Note: Descriptions are shown in the official language in which they were submitted.
WO 91/02370 PC~/GB90/01157
2 ~ ~ 3 ~
tlINIATURE CIRCUIT BREAKERS
This invention relates to miniature circuit breakers
of the kind for providing overload protection and selective
manual circuit breaking for domestic and like electri~ity
supply circuits, including installations for use in
business and commercial situations, but is to be
distinguished from industrial and other heavy duty circuit
breakers. An example of an embodiment of the invention
provides not only instantaneous circuit b-rea~ing-in the
event of a massive overload, but also a circuit breaking
response in the event of a creeping current rise above a
predetermined maximum. The invention also provides a
current sensing flux shifting device per se.
~ any proposals have been made for miniature circuit
brea~ers of the above kind, particularly in recent years.
Many of these can meet the practical requirement of a
relatively rapid current trip upon massive overload.
However, for circuit breakers applicable to the particular
field wit~l which the present invention is concerned
(miniature circuit breakers for domestic and the li~e
situ~tions) there is a furt`ner requirement concerning
dimensions. In short, there is a need for an absolute
minimum space requirement so that, if possible, a cassette-
like construction can be adopted so that a series of
minature circuit breakers can be assembled side-by-side in
a bank the width of each being preferably no greater than
half an inch (12.7 millimetres).
There are additional requirements raised by prior
proposals including a need for improved ease of manufacture
and assembly arising from the somewhat complex and muddled
layout adopted in certain cases.
We have determined that, surprisingly, in the low-duty
application with which the present invention is concerned,
the use of a double break circuit brea~er leads to
significant advantages in terms of rapidity of current
interruption while permitting a simplicity of layout and
WO91~02370 PCT/GB90/01157
construction leading to advantages in manufacture and
assembly.
In EP A 0 270 158 there is disclosed a circuit breaker
as defined ln the pre-charac~erising portion of claim l
hereof.
Other requirements and shortcomings in relation to the
above prior application and other prior art known to the
applicants include the following. ~irstly, an improved and
simplified means for adjusting the operation of a solenoid
used for sensing current overload and tripping a contact
breaker mechanism. Secondly, im~rovements in relation to
the mounting of one or more movable contacts in relation to
maintenance of adequate contact pressure during use.
Thirdly, improvements in runner design in order to achieve
rapid transfer of arc and thus minimise energy let-through
and contact wear. Fourthly, provision of a design
permitting the use of a large number of arc 2lates without
the need for large contact separation whereby rapid and
large increases in arc voltage can be obtained. Fifthly,
there is a need for a particularlv low moment of inertia
for the movable contact assembly, whereby delays in contact
opening are minimised. Sixthly, the provision of improved
means for adjusting the setting of the trip mechanism as a
whole and of the thermal trip in particular, particularly
having regard to manufacturing simplicity and ease of
calibration.
An object of the present invention is to provide a
miniature circuit breaker offering improvements in relation
to one or more of the matters discussed above, and
elsewhere herein, or generally.
According to the invention there is provided a
miniature circuit breaker as defined in the accompanying
claims.
In a preferred embodiment, a minature circuit breaker
comprises spaced connectors for connection to the circuit
to be controlled/protected. The circuit breaker
WO91/02370 PCT/GB90/01157
3 2~5Q31
interconnecting the connectors comprises o~erload detection
means and contact brea~er means actuable thereby, and
having arc extinguishing means. ~anual operating means for
the-contact breaker means is provided. The contact breaker
means comprises a pair of movable contacts and a pair of
relatively fixed contacts. Actuation means for the movable
contacts is provided to effect separation thereof. Each
contact of the pair of contacts is spaced apart from the
other thereof, one at each side of a central contact
carrier mounted for lengthwise movement about a
longitudinal carrier axis, to open and close the contacts.
The actuation means comprises a releasable latching
mechanism engageable with the contact carrier to hold same
in a latched "circuit closed" condition, and energy storage
means (for example in the form o~ a coiled tension spring
directly connected to the contact carrier means) to
releasably store energy for opening tha contacts when the
latching mechanism releases the contact carrier. A housing
is provided for the circuit breaker. The contact carrier
is mounted for guided sliding movement in the housing,
entirely independently of the latching mechanism, and under
the action of a tension spring generally aligned with the
longitudinal carrier axis. The tension spring acts between
the housing and one end of the contact carrier. The
latching mechanism is disposed to make latching engagement
with the other end of the carrier member. The arrangement
is such that after release of the contact carrier by the
latching mechanism, the contact carrier is free to move
under the sole action of the spring in the contact-opening
direction.
