Arc fault circuit interrupter without DC supply

Arcing faults are detected by sensing a voltage signal proportional to the rate of change, or di/dt, of the line current when the current steps into an arc fault. A current transformer is used to create the di/dt signal. The transformer has a selected core type, number of windings, and size which saturates at a pre determined level of primary current. The saturation acts to clamp the maximum di/dt voltage signal at the transformer output, and produce a constant output voltage. The constant output voltage eliminates a signal with a high di/dt, which may occur during a lamp burnout, from producing a much larger di/dt signal than that caused by an arc fault at a lower level of step current. The di/dt signal is passed through a high pass filter, which attenuates 60 hz sinusoidal signals, after which the signal is integrated. The integrator acts to delay circuit interruption means until a predetermined number of arcs has occurred. When the integrator voltage reaches a predetermined voltage level, a trigger device activates an electronic switch, which in turn activates circuit interrupting means. One of the novel aspects of this invention is the elimination of the need for a DC power supply.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an apparatus for arc faults in electrical power
 lines, and more particularly to apparatus that does not include a DC power
 supply.
 2. Description of the Prior Art
 A number of devices and methods have been used in the past to detect arc
 faults. Some of the prior art devices and techniques have involved the use
 of E and B field arc sensors, the detection of the amplitude of arc fault
 rate of change of current signals, the use of non-overlapping band-pass
 filters to detect white noise of arcs, and devices which detect the
 disappearance of arc faults near current zero crosses. Most of the prior
 art of arc detection occurs in circuit breakers where it acts as an
 enhancement to thermal-magnetic detection elements, which alone may fail
 to detect arc faults.
 There is a need for an economical arc fault detector which may be mounted
 into a wiring device which offers the same down stream protection as an
 arc fault detecting circuit breaker but at the similar cost advantage that
 currently exist between ground fault interrupting receptacles and ground
 fault interrupting circuit breakers. This invention provides that cost
 advantage.
 SUMMARY OF THE INVENTION
 It is an object of this invention to provide an arc fault circuit
 interrupter that is simpler and less expensive to manufacture than those
 previously known. Briefly stated, and in accordance with a presently
 preferred embodiment of the invention, an arc fault detecting circuit
 includes a saturating current transformer coupled to the electric power
 circuit to be protected that senses di/dt arc fault steps in current. A
 rectifier is preferably connected to the sensor, and an integrator is
 connected to the rectifier. The integrator voltage is used to activate a
 trigger device. The trigger device activates circuit interrupting means.
 The device does not require a DC power supply as found in the prior art.
 The saturating current transformer produces a constant output voltage at a
 pre determined level of arc fault current. This eliminates the effect of
 brief high current pulses of short duration from having the same effect on
 the detector as a series of smaller amplitude arc faults steps.
 In accordance with a further embodiment of the invention, a second
 integrator and trigger device is used instead of a saturating transformer
 to mitigate the effect of normal switching arcs and also the brief high
 current arc pulses.
 In accordance with another embodiment of the invention, an inhibit switch
 is placed between the integrator and trigger device, where it acts to
 inhibit the di/dt charging integrator from activating the trigger device
 unless the associated peak 60 Hz component of arc current exceeds a
 predetermined value.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 An arc fault detector in accordance with this invention is intended for
 incorporation into a receptacle, plug, or cord type device which is
 mechanically similar to ground fault interrupter devices such as those
 shown in U.S. Pat. Nos. 5,594,358, and 5,510,760.
 Referring now to FIG. 1a, one embodiment of the invention is shown in
 schematic diagram form, showing an electrical power line, comprising line
 side neutral and hot conductors 6 and 7 respectively, load neutral and hot
 conductors 50 and 51 respectively and a load 52. The load neutral 50 and
 hot 51 conductors are both protected by contactor mechanism 45. Contactor
 mechanism 45 is a spring loaded mouse trap type device, which is
 controlled by trip mechanism 44. Conduction of SCR 41 activates solenoid
 43 which activates trip mechanism 44. When the trip mechanism is
 activated, the spring loaded contacts 26 and 28 latch open, and stay open
 until they are manually reset.
 An arc fault sensing current transformer 1 is wrapped with a secondary
 winding 4, which surrounds the line conductor 7. The transformer has a
 core type, number of windings, secondary loading, and size which are
 selected to produce an output voltage proportional to step changes (di/dt)
 in current. The core is also selected to saturate at a predetermined level
 of primary current. The saturation acts to clamp the maximum di/dt voltage
 signal at the transformer output. This eliminates the effect of brief high
 current pulses of short duration from having the same effect on the
 detector as a series of smaller amplitude arc faults steps. Clamping the
 amplitude by saturation prevents false triggering on load events such as
 lamp filament burnouts which cause brief high current pulses.
