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Патент USA US3070738

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Dec. 25, 1962
w. H. EDMUNDs
» 3,070,728
SELECTIVE TRIP ARRANGEMENT UTILIZING CIRCUIT BREAKERS
WHICH ARE COORDINATED WITH CURRENT LIMITING FUSES
Filed Aug. 17s,' 1957
1045'
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2 sheets-sheet 1
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Dec. 25, 1962
w. H. EDMuNDs
3,070,728
sELEcTIvE TRIP ARRANGEMENT UTILIZING CIRCUIT BREAKERS
WHICH ARE cooRDINATED WITH CURRENT LIMITING FUSES
Filed Aug. 16, 1957
2 Sheets-Sheet 2
.IL-15.
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BY MEW/M
WQG%V
Unîœa States Paten-t» Ó frICC
3,070,728
Patented Dec. 25, 1962
2
g 1
vBecause of this operation, the total size of any circuit
interrupter, in view of the limitation of thermal and mag
netic stress, may be appreciably smaller than would a
corresponding circuit breaker which has to withstand the
available short circuit current of a particular system.
Therefore, if a circuit interrupting device of the type utiliz
ing current limiting fuses could be applied to a selective
trip arrangement for a plurality of series connected cir«
3 070 728
sELEcrrvE _TRIP AieRÁNGEMENr UrrLIzlNG
CIRCUÃT BREAKERS WHICH ARE COORDI
NATED WiTH CURRENT LIMITING FUSES
William Harold E_dmunds, Havertown, Pa., as_signor to
'I-TLE Circuit Breaker Company, Philadelphia, Pa., a
" corporation of Pennsylvania
'` Filed Aug. 16, 1957, Ser. No. 678,649
6 Claims. (Cl. 317-26)
cuit breakers, the current flow during a severe short cir->
10 cuit condition in any part of the system is limited to the
' My invention relatesto a selective. trip arrangement and
is more specifically related to the. use of circuit breakers
let-through current of the associated circuit interrupter.`-
Hence thermal and magnetic stresses in all of the circuits
between the faulted circuit and the source will be decreased
in view of the low let-through current as compared to the
selective `trip arrangement wherein the cross-_over point`
between the current limiting fuse and one of the trip ele 15 available short circuit current which normally would
iiow.
mentsof the circuit breaker is below the interrupting ca
The principle of my novel invention is to decrease they
pacity of the circuit breaker.
cross-over point of a circuit interrupting device utilizing
Selective trip> arrangements such asy the arrangement set
current limiting fuses to a point which is below the in-.
forth in UnitedStates Patent No. 2,704,311 to Thumim
and` assigned to the, assignee of the instant invention op 20 terrupting rating of the circuit breaker whereby the cur
rent limiting fuse operates before the trip elements of the
erate so that a plurality of circuit breakers may be _con
circuit interrupter next closer to the sources. It is to be
n_ected inl series wherein each of the circuit breakers is
pointed out that this constructionis diametrically opposite
capable lof interrupting any fault in its locality so that
to the presently used circuit breakers utilizing current
service isinterrupted only in that part ofthe system where
limiting devices where the cross-over point is appreciably`
the fault occurs while leaving the remainder of the system
which are, coordinated with current limiting fuses 1n a
in service.
,
.
higher than the interrupting rating of the circuit breaker,
the current limiting elements being operated when the
_,-Until the present time, it has not been possible to
rate of change of current during afault indicates that a
utilize circuit breakers which are coordinated with cur
severe fault current will be attained.
