Note: Descriptions are shown in the official language in which they were submitted.
BACXGROUND OF THE INVENTION
Before the present invention, a commercially practical current
limiting circuit breaker 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 thirty
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 ratingwithout 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 to meet appro-
priate standards af safety and performance established for circuit
.:. .. ..
breakers used in power distribution systems of 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
hundreds 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 millisecond when the available
short circuit current is 100,000 amperes or more.); (e) must be
compact enough to fit into existing circuit breaker panelboards ~ ~
(This requires that the ratio of interrupting rating to volume -
be equal to or greater than that for any prior circuit breaker.~
(f) must use non-toxic, non-hazardous materials; (g) must have a
response time which decreases proportionately as much as or
faster than available short circuit current is increased; (h)
must be economically competitive with present circuit protectiVe
2 -
.~. ~.
\ devices; and (i) must function without inducing severe transient
voltages. None of the prior current limiting circuit breakers
meets all the above requirements.
SUMMARY OF THE INVENTION
An object of the invention is to provide a current limiting
circuit breaker which meets all the above requirements.
Another object is to provide a current limiting circuit
breaker including a pair of main contacts, electromagnetically
and thermally operable tripping means for opening the pair of
main contacts, a pair of auxiliary contacts for current limiting
in series with the pair of main contacts, electromagnetically
operable means for opening the pair of auxiliary contacts, a field .
magnet associated with the pair of auxiliary contacts, and a ~
resistor connected in parallel with the pair of auxiliary contacts,
the resistor having a positive temperature coefficient of
resistance and the parallel circuit through the resistor including
a pair of conductor turns associated with the field magnet. ;
A further object is to provide an improved, fast acting
mechanism for opening the pair of auxiliary contacts of such a -
2Q current limiting circuit breaker.
Still another object is to provide an improved conductor-turn ;
arrangement for the electromagnetically operable means for opening
the pair of auxiliary contacts of such a current limiting circuit ;
breaker.
Yet another object is to provide an improved field magnet
structure for the pair of auxiliary contacts of such a circuit - -
limiting circuit breaker.
A still further object is to provide an improved electro-
magnetically and thermally operable tripping means for the pair
of main contacts of such a current limiting circuit breaker.
.. ..
.
;
Another object is to provide an improved movable contact
blade mounting arrangement for the pair of main contacts of
such a current limiting circuit breaker.
An additional object is to provide a current limiting circuit
breaker having means to rapidly increase the voltage drop across
the arc formed between the auxiliary contacts in the current
limiting section to a value which equals the supply voltage of the
source in substantially less than a quarter cycle and in about
one millisecond of time, thus checking any further rise in current
and almost simultaneously shunting the current through a current
limiting resistor connected in parallel with the current limiting
contacts. This action increases the power factor to near unity
thereby enabling interruption of a potentially high fault current
in less than one-quarter cycle of current.
An additional object is to provide a current limiting circuit
breaker wherein means to rapidly increase arc voltage between
auxiliary contacts to equal the voltage of the source includes
electromagnetic means to rapidly separate and lengthen the gap
between said contacts, first magnet means to simultaneously
produce magnetic lines of force to rapidly move said contacts
apart in divergent directions and to blow the arc between said ;
contacts in a third direction away therefrom, causing an addi-
tional lengthening of the arc and cooling thereof, thus rapidly
increasing arc resistance to raise the arc voltage to that of - -
the source, until saturation said electromagnetic means being -~
operative to increase speed of action proportional to the
increase in value of the square of the through fault current,
and likewise until saturation said field magnet means being
operative to increase the speed of action also with the square
of the increase in value of the through fault current.
- - :
'. . " .
, '' ,~
- . '' :: -
~V~4~
An additional object is to provide a current limiting circuit
breaker including means to prevent opening of the auxiliary con- . .
tacts below a threshold fault current of a selected magnitude.
Other objects and advantages will become apparent when the
following specification is considered along with the accompanying :
drawings. .. :.
BRIEF DESCRIPTION OF THE DRAWINGS .-
FIG. 1 is a longitudinal sectional view of a three-pole: ~ ,
current limiting circuit breaker constructed in accordance 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
circuit breaker of FIG. 1, taken generally along the line 2-2 of ~ .:
FIG. 1; ~
FIGS. 3, 4, 5, and 6 are enlarged perspective, top, 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 FIG. 7,
with portions broken away;
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 longitudinal 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
conductoxs associated with an electromagnet in a current limiting
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 axe 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 circuit .
