Note: Descriptions are shown in the official language in which they were submitted.
\ ~
~0485~1~
This invention relates to electric circuit inter-
rupting devices.
B~fore the present invention, a commercially practical
current limiting circuit bre~ker suitable for use in low voltage
power distribution systems of about 600 volts or less had been
sought by the power distribution and control industry for over
tAirty years. Various, sometimes conflicting requirements have
to be met. For example, a commercially practical current
limiting circuit breaker (a) must be repetitively operable at
its maximum short circuit interrupting rating without repair or
replacement of parts (This requirement precludes the use of
fuses, fused switches, or fused circuit breakers for achieving ;;
current limiting.); (b) must not have a temperature rise at the
terminals of more than 50 degrees Centigrade at rated steady
state current ~ meet appropriate standards ~ safety and perform-
ance established for circuit breakers used in power distribution
systemsof 600 volts or less (This requirement precludes the use
of a large built-in resistance to limit current.); (c) must have
a design applicable to a wide range of steady state current
ratings, from a few amperes to hundre~ of amperes; (d) must have ;~
current limiting capabilities competitive with those of the best
available other current limiting devices including fuses ~This
requires that the device will operate in a fraction of a milli-
second when the available short circuit current is 100,000 amperes
or more.); (e) must be compact enought to fit into existing cir-
cuit brea]cer panelboards (This requires that the ratio of inter-
rupting rating to volume be equal to or greater than that for
an~ prior circuit breaker.); (f) must use non-toxic, non-hazardous
materials; (g) must have a response time whic~ decreases propor-
tionately as much as or faster than available short circuitcurrent is increased; (h) must be economically competitive with
present circuit protective devices; and (i) must function without
inducing severe transient voltages. I~'one of the prior current
limiting circuit brea}cers meets all the above requirement~.
10~8~78
According to one aspect of the present invention,
there is provided in an electromagnetically operable circuit
interrupting device comprising a molded case, a pair of
separable contacts in the case, an elongated mova~le contact
arm to carry one of said contacts, between a contact open and
contact closed position, a magnetic core structure in the case :
substantially completely encircling the contacts and encircling
a substantial current carrying portion of said elongated mova~le
contact arm which extends rearwardly of said contacts, said :-
magnetic core including a pair of spaced apart upright magnet
walls, one of said magnet walls being on one side of said
contacts and the other of said magnet walls being on the oppo-
site side of said contacts, said walls extending upright in
the same direction as the direction of contact separation, said
walls being imperforate and unbroken defining a cavity there- - :
between a first unobstructed opening to said cavity defined by
corresponding longitudinal edges of said pair of magnet walls,
said first opening being spaced forwardly of said contacts in :
a direction to provide an outlet toward which an arc and shock
20 waves associated with such arc can move, a second opening to -
said cavity defined by corresponding longitudinal edges at the
opposite end of said pair of magnet walls, said second opening
b-ing spaced rearwardly of said contacts a distance substan-
tially equal to said portion of said elongated movable contact
arm extending rearwardly of said contacts which is encircled
by said magnetic core structure, said magnetic core thereby
including a first forwardly extending cavity portion extending
between said contacts and said~first opening to provide a
transverse magnetic field to coact with an arc for movement -
thereof fro~ said contacts toward said first opening, and a
second rearwardly extending cavity portion extending between
:' ~
- : - 2 -
- 1048578
said contacts and said second opening to provide a trans-
verse magnetic field to coact with said current carrying
portion of said elongated movable contact arm for movement
thereof in a contact opening direction.
BRIEF DESCRIPTIO~ OF THE DRP~WII~GS
Fig. 1 is a longitudinal sectional view of a three-
pole current limiting circuit breaker constructed in accord-
ance with the invention, taken generally along the line 1-1
of Fig. 2 and showing a center pole thereof with parts in
an ON position;
Fig. 2 is a cross sectional view of the current
limiting ciruit breaker of Fig.~ aken generally along
the line 2-2 of Fig. l;
Figs. 3, 4, 5, and 6 are enlarged perspective,
tops, side, and outer end views, respectively, of a line
terminal and stationary contact support assembly of any
one of the poles of the current limiting circuit breaker
of Fig. l;
Fig. 7 is an enlarged end view of an operating
mechanism in the center pole of the current limiting circuit
breaker of Fig. 1, with portions broken away and the parts
being shown in TRIPPED position;
Fig. 8 is a side view of the operating mechanism
of Fis. 7, with portions broken ~w~y;
.
'~
'
- 3 -
~ ' ' '.
.. . . . ~ . , .
1~48~
FIG. 9 is an enlarged cross sectional view through a blade
cross bar of the current limiting circuit breaker of FIG. 1,
taken between two blades;
FIG. 10 is an enlarged cross sectional view similar to FIG.
9, but taken at a blade of the center pole;
FIG. 11 is an enlarged fragmentary longtitudinal view of a
thermally actuated common trip bar and a fragmentary edge view
of an associated thermal trip lever of the current limiting
circuit breaker of FIG. l;
FIG. 12 is an actual size cross sectional view of the therm-
ally actuated common trip bar taken substantially along the line
12-12 of FIG. 11 and a side view of the associated thermal trip
lever;
FIGS. 13, 14, 15, and 16 are perspective, left end, side,
and right end views, respectively, of an assembly of electrical
conductors associated with an electromagnet in a current limit-
ing portion of any one of the poles of the current limiting
circuit breaker of FIG. 1, portions being broken away or omitted
in FIGS. 14, 15, and 16.
FIGS. 17, 18, and 19 are perspective, side, and end views,
respectively, of an electromagnet and contact blade assembly of :~
any one of the poles of the current limiting circuit breaker of
FIG. 1, the electromagnet being associated with the conductor ::
assembly of FIGS. 14-16 and having portions broken away in FIGS.