_
rent limiting fuses as the circuit interrupting device-in a
selective trip arrangement'because the operation of the 30 In the instant invention, however, I have found that
by reducing the cross-over point as above stated, I can>
trip elements of` an interrupting device closer to> the
now utilize current interrupting devices having current
source may.` closely coincide with the operation of the-trip
limiting‘means associated therewith in a trip arrangement
elements of the circuit breakers closer to the load respon
wherein' thermal and magnetic stresses throughout the sys
sive to a fault on the circuit closer to the load since -the
cross-over point at which the current limiting device of the 35
latter unit is at a point which is above the interrupting
tem'- are appreciably reduced.
rating or abilityv of its respective unit. This may be un
derstood with reference to my U.S. Patent No. 2,843,702.
issued July 15, 1958 entitled Protective Device With Cur
rent limiting devices in the selective trip arrangement of
i
'
While the circuit breakers which cooperate with cur
my invention can be provided with both thermal and mag-
netic trip elements (or overload and instantaneousv trip
rent Limiting Means and my copending applications Serial 40 elements respectively of any desired type) it is to be noted v
that the magnetic or instantaneous trip elements may be
‘ No. 461,378 iiled October ll, 1954 entitled Circuit Break
er Having Electrical and Mechanical Coordination With
a.jCurrent Limiting Device, `now Patent No. 3,032,631,
and Serial No. 622,042 tiled November 14, 1956 entitled
End Mounting of a Current Limiting Device Associated
With a Circuit Breaker, now Patent'No. 2,924,688 respec
tively, each of which are assigned to the assignee of the
instant invention.
` `
As set forth therein, circuit breakers which are co
ordinated with current limiting'fuses comprise a standard
circuit breaker which could have either a thermal trip
element or both a thermal and a magnetic trip element
which operate to trip the breaker on overload or relative»
omitted ‘with'the thermal trip element operating for all
conditions between rated load andthe cross-over point.
In. this case, and with the low cross-over point,` a
more active bi-metal may be utilized which would be lessy
sensitive to ambient conditions and also give better cali
bration characteristics. This is true since the bi-metal
will not nowkhave to withstand high short circuit cur
rents. _By way` of example, if a bi~metal only has>- to
withstand two thousand or three thousand amperes in
short circuit current, after which the duty of the breaker
is taken over by a current limiting fuse, then the bi-metal can have a reduced cross section to thereby make itmore
1y low fault conditions respectively. A current limiting
active than it would be if it had to withstand ten thou
fuse is then >connected in series with each phase of the 55 sand or tifteen thousand amperes in short circuit current. ‘
normal circuit breaker device wherein the current limit
Accordingly, a primary object of my invention is toy
ing fuse operates responsive to relatively severe short ciré
provide _a novel _selective trip system.
»
cuit conditions. The point .at which the current limiting
Another
object
of
my
invention
is
to
provide
a selec~
fuse takes over operation from the thermal trip element,
or the magnetic trip element when it is provided, is 60 tive trip system utilizing circuit breakers which are co
ordinated with current limiting devices.
known as'the “cross-over point” of the interrupter. It is
~ A further object of my 'invention is to provide circuit
to be noted that this cross-over point is usually beyond
breakers utilizing current limiting fuses in a selective trip
the capacity or interrupting ability of the circuit breaker.
system wherein the cross-over point of the circuit in
However, the current limiting fuse operates when the
rate of rise of current indicates that a subsequent severe
fault current will ñow. Thus the current limiting fuse
terrupting devices is substantially reduced from that which
is normally used.
'
`Still another object of my invention is to provide a
applications wherein the let~through current is substan
circuit breaker utilizing current limiting devices in a selec
tially lower thanthey available short circuit current of the
tive trip system wherein the cross-over point of the cir
System..
`
'
70 cuit interrupting devices are reduced to prevent interfer
begins to limit the current as set forth in the above noted
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4
v
ence between adjacent' series connected circuit interrupt
mean that at current values such as at three thousand
ing devices and the selective trip system.
cross-ove-r point of a combined circuit breaker and cur
amperes, both the instantaneous trip elements of circuit
breakers 106 and 104 would be substantially equally
affected whereby both of the breakers would be operated.
rent interrupting device to below the interrupting rating of
the circuit breaker and to utilize the adapted circuit in
terrupting device in a selective trip system.
ing circuit breaker 106, the operation of feeder breaker
104 would unnecessarily take the branches including
A still further object of my invention is to reduce the
Hence, because of a fault condition in the branch includ
These and other objects ofmy invention will become
breakers 108 and 110 out of service along with the
apparent from the following description taken in connec
branch including breaker 106.
tion with the drawings in which:
10
In accordance with my novel invention, however, I
FIGURE 1 shows a schematic diagram of an electrical
have moved the cross-over point to such a value that
network which may utilize combined circuit breakers and
there is no intersection between the current limiting de
current interrupting devices in accordance with' my in
vice characteristic and the thermal time delay and instan
taneous characteristics of adjacent breakers, and I assure
vention.