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 portion
in FIG. 21 indicating a portion which is cut away after electro-
plating,
FIG. 22 is an edge view of the resistor end portion, ~;
FIGS. 23, 24, and 25 are perspective, end, and side views, ~
30 respectively, of a field magnet assembly of any one of the poles : .
of the current limiting circuit breaker of FIG. l;
B `
':
4~
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. I; : :
FIGS. 30 and 31 are perspective and front views, respectively, . : .
of an arc chute adjacent the load terminal assembly of any one of
the poles of the current limiting circuit breaker of FIG. l;
FIG. 32 is a sectional view taken generally along the line :
32-32 of FIG. 31;
FIG. 33 is a perspective view of one of the arc plates in the
arc chute of FIGS. 30-32; and ~ ~.
FIG. 34 is a longitudinal sectional view of the current :
limiting circuit breaker of FIG. 1, taken generally along the - :.
line 34-34 of FIG. 2 and showing an outer pole thereof with parts -: : .
in an ON position.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT :
With reference to the drawings, a three-pole current limiting
circuit breaker 40 constructed in accordance with the invention
is shown in FIGS. 1,2,and 34. The circuit breaker 40 includes
a molded case comprising a molded base 41 and a complementary
molded cover 42 each having a pair of outer side walls and a pair
of spaced intermediate walls to provide three compartments 44, 45,
and 46 (FIG. 2~. The structure of a center pole of the cixcuit
breaker 40 disposed in the center compartment 45 is shown in .
FIG. 1. ~ .
A line terminal and stationary contact assembly 48 is shown
adjacent the left end of FIG. 1. The assembly 48 is better shown :-
in FIGS. 3-6 and includes a terminal member 49 and a stationary -:
contact mounting member 50 pivotally connected by a pin 51 and '~
30 electrically interconnected by a braided wire cable 52. The :;
terminal member 49 has a bight portion 49a and a pair of spaced
leg portions 49b and 49c as a first U-shaped portion, the leg
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
mounting tab 49h having an aperture 49i extending therethrough
projects at right angles from the bight portion 49a oppositely
from the leg portions 49b and 49c.
The member 50 has a bight portion 50a and a pair of spaced
leg portions 50b and 50c as a first U-shaped portion, the leg
portions 50b and 50c respectively having laterally extending ears
50d and 50e, a leg portion 50f of a second U-shaped portion
extending from the bight portion 50a to a bight portion 50g, and
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
pin 51. The cable 52 has one end welded to the tab 49f, one end
welded to the tab 49g, and a central portion welded to the bight
portion 50a. A stationary main contact 53 and an arcing contact
54 are secured to the leg portion 50h in abutting relationship to
each other. The leg portion 50f is provided with a threaded
aperture 50i for receiving a retaining screw 56 (FIG. 1) for a
contact pressure spring 57.
An internally threaded sleeve 58 is staked to the mounting
tab 49h at the aperture 49i and disposed in an apertured mounting
pad portion 41a of the base 41. A screw 59 threaded into the
sleeve 48 secures an apertured connector body 60 to the tab 49h.
The connector body 60 has suitable wire or cable receiving holes
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 tlle compartments 44 and 46.
A blade crossbar 63 extends transversely thr,ough the center
compartment 45 into the outer compartments 44 and 46. The inter-
mediate walls of the base 41 are slotted to receive the crossbar
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 sup-
port 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 provided 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). Portions 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. A pin 74 is mounted adjacent one end
in the ear 73c and adjacent the other end in a corresponding
mounting ear (not shown) of the frame member 72. A releasably
latchable cradle member or trip lever 76 is pivotally mounted
adjacent one end on the pin 74. Two inner toggle links 78 and 79
are pivotally mounted adjacent their inner ends respectively
on opposite end portions of a pin 80 mounted in the blade 67
of the center pole. Two outer toggle links 82 and 83 are
pivotally mounted adjacent their outer ends respectively 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 inner end
portions of the links 82 and 83 are pivotally interconnected
by an elongated toggle pin 85, the inner end portions of the
links 82 and 83 being offset to straddle the outer end portions
of the links 78 and 79.
