U.S. patent number 3,632,926 [Application Number 05/028,258] was granted by the patent office on 1972-01-04 for current-limiting circuit breaker having arc extinguishing means which includes improved arc initiation and extinguishing chamber construction.
This patent grant is currently assigned to General Electric Company. Invention is credited to Eldon B. Heft.
United States Patent |
3,632,926 |
Heft |
January 4, 1972 |
CURRENT-LIMITING CIRCUIT BREAKER HAVING ARC EXTINGUISHING MEANS
WHICH INCLUDES IMPROVED ARC INITIATION AND EXTINGUISHING CHAMBER
CONSTRUCTION
Abstract
Current-limiting circuit breaker with arc-extinguishing means
including a closed arc initiation chamber and a communicating
arc-extinguishing chamber, the sidewalls of the chambers each being
formed as a molded insert of high dielectric gas-generating
material, the sidewalls of the arc-extinguishing chamber including
means forming a controlled constriction through which the arc must
pass, and diverging arc runners together with strips of material
extending between the outer extremities of the arc runners, said
strips comprising a material which is normally nonconductive but
which becomes conductive when heated by the effect of an electric
arc.
Inventors: |
Heft; Eldon B. (West Hartford,
CT) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
21842424 |
Appl.
No.: |
05/028,258 |
Filed: |
April 20, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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592443 |
Nov 7, 1966 |
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Current U.S.
Class: |
218/90;
218/150 |
Current CPC
Class: |
H01H
9/346 (20130101); H01H 9/302 (20130101) |
Current International
Class: |
H01H
9/34 (20060101); H01H 9/30 (20060101); H01h
033/00 () |
Field of
Search: |
;200/149A,144C,148C,150,144,149 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Macon; Robert S.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of application Ser. No. 592,443
filed Nov. 7, 1966 by the same inventor.
Claims
What I claim as new and desire to secure by Letters Patent of the
United
1. An electric circuit interrupter comprising:
a. a support;
b. said support including an arc initiation chamber having a bottom
wall and opposed side- and end walls;
c. said support also including an arc-extinguishing chamber
connected to said arc initiation chamber by a passageway;
d. a pair of elongated arc runners supported on said support, each
of said arc runners having a first end thereof supported adjacent
said passageway, said arc runners extending from said passageway
into said arc-extinguishing chamber and diverging from said
passageway in the direction of said arc-extinguishing chamber;
e. means for creating an electric arc in said arc initiation
chamber between said first ends of said arc runners and moving said
arc outwardly along said diverging arc runners to a final path
extending between said outer ends of said outer arc runners;
f. said support comprising a first body of insulating material
having relatively high strength and heat-withstanding ability, said
first body of insulating material having said arc initiation
chamber and said arc-extinguishing chambers formed therein; and
g. a pair of protective insert members for said arc initiation and
said arc-extinguishing chambers comprising generally planar pieces
of insulating material preformed separately from said first body of
insulating material and conforming to the shape of said chambers
and fitting closely against the otherwise exposed walls of said
chambers, said inserts comprising an acetal resin compound having
the characteristic of generating an arc-extinguishing gas when
heated by the action of an
2. An electric interrupter as set forth in claim 1 wherein said
support also includes a baffle member of insulating material
extending substantially directly transversely across the outer ends
of said arc runners and substantially closing the outer end wall of
said arc-extinguishing chamber, said baffle member including a
plurality of elongated, relatively narrow holes extending
therethrough in a direction generally perpendicular to the line
interconnecting said outer ends of said arc runners, said baffle
member comprising a high-strength plastic
3. An electric circuit interrupter as set forth in claim 1, said
interrupter also comprising at least one narrow elongated strip
member of insulating material carried by said first body of
insulating material and positioned so as to extend substantially
the full distance between said outer ends of said diverging arc
runners and parallel to and closely adjacent said final path of
said arc, said strip member comprising a material which is normally
an electric insulator but which becomes conductive when heated by
the action of an electric arc whereby the magnitude of transient
high voltages across said arc runners at the
4. An electric circuit interrupter as set forth in claim 3 wherein
said elongated strip members comprise glass fiber filled molded
material.
Description
My invention relates to electric circuit breakers and particularly
to electric circuit breakers including means for extinguishing an
electric arc in a "current-limiting" manner.
The term "current-limiting" is used herein to refer to the action
of an electric circuit breaker in interrupting an electric current
in such a manner that when the circuit breaker operates in a
circuit of substantial voltage, such, for example, as the common
commercially used voltage of 120, 240, 480 or 600 volts, and of
substantial nominal current rating, (such, for example, as 15 to
100 amperes or more), to interrupt substantial short-circuit
"available" currents, (such, for example, as 10,000 to 100,000 or
more amperes), the following relationships hold true: the actual or
"let-through" current flowing through the circuit during the
short-circuit current interruption operation of the circuit breaker
never exceeds and is usually substantially less than the
"prospective" current. Prospective current is defined as that
current which would flow in the same circuit under short-circuit
conditions if the circuit breaker were to be replaced by an
equivalent impedance. Such current-limiting circuit breakers are
also characterized by the fact that in almost every instance,
complete interruption is accomplished in substantially less than
1/2 cycle of a 60-cycle-per-second time wave.
It has been discovered that in order to achieve such
current-limiting interruption, the circuit breaker must function in
the manner indicated as to the following three respects: (1) an arc
must be drawn in extremely short time, i.e., within about 0.500
milliseconds (one-half of one-thousandth of a second) from the
occurrence of the short-circuit condition, (2) the initially
created arc must be transformed into a high voltage drop arc,
developing a voltage drop across it exceeding the driving line
voltage of the circuit and this must occur also at extremely high
speed and in not substantially more than two-thousandths of a
second*(*Typical times in the present invention are 2 milliseconds
at 1,000 amperes available and 0.7 of 1 millisecond at 100,000
amperes available.) from the creation of the arc, and (3) the
high-voltage character of the arc must be maintained without
extreme or uncontrolled fluctuation or "retrogressions" until the
current is driven to zero and extinguished.
