a switch assembly that minimizes the possibility of arcing
between electrical
contacts and provides good electrical continuity through the switch assembly.
SUMMARY OF THE INVENTION
The present invention is an electrical switch, which generally includes an
insulative
housing having a wall defining an axial bore therein, a first electrical
contact disposed in the
housing bore and a second electrical contact movably disposed in the housing
bore between an
open position and a closed position. When the contacts are in their open
position, the second
electrical contact is spaced apart from the first electrical contact and when
the contacts are in
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their closed position, the second electrical contact is in electrical contact
with the first electrical
contact.
In a preferred embodiment, the switch further includes a viewing port disposed
in the
insulative housing wall adjacent the first electrical contact to permit
viewing of the first electrical
contact within the housing bore. The viewing port preferably includes a
transparent element
made from a clear insulative plastic material fixed within the housing wall.
The transparent
element may further be provided with a magnification feature to enhance
viewing and the
housing wall may include a protruding boss portion having a hole for receiving
the transparent
element.
The switch may further include a frangible insulative plate disposed in the
housing bore
between the first and second electrical contacts when the second electrical
contact is in its open
position, The frangible insulative plate is adapted to be broken by the second
electrical contact
as the second electrical contact is moved to its closed position. The
frangible insulative plate is
preferably made from a high dielectric strength glass material.
Another feature of the present invention is a high current electrical
connector system that
includes a male pin having a first end and a second end formed by a pair of
resilient legs that
define a slot; a female socket having a substantially cylindrical side wall
and a bottom surface
which define a cavity, an open end and a post that extends from the bottom
surface. The female
socket is configured to receive and electrically contact the second end of the
male pin and the
slot receives and electrically contacts the post. The male pin can be tapered
from the first end to
the second end and the post can have a base and a knurled end. Preferably, the
slot in the male
pin is configured to receive the knurled end. The cylindrical side wall of the
female socket can
include one or more apertures that are adapted to vent the cavity.
The high current electrical connector system can also include an electrically
insulated rod
and an actuating mechanism. The electrically insulated rod connects the
actuating mechanism to
the first end of the male pin.
Another feature of the present invention is an insulating seal ring for
electrically
insulating a movable energized contact in a housing for a high-current
electrical switch. The
insulating seal ring includes a generally annular body having an outer wall
with an outside
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diameter that defines an outer sealing surface, an inner wall with an inside
diameter that defines
an aperture with an inner sealing surface. The outer sealing surface is
adapted to be sealably
received by the housing and the inner sealing surface is adapted to sealably
receive an actuating
rod. The insulating seal ring also includes a generally annular core inside
the annular body. The
body has a first durometer (or hardness) and the core has a second durometer
and the materials
that form the body and core are selected so that the second durometer is
greater than the first
durometer. Preferably, the body is formed from an elastomeric polymeric
material, such as
natural rubbers, synthetic rubbers or fluoropolymers. The body can also be
formed from a
thermoplastic material, most preferably one that includes a polyethylene, a
polypropylene or a
polybutylene. The core is preferably formed from a thermoplastic material, an
elastic synthetic
polyamide material (Nylon), a polycarbonate, an acrylonitrile-butadiene
styrene, a polyester
terephthalate or a styrene-acrylonitrile.
The insulating seal ring preferably has an outside diameter that is greater
than or equal to
two times the inside diameter. The body can have a first substantially flat
surface and a second
substantially flat surface, wherein the distance between the first and second
surfaces defines a
thickness, and wherein the thickness is greater than or equal to the outside
diameter.
In another embodiment, the insulating seal ring includes a generally annular
body having
an inner concentric layer and an outer concentric layer, wherein the inner
concentric layer is
formed from a first elastomer material having a first durometer and the outer
concentric layer is
formed from a second elastomer material having a second durometer, wherein the
second
durometer is greater than the first durometer. The insulating seal ring can
also include a first
substantially flat surface and a second substantially flat surface, an outer
wall with an outside
diameter that defines an outer sealing surface and an inner wall with an
inside diameter that
defines an aperture having an inner sealing surface, preferably the outside
diameter is greater
than or equal to two times the inside diameter. The distance between the first
and second
surfaces defines a thickness which is preferably greater than or equal to the
outside diameter.
