Note: Descriptions are shown in the official language in which they were submitted.
CROSS REFERENCE TO RELATED APPLICATION
The invention disclosed in the instant application
is relsted to the following:
(1) U.S. Patent No. 3,991,391 issued November 9,
1976 to John A. Wafer, and
(2) Canadian Patent Application 206,938 filed
August 13, 1974 which is assig~ed to the assignee of
the present application.
BACKGROUND OF THE INVENTION
Field of the Invention:
This invention is related to circuit interrupters
and more particularly to a current limiting circuit inter-
; rupter wherein a movable contact arm is disposed within the
slot o~ a magnetic drive device and a pair of separable con-
tacts are disposed within the opening of an arc driving
yoke.
Description o~ the Prior Art:
It is common in the prior art to provide for
current limiting during circuit iault conditions. A common
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method of providing current limiting is through the use of
current limiting fuses in combination with a standard stored
energy type circuit interrupter as disclosed in U.S. Patent
No. 3,077,525 issued February 12, l963 to Dyer. U.S. Patent
No. 3,815,059 issued June 4, 1974 to L. A. Spollmen discloses
a circuit interrupter in series with an electromechanical
current limiting device which utilizes the force generated
by the overload current to drive the movable contact arm
open. In U.S. Patent No. 3,815,059 a current limiting in-
terrupter was provided with a movable contact arm which ispivoted around one end and which moves into the slot of a
magnetic drive device during circuit interruption.
When dealing with current limiting circuit inter-
rupters the term I t is often utilized to describe the ef-
fectiveness of a particular device. The meaning of I t can
best be understood by considering an electrical source feed-
ing a simple resistive load through the circuit interrupter.
The total energy seen by the load is given by: current
(I) flowing through the load times the voltage drop (IR)
across the load times the time (t) current flows. This can
be expressed mathematically by: energy dissipated equals
RI t. During normal operation this energy RI t is easily
dissipated by the system. When a fault occurs, however,
current (I) can become very large in value. For example~
with the present day power supplies potential fault current
up to lO0,000 amperes can occur. During fault conditions
the value of the load, R, will remain approximately the same
but the total energy input into the system will be very
large. In a practical system if the load is to be protected
the fault time should be limited to as small a value as
possible. A reduction of the time of fault current flow, t,
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will give a further reduction ln the energy input into the
load. This means that in any current limiting device the
total I2t, i.e., (the integrated fault current)2 x (tlme it
flows~ ls the important parameter to measure. The smaller
the I t value, the better the performance of the current
limiting device.
In the circuit interrupter art, it has long been
recognized that it is of a distinct advantage to provide
fast interruption of an established arc. It ls well known
by those skilled in the art that it is desirable to effect a
rapid extinction of the arc as quick as possible inasmuch as
the fault current flow through connected electrical equipment
will damage the equipment unless the fault current is limited.
Due to heating, voltage surges, and other harmful effects lt
is desirable to effect extinction of the fault current as
soon as possible after initiation.
In accordance with the teaching of` the present ln-
vention, a pair of spaced relatlvely station2ry contacts are
bridged by a movable brldging contact arm carrying two
movable contacts therewith which cooperate with the two
spaced stationary contacts to establlsh two serially related
arcs during circuit interruption. Each set o~ mating contacts
is disposed within the yoke of a magnetic arc driving struc-
ture which effects the rapid outward movement of the arc~ A
set of arc-extinguishing plates extends within the yoke in
proximity to each set of mating contacts. The magnetic
field set up by the current within the circuit interrupter
in the yoke affects a rapid lateral outward movement of the
arc into the arc-extinguishing plate structure.
Current limiting can be achieved by establishing a
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rapid rise of arc voltage. Arc voltage can be increased ra-
pidly by separating the contacts in a short period of time and/or
by elongating and rapidly driving the arc formed into a set of
spaced arc-extinguishing plates. In the disclosed invention,
a magnetic drive or linear slot motor is used to drive the con-
tact arm which bridges the two pairs of mating contacts, thereby
affecting rapid contact separation. A magnetic arc drive or
yoke is used at each pair of contacts to elongate and drive
the arc into T-shaped spaced arc-extinguishing plates. These
nonmagnetic plates can be formed of metal, metal sprayed with
insulation, or insulation. The combination of these two methods
of obtaining a rapid rise in arc voltage results in a current
limiting circuit breaker providing excellent current limitation.
