Note: Descriptions are shown in the official language in which they were submitted.
CA 02300344 2000-03-10
APPARATUS FOR DETECTING ARCING FAULTS AND GROUND
FAULTS IN MULTIWIRE BRANCH ELECTRIC POWER CIRCUITS
Related Application
This application is a continuation-in-part of Canadian Patent
Application Serial No. 2,248,486 filed on September 28, 1998.
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to apparatus providing protection in
multiwire branch circuits of electric power distribution systems, and in
particular, to circuit breakers providing protection from arcing faults and
ground faults in such circuits.
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 only permitted under certain conditions. Generally, they are permitted
as long as the two line conductors are energized by separate phases or by a
center tapped single phase to avoid overloading the neutral conductor, and
as long as all ungrounded conductors are disconnected simultaneously at
the panel board where the branch circuits originate. This simultaneous
disconnection 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 are provided with
short circuit and overcurrent protection by the tied single pole breakers or
the two pole breaker. Only the two pole breaker can also provide ground
fault protection by the addition of a common ground fault detector.
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
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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 to my knowledge they have not been adapted to multiwire
branches.
Parent application 08/939,263 filed on September 23, 1998
discloses apparatus for detecting faults in multiwire branch circuits where
both poles are tripped simultaneously in response to all faults, overcurrent,
arcing and ground faults, regardless of which line conductor is involved.
This arrangement complies with the general conditions for multiwire
branch circuits discussed above where all ungrounded conductors must be
opened simultaneously. This is because the two line conductors can be
connected to a single appliance, such as an electric stove for instance, to
provide 220 volts, or to two separate outlets in a common receptacle.
However, where the two line conductors, with the common neutral, are
connected to independent loads, both circuits do not have to be interrupted
simultaneously in response to an overcurrent condition (a short circuit or
overload) in one line conductor. By only interrupting current in the
affected line conductor, disruption of service is minimized. On the other
hand, an arcing fault occurs because of a wiring failure. This wiring failure
may be in the multiwire cable, and therefore, both poles should be tripped.
There is a need for a circuit breaker which can provide arc fault
protection as well as overcurrent, and ground fault protection for multiwire
branch circuits and which isolates overcurrent trips to the affected circuit
while tripping both circuits in response to arc faults or ground faults.
SUMMARY OF THE INVENTION
These needs and others are satisfied by the invention which is
directed to an apparatus for detecting faults in multiwire branch circuits. It
includes a two pole circuit breaker having a first pole connected to interrupt
current in the first line conductor and a second pole connected to interrupt
current in the second line conductor. The first and second poles have first
and second thermal magnetic trip devices which operate independently to
only interrupt an overcurrent condition in the affected pole. The apparatus
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further includes fault detection circuitry including a first arc fault
detector
connected to detect arc currents in the first line conductor and to generate a
first trip signal in response thereto, a second arc fault detector connected
to
detect arc currents in the second line conductor and to generate a second
trip signal in response thereto, and a ground fault detector connected to
detect ground faults between each of the line conductors and ground and
generate a third trip signal. The apparatus also includes means responsive
to any of these three trip signals to trip both poles of the circuit breaker.
Preferably, the ground fault detector detects neutral to ground faults in
addition to line to ground faults. It is also preferred that the ground fault
detector have a power supply fed by each of the line conductors so that it
remains operative even with one line unpowered.
Preferably, the arc fault detectors utilize the bimetal of the
respective pole which is connected in series with the associated line
conductor as a known impedance for monitoring the line current. In such
an arrangement, the first and second arc fault detectors are referenced to the
associated line conductor, and the trip signals are electrically isolated such
as by optocouplers from the common trip circuit. BRIEF DESCRIPTION OF
THE DRAWINGS
A full understanding of the invention can be gained from the
following description of the preferred embodiments when read in
conjunction with the accompanying drawings in which:
Figure 1 is a schematic diagram partly in block form of
apparatus in accordance with the invention.
Figure 2 is a schematic diagram of the arc fault detectors which
form part of the apparatus of Figure 1.
Figure 3 is a schematic diagram of the ground fault power
supply.
Figure 4 is a schematic diagram of the power supplies for the
arc fault detectors.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, a multiwire branch circuit 1 of an electric
power distribution system includes a first line conductor 3, a second line
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conductor 5 and a common neutral conductor 7. As previously mentioned,
the two line conductors 3 and 5 are energized by separate phases, or a center
tapped single phase supply voltage. Typically, the neutral conductor is
grounded as shown at 9. Protection in this multiwire branch circuit 1 is
provided by a two pole circuit breaker 11 which includes separable contacts
131 and 132 connected in series in the line conductors 3 and 5, respectively.