18 and 19;
FIG. 20 is a plan view of an unfinished current limiting
resistor for any one of the poles of the current limiting cir-
cuit breaker of FIG. 1, the unfinished resistor including end
portions to be cut off after electroplating;
FIG. 21 is a plan view of the end portion of the resistor
within the dotted enclosure 21 of FIG. 20, the broken line por-
tion
- i .. - ~ . . ~ . . -
' ll
1~41~S78
1 in FIG. 21 indicating a portion which is cut away after electro-
2 plating;
3 FIG. 22 is an edge view of the resistor end portion;
4 FIGS. 23, 24, and 25 are perspective, end, and side views,
respectively, of a field magnet assembly of any one of the poles
6 of the current limiting circuit breaker of FIG. 1;
7 FIGS. 26, 27~ 28, and 29 are perspective, top, inner end, and :~
side views, respectively, of an electrical conductor and load
terminal assembly of any one of the poles of the current limiting
circuit breaker of FIG. l;
11 FIGS 30 and 31 are perspective and front views, respectively '
12 of an arc chute adjacent the load terminal assembly o~ any one o~
13 the poles of the current limiting circuit breaker o~ FIG. l;
14 FIG. 32 is a sectional view taken generally along the line
32-32 of FIG. 31;
16 FIG. 33 is a perspective view of one of the arc plates in the
17 arc chute of FIGS. 30-32; and
18 FIG. 34 is a longitudinal sectional view of the current :~
19 limiting circuit breaker of FIG. 1, taken generally along the
line 34-34 of FIG. 2 and showing an outer pole thereof with parts
21 in an ON position.
22 DETAILED DESCRIPTION OF PREFERRED F,MBODIMENT
23 With reference to the drawings, a three-pole current limitinp
24 circuit breaker 40 constructed in accordance with the invention
is shown in FIGS. 1, 2, and 34. The circuit breaker 40 includes
26 a molded case comprising a molded base 41 and a complementary
27 molded cover 42 each having a pair of outer side walls and a pair~ ,f
28 spaced intermediate walls to provide three compartments 44, 45,
29 and 46 (FIG. 2). The structure Or a center pole of the circuit :
3o breaker 40 disposed in the center compartment 45 is shown in
31 FIG. 1.
32 A line terminal and stationary contact assembly 48 is shown
- '
.
. . . . . . . .
t ~ '
. 1~48S78
adjacent the left end of FIG. 1. The assembly 48 is better shown
2 in FIGS. 3-6 and includes a terminal member 49 and a stationary
3 contact mounting member 50 pivotally connected by a pin 51 and
4 electrically interconnected by a braided wire cable 52. The
terminal member 49 has a bight portion 49a and a pair of spaced
6 leg portions 49b and 49c as a first U-shaped portion, the leg
7 portions 49b and 49c merging respectively with a pair of spaced
extending leg portions 49d and 49e of a second U-shaped portion
having a split bight portion formed by two tabs 49f and 49g
extending respectively from the leg portions 49d and 49e. A
11 mounting tab 49h having an aperture 49i extending therethrough
12 pro~ects at right angles from the bight portion 49a oppositely
13 from the leg portions 49b and 49c.
14 The member 50 has a bigh~ portion 50a and a pair of spaced
leg portions 50b and 50c as a first U-shaped portion, the leg
16 portions 50b and 50c respectively having laterally extending ears
17 50d and 50e, a leg portion 50f of a second U-shaped portion
18 extending ~rom the bight portion 50a to a bight portion 50g, and
19 a leg portion 50h opposite the leg portion 50f. The leg portions
49d and 49e and the ears 50d and 50e are apertured to receive the
21 pin 51. The cable 52 has one end welded to the tab 49f, one end
22 welded to the tab 49g, and a central portion welded to the bight
23 portion 50a. A stationary main contact 53 and an arcing contact
24 54 are secured to the leg portion 50h in abutting relationship to
each other. The leg portion 50f is provided with a threaded
26 aperture 50i for receiving a retaining screw 56 (FIG. 1) for a
contact pressure spring 57.
28 An internally threaded sleeve 58 is staked to the mounting
29 tab 49h at the aperture 49i and disposed in an apertured mounting
3o pad portion 41a of the base 41. A screw 59 threaded into the
31 sleeve 48 secures an apertured connector body 60 to the tab 49h.
32 The connector body 60 has suitable wire or cable receiving holes
- 6 -
3~048~;'8
and is provided with an internally threaded hole for receiving
a clamping screw 61.
Similar terminal and stationary contact assemblies 48 are
provided in the outer poles of the compartments 44 and 46.
A blade crossbar 63 extends transversely through the center
compartment 45 into the outer compartments 44 and 46. The in-
termediate walls of the base 41 are slotted to receive the cross-
bar 63, but a bracket 64 (FIGS. 2 and 34) secured to the base 41
; in the compartment 44 and an oppositely formed bracket 55 (FIG.
2) secured to the base 41 in the compartment 46 are provided to
support the crossbar 64 for pivotal movement. Three movable
contact blades 67, one in each pole, are mounted directly in the
crossbar 63, which is made of molded plastic material and pro-
vided with a metallic reinforcing insert 68 (FIGS. 9 and 10).
Tooling recesses such as recess 63a and recess 63b (FIG. 9) are
provided at appropriate places in the crossbar 63 to hold it
while the blades 67 are being assembled therein. Each blade 67
is provided with a pair of shouldered portions 67a and 67b (FIG.
10) which abut the crossbar 63 and from which a tapered mounting
tongue portion 67c extends through the crossbar 63 and through
a retaining plate 69 staked thereto. A movable contact 70 is
provided on each blade 67 for engagement principally with an
associated one of the main stationary contacts 53.
Operating mechanism for the crossbar 63 and blades 67 is
disposed in the center compartment 45 (FIG. 1). P~rtions of the
operating mechanism are best shown in FIGS. 7 and 8, and are
there shown in a TRIPPED position. Two oppositely formed frame
members 72 and 73 (FIGS. 2, 7 and 8) are secured to the base 41
and contoured as at 73a (FIG. 8) which together with brackets 64
and 66 support the crossbar 63 for pivotal movement. The frame
member 73 is provided with an arm portion 73b (FIG. 7) extending
toward the frame member 72 and having a bent mounting ear 73c
at its free end.
- ............. : . , . ~. , :
. . . ~ . ~ .. .. .
~ I
1~48578
1 A pin 74 is mounted adjacent one end in the ear 73c and ad~acent
2 the other end in a corresponding mounting ear (not shown) o~ the
3 frame member 72. A releasably latchable cradle member or trip
4 lever 76 is pivotally mounted ad~acent one end on the pin 74. Two
inner toggle links 78 and 79 are pivotally mounted adjacent their
6 inner ends respectively on opposite end portions of a pin 80
7 mountedin the blade 67 of the center pole. Two outer toggle links
8 82 and 83 are pivotally mounted adjacent their outer ends respec-
9 tively on opposite end portions of a pin 84 mounted in the trip
lever 76. The outer end portions of the links 78 and 79 and the
11 inner end portions of the links 82 and 83 are pivotally inter- :
12 connected by an elongated toggle pin 85, the inner end portions of
13 the links 82 and 83 being offset to straddle the outer end portion
14 of the links 78 and 79.