A pair of arc chutes are located one on each side of
the contact carrier, and each one is connected at one end
by an arc runner to its own one of said contacts. The arc
chutes are interconnected at their other ends by a bottom
arc runner. The tension spring for the contact carrier
extends through an opening in the bottom arc runner.
W091/02370 PCT/GB9OtO1157
7,9~S~'J 4
Stop means is provided foL the contact carrier to
arrest movement of same in the contact opening direction.
The stop means comprises at least one pair of mutually
engageable fixed and movable complementary stop surfaces on
the contact carrier and on a relatively fixed structure
carried by the housing. The stop surfaces are disposed so
that they face in a direction inclined with respect to the
carrier axis, whereby rebound of the carrier on tripping is
minimised.
Further in the preferred embodiment, the mov2ble
contacts are mounted on the contact carrier by means of a
lateral contact support, carrying the contacts, and ~hich
can angularly adju~t itself with respect to the contact
carrier under the bias of a leaf spring engaging the
contact support on the convex surface of the s~ring, to
promote proper alignment of the fixed contacts.
The overload detection means comprises a solenoid
having a plunger extendable upon detection of an overload
to actuate the contact breaker means. The plunger is-
positioned to engage a leaf spring mounted externally of
the solenoid and operative to return the plunger after
extension on overload.
The latching mechanism comprises main and secondary
latch levers pivotally mounted for mutual engagement in a
position in which they co-operate to hold a main latch
lever in a position in which itretains the contact carrier
in its contact closed position. Said latch levers are
pivotally mounted on a bell crank lever. Said latch levers
and said bell crank and said contact carrier are all formed
of a synthetic polymeric material, preferably by moulding
techniques.
The overload detection means comprises thermal trip
means to cause the contacts to be opened. The thermal trip
means is electrically connected in series with said
CQntaCtS and positioned to act mechanically on a pivotally
mounted thermal trip lever itself positioned to act on said
WO91/02370 PCT/GB~/01157
2 ~
latch mechanism. The arrangement is such that the latch
mechanis~ can be tripped by said thermal trip lever upon
detection of a progressive overload. The thermal trip
lever is position-adjustable under the control of a screw-
threaded adjuster to vary the ~osition at which the latch
mechanism trips.
In the preferred embodiment, a large spring is
utilised to achieve rapid movement of the contact carrier
assembly. The magnetic and thermal forces required to trip
the breaker are minimised by the mechanical advantage
provided by a secondary latch lever. Re-latch of the
breaker after tripping is accomplished by rotation of the
linkage on the bell crank l-ever by means of the manually-
operable handle to t`ne re-latch position. A plastic guide
in the case forces the latch to reset. Previous peoposals
in this respect rely on a spring for re-latching of the
mechanism.
Fibre barriers placed in the throat of the arc chamber
serve to accelerate the movement of the arc along the
runners and to provide the necessary dielectric strength of
the arc chamber. The bimetals used in the thermal trip are
such that the temperature rise requirement for the
necessary thermal force and thermal deflection are
approximately equal. This ensures efficient use of bimetal
material and minimises the effects of variability of force
and deflection requirements on the calibration of the
breaker.
An embodiment of the invention will now be described
by way of example with reference to the accompanying
drawings in which :-
Fig l shows a minature circuit breaker in sideelevation and in the contact-closed position, and
surrounding the assembly are individual drawings of the
main parts thereof, some shown in perspective, for- purposes
of illustration;
Fig 2 shows the minature circuit breaker of Fig l in
WO91/02370 PCT/~B90/01157
~,o6~
its tripped position; and
~ ig 3-shows, on a larger scale, a flux shifter which
can be incorporated in the circuit ~reaker of Figs 1 and 2
in place of the solenoid there-used for tripping the
contact-opening mechanism.
As shown in Fig 1 a miniature circuit breaker 10
comprises a casing 12 having connectors 14, 16 for
connection to a circuit to be controlled. Interconnecting
the connectors 14, 16 is a circuit breaker circuit 18
comprising a lug plate 20, a solenoid coil 22 of a solenoid
24, a left arc runner 26, a contact carrier 28, associated
pairs of fixed and movable contacts 30, 32 and 34, 36, a
right arc runner 3a, a bimetal s,rip ~0, and a length of
flexible braid 42.