Prior application Ser. No. 457,557, R. L. Hurtle and H. G. Willard,
filed May 21, 1965, now abandoned and replaced by application Ser.
No. 768,963, filed Oct. 10, 1968 and application Ser. No. 866,083
filed Oct. 3, 1969, and assigned to the same assignee as the
present invention discloses a current-limiting circuit breaker
construction capable of operating in the above-described manner and
capable of providing a true current-limiting action at substantial
voltages such as 250 volts and substantial nominal currents such as
100 amperes nominal current-carrying capacity and 100,000 or more
amperes short-circuit current capacity. To applicant's best
knowledge and information, the circuit breaker constructed in
accordance with the aforesaid Hurtle and Willard application is the
first contact-operating circuit breaker to successfully and
repeatedly interrupt short-circuit currents with true
current-limiting action in circuits of commercial power voltages at
substantial available short-circuit current values. Prior thereto,
such interruption has been possible only by the use of
current-limiting fuses.
The present invention is an improvement on the Hurtle and Willard
circuit breaker, and a general object of the invention is to
provide an electric circuit breaker operating on the principles set
forth in the Hurtle and Willard aforesaid application and
satisfying a number of specific design and performance
requirements, and capable of being produced in physical sizes and
in range of ampere ratings such as to make it suitable for ready
adoption by circuit breaker users for use in applications where
non-current-limiting circuit breakers have heretofore been used.
For this purpose, such circuit breakers must comply with a number
of physical and electrical requirements set up by safety code and
inspection authorities and must be able to correlate with other
electrical equipment in a predetermined manner.
Circuit breakers constructed in accordance with the aforesaid
Hurtle and Willard application have been found to perform
remarkably well at high short-circuit current levels, as mentioned
above. Short-circuit currents of relatively low levels can be
adequately interrupted by conventional circuit breaker action. At
certain intermediate current ranges however, above the range of
conventional breakers but below the "high" short-circuit range, the
interrupting action of the circuit breaker of the aforementioned
application has been found to be erratic. At such levels,
"retrogressions" have been found to occur, which, although not
causing failure of the device, nevertheless cause rapid
deterioration of the materials of the circuit breaker and are
generally considered undesirable.
It is an object of the present invention to provide a
current-limiting circuit breaker of the type described which is
capable of providing a current-limiting interrupting action
throughout all ranges of available short-circuit current at least
up to the specified maximum.
It is another object of the invention to provide a current-limiting
circuit breaker having a nominal capacity of 100 amperes at 600
volts and capable of use on circuits with 100,000 amperes
short-circuit current available, and having a relatively small
size, such, for example, as about 4 inches wide by 41/2 inches high
by 17 inches in length.
It is a further object of the invention to provide an electric
circuit breaker of the current-limiting type which is capable of
performing a current-limiting function without the occurrence of a
voltage surge or "spike" at the termination of the current
interruption.
It is another object of the invention to provide an electric
circuit breaker having arc-interrupting means suitable for use with
effective sound and shock wave muffling means, so as to
substantially diminish the volume of sound, shock wave, and hot
gases otherwise emitted from the circuit breaker upon the
interruption of a high short-circuit current.
It is a general object of the invention to provide a
current-limiting circuit breaker of the type described which can be
manufactured by conventional manufacturing techniques, with
consistent, predictable performance characteristics and at
economical cost.
A number of other objects will in part become obvious, and in part
will be specifically pointed out in the following detailed
description of the invention, and its scope will be pointed out in
the appended claims.
In accordance with the invention, a current-limiting electric
circuit breaker is provided of the basic type as set forth in the
aforementioned Hurtle and Willard application, in which the current
interruption is initiated by the creation of two serially related
arcs which are elongated and merged into a single arc which is then
further elongated and maintained until the current is driven to
zero, which breaker is capable of interrupting short-circuit
currents with a smooth current-limiting action at all short-circuit
current values from the lowest value causing operation of the
high-speed opening means through the highest value which the
circuit breaker is capable of interrupting.
The circuit breaker includes a pair of spaced stationary contacts
having their generally planar contact surfaces inclined toward each
other, and a movable bridging contact member with corresponding
contact surfaces. The stationary contacts are supported in an "arc
initiation" chamber having its inner wall surfaces constructed of a
material having the characteristic that under the action of an
electric arc, it is ablated or transformed into a gas, the gas
having substantially no free carbon atoms. A suitable material is
an acetal resin material, and preferably a polyoxymethylene
material. Diverging arc runners extend from the stationary contacts
in one direction into an "arc extinction" chamber. The arc
initiation chamber is closed in the opposite direction by a
backwall, through which a contact operating rod extends. High-speed
operating means, such as a magnetic solenoid, is provided for
opening the movable contact at high speed.
In accordance with the invention, the arc initiation and arc
extinction chambers are formed so that the cross-sectional area of
the space available to the arc at first diminishes sharply as the
arc moves away, and then increases gradually. This has the effect
of retaining pressure in the arc initiation chamber for a period of
time and so avoiding "cavitation," which leads to arc
retrogressions as will be more fully described.
In addition, in accordance with the invention, the conventional
"arc-cooling plates" are omitted, and instead, the arc is elongated
in the arc extinction chamber in a substantially straight-line
condition between the arc runner tips and adjacent a slotted vent
control member or "baffle," to provide efficient arc control
action.