FIGURE 2 shows the tripping characteristics of the
that a fault in a branch circuit wlil not cause operation
of a breaker in a feeder circuit since the current limiting
device in the branch circuit is operated. Such a selective
current interrupting devices of FIGURE 1 on a current
time scale.
FIGURE 3 shows a' cross-sectional view through one
trip arrangement will provide lower magneticand thermal
phase of a typical molded case circuit breaker which is
stresses due to the current limiting effect of the current
coordinated with a current limiting device.
20 limiting device not only in the branch including the fault
FIGURE 1 shows a typical three-stage system which
but also in all lthe series connected devices and conductors
-consists of a main circuit breaker 100 which could be a
in the system.
generator breaker or a transformer breaker, feeder `circuit
Thus, where the feeder breakers 102 and 104 and the
breakers 102 and 104 and the branch circuit breakers such
m-ai'n breaker 100 are of the type utilizing mechanical
as circuit breakers 106, 108 and 110. Each of the circuit
timers which allow removal’ of the f-aulted breakers with
breakers 106, 108 and 110 may be of the type se-t forth in
out affecting breakers closer to the source, the reduced
FIGURE 3 while the main breaker 100` and- feeder
let-through current ofthe operating circuit breaker will
breakers 102 and 10‘4 could be either of this type or of
not subject the circuit breakers to as severe a condition
the normal or standard circuit breaker type. Clearly, any
of thermal :and magnetic stress as would the available
type of combined circuitbreake'r` and current limiting de 30 short circuit current of the faulted> circuit.
l
vice'could be used in FIGURE 1 so long as they are con
If desired, the branch circuit breakers 106, 108 and
structed accordingY to my invention.
110 need not' have both the thermal time delay _and in
stantaneo'us' trip units as indicated in FIGUREl 2, but
FIGURE 2 shows the characteristics- of the main
could operate with> only the thermal time delay units.
breaker, feeder breakers' and'branch breakers'when each
are ofthe type'which are coordinated with' current limit
Furthermore, lsince the cross-over point of the current
ing devices and adjusted in accordance withV my' _novel
limiting fuse is now lower, the bi-metal may be made
invention' whereby their cross-over> points are lsubstan
tially' reduced so that the current limiting‘brea'kers closer
to/ the load, such` as- breaker 106,v prevents interference
between the thermal or instantaneous tripping of adjacent
breakers (su‘ch'as feeder braker 104).
more' .active- since- it will now withstand -a lower short
circuitl current. Thus the cross-sectional area of thel bi
metal may be' madeV smaller vso that it will be less sensi
40 tive to ambient conditions and will also give better cali
ì ,
bration characteristics.
In a similar manner, the current'limiting device trip
ping means of breaker 104 is" so reduced as to prevent
interference with the thermal or instantaneous tripping
ofthe main breaker 100'_by the feedery breaker' 104.
,_
Inoperation, FIGURE 2‘shows that if a fault appears
,
FIGURE 3- shows for illustrative purposes only a cross
sectional view of one phase of one type` of circuit inter
rupting device comprising -a combined circuit breaker
45
and current limiting device (fuse 40)` which ycould be
utilized as the interrupters of> FIGURES 1' and 2. For
yfurther details of this structure, reference is made to my
on aA branch containing circuit’ interrupter 106 of the
copending application Serial No. 622,042 filed November
order of two hundred amperes, the circuit breaker will>
14, 1956.
trip in‘ approximately thirty-five seconds. If this fault
is increased to six-hundred amperes, then the tripping will 50
As seen in FIGURE- 3,_a tulip-clip type contact 62 is
occur in 019 cycle. Thus during overloads of a relativelyelectrically connected to contact 36 and is fastened within
housing 21 in anyl desired manner. Clearly, a set of
low value, the'thermal time delay of the'circuit breaker
of circuit interrupter 106 will cause the circuit to be
opened.v If, on the otherrhand, arelatively low fault
current flows in the branch containing breaker 106, then
the instantaneous or magnetic portion of the tripvunit
will cause the breaker to be opened.