A handle extension is formed by two handle plate members 86
and 87 pivotally mounted respectively on a pair of pins 88 and ~
89 disposed respectively in the frame members 72 and 73. The ~-
handle plate members are joined by a pair of spring anchoring
pins 91 and 92 and a reset pin 93. The toggle pin 85 has a pair
of spring hook members 95 and 96 pivotally mounted thereon
respectively adjacent opposite ends thereof. A tension spring 97
(FIG. 2~ is secured at an outer end to the pin 91 and at an
inner end (not shown) to the member 95 on one side of the toggle
pin 85, and a 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 toggle pin 85. Similarly, a tension spring 99
(FIGS. 2 and 8) is secured at an outer end to the pin 91 and at ;
an inner end to the member 96 on one side of the toggle pin 85, ~-~
and a tension spring lQ0 (FIGS. 7 and 8~ is secured at an outer
1 0 ''' "' ' ''
''' . ' '
- '
' -:
end to the pin 92 and at an inner end (not shown) to the member
96 on the other side of the toggle pin 85. The springs 97, 98,
99 and 100 maintain the toggle pin 85 in open-slotted inner ends
of the outer toggle links 82 and 83.
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 material 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 pivotally 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 lQ8
2Q (FIG. 8) having an enlarged head portion 108a (FIG. 7~ is received ~
in an aperture 106b (FIG. 12~ of the ther~al tXip lever lQ6 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~ extends ` -
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.
3Q 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
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 invert-
ed "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 bet~een 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 pai~ Of
opposite bent-over ears 112a, each ear 112a being spaced inwaxdly
2Q of a side portion of the respective magnetic core holder 111 ~nd
having an opening aligned with that of the opposite ear for
receiving the respective pin 107. As shown in Fig. 2, a right-
hand side portion of each holder 111 is outwardly offset at the
portion 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
~0 through the center compartment 45 into the outer compartments 44 -
. , - . , .
12
` . ...
. .-~
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 toward
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
respective armature plate 112 and at the other end to a respective
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 in appropriate grooves in the walls
of the base 41 and extending into the cover 42 adjacent the common
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 common
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
1 3 .- ~
- ~: .
.. ..
.~ . . .. , .... ~ .; . . . . . . . . .... - .. .
and described in U.S. Patent No. 3,895,205 dated July 15, 1975
and assigned to the assignee of this application, has a compre-
ssion 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
generally 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 No. 1,014,725
dated August 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 insulating material.
The arc extinguishing material referred to above is used to
coat other elements of this invention hereinafter described. ~ts
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 -
between 40% to 56% by weight of hydrated zinc borate in a dimethyl ~
silicone resin. A more complete description of such arc -
extinguishing material, and additional examples, are set forth
in Canadian Patent No. 1,014,725 referred to above and herein- -
3Q after by its Patent No. 1,014,725.
~ 4
. .
,
;s~
In each compartment, the movable contact blade 67 is connected
by a flexible braided cable 138 (FIGS. 1 and 34) to one leg
of a generally U-shaped conductor 139 secured at a bight portion
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 actuating leg 105a disposed in the compartment 45 being
shown in FIGS. 11 and 12. The free end portion of the bimetallic
strip 145 in each compartment is engageable with the respective
actuating leg 105a, and upon sustained moderate overload
current flow in the conductor 139, the bimetallic strip 145 is
heated sufficiently to pivot the actuating leg 105a counter-
clockwise 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 cOmmQn trip bar 118
under the influence of the tension spring 125 to piVot the
armature plates 112 about their respective pins lQ7 clockwise
as viewed in FIGS. 1 and 34. The trip lever 76 in the center
compartment 45 is thereby released to effect opening moVement of
the three movable contact blades 67. If a fault current higher ;~
than the moderate overload current flows through any of the con-
ductors 142, the respective magnet 113 attracts its associated
1 5
.:
.~ -.
3~
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 x
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 portion 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 "
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
20 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 ~h~
other end to auxiliary contact means for current limiting, i~`
including 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
there~on.
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.
30 17-19. A generally U-shaped laminated magnetic core 156 is ~ `
16
, .. . :
.