Each pole has a thermal-magnetic trip device 141, 142 which includes a
bimetal 151 and 152 connected in series in the associated line conductor.
The bimetals 15 respond to the heat generated by persistent overcurrent
conditions to trip a spring powered operating mechanism 161, 162 which is
mechanically connected to open the associated set of separable contacts 131
or 132. The thermal-magnetic trip devices also include a magnetic actuator
181, 182 which actuates the associated operating mechanism to
instantaneously open the corresponding contacts 131 or 132 in response to
very high overcurrents such as those associated with a short circuit. The
thermal-magnetic trip devices 141, 142 operate independently, and hence,
only interrupt overcurrents in the associated pole.
The two pole circuit breaker 11 is located in a load center (not
shown) which provides for distribution of power in various circuits such as
the multiwire branch circuit 1. In addition to the instantaneous and
delayed overcurrent protection provided by the thermal-magnetic trip
devices, the two pole circuit breaker also includes arc fault protection for
each of the line conductors 3 and 5 and ground fault protection for all three
conductors 3, 5 and 7. Separate arc fault detectors 171 and 172 are provided
for the line conductors 3 and 5, respectively. Each of these arc detectors
includes a current sensor to detect current in the associated line conductor.
In the preferred embodiment of the invention, the bimetal 15 is used as the
current sensor. As the low resistance of the bimetal 15 is known, the
voltage drop across the bimetal is a measure of the current in the associated
line conductor. The voltages across the bimetals are sensed through the
leads 191 and 192. Since the arc fault detectors 17 use the associated bimetal
15 as a current sensor, they need to be referenced to the associated line
CA 02300344 2000-03-10
voltage. Accordingly, the Line 1 arc fault detector 171 has a ground 211
referenced to the line conductor 3 while the Line 2 arc fault detector 172 has
a ground 212 referenced to the line 2 conductor 5. Each of the arc fault
detectors 171 and 172 has its own power supply 231 and 232 connected
5 between the associated line conductor and the neutral conductor 7.
Ground fault protection is provided by the ground fault
detector 25. In the preferred embodiment of the invention, the well known
dormant oscillator type ground fault detector is employed. A first ground
fault detector coil 27 encircles all three of the conductors 3, 5 and 7. In
the
absence of a ground fault, the resultant current through the coil 27 carried
by the three conductors will be zero. A ground fault on either of the line
conductors 3 or 5 will create an imbalance in the currents which will be
detected by the coil 27. As the neutral conductor 7 is grounded at 9 close to
the circuit breaker 11, a ground fault between neutral and ground will not
generate a sufficient imbalance in current for the coil 27 to detect. A second
coil 29 is used to inject a small voltage into the neutral conductor. If a
ground fault is present on the neutral conductor, a loop completed by this
ground fault will support an oscillation which will be detected by the
ground fault circuitry.
The ground fault detector 25 is powered by a ground fault
power supply 31. The ground fault power supply 31 is connected to both of
the line conductors 3 and 5 by the leads 331 and 332 so that the ground fault
detector 25 is operational if at least one of the line conductors 3 or 5 is
energized and the contacts 13 of the circuit breaker are closed. A common
lead 35 is connected between the ground fault power supply 31 and the
neutral conductor 7. As will be seen, with both line conductors 3 and 5
energized the output voltage of the ground fault supply provided on the
leads 371 and can be as high as about 300 volts dc. This voltage is used to
energize shunt trip coils 391 and 392 which results in the opening of both
sets of separable contacts 131 and 132 when a silicon controlled rectifier
(SCR) 41 is turned on in a manner to be discussed. A circuit 43 draws power
from the ground fault power supply 31 through the coils 391 and 392 to
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energize the ground fault detector circuit 25. The current drawn by this
power circuit 43 is insufficient to energize the coils to open the contacts
13,
but is sufficient to operate the ground fault detector 25. The circuit 43
includes a zener diode 45 which clamps the voltage across a capacitor 47 to
about 43 volts. A resistor 49 forms a filter with the capacitor 47 for this 43
volts dc. Another resistor 51 limits the current drawn by the circuit 43 to
below the level needed to energize the coil 37 and open the separable
contacts 13. The 43 volts dc is applied to the chip implementing the ground
fault detector 25 which contains an arrangement of zener diodes
represented by the zener diode 53 which generate 26 volts and other
voltages needed by the ground fault detector circuit 25. This 26 volt dc is
filtered by a capacitor 55.