If., however, the'
disconnects similar to disconnect 62 will be provided' for
the disconnects of each phase.
.
,
The circuit breaker 20v is provided with a rsimilar set
of disconnects such as disconnectl contact' 63, whe-rein
disconnects 62 and 63 cooperate for plug-in engagement
with s-tuds 64 and 65 respectively of mounting blocks 66
fault current through the breaker is of a relatively high
magnitude such as'approximately fourteen hundred am
and 67 which may be fastened' within a switchboard. In
peres` (which is above the reduced cross-over point of the' 00 a similar manner, each of the other phases will be pro
vided with similar sets of disconnects for cooperating with
circuit breaker) then the current limiting fuse will be
corresponding mounting blocks.
_
brought into operation to operate circuit breaker 106
within approximately 0.5 cycle.
As an alternative FIGURE 3 shows that axstraight
In a similar manner, circuit breakers 104 and 100
terminal 68 could` be brought out as seen in dotted lines,
will operate in the event of a fault in their respective cir 85 where connection to the current interrupting device is
made at leadconnecting means 69.
cuits wherein `the overload trip element ~will operate on
relatively small overloads, the instantaneous trip element
The current path of the phase shown in FIGURE 3
will operate on relatively low fault currents, while .the
Iincludes stud 65, circuit breaker tulip clip 63, a stationary
current Álimiter device will operate on potentially high
contact 70, movable‘contact of Contact arm 71, a pigtail
short circuit currents.
,
l
70 (not shown) which connects contact arm 71 to conductor
It is to be noted that if the current interrupting device
72, heater 73, coil 74, conductor 75, tulip clip 33, `stud
106 were constructed in they usual manner, that the cross-l
42, fuse 40, stud 41, tulip clip 36, tulip clip 62, and
over point would‘be considerably further to the right
in- lthe embodimentof FIGURE 2- andbeyond the inter
rupting rating of the breaker. This, however, would
stud 64.
,
y
`
Coil 74 has a pivotally mounted armature 76 and a
plunger 77 associated therewith, these components corn
3,070,728
5
6
prising the magnetic trip unit. Heater 73 has bi-metal
first magnitude; the other end of said first circuit being
78 and adjusting screw 79 associated therewith, these
connected to a source of power; the other ends of said
components comprising the thermal trip unit.
plurality of second circuits being corrected in series with
A tripper bar 80 is then seen in FIGURE 3 as having
member 81 fastened thereto which carries adjusting screw
respective loads to be energized from said source of
power; the current limiting device of each of said first
79 in cooperating alignment with bimetal 78 and is itself
circuit interrupters being adapted, upon operation there
of, to effect complete circuit interruption of its asso
aligned with plunger 77.
Member 81 has one end thereof protruding into slot
ciated first circuit independent of its associated series
82 of trip rod 83 and is rotatable in a counterclockwise
connected circuit breaker.
direction within slot 82.
2. A selective trip system comprising a first circuit
10
The operating mechanism 84 has a latch member 85
having one end thereof connected in series with one end
connected thereto and constructed to be latched by tripper
of a plurality of second circuits; the current carrying
bar S0. When tripper bar 80 is rotated in a counterclock
capacity of said first circuit being substantially the sum
wise direction, however, latch member 85 will be un
of the current carrying capacities of said second circuits;
latched to allow mechanism 84 to disengage the circuit 15 said first circuit having a first circuit interrupter asso
breaker cooperating contacts vof each phase since mecha
ciated therewith; said first circuit interrupter being adapt
nisni 84 is constructed to be common to each phase.
