Note: Descriptions are shown in the official language in which they were submitted.
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Ground fault protection for personnel and
electrical equipment is an ever increasing concern in both
home and industry. For residential circuit applications,
ground fault circuit interrupting (GFCI) devices are now
widely available in compact circuit breaker and receptacle
configurations for convenient installation in existing ser-
vice entry equipment and wall receptacle boxes. These
GFCI devices are primarily intended to protect people
from the hazards of electrical shock caused by leakage
current emanating from ground faults, however they do afford
a measure of equipment protection in terms of acting to halt
ground fault current which can be damaging to insulation.
For high current applications found in industry,
ground fault protection is available in basically two
configurations. With the event of so-called "static
trip" circuit breakers, it has become economically possible
to combine overcurrent and ground fault signal processing
circuitry in a compact electronic trip unit package which
can be integrated with the circuit breaker to achieve
comprehensive circuit protection. Alternatively,
traditional circuit breakers having thermal-magnetic and `
dual-magnetic trip units for overcurrent protection can be
utilized with so-called "ground fault relays" in providing
ground fault protection as well. These relays respond
to a ground fault signal developed by a suitable sensor,
such as a zero sequence transformer coupled with the load
carrying conductors, by energizing (or de-energizing) a
solenoid which, in turn shunts trips the breaker to initiate
circuit interruption.
U. S. Patent No. 4,044,395, issued August 23, 1977 `
and assignecl to the assignee of the instant application,
discloses a system for protecting relatively large power
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distribution circuits against ground faults by utilizing as
its principle operating component a conventional GFCI
device normally used in low power, residential circuit
applications. As disclosed and claimed therein, the GFCI
device is installed in a control circuit for an under-
voltage release solenoid adapted to a conventional circuit
breaker protecting the distribution circuit or in the
control circuit for the holding coils of a contactor
operating in the distribution circuit. A ground fault
sensor in the form of a zero sequence transformer coupled
with the load current carrying conductors of the distribu-
tion circuit develops a current signal in its secondary
winding in response to a ground fault on -the distribution
circuit. This current signal is injected into one side of
the control circuit to create a current imbalance of the
natu~e to which the GFCI device is responsive. The GFCI
device trips to interrupt the control circuit, and the
undervoltage release solenoid drops out to trip the circuit
breaker or the holding coil becomes de-energized to open
the contactor. In either case, the distribution circuit
is interrupted to clear the ground fault.
The system configuration of this patent, while
quite satisfactory for most application, has one drawback
in that it is sensitive to control circuit voltage fluxuations.
That is, should the control circuit voltage drop to an
abnormally low level, the undervoltage release solenoid
could drop out, causing the circuit breaker to trip despite
the fact there is no ground fault on the distribution circuit.
The same nuisance interruption of the distribution circuit
could likewise result from control circuit voltage dips
if a contactor is substituted for the circuit breaker.
It is accordingly an object of the present
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invention to provide an inexpensive ground fault protection
system suitable for high current, industrial circuit
applications.
An additional object of the present invention is
to provide a ground fault protection system of the above
character which utilizes as its fault clearing component
a circuit interrupting device of the more traditional, non-
static trip design.
A further object of the present invention is
to provide a ground fault protection system of the above
character which utilizes as one of its principle components
a low-cost ground fault circuit interrupting (GFCI) device
of the type presently enjoying wide use in the low-power
residential circuitry.
Other objects of the invention will in part be
obvious and in part appear hereinafter.
In accordance with the present invention, there
is provided a ground fault protection system ideally suited
for application to high current, high voltage power distri-
bution circuits, wherein the system utilizes as its
principle operating component a ground fault circuit
interrupting (GFCI) device of the type used in low voltage
residential-type circuits. As is well known, such GFCI
devices utilize a differential current transformer to
sense imbalances in the currents going out to and returning
from a load, as would be occasioned by leakage current
returning to the source through an unintended ground
path. The differential current transformer develops an
analog signal proportional to this leakage current which
is processed by electronic circuitry, and, if of a predeter-
mined magnitude and duration, an electronic switch is
triggered to complete ~n energization circuit for a shunt
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trip solenoid. Upon energization, the solenoid plunger
strikes a latch to release a spring powered mechanism
which is freed to open contacts,, clearing the ground
faul~ through which the leakage current originates.
In accordance with the present in~ention, the
GFCI device is equipped with switch means ln the form of
an auxiliary switch or a bell alarm switch for controlling
the energization of a shunt trip solenoid operative to
trip a circuit ~reaker protectiny the power distribution
circuit. The switch means is normally open, but is
operatively coupled to the GFCI device such as to be
closed incident to the tripping of the GFCI device. Closure
of the switch means completes an energization circuit
for the solenoid which then acts to trip the circuit breaker,
interrupting the distribution circuit.