The Line 1 arc fault detector 171 generates a trip signal on a
lead 571 in response to detection of an arc fault on the line 1 conductor 3.
Similarly, the Line 2 arc fault detectors 172 generates a trip signal on the
lead
572 in response to detection of an arc fault on the line 2 conductor 5. The
ground fault detector 25 generates a trip signal on the lead 59 in response to
detection of a ground fault between any one of the conductors 3, 5 and 7 and
ground. The ground faults detected by the detector 25 can be direct faults to
ground or themselves can be arcing faults between a conductor and ground.
A common trip circuit 61 responds to a trip signal generated by
any one of the arc fault detectors 171 or 172 or the ground fault detector 25.
This common trip circuit 61 includes the SCR 41, which when turned on
energizes the trip coils 391 and 392 to open the separable contacts 13. As the
arc fault detectors 171 and 172 are referenced to the line conductor on which
they are providing protection, the three detector circuits must be
electrically
isolated from one another. This isolation is provided by optocouplers 631
and 632 which convert the trip signals on the leads 571 and 572 to signals on
leads 651 and 652 of the common trip circuit 61. Drive current for the
optoisolators 631 and 632 is provided from the 26 volt dc supply of the
ground fault detector circuit 25 through resistor 67. The leads 651 and 652
from the optoisolators are connected in parallel which each other and with
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the lead 59 from the ground fault detector 25 to the gate of the SCR 41 so
that any one of the three signals can trip the contacts 131 and 132 of both
poles open.
As will be noted, the thermal-magnetic trip devices only open
the contacts in the associated pole, while the arc fault circuits and the
ground fault circuit open both poles. This arrangement is used where the
loads connected to the two poles are independent. Under such
circumstances, overcurrents and especially overloads result from
conditions at the load, and hence, disruption is minimized by only
interrupting current to the affected load. On the other hand, arcing and
ground faults are typically wiring problems which could be occurring in the
home run cable. Therefore, in response to these indications of a wiring
problem, both circuits are interrupted.
Figure 2 illustrates a suitable arc fault circuit 171. A similar
circuit can be provided for the arc fault circuit 172 keeping in mind that
each
must be referenced to the line conductor for which it is providing
protection since the bimetal in the conductor is being used for current
detection. The voltage across the associated bimetal 15 is provided on the
leads 191. The arc fault circuit 17 includes a pulse generator 69, a circuit
71
which provides a time attenuated accumulation of the pulses generated by
the pulse generator 69, and an output circuit 73 which provides a trip signal
on the lead 57.
The pulse generator 69 includes a high pass filter 75 formed by
the series connected capacitor 77 and resistor 79, followed by a low pass
filter
81 formed by the parallel connected capacitor 83 and resistor 85. The high
pass filter 75 and low pass filter 81 have a band pass in a range of about 400
to 590 Hz which generates pulses in response to the step increases in current
caused by striking of an arc.
An operational amplifier (op amp) 87 provides gain for the
pulses. A capacitor 88 reduces high frequency noise in the pulse signals.
The op amp 87 is biased at its non-inverting input by a 13 vdc supply
voltage. A resistor 89 and capacitor 91 delay application of the bias to
prevent false trip signals during power up. The positive and negative
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pulses generated by the band pass filter ride on the plus 13 vdc volt bias
applied to the op amp 87. This bias is removed by the ac coupling capacitor
93 which along with the resistor 95 forms another high pass filter stage.
The bi-polar pulse signal resulting is rectified by a rectifier circuit 97
which
includes another op amp 99. Positive pulses are applied to the
non-inverting input of the op amp 99 through the diode 101 while negative
pulses are applied to the inverting input through the diode 103. The output
of the op amp 99 is a pulse signal having pulses of a single polarity.