A handle extension is formed by two handle plate members 86
16 and 87 pivotally mounted respectively on a pair of pins 88 and 89
17 disposed respectively in the frame members 72 and 73. The handle
18 plate members are ~oined by a pair of spring anchoring pins 91 and
19 92 and a reset pin 93. The toggle pin 85 has a pair of spring
hook members 95 and 96 pivotally mounted thereon respectively
21 adjacent opposite ends thereof. A tension spring 97 (FIG. 2) is
22 secured at an outer end to the pin 91 and at an inner end (not :~
23 shown) to the member 95 on one side of the toggle pin 85, and a
24 tension spring 98 (FIG. 7) is secured at an outer end to the pin
92 and at an inner end to the member 95 on the other side of the
26 toggle pin 85. Similarly, a tension spring 99 (FIGS. 2 and 8) is
27 secured at an outer end to the pin 91 and at an inner end to the
28 member 96 on one side of the toggle pin 85, and a tension spring
29 100 (FIGS. 7 and 8) is secured at an outer end to the pin 92 and
3o at an inner end (not shown) to the member 96 on the other side of
31 the toggle pin 85. The springs 97, 98J 99 andlO0 maintain the
32 toggle pin 85 in open-slotted inner ends of the outer toggle links
33 82 and 83.
- 8 -
.
1~41~S7~
A retaining clip 102 is secured to the arm portion 73b of
the frame member 73 and to the corresponding arm portion (not
shown) of the frame member 72 for holding shock absorbing mat-
erial 103 (FIG. 8) for the blade 67 of the center pole at the
end of the opening movement.
The frame member 73 includes an arm portion 73d (FIG. 8)
having a semicircular recess 73e therein. The frame member 72
is similarly formed. A thermally actuated common trip bar 105
(FIGS. 1, 2, 11, 12, and 34) is pivotally mounted in the frame
member 73 at the recess 73e and in the frame member 72 at a
similar recess in an arm portion 72d (FIG. 2) of the frame member
72. A generally L-shaped thermal trip lever 106 best shown in
FIG. 12 is provided with an aperture 106a by which it is pivotal-
ly mounted on a pin 107 (FIGS. 1,2,7 and 8) having opposite end
portions disposed respectively in the frame members 72 and 73.
A pin 108 (FIG. 8) having an enlarged head portion 108a (FIG.7)
is received in an aperture 106b (FIG. 12) of the thermal trip
lever 106 and mounted in the frame member 72 to limit pivotal
movement of the trip lever 106 and prevent movement thereof
axially along the pin 107. -
A molded plastic operating handle 110 (FIGS. 1 and 2) ex-
tends through an aperture in the cover 42 and is recessed in an
enlarged inner end portion to receive the pins 91 and 92 and the
outer end portions of the handle plate members 86 and 87.
Each of the outer two poles is provided with a pin 107 (FIGS.
2 and 34~ identical to the pin 107 of the center pole but having
one end portion mounted in an appropriate groove in an outer wall
portion of the base 41 and an opposite end portion mounted in a
respective one of the brackets 64 and 66. Each of the three pins
107 has a magnetic core holder 111 and an armature plate 112
- pivotally mounted thereon. Each holder 111 carries a generally
U shaped magnetic core 113 having opposite leg portions secured
- ~ : : , ~
.: . ~ ' ' ~ ' :
57~
respectively to spaced opposite side portions of the holder. As
viewed in FIGS. 1 and 34, each side portion (only one being
visible) of the holder 111 is generally in the shape of an
inverted "Y" having one leg pivotally mounted on the pin 107
and the other leg connected to a corresponding leg of the other
side portion by a rear plate portion having an air gap adjusting
screw 115 threaded therein. Each screw 115 extends through a
rear wall portion of the base 41 and has a compression spring
116 mounted thereon. Turning of a screw 115 adjusts the air gap
between the free ends of the leg portions of the respective
magnetic core 113 and armature plate 112.
Each of the armature plates 112 is provided with a pair of
opposite bent-over ears 112a, each ear 112a being spaced inwardly
of a side portion of the respective magnetic core holder 111 and
having an opening aligned with that of the opposite ear for re-
ceiving the respective pin 107. As shown in FIG. 2, a right-hand
side portion of each holder 111 is outwardly offset at the por-
tion mounted on the respective pin 107, and a right-hand ear of
each of the armature plates 112 is similarly offset so as to be~
hidden by the holder 111. Thus, only the left-hand ear 112a of
each armature plate 112 is visible in FIG. 2. A free end portion
of each armature plate 112 is secured by a pair of rivets 117
(FIG. 2) to a common trip bar 118 of molded plastic extending
through the center compartment 45 into the outer compartments 44
and 46. The armature plate 112 of the center pole is apertured
to receive a free end portion of the trip lever 76 and thereby
releasably latch the trip lever, as shown in FIG. 1. The three
armature plates 112 and the common trip bar 118 are biased to-
ward latching position for the trip lever 76 by a pair of tension
springs 120 (FIGS. 2 and 34) disposed respectively in the two
outer compartments 44 and 46 and each secured at one end to a re-
spective armature plate 112 and at the other end to a respective
--10--
,
. .
~ 48S7~
one of the brackets 64 and 66.
Each of the compartments 44, 45 and 46 has a barrier plate
122 (FIGS. 1 and 34) mounted i~ appropriate grooves in the walls
of the base 41 and extending into the cover 42 adjacent the com-
mon trip bar 105 on the opposite side thereof from the respective
armature plate 112. As best shown in FIGS. 11 and 12, a latch
plate 123 is secured to the common trip bar 105 by a rivet 124.
The thermal trip lever 106 is provided with a hole 106c and a
lanced portion 106d, the hole being partly in the lanced portion.
A tension spring 125 (FIGS. 1 and 2) is anchored at one end on
the lanced portion 106d at the hole 106c and at the other end on
the barrier plate 122 in the center compartment 45 to bias the
thermal trip lever 106 clockwise about the pin 107, the barrier
plates 122 being omitted in FIG. 2. Each barrier plate 122 is
provided with a hook portion 122a (FIGS. 1 and 34), and in each
of the two outer compartments 44 and 46 a compression spring 126
(FIG. 34) is seated at one end on the respective hook portion
122a and at the other end on an appropriate portion of the com-
mon trip bar 105 to bias the trip bar 105 and the latch plate
123 mounted thereon toward latching position with respect to
the thermal trip lever 106.