Moreover, in accordance with the invention, the spacing of the arc
runner tips, and therefor the length of the arc in its most
elongated condition, is made substantially greater than otherwise
required, in order that the arc extinguisher may effectively coact
with an effective noise and flame "muffle" device. The creation of
excessively high voltage across the arc, and particularly the
creation of a terminal voltage surge or spike is avoided despite
this increased length of arc, by the inclusion of sidewall strips
of glass fiber-and-plastic compound such as "glass melamine" which
span the space between the arc runner tips at either side of the
arc. It has been found that by varying the exposed surface of such
strips, and concomitantly varying the exposed area of acetal resin
material, the arc voltage may be adjusted and maintained at the
desired level. It is speculated that the glass melamine strips
diminish the terminal arc voltage spike by reason of the glass
becoming partially conductive at at least the surface of the strips
when heated by the action of the arc and acting as a resistor in
parallel with the arc to discharge any high-voltage surges.
Applicant's best understanding as to how each of these aspects of
structure, and other more specific aspects to be described,
individually and collectively provide the advantages achieved by
the applicant's invention will be set forth in further detail
following a detailed description of the structure of a particular
embodiment of the invention.
In the drawings;
FIG. 1 is a side elevation view of the arc initiating and
extinguishing portion of an electric circuit breaker constructed in
accordance with the invention, a portion of the side-enclosing
casing being removed to show the interior parts;
FIG. 2 is a sectional view of the arc initiating and extinguishing
means of FIG. 1, taken substantially on the plane indicated by the
line 2--2 of FIG. 1, the arc muffle portion being omitted;
FIG. 3 is an exploded perspective view of the main parts of the arc
initiating and extinguishing structure of FIG. 1;
FIG. 3A is a semischematic representation of a complete circuit
breaker incorporating the invention;
FIG. 4 is a diagrammatic illustration representing undesirable
pressure conditions (cavitation) which it is believed may occur in
a circuit breaker arc chamber, leading to undesirable results;
FIG. 5 is a diagrammatic illustration representing pressure
conditions believed to occur in a circuit breaker arc chamber
constructed in accordance with the present invention and believed
to be desirable;
FIG. 6 is a fragmentary illustration showing a portion of the
arc-extinguishing means of the circuit breaker of FIG. 1;
FIG. 7 is a view similar to FIG. 6 but showing the comparable
portion of the structure of the circuit breaker of the aforesaid
Hurtle and Willard application;
FIG. 8 is a view similar to FIGS. 6 and 7 and showing a portion of
the arc-extinguishing means of another embodiment of the
invention;
FIGS. 9-12 are scale reproductions of oscillographic records of
tests of the illustrated embodiment of the present invention and of
other embodiments of the invention as described in the pertinent
portions of the specification.
In the drawings, the invention is shown as incorporated in an
electric circuit breaker including an arc initiating and
extinguishing assembly (see FIGS. 1-3) comprising a primary
insulating enclosure 10 made up of two cooperating halves 10A and
10B, see FIG. 2, having cooperating recesses therein and openings
therethrough.
Adjoining the outlet end of the arc initiating and extinguishing
assembly enclosure 10 is a muffle assembly 12 for the purpose of
reducing the magnitude of sound and the temperature of the gases
emitted from the arc-extinguisher enclosure 10. The construction
and operation of the muffle assembly 12 is described in copending
application (41D-490), and will therefore not be described in
detail here.
The enclosure 10 has a generally central, generally
hourglass-shaped chamber 14 therein formed by cooperating
conforming shaped recesses in each of the enclosure halves 10A and
10B. The recess 14 is made up of two parts: (1) an arc initiation
chamber 14A, and (2) an arc extinction chamber 14B. The chamber 14
including parts 14A and 14B, has its inner sidewalls provided with
inserts 18 and 20, see FIGS. 2 and 3.
The arc sideplate inserts 18 and 20 are so designed with respect to
the shape and dimensions of the recess 14 that the front edges 18D
and 20D respectively limit movement of these pieces within the
chamber 14 by engagement with the arc tip spacer members 60, which
in turn are held in place by the arc baffle plates 48 and 50,
which, like the succeeding elements are bolted in place by the
bolts 63. Thus the edge surfaces 18E and 20E are spaced away from
the corresponding portions of the enclosure halves 10A and 10B.
Because of this arrangement, no portion of the sideplates 18 and 20
is placed under tension at any time. If, on the other hand, the
conformance of the shape of these insert pieces with the chamber 14
were used to limit the motion of the sideplates 18 and 20 in the
direction of arc movement, the edges 18E and 20E would engage the
corresponding adjacent walls of the chamber 14. The restricted or
"neck" portion of these members would then be placed under tension
by the action of the arc gases acting on the inwardly sloping sides
at this point, resulting in cracking of the pieces at this
point.
The main housing members 10A and 10B may be constructed of any
suitable high-strength insulating material having high heat
resistance, such, for example, as glass-fiber reinforced polymer or
alkyd molding compound or other comparable material. The inserts 18
and 20, however, are constructed of an acetal resin material having
the ablating characteristics described above. Materials which have
been found suitable for this purpose, for example, include high
molecular weight polyoxymethylene materials such as the material
sold under the trademark "Celcon" by American Celanese Corporation,
and that sold under the trademark "Delrin" by the Dupont DeNemours
Company.
Also supported within the chamber 14, and overlying edgewise
portions 18A, 18B, and 20A, 20B of the inserts 18 and 20, are a
pair of angularly shaped elongated conductive straps 22 and 24. The
straps 22 and 24 each include a terminal portions 22A, 24A having
outgoing terminal straps 26 and 28 connected thereto respectively.