The outer sealing surface is adapted to be sealably received by the housing
and the inner sealing
surface is adapted to sealably receive the actuating rod.
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The inner concentric layer and the outer concentric layer are formed from
different
elastomeric materials selected from the group consisting of natural rubbers,
synthetic rubbers,
and fluoropolymers. The outer concentric layer material is selected so that
its durometer is
greater than the durometer of the inner concentric layer material. The outer
concentric layer
material can also be an elastic synthetic polyamide material (Nylon),
polycarbonate,
acrylonitrile-butadiene styrene, or styrene-acrylonitrile.
The switch assembly may further include method and apparatus for reducing
bending
forces on an electrical connection point. More specifically, the feeder
contact and current
carrying male pin are electrically coupled and include longitudinal axes which
are substantially
non-parallel. The feeder contact preferably includes a mechanically weakened
portion adjacent
the electrical connection. Upon closing of the switch,, high current flows
through the male pine
and feeder contact generating electromagnetic bending forces. These bending
forces tend to act
on the electrical connection thereby loosening or damaging the connection. The
mechanically
weakened portion directs the bending forces away from the electrical
connection to reduce
undesirable bending forces on the connection point.
The preferred embodiments of the switch of the present invention, as well as
other
objects, features and advantages of this invention, will be apparent from the
following detailed
description, which is to be read in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional view of the switch according to the present
invention.
Figure 2 is is a detailed cross-sectional view of the viewing port of the
present invention.
Figure 3 is a cross-sectional view of the housing central bore showing the
space between
the open contacts separated by glass insulating plates.
Figure 3a is a cross-sectional view of the housing central bore shown in
Figure 3 with the
contacts in a closed position and the glass plates broken.
Figure 4 is a cross-sectional view of the insulating seal ring and actuating
rod.
Figure 5 is a cross-sectional view of the feeder post contact and male pin
electrical
connection.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to Figure 1, in a preferred embodiment, the switch 10
according to the
preferred embodiment of the present invention is a medium-voltage, one-
operation switch. As
used in this disclosure with reference to apparatus, the term "medium voltage"
means apparatus
which is adapted to operate in electric utility power systems, such as in
systems operating at
nominal voltages of about 27 kv to about 29 kv, commonly referred to as
"distribution" systems,
as well as equipment for use in "transmission" systems. A high-voltage switch
of this type is
disclosed in commonly owned U.S. Patent No. 5,808,258.
The term "one-operation" generally means a device used to temporarily
interrupt power
between a "feeder" or "source" circuit and a "spur" circuit in order to safely
access or effect
repairs on the spur circuit. Upon successful repairs of the spur circuit, the
switch is closed to
restore power to the spur circuit and is replaced by a permanent connection at
a later low load
planned outage.
The switch 10 includes a housing 12 formed from a dielectric elastomer which
is
vulcanized under heat and pressure, such as ethylene propylene diene monomer
(EPDM)
elastomer. The housing 12 defines an elongated bore 14 extending in endwise
directions parallel
to an axis 16. The housing has a terminal end 18 and a second, opposite end
20, referred to
herein as the operating end. For reasons discussed below, the direction
parallel to axis 16 toward
terminal end 18 is referred to herein as the closing endwise direction,
whereas the opposite
endwise direction, towards operating end 20 is referred to as the opening
endwise direction.
The housing defines a tapered bushing 22 at the fixed end and a further
tapered bushing
24 extending perpendicular to the endwise axis. Bushing 24 has a cylindrical
metallic current-
carrying element 25 extending therein to the bore 14 in a direction
perpendicular to axis 16. This
current-carrying element 25 of the bushing 24 is generally adapted for
electrical connection to
the "spur" circuit of the power distribution system, as described above.
