Arc fault protected device

[0002] This application is a continuation-in-part of application Ser. No. 09/129,685, filed Aug. 5, 1998 now U.S. Pat. No. 6,259,996 which is a continuation-in-part of application Ser. No. 09/026,193, filed Feb. 19, 1998 now U.S. Pat. No. 6,246,556.

[0003] Each of the above applications has the same assignee as the present application, and is incorporated herein by reference.

[0004] The present invention relates to the protection of electrical circuits and, more particularly, to the detection of arcing faults in an electrical assembly for mounting on device.

[0005] The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These protection devices are typically circuit interrupters, such as circuit breakers and fuses, which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed.

[0006] Although circuit breakers are a preferred type of circuit interrupter because they protect a number of outlets simultaneously, ground fault detectors in electrical plugs, have become increasingly popular for devices used in areas where a short circuit caused by contact with water is a possibility, e.g., for hairdryers, which tend to be used in bathrooms.

[0007] Typically, ground fault detectors interrupt an electric circuit due to a disconnect or trip condition, such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor which could be caused by a leakage current or an arcing fault to ground.

[0008] Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector, between two conductors supplying a load, or between a conductor and ground. Arcing faults may not, however, cause a conventional circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does,,not contact a grounded conductor, object or person will not trip a ground fault protector.

[0009] There are many conditions that may cause an arcing fault, for example, corroded, worn or aged wiring, connectors, contacts or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and cause the conductor to reach an unacceptable temperature.

[0010] It is an object of the present invention to provide an arc fault detection system and method which reliably detects arc fault conditions which may be ignored by conventional circuit interrupters.

[0011] Another object of the invention is to provide an arc fault detection system which utilizes highly reliable electronic signal processing components, so as to be relatively simple and yet highly reliable in operation. More particularly, since some electrical systems do not include circuit breakers with arc fault detection systems, the present invention provides an arc fault detection system mounted to a device to protect the device, and anything downstream from the device, from arcing faults.

[0012] Other and further objects and advantages. of the invention will be apparent to those skilled in the art from the present specification taken with the accompanying drawings and appended claims.

[0013] In accordance with one aspect of the invention, there is provided an arcing fault protection assembly comprising a sensor, a broadband noise circuit, and a controller. The sensor detects a current flowing at a device and develops a corresponding sensor signal. The broadband noise circuit determines the presence of broadband noise in the sensor signal and produces a corresponding output signal. The controller processes the sensor signal and the output signal in a predetermined fashion to determine whether an arcing fault is present. The sensor, broadband noise circuit, and controller are mounted to the device.

[0014] In accordance with another aspect of the invention, there is provided a method of determining, at a device, whether arcing is present. The method comprises the steps of sensing a current at the device and developing a corresponding sensor signal, determining the presence of broadband noise in the sensor signal and producing a corresponding output signal, and processing the sensor signal and the output signal in a predetermined fashion to determine whether an arcing fault is present.

[0015] The above summary of the present invention is not intended to represent each embodiment or every aspect of the present invention. This is the purpose of the Figures and detailed description which follow.

[0016] Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.

[0017] FIG. 1 is a functional block diagram of an arc fault detection system embodying the invention.

[0018] FIGS. 2 and 3 are block diagrams of an arc fault detector system and ground fault sensor in accordance with the present invention.

[0019] FIG. 4 is a prior art tripping mechanism in the closed position.

[0020] FIG. 5 is a prior art tripping mechanism in the open position.

[0021] FIG. 6 is a prior art tripping mechanism in the closed position.

[0022] FIG. 7 is a prior art tripping mechanism in the open position.

[0023] While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form described but, on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0024] Referring to the drawings and initially to FIG. 1, there is shown in block form a novel arcing fault detector system in accordance with the invention and designated generally by the reference numeral 10. In the illustrative example, the arcing fault detection/protection system 10 is mounted to an electrical device 13 which is to be monitored for arcing faults. The arcing fault protection system 10 may include a circuit interruption device or “tripping mechanism,” in which case it may be termed an arc fault circuit interrupter (AFCI), and the device 13 may be termed an AFCI device. The arcing fault detection system 10 detects both series and parallel arcing at the device 13.

[0025] The device 13 may comprise a connector, such as an extension cord, a junction box, a switch or a dimmer, or the device may comprise a load, such as a lighting fixture, a smoke detector, or an appliance. The arcing fault protection system may be easily implemented with the existing circuitry of various devices, such as the dimmer, lighting fixture and smoke detector. The arcing fault detection system 10 may be mounted to the device 13 itself, to a cord used to connect the device 13 to a power source, or to a housing for the device 13. The arcing fault detection system 10 detects arcing faults originating at the device 13 and downstream from the device 43.