The straps 22 and 24 also include inwardly directed or converging
stationary contact support portions 22B and 24B, and outwardly
diverging arc runner portions 22C and 24C. Finally, the straps 22
and 24 also include outer termination or arc tip portions 22D and
24D on which are inserted "arc anchor" inserts 30 and 32 of high
refractory material, such, for example, as tungsten carbide.
Stationary contacts 25 are supported on the portions 22B, 24B of
the straps 22, 24.
The terminal portions 22A and 24A of the straps 22 and 24 are each
shielded from the internal portion 14A of the chamber 14 by means
of upstanding barriers 18C and 20C integral with the acetal resin
inserts 18 and 20 respectively. In addition, the inserts 18 and 20
include shield portions 18S and 20S which shield the otherwise
exposed portions of the bottom wall of the chamber 14A.
Also supported within the portion 14A of the chamber 14 is a
bridging contact member 34 having a pair of movable contacts 36 and
38 fixedly attached thereto. The bridging contact member 34 is
attached by means of a yoke retaining member 40 to a movable
operating rod 42 by which the contacts are operated between open
and closed circuit positions in a manner to be described. The
bridging contact member 34 is attached to the yoke member 40 by
means of a pin 34A extending into a pair of slots 34B in the yoke
member 40. The rod 42 is slidably guided by a low-friction bushing
or insert 44 trapped between the housing sides 10A.
The outer portions of the cooperating enclosure halves 10A and 10B
are cut back and serve to receive and support a baffle assembly
comprising two cooperating halves 46 and 48, (see FIGS. 2 and 3)
each having a number of slots 50 cut therein providing a series of
relatively wide thin exit passages for arc gas from the chamber 14
outwardly into the muffle assembly 12. The baffle assembly 46, 48
is preferably constructed of a suitable high-strength,
high-heat-resistance insulating plastic material such, for example,
as a glass-fiber-filled polymer material such as glass
melamine.
The cutback of the enclosure sides 10A and 10B provides a pair of
shoulders or shelves 52 and 54 against which the baffle blocks 46
and 48 are seated. The slots 50 in the baffle members 46, 48
preferably extend therein to a distance which extends at least to
the edge of the shelf 54. Thus the slots extend across the full
width of the chamber 14 at its outer most portion between the arc
tips 30 and 32.
In accordance with the invention, and for a purpose to be
described, the acetal resin inserts 18 and 20 are terminated short
of the baffle blocks 46, 48, and a pair of strips 58 and 60 are
provided which extend between the tips 22D and 24D of the
conductors 22 and 24, the end thereof being notched as at 60A to
prevent direct contact between these strips and the contact tips 30
and 32, for the purpose of improving the oversurface dielectric
condition. Likewise the arc runners 22 and 24 are terminated short
of the baffle blocks 46 and 48.
The two halves 10A and 10B of the enclosure 10 are provided with a
number of holes 62, by which these parts are securely bolted
together by bolts 63 which are surrounded by suitable insulating
tubing shields 64. Because of the large number and close proximity
of the bolts 62 and the high-strength material used, the chamber 14
is capable of sustaining extremely high pressures therein, which
pressures are estimated to exceed at least 300 pounds per square
inch during the interruption of short-circuit current.
The enclosure halves 10A and 10B, and the baffle blocks 46 and 48
each include elongated semicircular grooves 68 which cooperate to
provide circular holes in which elongated resilient sealing members
70 are inserted and compressed when these parts are in assembled
relation. The sealing members 70 provide a means for sealing
against the passage of gas under high compression from the chamber
14 outwardly through the crack between the members 10A and 10B and
the corresponding portions of the baffle blocks 46 and 48. Other
comparable sealing means, such as silicone plastic compounds which
are plastic when first inserted and later "cure" to a rubbery body
may of course also be used.
The movable contact member 34 is preferably retained in the closed
circuit position by operating mechanism including a biasing or
contact pressure means connected to the movable contact by means
which includes a breakaway-type connection, that is, one which can
be defeated or released. A mechanism of the type described is shown
in semischematic form in FIG. 3A.
As shown in FIG. 3A, the mechanism illustrated includes a generally
cylindrical collar member 90 which is fixedly attached to the
operating rod 42, and which has a peripheral groove 91 therein. A
yoke member 92 is slidably supported on the rod 42, and is
connected to the collar 90 by means of a pair of resilient spring
strip fingers 93, having return-bent end portions seated in the
groove 91. The yoke member 92, and thereby the movable contact
assembly including the collar 90, the rod 42, and the movable
contact member 34, is moved between open and closed circuit
positions by means of a pair of toggle links 94 (only one shown)
each of which has one end pivotally connected by means of a pivot
pin 95 to a releasable cradle member 96 which is supported on a
fixed pivot pin 97 in a suitable casing or support, not shown. The
other ends of the links 94 are connected to the yoke member 92 by a
lost motion connection comprising a slot, not shown, in each of the
links 94 receiving a pin 98 carried by the yoke member 92.
The links 94 are moved between open and closed circuit position
with a snap action by means of operating spring 99 having one end
connected to a pin 100 extending between the operating links and a
manually operable handle member 101, the line of action of the
spring 99 passing across the pivot pin 95 in the process.