A push-to-trip button 128 (FIG. 34), more completely shown
and described in U.S. Patent No. 3,895,205, issued July 15, 1975,
and assigned to the assignee of this application, has a compres-
sion spring 129 mounted thereon to normally maintain the button
flush with the surface of the cover 42 and is engageable with
the common trip bar 118 upon being pushed inwardly to move the
armature plate 112 of the center compartment 45 clockwise in
FIG. 1 toward unlatching position with respect to the trip
lever 76.
Each of the compartments 44, 45, and 46 is provided with a
laminated field magnet assembly comprising a plurality of gen-
,
..
1~485~8
erally O-shaped plates 130 and a plurality of generally U-shaped
plates 131 disposed around the respective stationary contacts
53-54 and movable contacts 70 of each pole. The field magnet
assemblies are coated with an arc extinguishing material such
as one of those disclosed in Canadian Patent 1,014,725, issued
October 2, 1977, and assigned to the assignee of this application.
Further, each compartment is provided with an arc chute 133
including a plurality of metal arc plates 134 best shown in FIG.
33 and a pair of venting plates 135 and 136 formed of insulat-
ing material.
The arcextinguishing material referred to above is used to
coat other elements of this invention hereinafter described.
Its function is to help create a medium in the arc chamber which
brings about a rapid rate of dielectric strength recovery of the
gap. A suitable material by way of example is a filler of be-
tween 40% to 56% by weight of hydrated zinc borate in a dimethyl
silicone resin. A more complete description of such arc ex-
tinguishing material, and additional examples, are set forth in
the Canadian Patent referred to above and hereinafter by its
Patent No. 1,014,725.
In each compartment, the movable contact blade 67 is con-
; nected by a flexible braided cable 138 (FIGS. 1 and 34) to one
leg of generally U-shaped conductor 139 secured at a bight por-
tion to the base 41 by a pair of screws 140 and 141. The other
leg of conductor 139 is secured to a conductor 142 secured to
the base 41 by a screw 143 and extending between the leg portions
of the U-shaped magnetic core 113 and along the barrier plate 122.
A generally L-shaped bimetallic strip 145 is secured at one end
to the bight portion of the U-shaped conductor 139. The common
trip bar 105 is provided with three actuating legs 105a, one in
each of the compartments 44, 45, and 46, only the center actuat-
ing leg 105a disposed in the compartment 45 being shown in FIGS.
11 and 12.
` ~U48578
The free end portion of the bimetallic strip 145 in each compart-
ment is engageable with the respective actuating leg 105a, and
upon sustained ~oderate overload current flow in the conductor `
139, the bimetallic strip 145 is heated sufficiently to pivot
the actuating leg 105a counterclockwise as viewed in FIGS. 1 and
34, the high expansion side of the bimetallic strip being on the
inside of the L-shape. The thermal trip lever 106 in the center ~ -~
compartment 45 is thereby released from the latch plate 123 on
- the thermally actuated common trip bar 105 and strikes the com-
mon trip bar 118 under the influence of the tension spring 125
to pivot the armature plates 112 about their respective pins 107
clockwise as viewed in FIGS. 1 and 34. The trip lever 76 in the
center compartment 45 is thereby released to effect opening move-
ment of the three movable contact blades 67. If a fault current
I higher than the moderate overload current flows through any of
the conductors 142, the respective magnet 113 attracts its as-
sociated armature plate 112 and all three of the armature plates
112 are pivoted clockwise to release the trip lever 76 and open
the contact blades 67. Pushing the button 128 also pivots the
common trip bar 118 and the three armature plates 112 clockwise
to release the trip lever 76 and open the contact blades 67.
The end of each conductor 142 opposite the end secured by
the screw 143 is connected by a screw 146 (FIGS. 1, 2, and 34)
to a flatwise L-shaped strap portion 148a of a box-like conductor
148 best shown in FIGS. 13-16. The conductor 148 includes the ;
strap portion 148a, an end portion 148b, a pair of spaced side
portions 148c and 148d, and a split end portion including a tab
portion 148e extending from the side portion 148c and a tab por-
tion 148f extending from the side portion 148d. The side portions
are generally square, except that the side portion 148c includes
a mounting tab 148g extending toward the base 41 when assembled.
In each of the compartments 44, 45, and 46, a conductor 150
-l3
~4~8
includes an edgewise L-shaped portion 150a secured at an end of
a longer leg thereof to a tab extending from a shorter leg of
the strap portion 148a and joined at an end of a shorter leg
thereof to an end of a strap portion 150b having an opposite end
secured to the tab portions 148e and 148f. A flexible braided
cable 151 is secured at one end to the conductor 150 and at the
other end to auxiliary contact means for current limiting, in-
cluding a movable contact blade 152 (FIGS . 1 and 34) having a
contact 153 mounted thereon. The blade 152 cooperates with
another movable contact blade 154 having a contact 155 mounted
thereon.
In each of the compartments 44, 45, and 46, the mechanism
by which the blades 152 and 154 are operated is best shown in
FIGS. 17-19. A generally U-shap~d laminated magnetic core 156
is disposed in an outer portion of the box-like conductor 148
(FIGS. 1 and 34) with a pair of spaced leg portions 156a and 156b
thereof ( FIG. 17) stradling the strap portion 148a and a pair of
oppositely extending shoulder portions 156c and 156d thereof (FIG.
17) respectively engaging the side portions 148c and 148d (FIG.
13). A generally U-shaped laminated armature 158 (FIGS. 17-19)
is disposed in an inner portion of the box-like conductor 148
(FIGS. 1 and 34) with a pair of spaced relatively short leq por-
tions 158a and 158b thereof ~ FIGS. 17 and 19) disposed respec-
tively opposite and in spaced relationship to the leg portions
156a and 156b. An armature pin support plate 160 is disposed
between the leg portions 158a and 158b. The armature 158 is
provided with a hole disposed centrally of a bight portion
thereof and aligned with a hole in the support plate 160 for
receiving an outer threaded stud portion of an armature pin 161
having a nut 162 threaded thereon to secure an inner, enlarged
; shouldered portion of the pin 161 against an inner side of the
armature 158. The armature pin 160 is provided with a pair of
opposed flats at its inner end and two spaced links 163 and 164
- ' ~
,
1t)4~7~
are pivotally mounted thereon by a pin 165. The links 163 and
164 carry a pin 166 engaged in a notch in an edge of the blade
152 facing the blade 154 and a pin 167 normally engaged with an
edge of the blade 154 facing the blade 152. The blade 152 is
pivotally mounted on a pin 168 received in a hole 169 (FIG. 17)
and the blade 154 is pivotally mounted on a pin 170 received in
a hole 171. The pivot pins 168 and 170 are disposed on opposite
sides of the armature pin 161 and opposite end portions thereof
are received respectively in a pair of molded inner casing por-
tions 173 and 174 (FIG. 19) secured together by a plurality of
rivets 175. A compression spring 176 disposed in the casing por-
tions 173 and 174 encircles the armature pin 161 and bears on
the blade 152 to urge it clockwise in FIG. 18 toward closed posi-
tion. The blade 152 bears on the pin 165 and causes the pin 167
to bear on the blade 154 to urge it counter-clockwise in FIG. 18
toward closed position. The spring 176 is also a return spring
for the armature 158 and armature pin 161. A shield 177 (FIGS.