In the event of a prolonged overload, heater 73 will
cause bimetal 78 to defiect into engagement with screw
79 to cause counterclockwise rotation of tripper bar 80
rent of a first magnitude; each of said second ‘circuits
ed to operate upon the occurrence of a short circuit cur
having respective second circuit interrupters associated
therewith; each of said second circuit interrupters com
prising a combined circuit breaker and current limiting
device; each of said circuit breakers including a thermal
and a magnetic trip means; said circuit breaker portion
of each of said second circuit interrupters comprising a
and tripping of the circuit breaker contacts.
Under relatively low fault conditions, coil 74 will create
a strong magnetic field to cause armature 76 to drive
plunger 77 into engagement with member 81, thus rotat
ing tripper bar 80 counterclockwise to effect instantane
ous tripping of the circuit breaker contacts.
trip unit including thermal and magnetic trip means for
opening their respective circuit breakers in response to
t
FIGURE 3 further shows the striker pin 85 of fuse 40
(as set forth in the above noted copending application
Serial No. 622,042.) as being positioned in registry with
prolonged overload and relatively low short circuit con
ditions respectively before said first circuit interrupter
operates; each of said current limiting devices being
aperture 56 in disconnect 42 and extension 60 of an 30 constructed to have a crossover point with respect to the
auxiliary tripper bar.
trip unit of their respective circuit interrupter which is
below the interrupting rating of their respective circuit
breaker; each of said current limiting devices being con
When fuse 40 is operated (on faults above a predeter
mined value), striker pin 85 is ejected into engagement
with extension 60 to rotate the tripper bar against the
biasing force of spring 61a. This rotation will then re
move extension 61 from tip 85a of trip rod 83 and allow
biasing spring S6 to drive trip rod to the left and' rotate
structed to operate to open their respective circuit in re
sponse to potentially high short circuit currents in their
respective circuits before said -first circuit interrupter op
crates on said potentially high short circuit currents _to
tripper bar 80 counterclockwi'se for tripping the -circuit
breaker contacts.
'
‘
'
.
Thus, upon operation of one of the current limiting
fuses due to a potentially severe fault current such as fuse'
4f), the circuit breakervcontacts will be disengaged to
prevent single phasing and to disconnect the circuit.
Since the fuse operates to limit the let-through cur
rent to a substantially small fraction of the available
short circuit current, the thermal and magnetic stresses
applied to the breaker components is appreciably re
duced.
40
thereby deenergize each `-of “said second circuits; the
magnitudes of said potentially high short circuit currents
in said second circuits being less than» said first magni
tude; the other end of said first circuit being connected
to a source of power; the other ends of said plurality of
second circuits being connected in series with respective
loads to be energized from said source of power; the cur
rent limiting device of each of said first circuit inter--
rupters being adapted, upon operation thereof, to effect
complete circuit interruption of its associated first circuit
independent of its associated series connected circuit
breaker.
ments of my novel invention, many variationsand modi 50
3. A selective trip system comprising a first circuit
ñcations will now be apparent to -those skilled in the
having one end thereof connected in series with one end
art and l prefer therefore to be limited, notby the
of a plurality of second circuits; the current carrying
specific disclosure herein, but only by the appended claims.