 The hot conductor 7 is used as the arc fault sensed conductor as any
 current arcing to neutral or ground will be supplied from this wire. The
 current transformer 1 is connected to bridge rectifier 10 by way of series
 capacitor 8. Capacitor 8, which in conjunction with resistor 8a, acts as a
 high pass filter for rejecting 60 hz sinusoidal signals. Capacitor 8 and
 resistor 8a may be eliminated without loss of arc fault detection but with
 an increased susceptability to false detection from large magnitude 60 HZ
 load currents such as may occur during motor start ups.
 When an arc fault occurs, step changes in current produce rectified pulses
 at the output of bridge rectifier 10. The rectified pulses are integrated
 by an integrator 24, for a predetermined time interval. The repeated
 charge pulses act to charge capacitor 16, and raise the voltage across
 capacitor 16 to the trigger threshold of diac 18. Diac 18 is a trigger
 device which triggers into conduction at one voltage, and which then shuts
 off at a lower voltage. The diac conduction discharges a portion of the
 charged stored in capacitor 16 into the gate 22 of SCR 41 triggering the
 SCR into conduction. SCR 41 conduction energizes solenoid 43, which
 activates trip mechanism 44, opening contacts 45.
 Capacitor 6 is used for transient suppression of noise at frequencies above
 that required for arc detection. Capacitor 20 acts to suppress noise
 voltage that may cause false triggering of SCR 41. Zener diode 10a is an
 optional voltage clamp, or limiter connected between the output of bridge
 rectifier 10 and circuit common 46, and performs the same function as the
 saturating current transformer in limiting the di/dt voltage pulses to a
 constant level. Resistor 14 discharges the integrator capacitor 16 after a
 pulse charging sequence has terminated. Resistor 8a is a bleeder resistor
 for capacitor 8 and performs the same function as resistor 14. An optional
 inductor 11, which negates the need for a saturating current transformer,
 as shown may be placed between the current transformer 1 and the
 integrator 24 which acts as an attenuating impedance for large di/dt
 pulses such as may occur during lamp burnout.
 Unlike most or all of the prior art, this design is very simple, and has
 the advantage of not requiring a DC power supply.
 FIG. 1b shows another circuit embodiment of the sensor transformer, high
 pass filter, and rectifier. This circuit performs the same functions of
 high pass filter and rectifier as shown in FIG. 1a with an additional
 voltage doubler feature. Capacitors 90 and 92 form a high pass filter with
 resistors 94 and 96 and also act as voltage doubler elements. An arc step
 in one direction causes a positive voltage at secondary output 91,
 developed across winding 4b, with respect to the transformer center tap 83
 held at circuit common. The same arc step also produces a negative voltage
 at secondary output 97, developed across winding 4a, with respect to the
 center tap 83. The positive voltage at output 91, drives part of the
 current through capacitor 92, which current is rectified by diode 99, and
 then passes out to the sensor pulse line 9. At the same time, the negative
 voltage at 97 causes a current to pass from the center tap 83 through
 diode 98 and capacitor 90 and back to the transformer secondary at 97.
 This current charges capacitor 90 with a positive polarity at junction 82,
 with respect to transformer secondary output 97, and causes the capacitor
 to store charge. When an arc step in the other direction occurs, output 97
 of secondary 4a becomes positive with respect to the center tap. This
 voltage drives a current through capacitor 90, then part of the current
 into diode 95, and out to signal line 9. The part current moves most of
 the stored charge in capacitor 90 plus the new charge arriving with the
 current from the transformer into the integrator capacitor 16, where it
 acts to pump the voltage above the level that would have occurred without
 the stored charge in capacitor 90. The next arc step in of the opposite
 polarity causes the same charge pump action out of capacitor 92. This
 action causes capacitors 90 and 92 to act as charge pump capacitors
 repeatedly charge pumping the integrator capacitor 16. Resistors 80 and 81
 are optional bleeder resistors for capacitors 90 and 92. In operation the
 circuit losses, along with the bleeder resistor discharge action on the
 capacitors, prevents actual voltage doubling. The voltage doubler action
 allows a sensor with fewer windings, when responding to lower arc sense
 pulse amplitudes, to produce the same diac trigger voltage as would occur
 without the doubler action. Those skilled in the art will understand that
 the components connected across one half of the secondary shown in FIG. 1a
 could be used with an untapped secondary winding, which produces a voltage
 doubler circuit that responds only to arc steps in one direction.