The releasable member or cradle 96 is normally held by a
combination armature-latch member 102 which is pivotally supported
at 103 on the aforesaid support or casing. The armature-latch
member 102 has a return-bent end portion 102' which is engaged by
the movable end of a bimetallic strip member 104 which has the
other end rigidly mounted on a combination terminal and support
member 105. The bimetallic strip 104 is further provided with a
magnetic field piece 106 which is rigidly attached thereto and
which serves to attract the armature member 102 upon the occurrence
of predetermined high-current conditions through the bimetallic
strip 104. An outgoing or load terminal member 107 is supported on
the terminal strap 105. The movable end of the bimetallic strip 104
is connected by a flexible connector 108 to the solenoid winding
109. On the occurrence of extremely high short-circuit current
conditions, the solenoid winding 109 causes the armature 110 to be
pulled toward the field piece 111, thereby moving the contacts 34
to an open position. The solenoid winding 109 is connected by means
of a conductor 112 to the contact assembly previously described
which in turn is connected to line terminal 113.
When the armature 110 is attracted by the high-speed solenoid 109,
the yoke member 90 is pulled from between the spring fingers 93,
permitting the contact rod 42 and its associated contact member 34
to move without restraint by the operating spring 99 after this has
occurred. The parts are also maintained in the open circuit
condition by the action of the spring fingers 93 following
interruption of the short-circuit current.
While one particular form of mechanism capable of use in the
invention has been described for illustration purposes, it will be
appreciated that more sophisticated mechanisms may be utilized,
such, for example as that shown in copending application (41D-417),
including operating means capable of generating high contact
pressure, and utilizing breakaway connections capable of
transmitting such higher contact pressures.
Low overload conditions, that is, for example, overloads within the
range of 125 percent-300 percent of the nominal rating of the
circuit breaker, cause deflection of the bimetallic strip 104 after
a predetermined time, moving the movable end of the bimetallic
strip 104 to the left as viewed in FIG. 3, and withdrawing the
armature-latch member from the releasable member 96. This permits
the releasable member 96 to rotate clockwise as viewed, moving the
pivot pin 95 across the line of action of the spring 99, and
reversing the bias of the spring 99 on the links 94, and rotating
these links in clockwise direction, moving the yoke member 92 back
toward open circuit position. Resetting of the mechanism is
accomplished by engagement between a projection on the handle 101,
not shown, and the cradle member 96, by which the cradle 96 is
rotated in counterclockwise direction until the latch end thereof
is once again held by the armature-latch member 102.
On the occurrence of high overloads or low short-circuit current
conditions, such, for example, in the range of 300-500 percent of
the nominal current rating, or such other range as may be selected,
the magnetic member 106 carried by the bimetallic strip 104 is
sufficiently energized to attract the armature-latch member 102,
rotating it clockwise about its pivot pin 103, and releasing the
releasable member 96.
On the occurrence of still higher short-circuit conditions, such as
those above 500 percent of the normal rating of the circuit
breaker, the high-speed solenoid 109 comes into operation. The
high-speed solenoid 109 remains in circuit at all times, but does
not operate at lower currents since the strength of its magnetic
field is not great enough to cause movement of the current-movable
contact assembly 42 against the bias force of the contact-holding
members 93.
Assuming the movable contact member to be in closed condition, a
short-circuit condition causes energizing of the solenoid winding
109 and high-speed movement of the movable contact in the opening
direction as previously described. When the contact pairs 36, 25
and 38, 25 separate, a pair of short, serially related arcs are
created. The pair of short arcs are very quickly elongated and
transformed into a single arc extending directly between the
stationary contacts 25. This longer arc is then further elongated
and moved, by the action of the general current path adjacent to it
and by the repelling action of the conductor portions 26, 28, out
along the arc runners 22, 24. During this action, gases are
generated from the arc chamber sideplates 18 and 20, which maintain
good dielectric or insulating conditions behind the arc and prevent
restriking or retrogressions until the current is completely
extinguished.
ANTICAVITATION STRUCTURE
As mentioned in the introductory portion of the specification, it
has been discovered by extensive testing that the circuit breaker
of the aforesaid Hurtle and Willard application, while performing
in excellent manner at extremely high short-circuit current
conditions, is subject to retrogressions at certain low
short-circuit current conditions which, at least in some instances,
might nevertheless lie above the values which are ordinarily
interrupted by the "magnetic" short-circuit interrupting means of
conventional circuit breakers.
Referring to FIG. 4, there is illustrated schematically a situation
which is believed to be illustrative of that causing retrogressions
at certain intermediate current levels in the aforementioned Hurtle
and Willard application structure. This difficulty is believed to
arise from "cavitation," that is, a region of extremely low
pressure created by movement of gases away from such region at
excessively high velocity as a result of action of the arc. Thus if
A in FIG. 4 is considered to represent a cross section of the arc
chute of a circuit breaker of the type described, the spot P may be
taken as representing a cross section of the arc as initially
formed in the lower part of this chamber. The action of the arc in
acting on the ambient air and upon the material comprising the
sidewalls, causes rapid expansion of the air and generation of
gases by means of volatilizing the material of the sidewalls
resulting in high pressure. This high pressure creates an immediate
movement of gases toward the open end of the arc chamber as
indicated by the arrow D. The pressure wave moves in the direction
of the arrow D as indicated by the successive diagrams B and C of
FIG. 4. The movement of the gases away from the region I where the
arc was initiated, at such high speed, in effect removes so much of
the gas from this region I so quickly that the region I then
becomes a region of extremely low pressure or in other words a
pressure "cavity." Because of this low-pressure region, the
high-pressure gases in the upper part of the arc chamber shortly
thereafter tend to surge backward as indicated in FIGS. C and D of
this figure. This action is believed to be detrimental for a number
of reasons. It will be recalled that one of the desired aspects of
a true current-limiting circuit breaker was stated to be its
ability not only to generate an arc at extremely high speed, but
also to maintain a high-voltage drop condition across this arc
until the current is driven to zero, without erratic action or
retrogressions. For this purpose it is not sufficient merely to
generate an arc having the required voltage drop across it. In
addition, the dielectric condition in the space between all
conductive parts available to the current as an alternate parallel
path must be maintained high enough to prevent the arc from
striking across at such alternate points and therefore
extinguishing in its high-voltage path. The most likely area for
such striking-over to occur is the area in which the arc was first
initiated.