capacity of said first circuit being substantially the- sum
l claim:
j
‘
of the current carrying capacities of said second circuits;
~ l. A selective trip system comprising a first circuit 55 said first circuit having a first circuit interrupter assoc
Although I have here described preferred embodi
having one end thereof connected in series with one end
of a plurality of second circuits; the current carrying
capacity of said first circuit being substantially. the sum
ciated therewith; said first circuit interrupter being adapt
ed to operate upon the occurrence of a short circuit
current of a first magnitude; each of said second circuits
of the current carrying capacities of said second cir
having respective second circuit interrupters associatedcuits; said first circuit having a first circuit interrupter 60 therewith; each of said second circuit interrupters com
associated therewith; said first circuit interrupter being
prising a combined circuit breaker and current limiting
adapted to operate upon the occurrence of a short circuit
device; said circuit breaker portion of each of said sec
current of a first magnitude; each of said second circuits
ond circuit interrupters comprising a trip unit including
having respective second circuit interrupters associated
thermal and magnetic trip means for openin-g their re
therewith; each of said second circuit interrupters com 65 spective circuit breakers in response to prolonged over
prising a combined circuit breaker and current limiting
load and relatively low short circuit conditions respec
device; each of said circuit breakers including a thermal
tively before said first circuit interrupter operates; each
and a magnetic .trip means; each of said current limiting
of said current limiting devices being constructed to op
devices being constructed to operate to open their re
spective circuit in response to potentially high short cir 70 erate to open their respective circuit in response to po
tentially high short circuit currents in their respective cir»
cuit currents in their respective circuits before said. first
cuits before said first circuit interrupter operates on said
circuit interrupter operates on said potentially high short
potentially high short circuit currents to thereby de-_
circuit currents to thereby deenergize each of said sec
ond circuits; the magnitudes of said potentially high short
energize each of said second circuits; the magnitudes of
circuit currents in said second circuits being less than said 75 said potentially high short circuit currents in said second
spr/aras
7
circuits being less than said first magnitude; each of
said respective current limiting devices limiting fault
current to a value below the available short circuit cur
rent of their said respective second circuit to limit thermal
and magnetic stress in both their respective second circuit
and in said first circuit; the other end of said first cir
high short circuit currents in their respective circuits be
fore their respective current interrupting means in said
plurality of sound circuits operates on said potentially
high short circuit currents to deenergize each of its re
spective third circuits; the magnitudes of said potentially
of said plurality of second circuits being connected in
series with respective loads to be energized from said
high short circuit currents in said third circuits being less
than said second magnitude; each of said current limit
ing devices of said second circuits being constructed to
operate to open their respective circuits in response to
source `of power; the current limiting device of each of
potentially high short circuit currents in their respective
cuit being connected to a source of power; the other ends
said first circuit interrupters being adapted, upon opera
tion thereof, to effect complete circuit interruption of
its associated first circuit independent of its associated
series connected circuit breaker.
4. A selective trip system comprising a first circuit
circuits before said first circuit interrupter operates to
deenergize each of said second circuits; the magnitudes
of said potentially high short circuit currents in said
second circuits being less than said first magnitude; the
having one end thereof connected in series with one
of power; the other end of said plurality of third cir
cuits being connected in series with respective loads to
be energized from said source of power; the current
limiting device of each of said first circuit interrupters
other end of said first circuit being connected to a source
end of a plurality of second circuits; the other end of
each of said plurality of second circuits being connected
in series with one end of a plurality of third circuits;
the current carrying capacity of said first circuit being 20 being adapted, upon operation thereof, to effect complete
substantially the sum of the current carrying capacities
circuit interruption of its associated first circuit inde
of said third circuits; each of said first circuit and Said
pendent of its associated series connected circuit breaker,
pluralities of second and third circuits having circuit
6. A selective trip system comprising «a first circuit hav
.interrupting means associated therewith; said first cir
ing one end thereof connected in series with one end of a
cuit interrupter being adapted to operate upon the occur
plurality of secondl circuits; the other end of each of said
rence of a short circuit current of a first magnitude; said
plurality of second circuits being connected in series with
interrupters in said second circuits being adapted to
one end of a plurality of third circuits; the current carry
operate upon the occurrence of a short circuit current of
ing capacity of said first circuit being substantially the sum
of the current carrying capacities of said third circuits;
a second magnitude smaller than said first magnitude;
each of said third circuit interrupters comprising a com 30 each of s-aid first circuit and said pluralities of second and
bined circuit breaker and current limiting device; each
third circuits having circuit interrupting means associated
of said circuit breakers including a thermal and a mag
therewith; said first circuit 4interrupter being .adapted to
netic trip means; each of said current limiting devices
operate upon the yoccurrence of a short circuit current
being constructed to operate to open their respective cir
of a first magnitude; said interrupters in said second cir
cuit in response to potentially high short circuit currents
cuits being yadapted -t'o operate upon the occurrence of a
in their respective circuits before their respective current
Short circuit current of a second magnitude smaller than
interrupting means in said plurality of second circuits
saidl first' magnitude; said interrupters in said third circuits
operates on said potentially high short circuit currents
beingl adapted to operate upon the occurrence of a short
tov deenergize each of its respective third circuits; the
circuit current of a third magnitude smaller than said
40
magnitudes of said potentially high short circuit currents
second magnitude; each of said second and third circuit
in said third circuits being less than said second magni
interrupters comprising -a combined circuit breaker and
tude; the other end of said first circuit being connected
current limiting device; each of said circuit breakers in
to a source of power; the other ends of said plurality of
cluding a'thermal and a magnetic trip means; each of said
third circuits being connected in series with respective
current limiting devices of said third circuits being con
loads to be energized from said source of power; the
current limiting device of each of said first circuit inter
rupters being adapted, upon operation thereof, to effect
complete circuit interruption of its associated first circuit
independent of its associated series connected circuit
breaker.