 FIG. 2 shows a further embodiment to the circuit of FIG. 1. All other like
 parts are like numbered. In this embodiment integrator 68 is of a lower
 timer constant than integrator 24. Diac 66 has a higher trigger voltage
 than diac 18. When integrator capacitor 64 quick charges on a large arc
 signal pulse, which causes the voltage of the capacitor 64 to reach the
 trigger threshold of diac 66, a pulse of charge is removed from capacitor
 64 by diac 66. In this manner large arc signal pulses caused by lamp burn
 out type events are mitigated by the charge dump action of diac 66. The
 diac 66 acts as a superior clamp to that of a zener diode as short
 duration arc pulses occurring from lamp burnout cause a discharge action
 on integrator capacitor 64 instead of only a clamping action In this way
 the charge effect on the detector of a short duration high current pulse
 is quickly mitigated. During actual high current arc faults which may also
 trigger the charge dump diac 66, the repetitive signal pulses pass enough
 charge into integrator 24 before the charge dump diac 66 is triggered to
 cause charging action of integrator 24. This activates diac 18 and SCR 41.
 Referring now to FIG. 3, an arc fault circuit interrupter in accordance
 with another aspect of this invention is illustrated in schematic diagram
 form. The interrupter is similar to the embodiment of the invention shown
 in FIG. 1 in many respects, and to that end, like components designated by
 like reference numbers. Arc fault sensing current transformer 1 is
 responsive both to the rate of change of current with respect to time
 (di/dt) and to the 60 Hz arc current produced by an arc fault.
 During an arc fault, a large di/dt pulse is typically produced along with a
 sustained 60 Hz arc current signal. The di/dt current pulse is rectified
 by bridge rectifier 10 and is conducted by blocking diode 101 to
 integrator 24 where it acts to charge integrator cap 16. Diac 18 is
 selected so that if the voltage across integrator capacitor 16 exceeds a
 predetermined magnitude, diac 18 conducts and a signal is applied to the
 drain of fet 106.
 The output of bridge rectifier 10 is also connected through decoupling
 resistor 118 to zener diode 116. Zener diode 116 clamps and strips the
 di/dt pulse off of the rectified 60 Hz arc current. The di/dt stripped
 rectified 60 Hz arc current is applied to integrator 114 where it acts to
 charge capacitor 115. When the voltage across the integrator capacitor 115
 rises above the clamp voltage of zener 118, the gate of fet 106 is
 enabled. In this way, both the the minimum di/dt value and the minimum 60
 Hz arc current value must exceed the predetermined thresholds before fet
 106 will conduct and provide a turn on signal to gate 22 of SCR 41, which
 activates solenoid 43 and trip mechanism 44, disconnecting the load from
 the source. Zener 118 may be replaced by trigger device, such as a second
 diac, for snap action triggering of the fet 106 gate.
 Preferably a zener diode 102 is connected across the output of integrator
 24 to limit the voltage applied to diac 18. The zener 102 is required to
 protect the drain source junction of fet 106 from any excessive voltage
 that may appear across capacitor 16 and diac 18 when fet 106 is held in
 the off state. Alternately, zener diode 104 is connected between the fet
 106 drain and circuit common, to limit the voltage that can be applied at
 the drain. The diodes 102 or 104 are required when di/dt pulses occur
 without an accompanying predetermined 60 hz current peak indicative of an
 arc fault. In this situation diac 18 could not prevent capacitor 16 from
 continuing to charge past the diac trigger voltage allowing an excessive
 voltage to build up at the fet 106 drain. These types of pulses can be
 generated by light dimmers or phase controlled motor controllers.
 The circuit shown in FIG. 3 is particularly well suited to discriminate
 between the high di/dt signals produced by lamp dimmers and the like,
 which are not accompanied by sustained 60 Hz arc current, from actual arcs
 which include both a high di/dt current and sustain 60 Hz arc current
 exceeding a preselected value.
 FIG. 4(a-e) illustrate waveforms generated in the circuit of FIG. 1. A
 typical arcing current waveform is shown at 102 in FIG. 4a. Step 104 in
 FIG. 4a shows one of the step increases in current that generates waveform
 106 shown in FIG. 4b at the secondary 4 of transformer 1. The arc current
 102 in FIG. 4a will have broad band noise shown at 105. Typically the arc
 extinguishes at the next current zero cross but in some cases may have a
 sharp extinguishing edge as shown at 102a in FIG. 4a which may generate a
 weak pulse shown at 102b, 102c and 102d in FIG. 4b, 4c, and 4d
 respectively. FIG. 4c pulse 108 shows the bridge rectified pulses. FIG. 4d
 charge waveform 112 shows the integrator capacitor 16 pulse charging. At
 114 in FIG. 4d, the diac triggers into conduction sending a pulse of
 current 118 into SCR gate 22 causing the SCR to conduct. When the SCR
 conducts solenoid 43 is energized and activates trip mechanism 44. This
 opens contact 45 disconnecting the load. Waveform 116 in FIG. 4d shows
 capacitor 16 discharging to the diac turn off voltage.
 While the invention has been described in connection with a number of
 presently preferred embodiments thereof, those skilled in the art will
 recognize that many modifications and changes may be made therein without
 departing from the true spirit and scope of the invention, which
 accordingly is accordingly is intended to be defined solely by the
 appended claims.