The circuit 71 generates a time attenuated accumulation of the
pulses in the pulse signal generated by the pulse generator 69. The pulses
are accumulated on a capacitor 105 connected to the 26 vdc supply. A bleed
resistor 107 connected across the capacitor 105 provides the time
attenuation. The pulses are applied to the capacitor 105 through a transistor
109. When no pulses are generated, both electrodes of the capacitor 105 are
at 26 volts. The pulses from the pulse generator 69 provide base drive
current for the transistor 109. A voltage divider formed by the resistor 111
and 113 connected at their common connection to the emitter of the
transistor 109 set the minimum amplitude for the pulses to turn on the
transistor 109. This threshold is selected so that pulses which could be
generated by some normal loads, such as for instance a dimmer switch
operating at normal loads, are not accumulated. The amplitude of the
pulses is set by the gain of the op amp 99 which in turn is determined by the
ratio of the feed back resistor 115 and input resistor 117. The amplitude and
duration of each pulse determine the amount of charge which is applied to
the capacitor 105. The successive pulses are accumulated through the
summation of the charge they add to the capacitor 105. The resistor 107
continuously bleeds the charge on the capacitor 105 with a time constant
determined by the values of the capacitor 105 and resistor 107 to time
attenuate the accumulation of the pulses. It can be appreciated that the
magnitude and time interval between pulses determines the instantaneous
voltage that appears across the capacitor 105.
The output circuit 73 monitors the voltage across the capacitor
105 representing the time attenuated accumulation of the pulses in the
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pulse signal generated by the pulse generator. Each pulse lowers the voltage
on the capacitor which is applied to the base of a transistor 119 in the
output
circuit. A voltage is applied to the emitter of the transistor 119 by the 13
vdc
supply through a resistor 121 and diode 123. With no pulses being
generated, the voltage on the base of the transistor 119 is 26 volts. Without
the diode 123, the 13 volt reverse bias would destroy the base to emitter
junction of the transistor 119. The diode 123 withstands this voltage.
When the voltage at the lower end of the capacitor 105, and therefore on
the base of the transistor 119, falls below the 13 volts minus the forward
drop across the diode 123, the transistor 119 is turned on. Feedback
provided through the lead 125 and the resistors 127 and 129 holds the
transistor 119 on by providing a continuous output of the op amp 99 which
holds the transistor 109 on. Turn on of the transistor 119 provides base
drive current for the transistor 131 which draws current limited by the
resistor 133 to generate an arc fault trip signal on the lead 571. The trip
signal actuates the optocoupler 631 which turns on the SCR 41 to trip the
separable contacts 13 open. The larger the pulses in the pulse signal
generated by the pulse generator 69 the harder the transistor 109 is turned
on, and hence, the faster charge is accumulated on the capacitor 105.
A circuit diagram of the ground fault power supply 31 is
shown in Figure 3. This power supply includes a bridge circuit 135 having
six diodes 137. Power is supplied to the bridge from the Line 1 conductor 3
through the lead 331 and from the Line 2 conductor 5 through the lead 332.
Output of the bridge is between the leads 371 and 372 and the ground fault
common 139. The lead 331 is connected to the mid-point of one leg of the
bridge 135 while the lead 332 is connected to the mid-point of another leg.
The neutral conductor 7 is connected to the mid-point of the third leg
through the lead 35. A pair of metal oxide varistors (MOVs) 141 protect the
power supply 31 from voltage surges on the line conductors. With both
line conductors energized the output of the power supply 31 across the lead
37 and common 139 is the line to line voltage. With only one line
conductor energized, the output of the power supply 31 is the line to
neutral voltage. As can be seen, the potential of the ground fault common
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139 changes. When the diode 1371 is conducting, ground fault common 139
is tied to the voltage on Line 1. With the diode 1373 conducting, it is tied
to
the voltage on Line 2, and if one line is not energized so that diode 1375
conducts on positive half cycles of the line voltage, the ground fault
5 common 139 is tied to neutral.
Figure 4 illustrates the power supply 231 for the Line 1 arc fault
detector 171. This power supply is connected on the hot side to the neutral
conductor 7 through the lead 143. The other side of the power supply is
connected to the Line 1 conductor 3 through the Line 1 arc fault detector
10 common 211. The diode 145 halfwave rectifies the neutral to Line 1 voltage
and the resistor 147 converts this rectified voltage signal to about a 6
milliamp current which charges a capacitor 149. The voltage across
capacitor 149 is clamped to 43 volts dc by the zener diode 151. Resistor 153
and capacitor 155 form a second filter. The voltage across the capacitor 155
is clamped at 26 volts dc by the zener diodes 157 and 159 to provide the 26
vdcl for the Line 1 arc fault detector 171. The common junction between
zener diodes 157 and 159 provides the 13 vdcl supply voltage for arc fault
detector 171. The power supply 232 for the Line 2 arc fault detector 172 has a
similar circuit configuration except that it is referenced to the common 212.
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 arrangement disclosed are meant to be illustrative only and not
limiting as to the scope of invention which is to be given the full breath of
the claims appended and any and all equivalents thereof.