18 and 19) having a forked end portion straddling the links 163
and 164 is disposed between the blades 152 and 154 and mainly
within the casing portions 173 and 174. Appropriate openings
are provided in the casing formed by the casing portions 173 and
174 for the armature pin 161, the cable 151, the contact blades
152 and 154, and a flexible braided cable 178 secured to the
blade 154. The sides 148c and 148d of the box-like conductor
148 respectively engage the casing portions 173 and 174, and the
mounting tab 148g (FIGS. 15 and 16) is disposed between a pair
of bosses on the casing portion 173, one sl~ch boss 173a being
shown in FIG. 19. The contact end portions of the blades 152
and 154 are disposed outwardly of the casing 173-174 and a piece
of shock absorbing material 180 (FIG. 18) is mounted in the
casing adjacent the blade 152 to cushion opening movement thereof.
A magnetic core structure 181 generally in the form of a
-~ 1048S78
1 rectangular tube surrounds the contact end portions of the blades
2 152 and 154 extending outwardly of the casing 173-174. The
3 magnetic core structure 181 is best shown in FIGS. 23-25 and com-
4 prises two identical, generally L-shaped, laminated magnetic cores
5 ¦ 182 and 183 arranged as shown with an end of a long leg portion
6 182a of the core 182 abutting an inner side of a short leg portion
183b of the core 183 and an end of a long leg portion 183a of
8 ¦ the core 183 abutting an inner side of a short leg portion 182a of
9 I the core 182. Each of the cores 182 and 183 is coated with an
10 ¦ arc extinguishing material such as disclosed in the aforesaid
11 ¦ CanadianPatent 1,014,725, and additional pieces of such
12 ¦ material are adhesively secured respectively to inner sides of
13 ¦ the L-shaped assemblies as shown in FIGS. 23 and 24. Alternativel Y/
14 the cores 182 and 183 could be generally U-shaped, C-shaped or
J-shaped.
16 In each of the compartments 44, 45, and 46, the cable 178
17 connected to the blade 154 is electrically connected at an
18 opposite end to one end of a terminal strap 184 best shown in
19 FIGS. 26-29 and having a terminal member 186 secured to an opposit ,
end. The terminal member 186 is similar to the terminal member
21 49 and has a bight portion 186a and a pair of spaced leg portions
22 186b and 186c as a first U-shaped portion, the leg portions 186b
23 and 186c merging at right angles respectively with a pair of
24 spaced leg portions 186d and 186e of a second U-shaped portion
25 having a split bight portion formed by two tabs 186f and 186g
26 extending respectively from the leg portions 186d and 186e. The
27 tabs 186f and 186g are secured to the terminal strap 184. A
28 mounting tab 186h having an aperture 186i extending therethrough
29 projects at right angles from the bight portion 186a oppositely
30 from the leg portions 186b and 186c.
31 An internally threaded sleeve 58 ~FIGS. 1 and 34) identical
32 to those staked to the tabs 49h is staked to the mounting tab 186h
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1~48S78
1 of each of the terminal members 186 at the aperture 186i therein
2 and disposed in an apertured mounting pad portion 41b of the base
3 41. A screw 59 threaded into the sleeve 58 secures an apertured
4 connector body 60 to the tab 186h. The connector body 60 is
identical to those secured to the tabs 49h and is provided with
6 an internally threaded hole for receiving a clamping screw 61. -
7 In each of the compartments 44, 45, and 46, a conductor 188
8 (FIGS. 13-16) has a tab 188a secured to the end of the strap por-
9 tion 150b adjacent the tabs 148e and 148f, a strap portion 188b
10 (FIGS. 1 and 34) extending between the leg portions 156a and 156b
11 of the magnetic core 156, an offsetting portion 188c extending
12 generally parallel to the tab 188a, and a strap portion 188d
13 extending through the magnetic core assembly 181 formed by the two
14 L-shaped magnetic cores 182 and 183 along the inner side of the
short leg portion 182b. A strip 189 of arc extinguishing material
16 such as disclosed in the aforementioned Canadian Patent, No.
17 1,014,725, is adhesively secured to the side of the strap
18 portion 188d facing the contact blade 152. A conductor 190
includes a tab portion 190a secured to an end of the strap portion
188d and extending and bent from a strap portion 190b. The strap
21 portion 190b extends parallel to an end face of the magnetic core
22 182 and is joined at right angles to a strap portion 190c extend-
23 ing somewhat diagonally across the outer side of the long leg
24 portion 182a. The strap portion 190c is joined at right angles
to a strap portion 190d extending along a rear wall of the base
26 41 and having an apertured offset connecting tab portion 190e
27 disposed in a hole extending through the rear wall of the base 41.
28 An internally threaded fastener 191 is secured to the connecting
29 tab portion l9Oe.
Opposite the compartments 44, 45, and 46, the rear wall of
31 the base 41 is pr~vided on the rear side with three shallow
32 recesses 44a, 45a, and 46a (FIG. 2) each having a resistor 192
l - 17 -
.
11
1048~i78
~;
. potted therein with potting material 193,pre~erably a ceramic
2 compound having properties of good thermal conductivity, such as
3 alumina or silica based ceramics. A thin plastic cover 194 is
-4 recessed in the base 41 and adhesively secured in place to cover
the potting material in all three of the recesses 44a, 45a, and
6 46a. The resistor 192 in each recess is made of material having
7 a positive temperature coefficient of resistance, is preferably
8 chromium-plated substantially pure iron wire, and is best shown
9 in FIGS. 20-22. An important feature of the resistor 192 is that
its resistance is transformable from a relatively low value to a
11 relatively much higher value, Other materials which have a posi-
12 tive temperature coefficient of resistance and can be used for the
resistor 192 in place of substantially pure iron include tungsten,
14 nickel, cobalt, and alloys or metallic compounds of these and
other elements such as cobalt-iron and zirconium diboride. In
16 these materials, the resistance is a direct function of tempera-
' 17 ture.