The magnetic arc drive rapidly moves any arc formed off of the
contacts which allows the use of silver cadmium oxide, AgCdO,
contacts which lower the temperature rise at the terminal. In
prior art circuit interrupters silver tungsten contacts are
normally required for high current interruption. In one embodi-
ment of the invention arc horns are formed extending outward
from the contacts towards the arc-extinguishing plates.
It is an ob~ect of this invention to teach a cir-
cuit interrupter having a movable contact arm which is
rapidly drawn into the slot of a magnetic drive device
during circuit interruption and having a yoke formed of a
magnetizable material disposed around the contacts which
are separated during circuit interruption.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention reference
may be had to the preferred embodiment exemplary of the
invention shown in the accompanying drawings, in which:
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Figure 1 is a side sectional view of a current
limiter utilizing the teachings of the present invention;
Figure 2 is a top view of the current limiter
shown in Figure l;
Figure 3 is an end view of the current limiter
shown in Figure l;
Figure 4 ls a view similar to Figure 1 but with
the current limiter in the open position;
Figure 5 is a side view of the bridging contact
arm utilized by the circuit interrupter shown in Figure l;
Figure 6 is a side view of a bridging contact arm
of another embodiment Or the invention wherein arcing
surfaces extend at an angle from the contacts;
Figure 7 is a top view of a T-shaped Deion plate;
Figure 8 are curves for system voltage, system
currents, current limiter arc voltage and clrcuit breaker
arc voltage for a circuit breaker protected by the disclosed
current limiter.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings and Figure 1 in
particular there is shown a current limiter 10 having a
molded housing 11 which utilizes the teaching of the present
invention. A movable bridging contact arm 12 having contacts
14 and 16 attached thereto provides a bridging current path
between stationary contacts 18 and 20. Stationary contacts 18
and 20 are aligned with movable contacts 14 and 16 respectively.
Contact arm 12 is movable between a closed position, as shown
in Figure 1, wherein contact 14 engages contact 18 and contact
16 engages contact 20, and an open position3 as shown in Figure 4,
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wherein contact 14 is spaced from contact 18 and contact 16
is spaced from contact 20. As shown in Figure 4, during
circu~t interruption when bridging contact arm 12 is in the
open position an arc 22 can form between the spaced apart
contact pairs 14-18 or 16-20. Terminals 24 and 26 are pro-
vided for connecting current limiter 10 in series in the
circuit to be protected.
A magnetic drive or linear slot motor 30 is provided
for moving contact arm 12 to the open position when current
flow through contact arm 12 exceeds a predetermined value.
Magnetic drive 30 is formed from a plurality of stacked
laminations formed of a ferromagnetic material. Slot motor
30 has a narrow slot 32 formed therein which is open at one
end. Bridging contact arm 12 is disposed within slot 32 in
proximity to the open end thereof.
When the current flow through contact arm 12 reaches a
certain threshold level the current flow in the contact arm
12 induces a magnetic force, in the laminated magnetic drive
30 surrounding the contact arm 12 which ^auses contact arm
12 to be pulled upward into slot 32. This construction
allows the bridging contact arm 12 to be pulled to open the
contacts very rapidly. The force generated is a function
of the let through current. Contact arm 12 during high
overload currents can be opened within 2 milliseconds.
Magnetic arc drives or yokes 40 and 42 are disposed
around contact pairs 14, 18, 16 and 20. Magnetic yokes 40
and 42 concentrate the magnetic field generated by the current
flow through the breaker to rapidly expel any arc 22 formed during
during circuit interruption into spacedrnetallic arc-extinguishing plates 52.
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When an arc 22 is formed dur~ng clrcuit interruptlon the arc
drivlng yokes40 and 4? rapidly dri~e the arc 22 into the
metalllc plates 52. When arc 22 ls driven into the
plates 52 it is stretched and rapidly extinguished.
nurlng high overload curnents ~ridging contact arm 12 can be
opened withln 2 milllseconds.