- To sense the existence of a ground fault on the
distribution circuit, a ground fault sensor is coupled with
the load current carrying conductors of the distribution
circuit for developing a current signal indicative of
the ground fault current magnitude. This current signal
is impressed across one side of a control power circuit
for energizing the GFCI device, thereby creating a current
imbalance sensible by the differential current transformer
of the GFCI device. The GFCI device trips, and the
switch means closes to fire the shunt trip solenoid,
thereby tripping the circuit breaker and clearing the ground
fault on the distribution circuit.
As an incidental, but nevertheless signi~icant
feature of the present invention, the control power circuit
may be utilized to power loads connected downstream from
the GFCI device. Such loads may include alarms, signal
lights, etc. traditionally found in motor control centers,
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for example. Since these loads are wired through the
GFCI device, they are afforded ground fault protection, a
significant advantage in terms of operating personnel
safety.
The invention accordingly comprises the features
of construction and arrangement of parts which will be
exemplified in the construction hereinafter set forth, and
the seope of the invention will be indicated in the claims.
For a better understanding of the nature and
objects of the invention, reference should be had to the
following detailed description taken in conjunction with
the accompanying drawing, in which the sole figure is a
circuit diagram, partially in bloek form, of a ground
fault proteetion system eonstrueted in accordance with
the present invention.
Turning to the drawing, the ground fault
protection system of the present invention is depieted
in its application to a high voltage, industrial-type
electrical distribution circuit ineluding a grounded neutral
source 10 supplying three-phase power over phase or line
eonductors 12 to a load 14. Ineluded in this distribution
circuit is a conventional three-pole eireuit breaker,
generally indicated at 16, having separable contacts 18
connected in series with each line eonduetor 12. The
cireuit breaker also ineludes, as diagrammatieally
illustrated in FIGURE 1, a trip unit 20 of known eonstrue-
tion responsive to the levels of eurrent flowing in the
three line conductors for effecting automatie opening
of the breaker contacts 18 under overload and short eir-
cuit conditions.
Operatively associated with eircuit breaker 16
in a well known manner is shunt trip solenoid 22. As is
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well understood in the art, a shunt trip solenoid in its
adaption to a circuit breaker is normally de-energized,
but when it is desired to trip the circuit breaker, its
coil is energized. Its plunger is magnetically attracted
from an inactive to an actuated position, in the process
striking a latch associated with the trip unit. The
latch releases the breaker mechanism which operates under
the power of a mechanism spring to abruptly open the
breaker contacts.
The system of the present invention further
includes a ground fault circuit interrupting (GFCI) device,
generally indicated at 26, of the type widely available
for use in low voltage residential-type circuits for pro-
tecting humans from the hazards of electrical shock due to
ground faults. As diagrammatically illustrated in FIGURE 1,
the GFCI device 26, energized from a conventional 120 volt
AC source 24, includes a differential current transformer
consisting of a toroidal core 28, a first single turn
primary winding 30a connected in series with the line
side of a control power circuit fed from source 24, a
second single turn primary winding 30b connected in series
with the neutral side of the control power circuit, and a
multi-turn secondary winding 32 connected to the input
of an electronic module 34. As is well understood in
the art, the differential~transformer develops a signal
in its secondary winding 32 in response to a differential
in the currents flowing in the primary windings 30a and
3Ob, as would be occasioned by an imbalance in the currents
flowing in the two sides of the control power circuit.
This signal is processed by electronic module 34, and,
if found to exceed a predetermined magnitude and duration,
an electronic switch is triggered to complete an energization
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41 PR 3217
circuit for a solenoid (not shown). Energiza-tion of this
solenoid initiates the opening of contacts 36 to interrupt
the control power circuit.
It will be understood that the GFCI device may
be constituted in either a single pole circuit breaker
configuration or a receptacle configuration. The GFCI
device is illustrated as being equipped to breaker both
sides of the control power circuit as is typical in
receptacle configurations. To break both sides of the
control power circuit using a circuit breaker configura-
tion, a conventional single pole GFCI circuit breaker is
equipped with one or more additional breaker poles which
may or may not have overcureent tripping capability. So-
called "switching neutral" GFCI circuit breakers are now
being offered by assignee's Circuit Protective Devices
Department, Plainville, Connecticut provide the capability
of breaking both sides of th`e control power circuit.
To sense a ground fault on the distribution
circuit downstream from circuit breaker 16, a ground
fault sensor, generally indicated at 38 and typically a zero
sequence transformer, is utilized. Thus, as illustrated
in FIGURE 1, sensor 38 comprises a toroidal core 40
which embraces the three line conductors 12 of the
distribution circuit. As long as the vectorial sum of
the currents flowing in the three line conductors 12
equals zero, the net flux induced in core 40 is also zero
and no voltage is induced on its multi-turn secondary
winding 42, which is connected across either primary
winding (winding 30b in the illustrated embodiment) of
GFCI device 26. It is seen that in the absence of an
induced voltage in sensor secondary winding 42, the GFCI
differential current transformer remains balanced. On the
41 PR 3217
~`4~i97
other hand, if a ground fault should exist on the dist~ibu-
tion circuit, such as illustrated at 44, the vertical sum
of the currents in line conductors 12 no longer equals zero,
and a voltage is induced in secondary winding 42. This
induced voltage, as impressed across primary winding 30b,
causes additional current to flow therethrough, and the
GFCI differential transformer becomes unbalanced, resulting
in the opening of the GFCI device contacts 36. While the
distribution circuit is illustrated as simply a three
phase, three wire circuit,`it will be appreciated that it
may also include a fourth, neutral wire, in which case
all four wires would be embraced by core 40. Moreover,
the sensor 38 may be constituted by separate current
transformers inductively coupled with each distribution
circuit wire and with their secondary windings connected
in parallel, residual circuit fashion.