18 As shown in FIG. 20, the resistor 192 terminates at each end
19 in a flattened, generally P-shaped portion which includes a
straight portion of length "X" to which an electrode is attached
21 for electroplating in a solution containing chromium. After
22 electroplating, the electrode terminal portions, as shown in
23 broken lines for one o~ the end portions in FIG. 21, are cut off,
i 24 and the remainder of the flattened end is aligned with the plane
j 25 containing the axis of the circular wire, as shown in FIG. 22.
-26 In each of the recesses 44a, 45a, and 46a, a screw 195 (FIGS.
27 1 and 34) secures an end portion 192a of the respective resistor
28 192 (FIG. 20), modified as described above, to the tab portion
29 192e (FIG. 13) of the conductor 190. A screw 196 secures an
3 opposite end portion 192b, modified as described, to an apertured
31 connecting tab portion 197a of a conductor 197 (FIGS. 26-29). An
,~ 32 internally threaded fastener 198 is secured to the connecting tab
- 18 -
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~048~78
portion 197a. The conductor 197 includes a strap portion 197b ex-
tending at right angles to the connecting tab portion 197a along
an end of the short leg portion 183b of the magnetic core 183 and
joined at right angles to a strap portion 197c extending along an
end face of the core 1~3. A bent tab 197d extending from the
strap portion 197c is secured to a conductor 199 having a strap
portion l99a extending through the magnetic core structure 181 a-
long the inner side of the short leg portion 183b of the magnetic
core 183. An offsetting portion l99b joins th~e strap portion ~
l99a to a tab portion l99c secured to the terminal strap 184 and -
having the cable 178 secured thereto~ A strip 200 of arc extin-
guishing material such as disclosed in the aforesaid Canadian
Patent No. 1,014,725, issued October 2, 1977, is adhesively
` secured to the side of the strap portion l99a facing the contact blade 154.
In each of the compartments 44,45, and 46, an arc chute 202
(FIGS.l and 34) for the contacts 153 and 155 is disposed adjacent
the magnetic core structure 181. The arc chute 202 is best shown
~ in FIGS. 30-32 and includes a pair of molded casing portions 203i.' 20 and 204 secured together by a plurality of rivets 205. Each of
the casing portions 203 and 204 is provided with a pair of re-
cesses on a side thereof facing the other casing portion, such as
~ an inner recess 203a and an outer recess 203b (FIG. 32~ in the
'~, casing portion 203, to provide a pair of passageways through thearc chute 202. Each of the casing portions is grooved on a wall
of each recess facing the other casing portion and each groove
has one of the arching plates 134, best shown in FIG.33, adhesive-
~i` ly secured therein. A venting plate 206 is adhesively secured to
l .
the casing portions 203 and 204 and is disposed in the base 41
~ 30 rearwardly of the respective connector body 60. A venting plate
; 207 is adhesively secured to the casing portions 203 and 204 andis disposed in the cover 42 of the assembled circuit breaker 40
forwardly of the respective connector body 60. From the contact side
_l9_
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:
of the arc chute, the arc plates 134 in the inner recess 203a
slant toward the rear wall of the base 41, and those in the outer
3 recess 203b slant toward the front wall of the cover 42. The arc
. 4 plates 134 in the casing portion 204 slant in a similar fashion,
but as best shown in FIG. 31, they are staggered with respect to
;; 6 those in the casing portion 203.
7 In each of the compartments 44, 45, and 46, when the contacts
. 8 153 and 155 are closed, part of the current from the conductor 142
.~ flows through the L-shaped portion 150a of the conductor 150 to
, 10 the cable 151 and the remainder flows by way of the strap portion
11 148a through the box-like conductor 148 and the strap portion 150b
12 of the conductor 150 to the cable 151. From the cable 151 the
13 total or recombined current flows through the contact blade 152,
14 contacts 153 and 155, contact blade 154, cable 178, and the
.~ 15 terminal strap 184 to the terminal member 186
. 16 The strap portion 148a and the magnetic core 156 in each
17 compartment form an electromagnet. Upon flow of a fault current
: 18 through the strap portion 148a greater than that at which the
.~ 19 magnetic core 113 attracts the armature plate 112, the magnetic
core 156 attracts the armature 158 along with the plate 160,
21 armature pin 161, nut 162, links 163 and 164, and pins 165, 166,
22 and 167. The pin 166 pivots the blade 152 about the pin 168
~ 23 toward an open position, and the pin 170 releases the blade 154 so
,~ 24 that it is free to pivot about the pin 170 toward an open position
25 under the influence of a repulsion force between the two blades du~
26 to the current path through the blades. The blades 152 and 154 ar~
. 27 also moved apart by magnetic forces induced by the current flow
28 therethrough, it being noted that they constitute partial conductol ,
29 turns for the magnetic core structure 181. The contacts 153 and
: 30 155 are thus separated to switch the current path through the
31 resistor 192.
~- The parallel circuits between conductor 142 and cable 151,
- 20-
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.. . . ~ . :
~V485~
comprising a circuit through conductor 150a in parallel with the
circuit through conductors 148a, 148, and 150b, provides by-pass
means for sufficient current to prevent opening the current limit-
ing contacts 153 and 155 until a threshold fault current above a -
~selected magnitude is present for magnetic core 156 to attract
armature 158 which opens contacts 153 and 155. sy way of example,
this circuit arrangement and electromagnet characteristics may be
adapted to prevent separation of the limiting contacts 153 and
155 below a threshold of 1,000 amps.
When the contacts 153 and 155 are separated, part of the cur-
. ~
rent from the conductor 142 flows through the L-shaped portion
150a and also through the strap portion 150b of the conductor 150
to the conductor 188, and the remainder flows by way of the strap
portion 148a through the box-like conductor 148 to the conductor
;~ 188. The recombined current then flows through the conductors 188
and l90,through the resistor 192, through the conductors 197 and
, 199, and through the terminal strap 184 to the terminal member 186.
The current limiter contacts preferably do not operate in the
i thermal overload range but only at relatively higher ranges of
fault current or short circuit conditions. Within the thermal
overload range, one or more of the bi-metallic strips 145 are
operable to trip the circuit breaker and open the sets of main
i contacts 53 and 54 as previously described. Immediately above the
thermal overload range, fault currents are still relatively low
but are of sufficient magnitude to cause attraction of one or more
,` of the armature plates 112 and open the sets of main contacts 53
..
' and 54 as previously described. Immediately above the thermal
overload range, fault currents are still relatively low but are of
sufficient magnitude to cause attraction of one or more of the
armature plates 112 and open the sets of main contacts 53 and 54
~ .
i as previously described. Such fault current are below the inter-
:
rupting ability of the sets of main contacts 53 and 54. Fault
,. .