Termlnal 24 is connected to a U-shaped conductor
60 having one leg extending wlthin yoke 40 to which ls
attached stationary contact 18. Terminal 26 is electrlcally
connected to U-s~aped conductor 62 whlch has one leg extendlng
within yoke 40 to which is attached stationary contact 20~
Thus wlth the current ll~iter 10 in the closed position as
shown ln Flgure 1 a continuous current path exlst from
termlnal 24, through cond~ctor 60, through contact palr 18-
14, through bridging contact arm 12, through contact pair
16-20, through U-shaped ¢onductor 62, to terminal 260
Contact arm 1~ is pinned ~o sliding member 64 whlch can move
in bushln~ 66 disposed ln an openlng 68 through slot motor
30. Contact closing force ls supplied by a palr of concentric
compression springs 70 and 72 dlsposed between a retainer 74
and slidable member 64. Retalner 74 is rigidly connected to
a shaft 76, which passes through slidlng member 64 and is
connected to a keePer 78. Xeeper 78 is latched in positlon
by permanent magnets 80 and 82. The posltlon of the keeper
on shaft 76 can be ad~usted to supply the correct contact
force. The magnetic latch utilizes two permanent magnets 80
and 82 and two pole pieceQ dis~osed therebetween. Keeper 78
is latched in place by a force greater than the required
contact force suppliçd by sprlngs 70 and 72. When the
driving force of l~near slot motor 30 exceeds the latching
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force keeper 78 is released and the whole contact assembly
is free to move. A retainer 84 is attached to the shaft 76
and makes contact witn bridging contact arm 12 when the
~eeper is released and limits the travel of shaft 76 with
respect to sliding member 64. A light compression spring 86
is used to prevent the contact arm from reclosing after it
bounces off the top of the linear slot motor 30. Two arc
barriers 90 and 92 prevent arcs formed during circuit inter-
ruption from coming down the slot 32 as the bridging contact
arm 12 moves. The bridging contact arm 12 forms the armature
of the linear slot motor and a stack of laminations 31 forms
the stator. A bumper 94 is provided at the top of the slot
to prevent damage when bridging contact arm 12 is rapidly
moved to the open position. The current limiter 10 is
syrnmetrical so either end can be the input. Magnetic arc
drives 40 and 42 are the same at each end.
Magnetic arc drives 40 and 42 comprise a series of
window frame laminations. An insulating liner 96 is formed
around the arc driving yoke 40. T-shaped spaced arc-extinguish-
20 ing plates 52, as shown in Figure 7, are disposed within theopening of yoke 40. T-shaped plate 52 has a top portion 53 and
a leg portion 54 which when installed extends within magnetic
yoke 40. The free end of T-shaped plate 52 can be plated or
coated with tungsten to lessen deterioration when exposed to
arcing. The outer top of the T-shaped plates 52 are cut
at a 45 angle to allow venting in the vertical direction.
The shape of the plates 52 can be varied as desired. The
plates 52 can be formed from metal, metal coated with
insulation, or insulation. It is desirable to keep the weight
of contact arm 12 to a minimum since the acceleration
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of contact arm 12 ls portional to the sl~t motor ~orce
divided by the mass of the brldglng contact ~rm 1~ assombly.
Referring now to Flgure 6 there ls shown a contact
arm 13 for another embodiment o~ invention. Operation o~
the current limiter 10 wi~h contact arm 13 is as descrlbed
above and will not be described agaln in detall. When
bridgin~ contact arm 13 is utllized T-shaped spaced arc-
extinguishlng plates 53 having a shorter leg portion 54 are
required-. An arcing surface 15 is formed extending at an angle
~rom the contacts 14 and 16 supported on brid~ng contact arm 13.
Referrlng now to Figure 8 there is shown the
result of tests utllizlng the contact arm 13 and the shorter
plates 53. A small amount of tungsten was brazed to
the free ends Or the plates 53 in proxim~ty to the
~rc. These tests were conducted with current limiter 10
connected in series with a breaker. The results shown ln
Figure 8 show that current limiter 10 will protect downstream
clrcuit breakers for potçnt~al fault currents up to 100,000
amperes. Once a circuit breaker is connected ln series with
the current limiter the peak let through current is reduced
to a lower value than that associated with a circuit llmiter
alo~e slnce the added impedance of the circult breaker
itself reduces peak current. Peak let through current 18
not the only criteria in dqterm~ning if a downstream breaker
will be protected. I t and arc watt-seconds in the serle~
breaker are also important parameters. If the serles breaker
is much slower in opening its contacts than the current
limiter then the contacts m~y remain closed durlng most Or
the pulse current flow. In this case, the I t is lmportant
with respect to the breaker staying intact but the arc watt-
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seconds may be very small and consequently the gas pressure
in the breaker may also be small. I~ the series breaker
opera~es to~ quickly, ~he arc watt seconds may become greater.
The important result demonstrated, however, ls that it i8
posslble to protect series breakers at their appropriate
voltage ratings. With further optimlzation, lt is anticlpated
that the protection level can be increased.