[0026] The most extensive protection from arcing faults is provided in electrical systems with an arcing fault detection system 10 closest to the power source, i.e., at a circuit breaker. The present invention provides protection from arcing faults in systems without such arcing fault detection systems 10 at the circuit breakers. The arcing fault detection systems 10 may advantageously be placed at a device 13 wired directly from a branch circuit originating at the circuit breaker box, and having the rest of the branch circuit extending downstream from the device 13 so as to detect arcing faults occurring throughout the system.

[0027] At least one sensor 16 is provided in association with the device 13 for producing a signal representative of a signal condition, such as power, voltage or current in the circuit 12. In the illustrated embodiment, this sensor 16 comprises a current rate of change sensor (di/dt). A line conductor 14 of the circuit 12 passes through the rate of change (di/dt) current sensor 16 which produces a signal representative of the rate of change of current flow in the line conductor 14. In the illustrative embodiment, both the line conductor 14 and a neutral conductor 15 pass through a ground fault detector or sensor 20 which is responsive to the current flowing through the line 14 and neutral 15 sides of the circuit 12 for producing an output signal at an output 22. If the current flow through the line 14 and neutral 15 conductors is different, this is indicative of a ground fault.

[0028] Preferably, the di/dt sensor 16 and the ground fault sensor 20 each comprise a toroidal coil having an annular core which surrounds the relevant conductors, with a toroidal sensing coil wound helically on the core. In the di/dt sensor 16, the core may be made of magnetic material, such as ferrite, iron or molded permeable powder, such that the sensor is capable of responding to rapid changes in flux. An air gap may be cut into the core in certain instances to reduce the permeability, and the core material is such that it does not saturate during the relatively high current produced by some forms of arcing so that arc detection is still possible. The particular requirements for the construction of the toroidal coil and core for the ground fault sensor 20 may differ somewhat for those from the di/dt sensor 16, such ground fault sensors or transformers being generally known in the art.

[0029] During operation, the current in the monitored device 13 generates a field which induces a voltage in the di/dt sensor 16. The voltage output of the sensor 16 is primarily proportional to the instantaneous rate of change of current. The calibration of the sensor 16 may be chosen to provide a signal in a range and frequency spectrum in which arcs can be most easily distinguished from loads 48. This range and spectrum may vary with the application.

[0030] Is The di/dt sensor 16 provides an input to an arcing fault detector circuit 24, which may include a broadband noise detector circuit, and to a current measuring circuit 26. In one embodiment, the components of the arcing fault circuit detector 24 and the current measuring circuit 26 are provided on an application specific integrated circuit (ASIC) 30. Suitable output signals from the ASIC 30 are fed to a microcontroller or microprocessor 40 (e.g., PIC16C73A) which, based on analysis and further processing of the signals provided by the ASIC 30, makes a decision as to whether to send a trip or “arc detected” signal 315 to an output 42. This trip signal 315 may be used for activating a trip circuit (not shown) which will, in effect, switch the line side conductor 14 of the 120 VAC circuit 12 to an open circuit condition to remove power from the circuit(s) in which arcing has been detected.

[0031] The broadband noise detector 24 comprises one or more band-pass filter circuits 50 which receive the rate of change of current signal from the di/dt sensor 16. The band-passes of these circuits 50 are selected to detect the presence of broadband noise in frequency bands which are representative of a frequency spectrum typical of arcing faults. Each of the band-pass filter circuits 50 feeds a filtered signal, comprising those components of an input signal from the di/dt sensor which fall within their respective band-pass frequency bands, to a signal detector circuit 52.

[0032] The output of the sensor 16 may also feed a time-integration or integrator circuit 18. The integrator may be a passive resistor-capacitor circuit followed by an amplified integrator, the output of which is proportional to the AC current. The integrator 18 provides a signal to be sampled by an analog-to-digital A/D converter 19. In one embodiment, the output of the A/D converter 19 is a series of 8 bit (minimum) values representing the current at a rate of 32 samples per half cycle. The A/D converter may be a part of the microprocessor or microcontroller 40. As the frequency drifts from nominal, the time between voltage zero crossings, detected at a zero crossing detection circuit 21, is measured using internal timers and used to vary the sample rate to achieve a constant number of samples per cycle.

