Note: Descriptions are shown in the official language in which they were submitted.
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STANDARD SIZED APPARATUS FOR DETECTING ARC FAULTS
AND GROUND FAULTS IN MULTIWIRE BRANCH
ELECTRIC POWER CIRCUITS
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to apparatus for
providing protection in multiwire branch circuits of
electrical power distribution systems, and in particular, to
two pole circuit breakers that share a common neutral, for
providing protection for arc faults and ground faults in
such circuits, that are capable of being mounted in two
adjacent standard 3/4-inch slots of a load center panel.
Background Information
Branch circuits in electric power distribution
systems often share a common neutral conductor to reduce the
wiring required. Such multiwire branch circuits are often
referred to as "home runs". Home runs are permitted as long
as the two line conductors are energized by 120 volt
separate phases of a center tapped single-phase transformer
to avoid overloading the neutral conductor. This
simultaneous disconnection of both of the ungrounded
conductors can be accomplished with a two pole disconnect,
two single pole circuit breakers with a handle tie, or a two
pole circuit breaker.
Presently, such multiwire branch circuits can be
provided with short circuit and overcurrent protection by
the tied single pole breakers or the two pole breaker. Only
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a two pole breaker can also provide ground fault protection
along with the addition of a common ground fault detector.
Ground fault protection can be either personnel protection
or equipment protection.
Recently there has been an increased interest in
providing protection from arc faults. Arc faults are
intermittent high impedance faults which can be caused for
instance by worn insulation, loose connections, broken
conductors, and the like. Because of their intermittent and
high impedance nature, they do not generate currents of
sufficient instantaneous magnitude or sufficient average
current to trigger the thermal-magnetic trip device which
provides the short circuit and overcurrent protection.
Various types of arc fault detectors have been proposed, but
except for the circuit described in Patent Application
Serial No. 08/939,263, filed September 29, 1997 (attorney
docket 97-PDC-312) such detectors have not been adapted to
multiwire branches employing a common ground. A further
need exists for an arc fault breaker in a two pole design
with a shared neutral for application in common household
load centers employing standard face panels with 3/4-inch
knockouts.
The prior art has taught employing multiple pole
breaker designs such as are taught in Patent Number
5,483,211, filed January 9, 1996, and Patent Number
5,453,723, filed September 26, 1995, where the electronic
trip circuits are housed in separate compartments from the
thermal-magnetic tripping units and span several breaker
slots in a load center. However, none of these arrangements
are suitable for a home run configuration. Thus, the art
evidences the further need for an arc fault breaker in a two
pole design with a shared neutral for application in common
household load centers employing standard face panels with
3/4-inch knockouts.
SUMMARY OF THE INVENTION
These and other needs are satisfied by the
invention which is directed to an apparatus for detecting
faults in multiwire branch circuits, that is capable of
fitting within standard load center 3/4-inch knockout
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panels. It includes a two pole circuit breaker (providing
standard circuit protection) having a first pole connected
to interrupt current in the first line conductor and a
second pole connected to interrupt current simultaneously in
the second line conductor. The apparatus further includes
fault detection circuitry including a first arc fault
detector connected to detect arc currents between the first
line conductor and neutral/or ground and to generate a trip
signal in response thereto, and a second arc fault detector
connected to detect arc currents between the second line
conductor and neutral or ground and to generate a trip
signal in response thereto, and a ground fault detector
connected to detect ground faults between each of the line
conductors and ground. The apparatus also includes a
conventional thermal-magnetic trip mechanism and means
responsive to each of the trip signals to trip the two pole
circuit breaker.
Preferably the thermal-magnetic trip unit for
both poles is maintained within a standard 3/4-inch circuit
breaker case having a face designed to protrude through the
standard 3/4-inch load center panel knockouts. The
electronic processing circuits for the ground fault and arc
faults detection circuits are maintained within a second
standard 3/4-inch circuit breaker housing that abuts the
first housing on its side so the two housings are aligned
and connected at their juncture in a manner that permits
them to protrude through adjacent knockouts in the load
center panel. In one embodiment the breaker switches
protrude through the face of the first housing while the
trip indicator and duel trip test button protrude through
the face of the second housing. Desirably, electrical
shielding is provided at the interface between the first and
second housings.
These, together with other objects and
advantages which will be subsequently apparent, reside in
the details of construction and operation as more fully
hereinafter described and claimed, reference being had to
the accompanying drawings forming a part hereof.
BRIEF DESCRIPTION OF THE DRAWINGS
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The foregoing features will be more fully
appreciated in view of an illustrative example thereof taken
in conjunction with the accompanying figure of which:
Figure 1 is a top prospective view of a circuit
breaker housing constructed in accordance with this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 illustrates a circuit breaker housing
constructed in accordance with this invention. The
10 housing 10 forms two interior compartments 12 and 14. Each
of the compartments 12 and 14 are sized to be substantially
equivalent in volume and design to a standard 3/4-inch
circuit breaker housing. The compartment 12 houses the
thermal-magnetic trip mechanism of the circuit breaker which
is well known in the art. The thermal-magnetic trip
mechanism includes separate thermal-magnetic trip assemblies
for each pole, which are tied together by a common trip bar.
Each pole can be reset by using the switch levers 16 and 18
which are connected by handle link 17 so both poles reset
simultaneously. Where a common neutral conductor is
employed, it is important that both poles are disconnected
simultaneously. Though not shown, a male clip is provided
under each of the housings 12 and 14 to connect to the hot
side of the line respectively for phases A and B. A female
clip is also provided on the underside of the breaker and
aligned to connect to the line ground in the back panel of
the load center. The phase A load wiring is connected to
terminal 24 and the phase B load wiring is connected to
terminal 26; both terminals being positioned on the
periphery of the housing 12.
The second compartment 14 houses the electronics
for the ground fault detection circuit and the arc fault
detection circuit. The electronics and the operation of the
fault detection circuits can be better appreciated by
reference to co-pending application serial number 08/939,263
(attorney docket number 97-PDC-312). Preferably electrical
shielding is provided between the housings 12 and 14 so that
any arcing that occurs within compartment 12 does not affect
the electronics housed in compartment 14. A duel test
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button 22 protrudes through the face of the compartment 14
for testing the ground fault and/or arc fault detection
circuits. In addition, a trip indicator is provided on the
5 face of compartment 14. Both compartment 12 and compartment
14, respectively, have a raised portion on their face which
fits through the knockout in the load center panel. A
recess 32 is provide between the two raised faces 28 and 30
to seat the crossbar spanning between the knockouts on the
load center panel.
Thus, the housing of this invention permits a
two pole breaker employing a shared neutral and having arc
fault and ground fault trip capability to be seated within a
standard 3/4-inch breaker panel in a manner that enables the
electronics to be shielded from arcs generated within the
thermal-magnetic trip mechanism portion of the housing.
While specific embodiments of the invention have
been described in detail, it will be appreciated by those
skilled in the art that various modifications and
alternatives to those details could be developed in light of
the overall teachings of the disclosure. Accordingly, the
particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of the
invention which is to be given the full breath of the claims
appended and any and all equivalents thereof.