With this in mind, it will now be observed that when the arc is
initially created, a certain amount of metallic material from the
contacts is necessarily vaporized, and such metallic particles or
ions are, in the initial high-pressure gas, generated by the arc
and accordingly, "contaminate" this gas. This initial high-pressure
contaminated gas initially moves away from the arc initiation point
at high speed, leaving the space where the arc was initially
created relatively clean and with good dielectric condition.
At the same time that the contaminated gas is being moved outwardly
as illustrated in FIG. B, the relatively clean, high-dielectric gas
which has been generated from sidewalls of the acetal resin
material has filled the lower portion of the arc chamber I and
created a relatively good dielectric situation at this point. If,
however, cavitation occurs as illustrated in FIG. 4, some of the
contaminated gas in the forefront of the wave is returned and mixed
in turbulent fashion with the less contaminated gas or vapor
generated from the sidewalls of the arc chamber adjacent the arc
initiation point, thereby lowering the dielectric strength in this
region and leading to the creation of arc retrogressions or
striking-over at this point.
In accordance with the invention, the sidewalls of the chamber 14
are modified so that, proceeding from the end of the chamber where
arc initiation occurs at the region I in FIG. 5A, for example, the
sidewalls first converge sharply because of the built-up portions
18M, 20M, providing a narrow passage or constriction 14C.
Thereafter, the walls diverge to a maximum spacing, the last
portion of the sidewalls extending parallel to each other for a
short distance adjacent the spacer piece 60.
It is believed that the illustrated and described construction
prevents cavitation in the following manner. Referring to FIG. 5A
assuming that the arc is initially created in the region I and
creates a high pressure in this region, and thereafter is moved by
magnetic forces outwardly and upwardly as viewed as indicated in
5B, the restriction provided by the converging walls at 14C
prevents the overly rapid release of the pressure from the chamber
I. Accordingly, as the arc moves upwardly in chamber 14, the
differential of the pressure between the upper and lower portions
of the arc chamber is not so great as to cause the turbulent
resurgence of contaminated gas as previously experienced. Since the
pressure at the upper portion of the arc chamber also diminishes
quickly because of the escape of the gases through the open end of
the arc chamber, a more gradual and equally distributed diminishing
of the pressure throughout the chamber occurs. This permits the
uncontaminated gas generated from the acetal resin sidewalls of the
initiation chamber to remain in this portion of the chamber and to
maintain the desirable dielectric conditions here while very high
voltages are being generated across the arc. Thus, for example, in
circuits of 600 volts, an arc voltage of at least 1,200 volts must
be attained by the arc, and voltages of at least 1,250 are commonly
developed by circuit breakers constructed in accordance with the
invention. Moreover, as indicated in the oscillograms reproduced in
FIGS. 9-12, and in particular in FIGS. 10, 11 and 12, this high
voltage is maintained for a substantial period of time while the
current is driven to zero.
Referring to FIG. 9, there is reproduced an oscillographic record
of a short-circuit interruption test performed on a circuit breaker
of the type described herein but having an arc chamber with
straight sidewalls. The curve 80 represents the current in the
circuit while the curve 81 represents the voltage drop across the
arc. It will be observed that under the current and voltage
conditions existing, a series of 10 retrogressions 81A occurred
before interruption was finally completed.
In FIG. 10 there is shown the oscillogram of a circuit interruption
with a circuit breaker having a construction generally similar to
that of the breaker tested in the test illustrated in FIG. 9,
excepting that the sidewalls of the arc chamber were additionally
provided with the described constriction adjacent the arc
initiation portion of the chamber, in accordance with the present
invention. In this figure, the curve 84 represents the current of
the circuit, while 85 represents the arc voltage. As illustrated,
under similar voltage and current conditions, the high voltage
generated by the arc was maintained without retrogressions until
the current was driven to zero. Note also that the interruption was
completed in a substantially shorter time. This means also that the
"let-through" energy was substantially less. This in turn means
that the interrupter suffered less destructive "wear and tear" and
also that other circuit components in the circuit experienced less
stress.
CONTROL OF ARC VOLTAGE
Referring to FIGS. 6 and 7, a portion of the arc-interrupting
structure of the present invention and of the aforesaid Hurtle and
Willard application are shown respectively. As described in the
Hurtle and Willard application, and referring to FIG. 7, the
structure disclosed therein includes a pair of diverging arc
runners 15A', 16A' terminating in arc tips 16A" and 15A", and a
series of spaced metallic arc-cooling plates 68', together with an
arc baffle member of insulating material 67' having slots 67B'. The
longest arc path in this structure, and the path which the arc
occupies just prior to extinction, is that indicated in dotted
lines in FIG. 7, and extends from one of the arc tips 15A" to one
of the end plates 68', thence across the spaces between plates from
one plate to another, to the opposite end plate, and from the
opposite end plate back to the other arc tip 16A". It will be
observed therefore that the total arc column length L.sub.T may be
represented as
L.sub.T = 2L.sub.1 +(N- 1)L.sub.2
where capital N equals the number of plates 68', L.sub.1 represents
the distance from the end arc chute plates to the adjacent arc
runner tips, and L.sub.2 is the distance between adjacent plates.
Since each individual arc has an "anode drop" and a "cathode drop"
which together total about 15 volts, the total voltage drop across
this arc path may be represented as
E.sub.T =15(N+ 1)+eL.sub.T
where small e is the voltage drop along the column of the arc in
volts per inch.
