Abstract:
An arc fault circuit interrupter which protects an AC circuit subjected to a possible arcing fault includes a sensor which produces a signal that can be evaluated to see if it is associated with arc fault current. The evaluation is done by a discrimination circuit which identifies a characteristic of the sensor signal associated with cessations of the arc fault current. An accumulator counts the number of occurrences of the cessations, and if the number of cessations counted within a predetermined time interval exceeds a predetermined number, the arc fault circuit interrupter interrupts the AC circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. Provisional Application Ser. No. 60/344,457 filed Nov. 9, 2001 and entitled AFCI WHICH DETECTS THE CESSATION OF ARCS OF AN ARC FAULT, incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to the field of arc fault detectors and interrupters, and more particularly to an arc fault interrupter which ignores signal associated with arc make which is mimicked by phase controllers, while detecting and interrupting signal that is uniquely associated with arc cessation, in order to eliminate false operation of the arc fault interrupter. 
   BACKGROUND OF THE INVENTION 
   A percentage of fires each year are caused by electrical branch circuit wiring arcing faults involving currents below the trip level of a conventional circuit breaker or OCPD (over current protection device) as well as below the handle rating of the breaker. Basic overcurrent protection afforded by circuit breakers is designed to prevent I 2 R heating of the wiring in an electrical distribution system, which is typically caused by circuit overloading due to short circuits and not arcing faults. A true short circuit is a rarity in an electrical system. In fact, it is more accurate to think of electrical faults as being an arc fault with some level of impedance (low current) or with a low impedance (high current). Many electrical faults begin as high impedance breakdowns between the line and neutral conductors or between the line conductor and the ground wire or device components. AFCI (Arc Fault Circuit Interrupter) technology affords protection from conditions that may not necessarily be an immediate threat but could become hazardous if left unattended. 
   In order to start a fire, three elements must be present: fuel, oxygen (air), and energy to ignite the fuel. Arcing is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can reach temperatures of several thousand degrees Celsius. Arcing produces sufficient energy to reach the ignition point of nearby combustible material(s) before a circuit breaker can respond. Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers or OCPD&#39;s, which alone usually do not detect and respond to arc faults. 
   There are different types of arc faults such as those that are known as “A-type” which occur across a break in the line or neutral conductors or at a loose terminal in a branch circuit of a distribution network. The conductors are carrying current to a load derived from the line voltage. The arc could likewise occur as a break in the conductors or at a loose terminal associated with an extension cord deriving power from line voltage, thereby completing the circuit to the load. Since the current through the A-type fault is limited by the impedance of the load itself, i.e., because the fault is in series with the load, an A-type fault is also known as a “series fault.” 
   “B-type” arc faults are a second arcing condition. In a B-type fault, the arc occurs between two conductors in the branch circuit or extension cords plugged into it, at a site where the insulating media separating the two conductors has been compromised, e.g., by a staple penetrating the middle of an extension cord causing part of the insulation between the conductors to be nullified. The arc may occur across the line and neutral conductors or the line and ground conductors, or in the case of reverse polarity where the line voltage is reverse-polarized, between the neutral and ground conductors. The current through the B-type fault is not limited by the impedance of the load, but rather by the available current from the supply, as established by the impedance of the conductors and terminals between the source of line voltage and the position of the parallel fault, i.e., the conductive members carrying the current. Since B-type faults are effectively across the line, they are also known as “parallel faults.” 
   A primary problem in AFCI design is identifying an arc fault, such as A-type or B-type arc faults, without falsely identifying normal loads, such as phase controllers such as light dimmers that commonly employ a solid state switching device such as a triac. These devices tend to mimic certain characteristics of arc faults. The user varies the current delay angle of the switching device, which is the particular phase angle on each half cycle of the line voltage when the switching device becomes abruptly conductive. Once the switching device is conductive, the light or other controlled load is electrically connected to the line voltage for the remaining portion of the half line cycle, at which time the instantaneous current through the controlled load is at or near zero and the switching device turns off. The process of the switching device turning on at the current delay angle and turning off the end of the half cycle is repeated for subsequent half cycles. Each time the switching device turns on there is a high rate of change of current through the controlled load. The repetitive abrupt appearance of current is not unlike the behavior of arcing faults, wherein a sufficient instantaneous line voltage is necessary in order for the arc to strike. The current through the arc fault abruptly commences when the arc is struck, producing a high rate of change of load current similar to the switching device turning on at the current delay angle. 