[0033] The broadband noise circuit 24 determines if there is simultaneously a trigger level signal in two, or more frequency bands. In order to do this, a portion of the signal from the di/dt sensor 16 is routed to band-pass filters 50. The minimum number of band-pass filters is two. The frequency bands of the filters are chosen across the spectrum from 10kHz to 100 kHz. In one example, for a two-band implementation, the center frequencies are 33 kHz and 58 kHz. In this example, the output signals from the band-pass filters 50 are detected (rectified) and filtered with a low pass filter with a corner frequency of 5 kHz. The signal output of each frequency band is routed to a comparator (signal detector) 52, where it is compared with a reference voltage level and, if sufficient, causes an output pulse. The “trigger level” of the signal from each band required to produce an output pulse from the comparator is determined by analyzing the non-arcing load-generated signature of the application. Additional comparators (AND gates) are used to send a pulse whenever multiple filter bands simultaneously receive a triggering signal in their band. The resulting pulses indicating signal acquisition in multiple bands are counted by the microprocessor 40 and used in some arc detection algorithms.

[0034] The samples of current are converted into current_peak, current_area, and max(di/dt). These values are stored for each half cycle of voltage.

[0035] The use of the terms “band-pass filter,” “comparator,” “AND gate,” and “Integrator” does not limit the invention to hardware equivalents of these devices. Software equivalents of these functions can be implemented, provided the di/dt signal (from sensor 16) is first amplified and converted to digital values.

[0036] In the illustrative embodiment, a voltage sensor 25 is implemented as a resistor divider (not shown) which provides an attenuated voltage level compatible with the solid state logic devices. A zero crossing circuit 21 is implemented with a low pass filter (corner frequency 1 kHz) and comparators to provide a digital “1” when the voltage is above zero volts and a digital “0” when the voltage is below zero volts. The microcontroller 40 accepts the logic levels and incorporates timers to determine if the system frequency has increased or decreased from the previous cycle. The A/D sample rate is then adjusted faster or slower to maintain 64±1 samples per cycle.

[0037] The ground fault sensor 20 feeds a ground fault amplifier 120 and an absolute value circuit 122 which form the ground fault detector circuit 28. The ground fault. amplifier 120 essentially amplifies the low level difference in the current flow between the line 14 and neutral 15 conductors as detected by the ground fault sensor 20. The absolute value circuit 122 turn is negative-going signals into positive signals and passes positive-going signals through unchanged.

[0038] FIG. 1 illustrates one embodiment of an ASIC 30 for performing the above-described operations. Further details of an ASIC 30 and equivalent circuits which may be utilized may be found in the above-referenced co-pending application Ser. No. 09/026,193. Details regarding an algorithm used to analyze the current waveforms and broadband noise to determine whether arcing is present may be found in the above-referenced co-pending application Ser. No. 09/129,685.

[0039] As further illustrated in FIGS. 2 and 3, the ground fault sensor 20 comprises a toroidal coil having a grounded neutral winding 300 and a ground fault current winding 302. The di/dt sensor 16 is a toroidal coil having an annular core surrounding the line conductor 14, and a di/dt winding 304 and a self-test winding 306. The respective windings 300, 302, 304 form like-designated inputs to arcing fault and ground fault circuit 308.

[0040] FIG. 2 depicts the arcing fault circuits and ground fault circuits on one integrated circuit 308, while FIG. 3 depicts the arcing fault circuits and self-test circuit on a first integrated circuit 310 with the ground fault circuits on a second integrated circuit 312. Although a trip circuit block 314 in FIG. 3 is separate from the ground fault sensor 20 and arcing fault detector system 10 blocks, it may also be integrated into either block.

[0041] In an effort to conserve space, both chip-on-board technology and resistor-on-board technology may be implemented in the arcing fault detector system 10 of the present invention. The chip-on-board technology takes the silicon die of a chip, places it on a circuit board, and covers it with a plastic-like coating. This saves space over the traditional method of using a standard package. A similar idea is to use ball grid array (BGA) packages. These packages save as much space as the chip-on-board technology, but have the advantage of not requiring a clean room. They do, however, require X-ray equipment to inspect the chips. Many manufacturers of silicon chips, including SVI Public Co., Ltd., and Argo Transdata Corp., are now supporting BGA's.

[0042] Resistor-on-board technology is a screening process where standard resistors are screened onto a circuit board. Although screened-on resistors are not smaller length-wise or width-wise, they are flat. Therefore, other components can be placed over them. Multek (a DII company) manufactures boards with screened-on resistors.