To translate the tipping of GFCI device 26 in
response to a ground fault on the distribution circuit
into tripping of circuit breaker 16 to clear the ground
fault, the GFCI device is adapted with a switch 50. This
switch may be in the form of an auxiliary switch mechani-
cally coupled with the GFCI circuit breaXer mechanism and
operated thereby to assume a normally open condition while
the GFCI breaker contacts are closed and to assume a
closed condition whenever the GFCI breaker contacts are
opened. Preferably however, switch 50 is of the so-called
"trip or bell alarm" type physically adapted to a GFCI
circuit breaker in the same manner as currently being adapted
to conventional residential-type circuit breakers. A
bell alarm switch is mechanically adapted to a circuit
breaker so as to be insensitive to manual opening of the
circuit breaker, but is actuated to i-ts closed condition
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in response to tripping of the circuit breaker. In this
connection, reference is made to the disclosure in U.S.
Patent No. 3,256,407 - K.W. Klein, dated June 14, 1966.
Switch 50 may be adapted to a GFCI receptacle by
positioning its contacts for closure in response to
engagement by the receptacle movable contact carrier as
it springs to its open circuit position incident to a
ground fault trip function. In either case, switch 50
is mechanically coupled to the GFCI device contacts 36
such that the switch is left open as long as the device
contacts are closed. When the device contacts are
tripped open, the switch is incidentally actuated to its
closed condition. Switch 50 is wired into an energization
circuit for the shunt trip solenoid 22 which, in the
illustrated embodiment of the invention, is tapped
into the control power circuit at the line side of
the GFCI device, but could be fed from a source separate
from source ~4. It is thus seen that until the GFCI device
is tripped, the switch is open and -thus energization of
the shunt trip solenoid from source 26 is inhibited.
However, when the GFCI device trips in response to a
ground fault on the distribution circuit, switch 50
closes to effect energization of the shunt trip solenoid
and tripping of circuit breaker 16.
From the foregoing description of the disclosed
embodiment of the invention, it is seen that by using the
combination of three low-cost components, namely, a
residential-type GFCI device equipped with a trip alarm
or auxiliary switch, a shunt trip solenoid and a ground
fault sensor, to control a conventional industrial circuit
breaker, economical ground fault protection is afforded
to a high current power distribution circuit.
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As an additional feature of the present inven-
tion, the control power circuit is utilized to power an
electrical load 52 connected downstream from the GFCI
device. This load may include indicator lights, alarms,
and other devices as may be incorporated in typical
industrial electrical power delivery installations.
Since load 52 is fed via the GFCI device, it, as well as
the control power circuit downstream from the GFCI device,
is afforded ground fault protection. This is a significant
feature as well from a personnel safety standpoint. If
the GFCI device is in a circuit breaker configuration,
then load 52 is also afforded overcurrent protection.
It will be appreciated that control power source
24 may be in the form of a 120/240 AC volts supply feeding
two line conductors and a grounded neutral conductor. In
this case, the GFCI device 26 would be provided by a two
pole GFCI circuit breaker, such as is offered by assignee's
Circuit Protecti~te Devices Department, Plainville, Connecti-
cut. This can be desirable from the standpoint that load
52 may require a 240 volt supply or be constituted by a ^
variety of 120 and 240 volt loads. In practice, it
would be preferable to connect secondary winding 42 of
sensor 38 directly across one of the primary windings 30a,
30b, rather than across the one primary winding and one set
of GFCI contacts 36 as shown, so as to avoid open-circuiting
the sensor secondary winding.
Also to be noted is the fact that the GFCI device
contacts 36 are not electrically involved in the shunt
trip solenoid energization circuit and need not be directly
mechanically involved with switch 50, as in the case of a
bell alarm switch. Consequently, if a particular installa-
tion does not call for a load 52 or that such load and its
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41 PR 3217
associated circuit does not require ground fault, then
the GFCI device may be devoid of contacts 36. I~ this
case, the shunt trip solenoid energization circuit
including switch 50 could be connected to the load side
of the GFCI device.
It will thus be seen that the objects set
forth, among those made apparent in the preceding descrip-
tion, are efficiently attained and, since certain changes
may be made in the above construction without departing
from the scope of the invention, it is intended that
all matter contained in the above description or shown
in the accompanying dra~ings shall be interpreted as
illustrative and not in a limiting sense.
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