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The portion of the housing 12 disposed between the tapered bushing 22 and the
operating
end 20 has a generally cylindrical exterior surface, so that the wall of the
housing in this region is
generally in the form of a cylindrical tube. The housing is provided with an
electrically
conductive insert 26 formed from a mixture of the same elastomer used for the
remainder of the
housing and an electrically conductive material such as carbon black. Insert
26 covers the
interior wall of bore 14 from the operating end 20 to a point adjacent the
bushing 24.
Overlying the majority of the exterior surface of the housing 12 is a
conductive jacket 28.
The bushing 24 extends from the housing through a hole in the conductive
jacket 28. The
conductive jacket 28 may also be formed from a mixture of the same elastomer
used for the
remainder of the housing and an electrically conductive material such as
carbon black. The
exterior conductive jacket 28 is in intimate, void-free contact with the
outside of the housing 12,
and is securely bonded thereto. Likewise, the semiconducting lining 26 is
intimately bonded to
the dielectric elastomer of the housing 12. These components may be fabricated
by insert
molding, as described in U.S. Patent No. 5,808,258.
Fixed at the terminal end 18 of the housing 12 is a metallic terminal end
closure 30,
which seals the central bore 14 at the terminal end. A fixed contact 32 is
mounted to the terminal
end closure 30 and projects into the central bore 14 of the housing 12. The
fixed contact 32
includes an engagement end 33 and further includes a terminal end stub contact
34 formed
integrally with the fixed contact, which projects outwardly from the central
bore 14 beyond the
terminal end closure 30.
The switch 10 further includes an actuating device 38 mounted to the operating
end 20 of
the housing 12. The actuating device 38 is connected to a moveable or
operating-end male
contact pin 40 extending into the central bore 14 of the housing 12. The
contact pin 40 is in
electrical contact with the first cylindrical metallic current-carrying
element 25 disposed in the
second bushing 24. More specifically, the first current carrying element 25
includes a threaded
end 29 which is received in a threaded bore 31 of a donut contact 27. The
donut contact 27
includes an axial bore to slidingly electronically communicate with the
contact pin 40. The first
current carrying device or post contact 25 includes a central axial bore
therein to receive the post
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of the high voltage connector, such as an elbow connector (not shown). The
contact pin 40 is
driven by the actuating device 30 in the closing endwise direction from an
open position, as
shown in Figure 1, to a closed position, wherein the contact pin engages the
fixed contact 32.
The actuating device 38 moves the contact pin 40 rapidly between opened and
closed positions
so as to minimize arcing.
The actuating device 38 is preferably extremely compact and accommodated in a
tubular
housing 39 of essentially the same diameter as the switch housing 12. An O-
ring or other
conventional seals (not shown) can be provided between the actuator housing 39
and the switch
housing 12 so as to provide a weather-tight seal protecting the elements of
the actuating
mechanism 38. Any of the numerous drive mechanisms known in the art for moving
switch
contacts can be used in the switch 10. For example, pneumatically-operated
devices, solenoid-
actuated devices, spring-operated devices and other known mechanisms can be
used. Moreover,
these can be either manually activated or automatically activated by a control
system or by a
sensor associated with the switch for detecting a condition in the circuit.
The interior central bore 14 surrounding the fixed contact 32 and the contact
pin 40 has a
controlled atmosphere therein. As used in this disclosure, the term
"controlled atmosphere"
means an atmosphere other than air at normal atmospheric pressure. Most
preferably, the
atmosphere within the central bore 14 is under a subatmospheric pressure. The
composition of
the atmosphere may also differ from normal air. For example, arc-suppressing
gases such as SF6
may be present within the bore.
The switch 10 further includes a terminal end cover 42 formed from a
dielectric
elastomer similar to the housing 12. The cover 42 may include a terminal end
electrical stress
relief element 44, formed from a semiconducting elastomer, disposed therein.