[0043] Conventional tripping mechanisms for ground fault circuit interrupters (GFCI) in receptacles may be implemented in conjunction with the arcing fault detector system 10. For example, FIGS. 4 and 5 illustrate a prior art GFCI receptacle tripping mechanism. FIG. 4 illustrates a tripping mechanism for a GFCI and/or AFCI receptacle in the closed position where current can flow though the receptacle to the load 48. The current originates through a pigtail or other flexible conductor 316, through a movable contact arm 318, through mating contacts 320 and to a stationary contact arm 322. The contacts 320 are held closed by a spring 324, while one end of the movable contact arm 318 is supported by and pivoted about a latch 326. A clapper arm 328 is held biased away from a trip coil 330 by a spring 332 with sufficient force to support the movable contact arm 318 in the latched position. A reset and trip indicating button 334 is normally biased into a receptacle housing 336 by a spring 338.

[0044] When the trip coil 330 is energized by turning on an SCR or equivalent, to short it across the line voltage, the clapper 328 is momentarily pulled to the closed position against a pole piece 340, thereby releasing the latch 326. As seen in FIG. 5, when the clapper 328 closes and removes the support from the latch end of the movable contact arm 318, the arm 318 is free to rotate about the end of the reset button 334 at end 342. The arm 318 then rotates until it contacts stationary surface 344 and opens the contacts 320 to disconnect the receptacle load 48 from the line 14. The trip coil 330, being connected to the load side of the contacts 320, is de-energized when the contacts 320 open.

[0045] Although only one set of contacts 320 is shown in FIGS. 4 and 5, a second set of contacts and contact arms are typically provided to open both the line conductor 14 and neutral conductor 15.

[0046] In the tripped state, the trip indicator 334 is biased out of the receptacle housing 336, indicating that the device has tripped until the spring 338 is fully compressed. To reset the mechanism to its closed state, the reset button 334 is pushed into the housing 336. This causes the contact arm 318 to pivot about the stationary surface 344, raising the latch end of the contact arm 318 such that spring 332 pulls the clapper 328 back into the latched position. The reset button 334 is then released and the contacts 320 close, while latch 326 once again supports the contact arm 318.

[0047] FIGS. 6 and 7 illustrate an alternate prior art embodiment for a GFCI tripping mechanism. In the closed position, as shown in FIG. 6, when current flows through the receptacle 11 to the load 48, the current path is through a pigtail or other flexible connector 346, through a movable contact arm 348, through mating contacts 350 and to a stationary contact arm 352. A spring 354 forces a trip indicator/reset button 356 in a direction outward from the receptacle housing 358. The trip indicator/reset button 356, in turn, pushes on lever 360 in a direction to force the contacts 350 to close. A notch on the lever 360 forms a latch 364 which mates with the contact arm 348. A plunger bias spring 366 biases the plunger 368 out of a trip coil 370 and holds the lever 360 against the movable contact arm 348 in a latched position. A spring 372 biases the movable contact arm 348 toward the open position, however, latch 364 holds the contacts 350 closed.

[0048] When the trip coil 370 is energized by turning on an SCR or equivalent to short the trip coil 370 across the line voltage, the plunger 368 is drawn into the trip coil 370 against the plunger bias spring 366, as shown in FIG. 7, and moves the lever 360 so that the latch 364 is released. With the latch 364 released, the movable contact arm 348 is free to rotate about a pivot 374 under the influence of the spring 372. The contacts 350 are thereby separated, disconnecting the receptacle load 48 from the line. The trip coil 370, being connected to the load side of the contacts 350, is de-energized when the contacts 350 open.

[0049] Although only one set of contacts 350 is shown in FIGS. 6 and 7, a second set of contacts and contact arms are typically provided to open both the line conductor 14 and neutral conductor 15.

[0050] In the tripped state, as shown in FIG. 7, the trip indicator/reset button 356 extends from the receptacle housing 358, being forced outward by spring 354, to indicate that the device has tripped. To reset the mechanism to its closed state, the trip indicator/reset button 356 is pushed into the housing 358. This causes the lever 360 to move in a direction toward the movable contact arm 348. With the trip coil 370 de-energized, the plunger 368 and lever 360 are biased away from the trip coil 370 by the plunger bias spring 366. As the latch 364 on the end of the lever 360 moves past the latch surface on the movable contact arm 348, the latch 364 is engaged. After the reset button 356 is released, the spring 354 once again forces the contacts 350 together, as described above.

[0051] While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.