1~48578
currents immediately above this range are just sufficient to
;; cause magnetic core 156 to attract armature 158 and pin 161 which
cause limiter contacts 153 and 155 to open. As the current de-
cays, the magnetic forces also decay. The compression spring 176
in urging contacts 153 and 155 to a closed position tends to
dominate over the decaying current causing those contacts to re-
close while a short arc still exists in a small air gap between
them. This action often leads to contact welding. To solve
this problem, an additional or supplemental magnetizing turn 188b
is provided in series with current limiting resistor 192. Thus,
while fault current still flows in resistor 192, magnetic core
156 will be sufficiently energized to attract armature 158 to
hold contacts 153 and 155 apart.
In each compartment, the strap portion 148a is the only ef-
fective conductor turn for the magnetic core 156 when the con-
tacts 153 and 155 are closed, and only part of the current flows
therethrough, the remainder flowing through the by-pass conductor
provided by the L-shaped portion 150a. When the contacts 153
; and 155 are open, the strap portion 188b provides an additional -~
conductor turn, and it carries the total current while the strap
portion 148a is effective as a conductor turn carrying part of
, . ~
the current. The additional conductor turn 188b enables the
blades 152 and 154 to be maintained in an open position with
less current than is required to move them to an open position
; originally. By the time the blades 152 and 154 move back to
closed position under the influence of the spring 176, the fault
current will have been dissipated in the resistor 192 and the
blades 67 will have been opened.
In each of the compartments 44, 45, and 46, the strap por-
tions 188d and l99a are conductor turns for the magnetic core
structure 181. Further, portions of the contact blades 152 and
154 are partial conductor turns for the magnetic core structure
: . .
. ~ ,. . . . .
:,. '- . : ' . . ~ - ``
, 1048S~
181. When the contact blades 152 and 154 are moved to open posi-
tion and an arc 208 forms between the open contacts 153 and 155,
; the magnetic field set up as a result of current flow through
::
the partial conductor turn portions of the contact blades 152
and 154 acts on the arc 208 to force it toward the arc chute 202
with its staggered, slanting arc Plates 134. Once the arc is
interrupted, the current flow shifts to the previously described
path through the resistor 192, and the flow through the conductor
turns 188d and l99a maintains the magnetic field, aids the di-
electric strength recovery of the gap, and thereby guards against
re-ignition. Any re-ignition of the arc would also take place
in a magnetic field, which would force the arc out again.
; The device of this invention is compact enough to fit into
existing circuit breaker panelboards and yet it is capable of
~; repeatedly interrupting currents in excess of 100,000 amperes
~: root-mean-square (RMS) symmetrical. With such currents avail-
::,~,.
able, the arc which forms between the contacts 153 and 155 upon
their opening must be extinguished in about a millisecond or
; less. This is accomplished by the generation of a sustained arc
voltage which reaches the magnitude of the impressed supply volt-
age in about a millisecond or less. The ~tructure used to ac-
complish this result includes the fast operating mechanism for
opening the blades 152 and 154 with their contacts 153 and 155,
the magnetic core structure 181, the coating of the arc chamber
with arc extinguishing material, and the resistor 192 connected
in parallel with the contacts 153 and 155.
The magnetic core structure 181 encloses the contacts 153
- and 155 and a substantial portion of the blades 152 and 154 and
provides a magnetic field with the maximum practical value o
magnetic flux density normal to the blades 152 and 154 and also
normal to the arc. The magnetic field exerts a force on each
. . ~
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:
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48S78
:'
:,
. 1 blade tending to "blow" them apart, and also exerts a force on the
- 2 arc/tending to "blow" the arc out toward the arc chute 202. The
3 ~orce is proportional to the product of the current and the magne-
4 ¦ tic flux density. Since the magnetic flux density is derived from
the current, the force is proportional to the square of the cur-
. 6 rent, and the higher the available current is, the faster the
7 blades open and the faster the arc is blown out The response of
8 the current limiting device is thus proportional to the severity
: 9 of the short circuit. The magnetic core structure 181 and blades
152 and 154 are so arranged that the lines of force in the
11 magnetic field intersect blades 152 and 153, through which current
12 flows in opposite directions, from the direction which will force
13 said blades apart. As viewed in Fig. 1, when current flows in
, 14 the direction from cable 151, forward through contacts 153 and 155
then from the contact end of blade 154 back through blade 154 and
16 out through cable 178, then during such current flow the magnetic
17 flux and lines of force in the transverse magnetic field extend
;~ 18 from leg 183a (Fig. 23) of magnetic core 183 to leg 182a (Fig. 23)
; of magnetic core 182 (Figs. 1 and 23). This arrangement of curren
20 flow through blades 152 and 154, and magnetic flux across said
21 blades tends to force blades 152 and 154 apart.
22 Furthermore, when blades 152 and 154 separate and an arc 2~ ,
23 forms between contacts 153 and 155, current flows through said
24 arc from contact 153 to contact 155. The transverse magnetic
25 field, with lines of force from leg 183a to leg 182a, acting on
26 such arc with current flow as described, will therefore "blow" the
27 arc forward toward arc plates 134. This "blowing" action effec-
. 28 tively increases the arc length and resistance and therefore arc
voltage, consequently limiting the current as well as extinguish-
~: 30 ing the arc. The magnetic field also aids the rate of dielectric
31 strength recovery of the gap across contacts 153 and 155 followin
32 arc extinction and the subsequent continued rise of the impressed
- 24 -
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'
1048S78
voltage across the gap after current trans~er~ It should also
be noted that by increasing arc voltage the transverse magnetic
field has the. effect of increasing the power factor of the cir-
cuit by inserting resistance into the essentially inductive
short circuit thereby reducin~ the lag of current behind voltage~
The power factor is increased almost to unity.
, Blades 152 and 154 are elongated and pivotally mounted at
. respective points 151 and 170, which provides leverage effect to
i increase speed and resistance at the contact ends thereof when
;r? 10 actuated by magnetic core 156~ Thus, when core 156 is energized
to raise axmature pin 161 a given distance within a given time,
the contact ends of blades 152 and 154 and xespective contacts
153 and 155, will move apart a greate~ distance within a shorter
time than the corresponding displacement and rate of speed of
,~ armature pin 161.
t,,
: The contact blades 152 and 154, and contacts 153 and 155,
~;
are shaped and dimensioned to provide structures of relatively
... low mass and minimum inertia to respond quickly and open rapidly
,
when the electromagnet is energized~ -
,~ 20 The contact blades 152 and 154 are constructed, dimensioned
and mounted with respect to the actuating electromagnet (magnetic
core 156, armature 158) to provide a gap on the order of one-
quarter inch within one sixteenth cycle of current flow or about
0.001 seconds (within one millisecond).