The terminal end
cover 42 is positioned on the housing 12 so that an internal taper in the
cover is firmly engaged
with the conical seat 22 at the terminal end 18 of the housing and so that the
electrical stress
release element 44 surrounds the contact stub 34 extending out of the terminal
end of the
housing. The terminal end cover has a second cylindrical metallic current
carrying element 46
mounted therein, which is electrically coupled to the contact stub 34. This
second current-
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carrying element 46 of the end cover 42 is generally adapted for electrical
connection to the
"feeder" or "source" circuit of the power distribution system, as described
above.
In operation, the switch 10 is connected in the circuit through current-
carrying elements
25 and 46, and hence through terminals 40 and 34. In the position illustrated
in Figure 1, the
switch is open. To close the switch, the actuating device 38 is activated to
axially translate the
movable contact pin 40 in the closing direction toward the fixed contact 32
until the two are
mechanically and electrically engaged. As mentioned above, this movement
occurs suddenly,
thereby minimizing any possibility of arcing between the contacts.
Referring additionally to Figure 2, to visually confirm the condition of the
internal
contacts 32 and 40 with respect to each other (i.e., open or closed), the
switch housing 12 is
provided with a viewing port 48 positioned directly adjacent the engagement
end 33 of the fixed
contact 32. The viewing port 48 is preferably in the form of a transparent
element 50 fixed
within the insulative material of the housing 12 so as to provide visual
access into the interior
bore 14 of the housing at a point 51 directly adjacent the engagement end 33
of the fixed contact
32. The transparent element 50 is preferably made of a clear insulative
plastic material and may
be provided with a magnifying feature to enhance viewing. However, any
insulating material
having a sufficient level of transparency can be used for the transparent
element 50.
The transparent element 50 may be press-fit or bonded within a hole of the
housing 12
formed during molding of the housing. In this regard, it is preferred to form
the housing 12 with
a protruding boss portion 52 having a hole for receiving the transparent
element 50. By
providing the boss portion 52, the depth of the hole can be increased, thereby
increasing the
contact surfaces between the hole and the transparent element 50 to enhance
the hold
therebetween. An electrical stress grading coating can also be applied between
the hole surface
and the transparent element 50 to ensure adequate electrical interface
therebetween.
The conductive jacket 28 of the switch housing 12 preferably extends upwardly
to cover
the side walls of the boss portion 52 and defines an opening for the end face
54 of the boss
portion. Thus, the transparent element 50 penetrates the insulation wall of
the housing 12 while
maintaining the insulative layer between the energized contacts 32 and 40 and
the external
CA 02508989 2005-06-01
grounded shield 28 of the housing. A cap (not shown) is provided to cover the
viewing port to
keep it free from debris.
Referring now additionally to Figure 3, to further minimize arcing between the
movable
contact pin 40 and the fixed contact pin 32, the housing 12 of the present
invention further
preferably includes at least one frangible insulative plate 56 fixed in the
housing central bore 14
between the contacts. The plate 56 is preferably made from a high dielectric
strength glass,
about 1/8" thick, which can be fixed in the central bore 14 during molding of
the housing 12. In
the preferred embodiment, the housing 12 includes two glass plates 56 disposed
adjacent
respective contacts 32 and 40.
As a result, the contacts 32 and 40 can be separated by air without the need
for a large
volume. Moreover, the contacts 32 and 40 can be placed closer together since
the glass plates 56
serve to increase the static dielectric strength between the contacts to
control the arcing during
closure. The glass plates 56 provide the limited arc time needed for a
successful metal to metal
connection to extinguish any arc.
In operation, the normally open switch 10 can be installed between a faulted
spur circuit
and a source circuit after shutting off the voltage source. The spur circuit
is grounded via the
first current carrying element 25 of the switch while the source circuit is
connected to the second
current carrying element 46. Grounding of the spur circuit and disconnection
of the source
circuit is easily confirmed by viewing the open position of the contacts 32
and 40 within the
housing bore 14 through the viewing port 48. The faulted spur circuit can now
be safely
repaired.