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 i -~
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 con-
ductor 148 (FIGS. 1 and 34) with a pair of spaced relatively
short leg portions 158a and 158b thereof (FIGS. 17 and 19)
disposed respectively 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 arma-
ture 158 is provided with a hole disposed centxally 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 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 portions 173 and 174 (FIG. 19)
secured together by a plurality of rivets 175. A compression
'' .: " '
1 7
. . .. ......
spring 176 disposed in the casing portions 173 and 174
encircles the armature pin 161 and bears on the blade 152 ~, -
to urge it clockwise in FIÇ. 18 toward closed position. The
blade 152 bears on the pin 166 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. Appro- ;;
priate 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 such 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
rectangular tube surrounds the contact end portions of the -
blades 152 and 154 extending outwardly of the casing 173-174.
The magnetlc core structure 181 is best shown in FIGS. 23-25
and comprises two identical, generally L-shaped, laminated
magnetic cores 182 and 183 arranged as shown with an end of a
long leg portion 182a of the core 182 abutting an inner side
, , ' ' '. ','
~8 .-
of a short leg portion 183b of the core 183 and an end of a
long leg portion 183a of the core 183 abutting an inner side
of a short leg portion 182a of the core 182. Each of the cores
182 and 183 is coated with an arc extinguishing material such
as disclosed in the aforesaid Patent 1,014,725, and additional
pieces of such material are adhesively secured respectively
to inner sides of the L-shaped assemblies as shown in FIGS. 23
and 24. Alternatively, the cores 182 and 183 could be generally
U-shaped, C-shaped or J-shaped.
In each of the compartments 44, 45, and 46, the cable 178
connected to the blade 154 is electrically connected at an
opposite end to one end of a terminal strap 184 best shown in
FIGS. 26-29 and having a terminal member 186 secured to an
opposite end. The terminal member 186 is similar to the ~,
terminal member 49 and has a bight portion 186a and a pair of
spaced leg portions 186b and 186c as a first U-shaped portion,
the leg portions 186b and 186c merging at xightangles
respectively With a pair of spaced leg portions 186d and 186e
of a second U-shaped portion having a split bight portion
formed by two tabs 186f and 186g extending respectively from
the leg portions 186d and 186e. The tabs 186f and 186g are
secured to the terminal strap 184. A mounting tab 186h having
an aperture 186i extending therethrough projects at right
angles from the bight portion 186a oppositely from the leg
portions 186b and 186c.
An internally threaded sleeve 58 (FIGS. 1 and 34~ identi-
cal to those staked to the tabs 49h is staked to the mounting
tab 186h of each of the terminal members 186 at the aperture
186i therein and disposed in an apertured mounting pad portion
41b of the base 41. A screw 59 threaded into the sleeve 58 ~ ;
.~
~ 9
. .
' ~.
'''' ~'
: . . . . . , . ,. ,. ,.. ... . . - .. . . ... ... ..... ... .. .
secures an apertured connector body 60 to the tab 186h. The con-
nector body 60 is identical to those secured to the tabs 49h and
is provided with an internally threaded hole for receiving a
clamping screw 61.
In each of the compartments 44, 45, and 46, a conductor 188
(FIGS. 13-16) has a tab 188a secured to the end of the strap por-
tion 150b adjacent the tabs 148e and 148f, a strap portion 188b
(FIGS. 1 and 34) extending between the leg portions 156a and 156b
of the magnetic core 156, an offsetting portion 188c extending
.
generally parallel to the tab 188a, and a strap portion 188d ^
extending through the magnetic core assembly 181 formed by the
two L-shaped magnetic cores 182 and 183 along the inner side of ~ -
the short leg portion 182b. A strip 189 of arc extinguishing
material such as disclosed in the aforementioned Patent No.
1,014,725 is adhesively secured to the side of the strap portion
188d facing the contact blade 152. A conductor 190 includes a
tab portion 190a secured to an end of the stxap portion 188d and
extending and bent from a strap portion 19Qb. The strap portion
190b extends parallel to an end face of the magnetic core 182
and is joined at right angles to a strap portion 190c extending
somewhat diagonally across the outer side of the long leg portion
182a. The strap portion 190c is joined at right angles to a
strap portion 190d extending along a rear wall of the base 41
and having an apertured offset connecting tab portion 190e ;
.
disposed in a hole extending th~ough the rear wall of the base 41.
.: .