Solenoid 24 constitutes overload detection means and
is arranged to actuate contact breaker means ~4 including
the contacts 30, 32 and 34, 36 together with actuation
means for the movable contacts, which will be described
below.
Arc extinguishing means 46 is provided for the contact
breaker means 4~ in the form of a pair of arc chutes 48, 50
located one each side of the contact carrier 28 and
interconnected by a bottom arc runner 52.
~ anual operating means for the contact breaker
comprises a handle 54 pivotally mounted on casing 12 and
connected by a lin~ 56 to a latch mechanism 58 to be more
fully described below.
The movable contacts 32, 36, mounted on contact
carrier 28 are located one at each side of a carriage
member 60 which is mounted for lengthwise movement about a
longitudinal carrier axis 62, to open and close the
contacts. Carriage 60 is guided for sliding movement by
guides 64, 66. As shown in ~he perspective view of the
carria~e, it is formed as a moulding of ~olymeric material.
A recess 68 is provided at one end of the carrier to
receive one end 70 of a main latch lever 72, which is
WO 91/02370 PCI'/GB90/01157
2~5~vl
pivoted at 74 and has a notch 76 at its other end for
engagement with a complementary formation 78 on a secondary
latch lever 80, pivotally mounted at 82 on a bell crank
lever 84 which is itself pivotally mounted on housing 12 at
86. Main latch lever 72 is likewise pivotally mounted on
bell crank 84. The bell crank itself is connec~d by link
56 to handle 54, for re-latching purposes.
A thermal trip lever 88 is pivotally mounted at 90 on
a pin 92 of a calibration brac~et 94 which is position-
adjustable by means of an calibration screw 96 having a
threaded stem 98 engaging an internal threaded bore 100 in
bracket 94. Thermal trip lever 88 has an in-turned end 102
for engagement with bimetal strip 40. Its other end 104
engages a recess 106 formed in secondary latch 80, whereby
position-adjustment of calibration bracket 94 varies the
sensitivity of the latch mechanism 58.
Carriage 60 has a spigot 106 to which is directly
connected a coiled tension spring 108. The other end of
spring lOa is connected at 110 to housing 12, whereby the
spring for¢e is applied directly to carriage 60 and contact
carrier 28, directly in-line with same, and without any
intermediary structure.
Contact carrier 28 is connected to carriage 60 by
being inserted through an opening 112 therein, in which it
is a relativley loose fit, so as to be pivotable about an
axis generally parallel to spigot 106. A convex leaf
spring 114 has its ends located in lips at the ends of
lateral brackets 116, 118 formed integrally with carriage
60. The spring's convex surface engages contact carrier 28
and biases same in a contact-closing direction.
Stop means 120 is provided ~or carriage 60 to arrest
movement of same in the contact-opening dire~tion. The
stop means comprises t~o pairs of mutually engageable fixed
and movable stop surfaces, one pair 122 at the underside of
the brackets 116, 118, and another pair 124 provided on a
fixed structure mounted via bottom runner 52 on housing 12.
WO91/02370 PCT/GB90/01157
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The stop surfaces are disposed so that they face in
directions inclined with respect to the carrier axis 62 so
as to minimise rebound on contact-opening. This fixed
structure may li~ewise be moulded from a polymeric
material, likewise carriage 60, levers 72 and 80 and handle
54.
Solenoid 2~ comprises coil 22, a cas:ing 124, and
plunger 126, an insu~ator tube 128 and an actuator pin 130
positioned to actuate the end 132 of secondary latch lever
80. A generally L-shaped leaf spring 134 is fixed to a
mounting 136 and has its end 138 positioned to engage pin
130 so as to be resiliently deflected thereby and to
provide a return function therefor. ~y virtue of the
location of spring 134 externally o~ solenoid 24 and its
accessibility for manipulation, it permits manual
adjustment (by deflection), of its re~urn function.
In use, with the circuit breaker in the condition
shown in Fig 1, the contacts 30 to 36 are closed and the
circuit is made between connectors 14, 16.