Once repaired, the actuating mechanism 38 can be activated to translate the
movable
contact pin 40 forward toward the fixed contact 32. As the contact pin 40
travels, it breaks the
nearest glass plate 56 but is still insulated by arcing by the far glass plate
situated directly in front
of the fixed contact 32. Only when the second glass plate 56 is broken will an
arc strike, but by
this point, the pin 40 is already into engagement with the engagement end 33
of the fixed contact
32, as shown in Figure 3b. Engagement of the contacts 32 and 40 is also easily
confirmed with
the viewing port 48.
Thus, power is restored to the spur circuit without interruption of power in
the source
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circuit. The advantage of the switch 10 is that service is maintained to the
majority of power
customers on other spur circuits during the repair of the faulted spur circuit
and a second
interruption is prevented to restore power to the faulted spur circuit during
a high load period.
The switch 10 can be subsequently removed and replaced with a permanent
connection during a
low load planned outage.
Figure 3 shows an electrical contact system that includes a movable male pin
40 and a
stationary female socket contact 32. In Figure 3, the male pin 40 and the
female socket 32 are in
the open position and in Figure 3a the contacts 32, 40 are in the closed
position. The pin contact
40 is segmented into sections 41 and a portion of its longitudinal axis is
bored out to form a slot
43 which accepts a post 35 provided in the center of the female contact 32.
Preferably, the
segmented section of the pin contact 40 is tapered and the segmented sections
or fingers provide
some resilient spring when engaging the female socket. The female contact 32
is cylindrically-
shaped with a bottom surface 45 and an inner side wall 47 extending from the
bottom surface. In
addition, the internal post 35 inside the female socket 32 extends from the
bottom surface 45 and
is configured to be received by the axial bore or slot 43 in the male pin 40
so that the pin 40 is
trapped between the inner wall 47 of the female socket and the outer wall of
the internal post 35
to prevent any movement upon coupling of the pin and socket. The inside wall
47 of the socket
32 and outer surface of the post 35 preferably include a roughened surface,
such as being
serrated or knurled. Multiple contact surfaces and the scraping action of the
serrated surfaces
provide good high current transfer and prevent broken shards of glass from
interfering with the
connection.
In a preferred embodiment, the stationary female contact 32 is connected to
one 600A
separable connector rod contact 46 and the movable male pin 40 is physically
connected to, but
electrically insulated from, the actuating mechanism 38. The pin 40 passes
through and is
slidingly electrically coupled, preferably by means of a spring contact, to
the donut contact 27.
As earlier described, the donut contact 27 includes a threaded bore 31 to
receive the threaded end
29of the first current carrying contact 25.
In the open position of the preferred embodiment, the electrical contact
system has
approximately 3.5 inches separating the male pin contact 40 and the female
socket contact 32.
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However, in other embodiments, the separation distance can vary from about 2
to 6 inches or
more. The insulation medium between the contacts is air and glass. The two 1/8-
inch thick glass
plates 56 provide a dual function of maintaining dielectric strength across
the open contacts, and
controlling the arc distance and time between the closing contacts. One 1/8-in
thick glass plate
56 provides sufficient dielectric strength to prevent an arc strike until the
glass plate 56 is broken
by the closing pin contact 40. Considering the contact chamber is a closed
vessel, and the
current can be a maximum of 40 kA symmetrical, it is critical to limit the arc
energy for a
successful close. Excess arc energy will cause a rapid increase of pressure
and excess erosion of
the contacts. This will result in a housing rupture and fault to ground. With
the 1/8-in thick
glass plate and a contact closing speed of 387 in/sec, the arcing time is
limited to approximately
0.32 milliseconds. Fault-close tests at 40 kA have demonstrated successful
closure with minimal
damage to the contacts.