An internally threaded fastener 191 is secured to the connecting ;
tab portion 190e.
Opposite the compartments 44, 45, and 46, the rear wall of ,
. ..
the base 41 is provided on the rear side with three shallow
recesses 44a, 45a, and 46a (FIG. 2) each having a resistor 192
... ..
2 0 ~ - :
., .: .
'..' . . .':
..-. ::
.- .
potted therein with potting material193, preferably a ceramic
compound having properties of good thermal conductivity, such as
alumina or silica based ceramics. A thin plastic cover 194 is
recessed in the base 41 and adhesively secured in place to cover
the potting material in all three of the recesses 44a, 45a, and
46a. The resistor 192 in each recess is made of material having
a positive temperature coefficient of resistance, is preferably
chromium-plated substantially pure iron wire, and is best shown ;
in FIGS. 20-22. An important feature of the resistor 192 is that
its resistance is transformable from a relatively low value to a
relatively much higher value. Other materials which have a posi-
tive temperature coefficient of resistance and can be used for the
resistor 192 in place of substantially pure iron include tungsten,
nickel, cobalt, and alloys or metallic compounds of these and -
other elements such as cobalt-iron and zirconium diboride. In
these materials, the resistance is a direct function of tempera-
ture.
As shown in FIG. 20, the resistor 192 terminates at each end
in a flattened, generally P-shaped portion which includes a
straight portion of length "X" to which an electrode is attached
for electroplating, the electrode terminal portions, as shown in
broken lines for one of the end portions in FIG. 21, are cut off,
and the remainder of the flattened end is aligned with the plane
containing the axis of the circular wire, as shown in FIG. 22.
In each of the recesses 44a, 45a, and 46a, a screw 195 (FIGS.
1 and 34) secures an end portion 192a of the respective resistor
192 (FIG. 20), modified as described above, to the tab portion
192e (FIG. 13) of the conductor 190. A screw 196 secures an
opposite end portion 192b, modified as described, to an apertured
connecting tab portion 197a of a conductor 197 (FIGS. 26-29~. An
21
.. . . . . . . . . . . .
internally threaded fastener 198 is secured to the connecting tab
portion 197a. The conductor 197 includes a strap portion 197b -
extending 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 183. 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
along the inner side of the short leg portion 183b of the magnetic
core 183. An offsetting portion 199b joins the 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 Patent No.
1,014,725 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. 1 and 34) for the contacts 153 and 155 is dis ~ sed
adjacent the magnetic core structure 181. The arc chute 202 is
best shown in FIGS. 30-32 and includes a pair of molded casing
:, . . ...
20 portions 203 and 204 secured together by a plurality of rivets `
205. Each of the casing portions 203 and 2Q4 is provided with
a pair of recesses 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 passage-
ways through the arc 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 arcing plates 134, best
shown in FIG. 33, adhesively secured therein. A venting plate
206 is adhesively secured to the casing portions 203 and 204
and is disposed in the base 41 rearwardly of the respective
2 2 ~ :-
.
.. . .. ..... .. ;. ;;.. `. ~. -, ` `. ` - . ` . .. ... . ~. , .. - . - . .
4~ :
connector body 60. A venting plate 207 is adhesively secured
to the casing portions 203 and 204 and is disposed in the cover
42 of the assembled circuit breaker 40 forwardly of the respective
connector body 60. From the contact side 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 recess 203b slant toward
the front wall of the cover 42. The arc 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 those in the casing
portion 203.
In each of the compartments 44, 45, and 46, when the
contacts 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 the cable 151 and the remainder flows by way of `
the strap portion 148a through the box-like conductor 148 and ;~
the stxap portion 150b of the conductor 150 to the cable 151.
From the cable 151 the total or recombined current flows through
the contact blade 152, contacts 153 and 155, contact blade 154,
cable 178, and the terminal strap 184 to the terminal member 186.
The strap portion 148a and the magnetic core 156 in each
compartment f~rm an electromagnet. Upon flow of a fault current
through the strap portion 148a greater than that at which the ;;~
magnetic core 113 attracts the armature plate 112, the magnetic
core 156 attracts the armature 158 along with the plate 160,
armature pin 161, nut 162, links 163 and 164, and pins 165, 166, ~-
and 167. The pin 166 pivots the b~ade 152 about the pin 168 ~-
toward an open position, and the pin 170 releases the blade 154
so that it is free to pivot about the pin 170 toward an open -
position under the influence of a repulsion force between the -
two blades due to the current path through the blades. The
23
: .