On the other hand, in a structure in accordance with the present
invention as shown for example at FIG. 8, where the arc exists
merely in a straight line directly between the arc tips 22D, 24D,
the voltage drop may be represented as
E= 15+ eL.sub.8
where small e is again the voltage drop of the arc column in volts
per inch, and L.sub.8 is the overall length of the arc as marked in
FIG. 8.
Consideration of these formulas would ordinarily lead to the
conclusion that the voltage drop of the FIG. 8 form (the present
invention) would be less than the voltage drop of the FIG. 7 form
(the Hurtle and Willard application) by an amount equal to 15N
volts. It has been discovered, on the contrary, however, that for
the same total length of arc, (i.e., L.sub.T of FIG. 7 equal L of
FIG. 8) the voltage drop in the case of the FIG. 8 structure is
very much greater than the voltage drop of the FIG. 7 structure.
While the reasons for this are not completely understood, it is
speculated that this may be explained by the situation being such
that the portion of the space between plates which may be
considered "arc column" length is not as great as ordinarily
supposed, perhaps, for example, because of the intrusion into this
space of metallic vapor from the surface of the plates.
Whatever the explanation may be, applicant has discovered that
whereas with a structure as shown in FIG. 7, a total arc length
(not counting the thickness of the plates used) of 21/2inches is
required to provide an arc voltage drop of 1,200 volts as necessary
to "override" line voltage of a 600-volt circuit and thereby
achieve current-limiting action, on the other hand, with a
structure such as shown in FIG. 8, a total arc length of only 11/4
inches is required to do this. It will at once be observed that the
structure of FIG. 8 is very much more compact than that of FIG. 7,
and that space is saved not only in the direction of the arc length
which is reduced by about 60 percent, but also, and equally
important, in the direction at right angles to the arc length,
because of the total omission of the plates 68, of about 32
percent.
It is the uniform teaching of the prior art in connection with arc
interruption in air that the use of spaced metallic plates is
desirable for the purpose of cooling and therefore "deionizing" the
arc to aid in its extinction. To the extent that it may have been
appreciated in the prior art that greater arc voltage drop could be
achieved across a given space by the omission rather than the
inclusion of metallic plates (although this is nowhere suggested,
to applicant's knowledge), it is believed that this was not
feasible in such prior art conventional circuit breaker because the
arc-extinguishing environment in which the arc was created and
contained was not such as to afford a true current-limiting
interruption. Since the other aspects of such interrupters did not
make possible a current-limiting interruption, the creation of such
high-voltage arcs would only lead to to occurrence of
retrogressions with consequent need for dissipation of increased
amounts of energy in the arc, increased degradation of the arc
chamber components, and probably complete failure of the device.
The present applicant, on the other hand, has discovered that if
the dielectric conditions are maintained in the arc chamber such as
by the structure described, substantial advantages are achieved by
the omission of metallic arc plate grids.
A further unsuspected advantage of omitting the metallic arc grid
plates is that their omission tends to render less severe the
terminal voltage spike or surge which occurs upon the extinction of
the arc. It is believed that the explanation for this phenomenon is
as follows. It is well understood that the anode and cathode
voltage drop of an arc in air add up to about 15 volts. Stated in
another way, once an arc through air has been created by means of a
high voltage, if the voltage is thereafter decreased, the arc
cannot continue to exist if the applied voltage drops below 15
volts. If a number of such arcs are connected electrically in
series, such, for example, as may be the case in a circuit breaker
including a number, such as 10, spaced arc grids, the total applied
voltage necessary to sustain a power arc once created therein will
be about 10.times. 15, or 150 volts. It will therefore be
appreciated, that, quite apart from other considerations, as the
voltage across the arc decreases toward zero, the arc in a circuit
breaker having spaced metallic grids will extinguish at a higher
point in the applied voltage wave (and hence at a slightly earlier
point in time) than in a similar construction not including any
such spaced grids. Conversely, final extinction of the arc occurs
at a slightly later point in time in the situation where no
intermediate grids are present. Since the current is decreasing
with time, it can be said that the current will extinguish and the
current will suddenly go to zero from a higher point in the
decreasing current wave when intermediate grids are involved than
in the comparable circuit breaker without such metallic grids.
The terminal voltage spike referred to is caused by the high rate
of change of current or negative di/dt which occurs at the time of
extinction of the arc. Since extinction takes place in
substantially the same short space of time regardless of the
current value just prior to extinction, it will be readily
appreciated that a much higher rate of change of current, (di/dt),
results if extinction occurs at a time when the current is at a
relatively high level.
Stated in another way, in the circuit breaker constructed in
accordance with the present invention, since there is only a single
anode and cathode voltage drop involved, the current is permitted
to decrease to a much lower value before the sustaining voltage
becomes unable to maintain the arc and the current is extinguished
and immediately goes to zero. Accordingly, a much lower di/dt is
involved, and the corresponding terminal voltage spike is much
less.
In accordance with another aspect of the invention, the overall
spacing of the arc runner tips 22D, 24D is not established at the
minimum spacing required to generate the desired arc voltage. On
the contrary, a wider spacing is used, although still substantially
less than the spacing which would be required if metallic plates
were used. The wider spacing is utilized in order to make it
possible for the arc-extinguishing assembly to cooperate in the
most desirable manner with the arc muffle assembly 12. Thus it has
been discovered that optimum muffling action of the sound, shock
wave, and hot arc gases requires a given optimum input opening for
the muffle assembly.