The male pin 40 is electrically isolated from the actuating mechanism 38 by a
non-
conductive coupling (or actuating) rod 80, preferably made of fiberglass. The
first end 82 of the
rod 80 is connected to the actuating mechanism 38 and the second end 84 is
connected to the pin
contact 40. When the contacts 32, 40 are open, the pin contact 40 side is
connected to the feeder,
which is grounded, and voltage withstand need not be considered. When the
contacts 32, 40 are
closed and energized, the pin contact 40 is insulated from the grounded
actuating mechanism 38
by the insulated coupling rod 80.
Another feature of the present invention, is an insulating seal ring 70 as
shown in Figure
4. Any medium or high voltage switch having an electrically grounded mechanism
that is
mechanically connected to and operates an energized contact must have an
insulating barrier
between the two to prevent flashover or creep. The insulating barrier must
maintain a continuous
seal when the switch is actuated without interfering with the travel of the
actuating mechanism of
the switch. By controlling the frictional interference level between the
sealing surface of the ring
and the rod, the seal can be maintained over the entire travel of the rod.
This concept can be
used in most types solid dielectric switches.
Figure 4 shows an insulating seal ring 70 for electrically insulating the
movable
energized contact 40 in the housing 12 of the high current switch 10. The
insulating seal ring 70
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is generally donut shaped with sealing surfaces 71, 73 on the respective outer
and inner
circumferences of its annular body. The insulating seal ring 70 has a ring-
shaped core 72 that is
covered with an insulating layer 74. The core 72 is formed from material that
is harder than the
insulating layer 74 material so that the core 72 has a stiffening effect on
the insulating layer 74.
In another embodiment, the insulating seal ring 70 is formed from two
concentric rings of
different materials, wherein the material that forms the outer ring is harder
than the material that
forms the inner ring. This allows the inner sealing surface 73 to be less
stiff and have different
sealing properties from the sealing surface on the outside surface.
The actuating rod 80 that connects the energized contact pin 40 and the
actuating
mechanism 38 of the switch 10 shown in Figure 1 preferably has an insulating
barrier between
the contact pin 40 and the actuating mechanism 38, which allows about 4 inches
of movement.
The insulating seal ring 70 provides an electrically insulated barrier that
permits the rod 80
substantially unrestricted travel over most of its length.
The inner diameter of the insulating seal ring 70 has an inner sealing surface
73 which is
sized based on the diameter of the rod 80 that connects the pin contact 40 and
the actuating
mechanism 38. The rod 80 is formed from an insulating material and has a
diameter configured
so that the inner sealing surface 73 does not sealably engage the rod 80 until
it has substantially
reached the end of its travel. The frictional interferences of the outer
sealing surface 71 and the
inner sealing surface 73 provide an electrically insulating seal between the
switch contacts 32, 40
and the actuating mechanism 38. The stiff core 72 of the insulating seal ring
70 allows the inner
and outer sealing surfaces 71, 73 to operate independently, without a
significant transfer of the
forces from one surface to the other surface. Thus, tracking on the surface of
the rod 80 is
prevented by the inner sealing surface 73 of the insulating seal ring 70 which
provides electrical
insulation around the rod 80. Similarly, the outer sealing surface 73 of the
insulating seal ring 70
provides electrical insulation with the inner surface of the housing chamber
12. The insulating
seal ring 70 provides the required AC, DC and BIL withstand levels between the
open contacts
and between the contacts and case ground.
In a preferred embodiment, the insulating seal ring 70 is formed from a
plastic ring-
shaped core 72 that is overmolded with an insulating layer 74 of an elastomer
material,
14
CA 02508989 2005-06-01
preferably rubber. The outer diameter ("OD") of the insulating seal ring 70
defines an outer
sealing surface 71 that is configured to sealably contact the generally
cylindrical, inside wall of
the switch housing 12. The aperture 75 in the insulating seal ring 70 has an
inner diameter
("ID") which is configured to sealably receive the rod 80 and provide
electrical isolation between
the actuating mechanism 38 and the high current pin 40 and socket 32
electrical contact system.