The electromagnetic means (magnetic core 156, axmature 158,
pin 161, and connecting links), the f.ield magnet structure 181,
blades 152 and 154, and the particular way in which they are
positioned and associated as described, serve to open the cur-
rent limiting contacts 153 and 155 in about 0.0002 seconds (.2
of a millisecond) from initiation of a fault current in the cir-
cuit above the threshold selected for operation of the current
limiting section, or within one-eightieth cycle of current flow~
.,
.;,
:
1048578
Under conditions of high available short circuit currents,
the limiter contacts 153 and 155 are open in as little a time as
.2 milliseconds (one-eightieth of a cycle~ from current initia-
tion. As the contacts open an arc is formed between them. The
arc between the limiting contacts is ordinarily extinguished
within one millisecond by the structure and mechanism of this
invention. It should be borne in mi~d that the mechanism describ-
ed responds with the square of the magnitude of fault current so
the larger the fault current, the faster the current limiting
; 10 response. This accelerating responsiveness includes not only
.'
the speed of contact separation, but the effective responsive-
ness of the transverse magnetic field ~enerated by field magnet
structure 181 on the arc formed between contacts 153 and 155
which raises the arc voltage almost instantaneously to equal the
voltage of the source by the means described (essentially by
lengthening the arc through faster and greater contact separa-
tion plus bowing forwardly, plus cooling, all of which increase
resistance of the arc and arc volta~e~ ~hen the arc voltage
equals the supply voltage, current can no longer continue to
rise and is forced to transfer completely into the current
limiting resistor 192 where its energy is dissipated.
The main breaker contacts 53 and 70 open within 0.004 se-
`conds of fault current initiation) or within 1/4 cycle of cur-
rent flow at 60 cycles per second by which time the fault cur-
rent has been fully shunted into current limiting resistor 192
;`~and its energy dissipated~ The main contacts 53 and 70 being
opened, current has ceased to flow in the protected circuit in
less than 1/4 cycle or less than 4 milliseconds after appearance
of the fault current above the threshold selected for the limit-
ing section of the circuit breaker to become operable.
The effective current limiting responsiveness of the follow-
ing combination, (1) speed of contact separation plus (2
- æc- ~ -
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~ . . . - . , : . :
, .. .. . .
` ` i~l48S78
increasing arc voltage to equal source voltage, occurs within
' about a millisecond or less by means of the invention described
herein. This is important because symmetrical short circuit cur- '
'', rents have their maximum growth rate during the first millisecond
`; immediately following current zero~ The current limiting means
in accordance with this invention intercepts the short circuit
~ current before it achieves a significant growth following cur-
`' rent zero and shunts it into limiting resistance 192 having a
positive temperature coefficient of resistance~
The mechanism as described can be mounted in compact cases
to fit in standard panelboards. The compactness may be measured
in terms of the ratio of short circuit amperes of interrupting
rating to circuit breaker volume~ The table below provides a
^ reasonable illustration of the volumetric efficiency of short
' circuit interruption of the subject breaker. The volume of five
representative circuit breakers is given in the second column
, and the interrupting rating shown in column 3~ The first circuit
' breaker in the following table is the subject matter of this
application.
(1) (2) (3) (4)
' Breaker Maximum
Breaker Volume Interrupting Volumetric
Ampere Cubic Inches Rating,480V, Efficiency
Rating* (Typica'l Brkr.) 30,K-Amps rms KVA/in.3
: . .
- Instant Inven. 100 138 100-200** 347-694 ~-
, Representative ( 100 85 25 142
', Circuit ( 225 131 35 128
Breakers ( 400 273 35 61.5
for (1000 569 35 29.5
comparison (2500 1994 85 20
'~ * This is the steady state current rating, all
breakers listed are molded case circuit breakers.
~, ** The 100 KA ratin~ is an established but not a
maximum figure.
~ o2, ~--
,, .
1048578
1 An additional feature of this invention which aids in fitting
2 a mechanism of high interrupting capacity within a circuit breaker
of minimum volume, ar~ plates 134 positioned forward of limiting
4 contacts 153, 155 and blades 152, 154. One of the current limitin
5 features of this invention is the rapid increase of arc voltage to
6 equal source voltage. However, when high arc energy is applied
7 to the air slab in the arc chamber, the air temperature rises very
8 rapidly which creates shock waves and large pressure gradients
which must be dissipated. The devices which have attempted to
limit current by generating high arc voltage have accordingly been
11 bulky. They have had to include a large volume chamber in which
12 to dissipate the shock waves and pressure gradients created by
13 this means of current limiting. The invention herein combines
14 arc voltage increase with other current limiting means, so the
degrees of shock waves and pressure gradients are substantially
16 less than in those devices which rely on the arc voltage means
17 alone. Furthermore, plates 134 are particularly shaped, dimension
18 ed and mounted as described above with respect to the arc, its
19 path of movement, plus the direction of shock waves and air
pressure gradients created, to intercept and effectively dis-
21 sipate such forces without requiring a relatively large volume
22 chamber.
23 The arc extinguishing material which coats the magnetic core
24 structure 181 and lines the inside of the rectangular tube formed
thereby and the inner sides of the conductor turns 188d and l99a
26 to a large extend determines the rate of dielectric strength
27 recovery across the contacts during and immediately following arc
28 extinction. The dielectric strength recovery is essential to the
29 current limiting process and is further aided by the magne~ic
field. The arc extinguishing material is selected in accordance
31 with the disclosure of the above mentioned Canadian Patent No.
321,014,725.
- 28 -
' ~''' ' . .'.~ ~,. ~'. , ,
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. ,
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`:1~48578
1 The resistor 192 should have a positively transformable
2 resistance, capable of changing from an extremely low value to a
?':` 3 much higher value after the arc across the contacts 153 and 155
4 is extinguished and the total current is forced to flow through
.the resistor and bypass the contacts. The transformation of the
resistance increases the circuit power factor, aids interruption,
and limits the "through" i2t (product of the square of the current
8 and the time) factor of the short circuit.
9 Various modifications may be made in the structure shown and
-10 described without departing from the spirit of the invention and
11 scope of the attached claims.
12
13
14
~,''/, 19
21
22
23
24
26
27
28
29
3l
32
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