:: .
blades 152 and 154 are also moved apart by magnetic forces
induced by the current flow therethrough, it being noted that
they constitute partial conductor turns for the magnetic core
structure 181. The contacts 153 and 155 are thus separated to
switch the current path through the resistor 192.
The parallel circuits between conductor 142 and cable 151,
comprising a circuit through conductor 150a in parallel with the
circuit through conductors 148a, 148, and 150b, provides by-pass
means for sufficient cur~ent 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. By 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
current from the conductor 142 flows through the L-shaped portion
150a and also through the strap portion l50b 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 190, 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 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
contacts 53 and 54 as previously described. Immediately above
' 2g :
,
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 as previously described. Such fault currents are below
the interrupting ability of the sets of main contacts 53 and 54.
Fault 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
decays, 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
reclose 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
effective conductor turn for the magnetic core 156 when the
contacts 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 con-
tacts 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
..
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: .:
, , ` ,, .-. . ; ` . .... . -: ..
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
orginally. 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
portions 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
181. When the contact blades 152 and 154 are moved to open
position and an arc 208 forms between the open ocntacts 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 des-
cribed path through the resistor 192, and the flow through the
conductor turns 188d and l99a maintains the magnetic field, aids
the dielectric 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 available,
.
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
2 ~ -
. ~ .
..
. . .
.- , . .. . . .... ... .. . . . . . . .. ..
which reaches the magnitude of the impressed supply voltage in
about a millisecond or less. The structure used to accomplish
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 of
magnetic flux density normal to the blades 152 and 154 and also
normal to the arc. The magnetic field exerts a force on each
blade tending to "blow" them apart, and also exerts a force on ;
the arc 208 tending to "blow" the arc out toward the arc chute
202. The force is proportional to the product of the current
and the magnetic flux density. Since the magnetic flux density
is derived from the current, the force is proportional to the
square of the current, and the higher the available current is,
the faster the blades open and the faster the arc is blown out.
The response of the current limiting device is thus proportional ;
to the severity of the short circuit. The magnetic core structure
181 and blades 152 and 154 are so arranged that the lines of
force in the magnetic field intersect blades 152 and 153, through
which current ~lows in opposite directions, from the direction
which will force said blades apart. As viewed in Fig. 1, when
current flows in the direction from cable 151, forward through
contacts 153 and 155, then from the contact end of blade 154
back through blade 154 and out through cable 178, then during
such current flow the magnetic flux and lines of force in the
transverse magnetic field extend from leg 183a (Fig. 23~ of
magnetic core 183 to leg 182a (Fig. 23) of
2 7
. ~ .
.
.- ..
.. . ...
. . . .. ... , . .. , . . . .. . . .. ... ; ..
.. . . ... - .. ~ . .. . . ... . . . . . . - . ..
magnetic core 182 (Figs. 1 and 23). This arrangement of current
flow through blades 152 and 154, and magnetic flux across said
blades tends to force blades 152 and 154 apart.
Furthermore, when blades 152 and 154 separate and an arc 208
forms between contacts 153 and 155, current flows through said
arc from contact 153 to contact 155. The transverse magnetic
field, with lines of force from leg 183a to leg 182a, acting on
such arc with current flow as described, will therefore "blow"
the arc forward toward arc plates 134. This "blowing" action
effectively increases the arc length and resistance and therefore
arc voltage, consequently limiting the current as well as
extinguishing the arc. The magnetic field also aids the rate of
dielectric strength recovery of the gap across contacts 153 and
155 following arc extinction and the subsequent continued rise
of the impressed 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 circuit by inserting resistance into the essentially
inductive short ciruit thereby reducing 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
increase speed and resistance at the contact ends thereof when
actuated by magnetic core 156. Thus, when core 156 is energized ~-
to raise armature pin 161 a given distance within a given time,
the contact ends of blades 152 and 154 and respective cantacts '
153 and 155, will moVe apart a greater distance within a shorter
time than the corresponding displacement and rate of speed of
armature pin 161. `
The contact blades 152 and 154, and contacts 153 and 155,
-:
~ 8 ~
. . .. .. .. .. , . . . .. ~ . . . .. ~ . " ... - . .. , .~ . . . . . .. . . .. . . ..