More specifically, the rod 80 is formed from an insulating material, such as
fiberglass, a
thermoplastic material or other non-conductive material with sufficient
hardness to maintain
structural integrity during the operation of the switch 10. The rod 80 has a
first end 82 that is
connected to the electrical pin connector 40 and a second end 84 that is
connected to a actuating
mechanism 38. Actuation of the switch 10 moves the rod 80 through the aperture
75 in the
insulating seal ring 70. The rod 80 is shaped so that the outside diameter of
the rod 80 at the
second end 84 allows it to pass through the aperture 75 in the insulating seal
ring 80 without
sealably contacting the inner sealing surface 73 of the ring 70 over most of
its travel. At the
point where the rod 80 nears the end of its travel, the diameter near the
first end 82 of the rod 80
passing through the aperture 75 in the ring 70 increases so that the rod 80
sealably contacts the
inner sealing surface 73 of the ring 70 that prevents arcing from one side of
the ring 70 to the
other.
Preferably, the rod 80 has at least a first diameter, which allows it to
unobstructively pass
through the aperture 75 in the ring 70, and a second diameter which sealably
contacts the inner
sealing surface 73 of the ring 70. However, other configurations of the rod 80
such as a tapered
construction or more than two different diameters are also contemplated by the
invention.
The outer and inner sealing surfaces 71, 73 of the insulating seal ring 70
provide
electrically insulating seals between the ring 70 and the housing 12 and the
ring 70 and the rod
80. The insulating seal ring 70 can withstand the voltage gradient that occurs
when the switch 10
closes and isolates the switch contacts 32, 40 inside the housing 12. The
rigid core 72 allows
independent frictional interference levels at the sealing surfaces 71, 73 and
prevents the force
applied on one sealing surface from being transferred to the other sealing
surface. In addition to
minimizing the transfer of forces between the two sealing surfaces 71, 73, the
ring-shaped core
72 evenly distributes any force that is transferred.
CA 02508989 2005-06-01
The configuration and dimensions of the core 72, as well as the thickness of
the
insulating layer 74 on either side of the core 72, provides adjustable levels
of friction at the
sealing surfaces 71, 73. The harder material of the core 72 acts as a
stiffener for the insulating
layer 74 on either side of the ring 70. The closer the core 72 is to the
sealing surfaces 71, 73, the
greater the stiffening effect on the insulating layer 74. A thicker core 72
results a less flexible
insulating layer 74 and hence more friction at the sealing surfaces 71, 73.
While a smaller core
72 results in an insulating layer 74 with more flexibility and movement and
hence less friction on
the rod 80.
The insulating seal ring 70 engages the rod 80 at its inner sealing surface 73
and the
switch housing 12 at its outer sealing surface 71. The frictional interference
level required to
properly seal these two surfaces is different. The rigid plastic core 72
allows the stiffness of each
sealing surface 71, 73 to be designed for the specific application and
controlled independently.
Preferably, the core 72 is designed to provide an insulating seal ring 70
having a higher frictional
interference level with greater stiffness at the substantially stationary
outer sealing surface 71
and a lower frictional interference level with less stiffness at the inner
sealing surface 73. The
lower frictional interference level of the inner sealing surface 73 allows
substantially unrestricted
movement of the rod 80 through the aperture 75 in the insulating seal ring 70.
Without the
plastic core 72, forces on one of the sealing surfaces would be transferred to
the other sealing
surface.
Alternatively, as will be understood by those skilled in the art, the
insulating seal ring 70
can be formed from an elastomer without a core, preferably a rubber, which
sealably contacts the
switch housing at the outer sealing surface and sealably contacts the rod at
the inner sealing
surface. In one embodiment, the insulating seal ring can be made from two
concentric rings
formed from elastomer materials having different durometers (hardness). The
elastomer that
forms the outer ring preferably has a higher durometer and is stiffer, while
the inner ring is
formed from a lower durometer elastomer which is less stiff and facilitates
the travel of the rod
through the insulating seal ring. The two elastomer rings are bonded together
using methods
well known to those skilled in the art.