In the event of a progressive slight overload, bi~etal
40 progressively heats, deflects, and causes thermal trip
lever 88 to pivot clockwise as seen in Fig 1 and, at a
predetermined deflection of the bimetal, secondary latch
lever 80 is pivoted anti-clockwise as seen in Fig 1,
thereby releasing main latch lever 72 and thus likewise
releasing c~rriage 60 under the action of spring 108, which
retracts the carriage and opens the contacts. Stop means
120 inhibits carriage bounce.
The circuit can then be re-made by actuating handle
154. The point at which ~he mechanism trips can be
adjusted by means of calibration screw 96 which is readily
manually adjustable after removing calibration cover 140.
As regards instantanous tripping under massive
overload, the function is as follows. Solenoid 24 detects
the overload, and pin 130 deflects spring 134 and engages
end 132 of secondary latch lever 80, thereby pivoting it
WO91/02370 PCT/~B90/01157
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anti-clockwise and releasing main latch lever 72, which
likewise releases carriage 60, as described previously. As
the contacts open, an arc is struck and transferred ~o the
arc chutes ~8, 50, ~hich rapidly extinguish same.
In the embodiment of Fig 3 there is shown a ~lux
shifter which can be substituted forthe solenoid 24 in the
above-described embodiment.
As shown in Fig 3,flux shifter 140 comprises a casing
142, a` conductor 144, a shunt 145, a magnet 146, legs 147
and 151, an insulator 148, an armature 150, and a return
spring 152 acting between casing 1~2 and a pin head 154.
The magnetic flux from the magnet 146 flows through the
magnetic circuit formed by legs 147 and 151, armature 150
and shunt 145. The magnetic circuit elements should be
made from high permeability materials. As air gap (not
shown) between shunt 145 and legs 147 and 151 is introd~ced
to bias the magnetic circuit so that most of the flux
passes through the armature 150. The resulting magnetic
force between the legs 147 and 151 and t`ne arma~ure 150
holds said armature in place against the force from spring
152. Holes (not shown) in legs 147 and 151, placed in the
region of said legs that lies between the magnet 146 and
the armature 150, reduce the cross-section of material in
said region. The magnet 146 and magnetic circuit cross-
sectional areas are chosen so that in this region the
material is at the knee in the hysteresis curve, just
below saturation.
When current passes through the conductor 144 the
magnetic field intensity is increased on one side of each
hole and decreased on the other. Because of the
nonlinearity of the hysteresis curve, the side of the hole
with the increase in magnetic field intensity has a small
increase in flux, while the other side has a much larger
decrease in flux. The net result is a shifting of some of
the flux from the armature 150 to the shunt 145. Should
sufficient current pass through the conductor the magnetic
WO91/02370 PCT/GB90/01157
~3~ ~ 10
force holding the armature 150 will be exceeded bv the
force of the spring 152. The armature 150 is then fo~ced
away from the legs 147 and 151 which increases the
reluctance of the magnetic circuit through the armature
150. ~ost of the flux is shifted through the shunt 1~5
since it is now the low reluctance path and as a result
the magnetic force on the armature 150 decreases rapidly.
The loss of magnetic force allows rapid motion of the
armature 150 and pin 154 through the action of the soring
152. Resetting of the trip unit 140 is accomplished by
pushing the armature 150 bac~ to its position ne.Yt to t-he
legs 147 and 151.
Amongst the advantages provided by the above
embodiment are the following. ~irstly, by the use of a
double-break contact~assembly, the more stringent modern
requirements for rapidity of current interruption are more
readily met. By arranging for the carriage 60 and contact
carrier 28 to be mounted entirely independently of the
latching mechanism 58, none of the structure of the latter
is carried on the carriage, and the latter is tnus quite
free to retract rapidly under the action of spring 108.
The latter has nothing else to move.
The calibration arrangement for the trip mechanism is
less prone to mis-adjustment than previous proposals due to
the direct and simple way in which adjustment is achieved.
By adoption of plastic materials (synthetic polymers)
for ajsignificant number of the major components of the
circuit breaker, a low coefficent of friction is proYided
for the trip mechanism, whereby a lower trip point can be
achieved.
The anti-bounce arrangement for the carriage and
contact c~rrier provides the significant advantage of
avoiding any tendency for the arc to re-strike after
contact opening due to rebound of the contact carrier in
the direction of the stationary contacts.