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.
The contact blades 152 and 154 are constructed, dimensioned
and mounted ~Jith 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, armature 158,
pin 161, and connecting links), the field magnet structure 181,
blades 152 and 154, and the particular way in which they are
positioned and associated as described, serve to open the current
limiting contacts 153 and 155 in about 0.0002 seconds (.2 of a
millisecond) from initiation of a fault current in the circuit
above the threshold selected for operation of the current
limiting section, or within one-eightieth cycle of current flow.
Under high 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
initiation. 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 mind that the mechanism
described responds with the square of the magnitude of fault
current so the larger the fault current, the faster the current
limiting response. This accelerating responsiveness includes
not only the speed of contact separation, but the effective
responsiVeness of the transverse magnetic field generated by
field magnet structure 181 on the arc formed between contacts ~ `
153 and 155 which raises the arc voltage almost instantaneously
29 ~ ~:
.' .
~`.. "'.'.
:`,.:'..
~ . ~ . . .
to equal the voltage of the source by the means described
(essentially by lengthening the arc through faster and greater
contact separation plus bowing forwardly, plus cooling, all of
which increase resistance of the arc and arc voltage). When 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 seconds
of fault current initiation, or within 1/4 cycle of current flow
at 60 cycles per second by which time the fault current 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 limiting section of -
the circuit breaker to become operable.
The effective current limiting responsiveness of the
following combination, (1) speed of contact separation plus (2)
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
currents have their maximum growth rate during the first milli-
second 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
current zero and shunts it into limiting resistance 192 having
a positive temperature coeffecient 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.
tl) (2) (31 (4)
Breaker M~m
Breaker Volume Interrupting Vblumetric
Ampere Cubic Inches Rating, 480V, Efficiency
Rating* (Typical Brkr.)30,K-Amps rms KV~4in.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 rating is an established butnota m~m figure. `
An additional feature of this invention which aids in fitting
a mechanism of high interrupting capacity within a circuit breaker
of minimum volume, are plates 134 positioned forward of limiting
contacts 153, 155 and ~lades 152, 154. One of the current limiting
features of this invention is the rapid increase of arc voltage - -
to equal source voltage. However, when high arc energy is applied
to the air slab in the arc chamber, the air temperature rises
very rapidly which creates shock waves and large pressure grad-
ients which must be dissipated. The devices which have attempted
to limit current by generating high arc voltage have accordingly
~ 31
.
-, ...... . .. .. . . - ,.. :. . .. .. . ,- , - ., . .. : . . ... ... . . .. . . . :
... . .. ... . . . . . ., ... . . .. ~ . . ... . .. `- . - ` . - ........... . - . -.: ..
, .. . . ... . ~ . . -. ... .. ; .. .. . ; . . . . . . ~ .: ,
4~ ~
been bulky. They have had to include a large volume chamber in
which to dissipate the shock waves and pressure gradients created
by this means of current limiting. The invention herein combines
arc voltage increase with other current limiting means, so the
degrees of shock waves and pressure gradients are substantially
less than in those devices which rely on the arc voltage means
alone. Furthermore, plates 134 are particularly shaped, dimension-
ed and mounted as described above with respect to the arc, its
path of movement, plus the direction of shock waves and air
pressure gradients created, to intercept and effectively dis-
sipate such forces without requiring a relatively large volume -
chamber.
The arc extinguishing material which coats the magnetic core
structure 181 and lines the inside of the rectangular tube formed
thereby and the inner sides of the conductor turns 188d and l99a
to a large extent determines the rate of dielectric strength
recovery across the contacts during and immediately following arc
extinction. The dielectric streng~h recovery is essential to the
current limiting process and is further aided by the magnetic
field. The arc extinguishing material is selected in accordance
with the disclosure of the above mentioned ~atent No. 1,014,725.
The resistor 192 should have a positively transformable
re8istance, capable of changing from an extremely low value to a
much higher value after the arc across the contacts 153 and 155
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" i t ~product of the square of the current
and the time~ factox of the short circuit.
Various modifications may be made in the structure shown and
described without departing from the spirit of the invention and
scope of the attached claims.
32 :
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