16
CA 02508989 2005-06-01
Yet another feature of the present invention is the provision of a mechanical
weak point
on the spur side first current carrying contact 25 to accommodate
electromagnetic forces
generated upon electrical connection. As shown in Fig. 5, the switch 10
includes a contact pin
40 located within a central bore 14 thereof. As previously discussed, the
contact pin 40 is
provided to be axially movable within the bore 14 and makes electrical contact
with a contact
donut 27. The contact donut 27 includes a threaded bore 31 to receive the
threaded end 29 of
first current carrying contact 25 to provide a current path from the first
current carrying contact
to the contact pin 40. The first current carrying contact 25 extends at
approximately a 90 angle
with respect to the contact pin 40 and provides a current path through the
switch. The first
current carrying contact 25 is housed within the bushing 24 and includes a
central axial bore 87
therein adapted to receive an electrical contact from a spur side separable
connector (not shown).
The separable connector may preferably take the form of a high voltage elbow
connector such as
an Elastimold (please provide model # and rating) available from Thomas &
Betts Corporation,
Memphis, Tennessee.
The mechanical weak point of the present invention is provided on the first
current
carrying contact 25 near the threaded end 29 thereof. The mechanical weak
point is preferably in
the form of a recessed portion 85 of the contact 25. The purpose of the
mechanical weak point is
to permit some degree of bending to accommodate electromagnetic forces from
distorting and/or
loosening the connection between the threaded end of the contact 25 and the
donut contact 27.
More specifically, during high current flow as illustrated by arrows I1 and
12, electromagnetic
forces illustrated by arrows F1 and F2 are produced on the current carrying
members. It has been
found that such forces applied to an unsupported electrical contact point,
such as a rigid threaded
contact 25 not including the recessed portion tended to distort and/or loosen
the threaded
connection between the contact 25 and donut contact 27. This distortion or
loosening of the
connection has been found to weaken the electrical connection and lead to
possible failure of the
device.
The electromagnetic forces generate a bending force because the current flows
through
the first current carrying contact into the switch device and makes a right
angle turn to the
contact pin 40 and socket contacts 32. Accordingly, the electromagnetic force
generated by the
current flowing through the contact 25 is in a direction different from the
electromagnetic forces
17
CA 02508989 2005-06-01
generated by current flowing through the contact pin 40 and socket contacts 32
as shown by
arrows F1 and F2.. These electromagnetic forces act in different directions
and tend to try to
straighten the current flow path creating undesirable bending forces on the
electrical system
assembly components and especially at the juncture between the first current
carrying contact 25
and contact pin 40.
The present invention provides a solution to accommodate these electromagnetic
forces
and maintain a good electrical connection during high current operation. The
first current
carrying contact is provided with a recessed portion 85 such that the bending
forces are directed
to the mechanical weak point of the contact relieving the stress on the
threaded connection.
Stated differently, the bending forces will tend to bend the contact 25 in the
recessed portion
thereby reducing the stress on the electrical connection point. In a preferred
embodiment, the
first current carrying contact may be formed of a conductive material having
increased
maleability so that forces generated on the post contact tend to bend the
contact at the
mechanical weak point or undercut, not tend to loosen or distort the
electrical connection point.
The mechanical weak point or recessed portion of a contact to permit some
bending in
the region can be applied to any high current application where limiting
bending forces is
desirable. Such an electrical contact system is particularly useful in
reducing electromagnetic
bending forces to prevent damage or failure of a connection point wherein the
longitudinal axes
of the contacts is substantially non-parallel. The provision of the recessed
portion on the contact
can be used with a variety of different connections, such as threaded, welded,
soldered, sliding,
crimp or any other known electrical connection method to direct bending forces
away from the
connection point.
Although preferred embodiments of the present invention have been described
herein
with reference to the accompanying drawings, it is to be understood that the
invention is not
limited to those precise embodiments and that various other changes and
modifications may be
affected herein by one skilled in the art without departing from the scope or
spirit of the
invention, and that it is intended to claim all such changes and modifications
that fall within the
scope of the invention.
18