When the arc runner tips 22D, 24D are spaced at the relatively
wider spacing as shown in FIG. 6, however, if no further provision
were made, the resulting arc voltage and its associated terminal
voltage spike would be such as to be undesirable since such a high
voltage spike might cause "high-potting" or "voltage puncturing" at
some point in the circuit. In accordance with the invention,
therefore the relatively wider spacing is made possible and the
terminal voltage spike situation is nevertheless controlled, by the
use of special bridging members 58, 60 which extend between the arc
runner tips 22D, 24D. The bridging members 58, 60 which extend at
each side of the arc runner tips, are constructed of a material
having a substantial portion thereof of glass. Since the presence
of these pieces has been found to bring about correction of the
voltage spike, it is believed that these pieces become temporarily
conductive and thereby dissipate the voltage spike or surge.
FIGS. 9-12 are reproductions of oscillographic records of current
and voltage conditions during each of a series of tests of
current-limiting circuit breakers. FIG. 9 is an oscillographic
record of the current and voltage conditions existing during a
short-circuit test of a current-limiting circuit breaker
constructed in accordance with the aforesaid Hurtle and Willard
application, the line voltage and short-circuit current
availability being 240 volts and 10,000 amperes respectively, at a
closing power factor of 35.degree.. It will be observed that in
this instance a number of arc retrogressions (10 in all) occurred
before the current was finally driven to zero.
Each of the retrogressions appearing in FIG. 9 is evidenced by a
sharp drop in the voltage appearing across the arc. This indicates
that the arc which has once been elongated to a high-voltage
condition restrikes at a previous location and at a shorter length,
is once again lengthened to its high-voltage condition and once
again reignites at the former location, this process being repeated
a number of times. As previously stated, this action is undesirable
because it causes deterioration of the conditions within the arc
chute and is likely to lead to complete failure to interrupt.
It is believed that these retrogressions occurred because the
device was tested at an intermediate short-circuit current
availability range rather than at a high short-circuit current
availability range. Thus it is a unique characteristic of a true
current-limiting circuit breaker that it operates most effectively
on extremely high short-circuit availability currents, and that
within certain limits, the higher the amount of short-circuit
current available, the quicker it is interrupted. Conversely,
current-limiting interruption becomes, at least in some respects,
more difficult at low and intermediate short-circuit current
availability ranges.
FIG. 10 is a reproduction of an oscillogram of a test of a circuit
breaker constructed substantially similar to that tested in the
test illustrated in FIG. 9, with the exception that the sidewalls
of the arc chamber were constructed in accordance with the
"anitcavitation" structure described above, i.e., the arc chamber
was made to taper inwardly sharply adjacent the arc initiation
region and then to gradually widen in the direction in which the
arc moves away from the arc initiation region. The circuit
conditions were substantially the same as those in the test of FIG.
9. It will be observed that in this test, no retrogression
occurred. In addition, the current was interrupted in a much
shorter total time.
FIG. 11 is a reproduction of an oscillogram of a test of a circuit
breaker substantially similar to that tested in the test
illustrated in FIG. 10, with the exception that the arc-cooling
grids were removed. In this figure, the curve 88 represents the
current, and the curve 89, represents the arc voltage. Again, it
will be observed that no retrogressions occurred. However, it will
be noted that in both the FIG. 10 and FIG. 11 tests, the voltage
appearing across the arc rose very sharply just prior to complete
interruption of the current. As previously mentioned, this is
undesirable, since it applies a high voltage throughout the load
circuit and may cause insulation puncturing, flashovers, or other
undesirable phenomena.
In FIG. 12 there is illustrated a reproduction of an oscillogram of
a test of a circuit breaker constructed substantially similar to
that tested in the test illustrated in FIG. 11, but with the
addition of two strips of glass melamine molding material, one
closely adjacent each side of the path extending between the arc
runner tips, that is, pieces similar to the bridging pieces 60
illustrated in the drawings, see FIG. 6 and FIG. 3. In this figure,
the curve 90 represents the current, while the curve 91 represents
arc voltage. It will be observed from this figure that the terminal
voltage spike or surge is drastically reduced and virtually
eliminated. As previously stated, it is believed that the glass
included in these pieces at least on the outer surfaces exposed
directly to the arc becomes partly conductive, and serves as a
resistor to discharge the high voltage which tends to build up
between the arc runner tips.
The glass melamine bridging means 60 are notched as indicated at
60A so that they do not directly contact the ends of the arc
runners 22C, 24C, or the arc contact tip inserts 30, 32,
respectively. This is for the purpose of avoiding a continuous path
between the arc runner tips of material other than acetal resin
material which has been directly exposed to the action of the arc.
It is desirable to avoid such a continuous path in order to
maintain the dielectric conditions between the arc runner tips at a
required level, such, for example as to be able to withstand an
applied test voltage of 2,200 volts for 60 seconds following
short-circuit operation, as required by Underwriters' Laboratories
tests. The arc runner tips are also separated from the baffle
blocks 46, 48 for the same reason.
The arc baffle blocks 46, 48 are preferably constructed of glass
fiber filled melamine resin compound. This material is used, rather
than an acetal resin material, since it has the required strength
to withstand the high shock wave generated by the interrupter, and
since it has been found by test that the use of this material does
not detract from the desired current-limiting performance of the
circuit breaker when constructed in accordance with the present
invention. The casing halves 10A and 10B are preferably constructed
of a high-strength, high-dielectric material, such, for example, as
the glass fiber filled alkyd molded compound. Other materials
having equally good electrical and physical characteristics may of
course be utilized.
While the invention has been described in only specific
embodiments, it will be readily appreciated that numerous
modifications and variations may be made without departing from the
spirit of the invention. It is accordingly intended by the appended
claims to cover all such modifications and variations whether in
substitution of materials or variation of size, dimension, etc. as
fall within the true spirit and scope of the invention.
* * * * *