5. A selective trip system comprising a first circuit
having one end thereof connected in series with onev
end of a plurality of second circuits; the other end of
each of said plurality of second circuits being connected
in series with one end of a plurality of third circuits; the
current carrying capacity of said first circuit being sub-i
stantially the sum ofthe current carryingv capacities of said
thirdcircuits; each of said ñrst circuit and said plu
ralities of second and third circuits having circuit in
terrupting means associated therewith; said first circuit
interrupter being adapted to operate upon the occur
rence of a short circuit current of a first magnitude; said
interrupters in said second circuits being adapted to op
erate upon the occurrence of a short circuit current of a
second magnitude smaller than said first magnitude; said
interrupters in said lthird circuits being adapted to Operate
upon the occurrence of a short circuit current of a third
magnitude smaller than said second magnitude; each of
said second' and third circuit interrupters comprising a
combined circuit breaker and current limiting device;
structed to operate to open »their respective circuit in re~
spouse to potentially high short circuit currents in their
respective circuits before their respective current i-nter
rupting means in said plurality of second `circuits operates
on said potentially high short circuit currents to deener
gize each of its respective third circuits; the magnitudes of
said potentialy high short circuit currents in said third
circuits being less than said second magnitude; each of
said current limiting devices of said second circuits being
constructed to operate to lopen their respective circuits in
response l’to potentially high short circuit currents in their
respective circuits before said first circuit interrupter op
erates to deenergize each of said second circuits; the mag
nitudes of said potentially high short circuit currents in
Said second circuits being less than said first magnitude;
the thermal and magnetic stress in said firs-t circuit being
reduced by the current limiting action of said current
limiting devices in said second and third circuits, the
thermal and magnetic stress in said second circuits being
reduced by current limiting action of said current limiting
devices in said third circuits; the ‘other end of said first
circuit being connected to la source of power; the other
ends of said plurality of third circuits being connected in
series with respective loadsV to Ibe energized from said
source of power; t-he current limiting device of each of
of said third circuits being constructed to operate to
.said first circuit interrupters being adapted, upon opera
tion thereof, to effect complete circuit interruption of its
associated first circuit independent of its associated series
connected circuit breaker.
open their respective circuit in response to potentially
(References on following page)
each of said circuit breakers including a thermal and a
magnetic trip means; each of said current limiting devices
3,070,728
References Cited in the file of this patent
UNITED STATES PATENTS
1,479,465
1,919,466
2,140,360
2,439,165
Greenwood ____________ __ Iain. 1, 1924
Jansson ______________ __ July 25, 1933
Jennings ____________ __ Dec. 13, 1938
Graves ______________ __ Apu'. 6, 1948
2,473,196
2,734,111
2,770,757
2,823,338
2,919,328
2,920,241
10
Y
Dannenburg __________ __ June 14, 1949
Kozacka ______________ __ Feb. 7, 1956
Edsall ______________ __ Nov. 13, 1956
Eds'all ______________ __Feb. 11, 1958
Kozaca ______________ __ Dec. 29, 1959
Jacobs ________________ .__ Jan. 5, 1960
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