   There is a need for an arc fault circuit interrupter that improves upon prior art devices that have detected arc faults on the basis of changes in the current which are unable to distinguish between signals produced by arc faults from those produced by phase controllers such as light dimmers. Methods have been devised that try to distinguish the two origins of changes in the current, for example, on the basis of the steepness of the edge of the first derivative of the current, i.e., the current di/dt, and on di/dt pulse amplitude or repetition rate patterns, but an AFCI using these methods is still prone to either a failure to detect and interrupt a true arc fault hazard or prone to false interruption due to phase controllers depending on the chosen sensitivity of the AFCI&#39;s detector. 
   SUMMARY OF THE INVENTION 
   Briefly stated, an arc fault circuit interrupter which protects an AC circuit subjected to a possible arcing fault includes a sensor which produces a signal that can be evaluated to see if it is associated with arc fault current. The evaluation is done by a discrimination circuit which identifies a characteristic of the sensor signal associated with cessations of the arc fault current. An accumulator counts the number of occurrences of the cessations, and if the number of cessations counted within a predetermined time interval exceeds a predetermined number, the arc fault circuit interrupter interrupts the AC circuit. 
   According to an embodiment of the invention an arc fault detector for protecting a circuit on an AC power line for distribution of an electrical current to a circuit includes a sensor for sensing step transitions in the current; a discriminator receiving an output from the sensor and providing an output signal in response to a portion of sensed step transitions having proper polarity; and an accumulator that receives signal from the discriminator and that counts the portion of step transitions for a first predetermined time interval; wherein the accumulator is responsive to the count exceeding a first predetermined number. 
   According to an embodiment of the invention, an arc fault detector includes detection means for detecting an arc fault rate of current change in an electrical current delivered from an AC power source, wherein a polarity of the arc fault rate of current change is opposite to a polarity of the electrical current containing the arc fault rate of current change; gate means for permitting acceptance of the arc fault rate of current change during one or more predetermined intervals of one or more cycles of the AC power source into an accumulator; determining means for determining when an accumulation of the accepted arc fault rate of current changes in the accumulator in a predetermined pattern, and during a predetermined time interval; and signaling means, responsive to the determining means, for signaling a circuit interrupter to interrupt the electrical current. 
   According to an embodiment of the invention, an arc fault circuit interrupter for protecting an AC circuit subjected to a possible arcing fault includes a sensor for producing a signal associated with the arc fault current; a discrimination circuit for identifying a characteristic of the sensor signal associated with cessations of the arc fault current; and an accumulator for counting a number of occurrences of the cessations; in which when the number of cessations exceeds a first predetermined number during a first predetermined time interval, the arc fault circuit interrupter is caused to interrupt the AC circuit. 
   According to an embodiment of the invention, an arc fault detector for detecting arc faults which have an associated electrical current in an AC power line includes means for sensing said electrical current; and means for detecting step changes in said electrical current indicative of arc cessation. 
   According to an embodiment of the invention, a method for detecting arc faults that have an associated electrical current in an AC power line includes the steps of sensing said electrical current; and detecting step changes in said electrical current indicative of arc cessation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic circuit diagram of an embodiment of the present invention; 
       FIG. 2A  shows a line voltage waveform as reference for the waveforms of  FIGS. 2B–2G ; 
       FIG. 2B  shows the waveform for the arc fault current; 
       FIG. 2C  shows the waveform for the rectified and clamped line for producing a zero cross reference; 
       FIG. 2D  shows the waveform for the sensed arc fault di/dt; 
       FIG. 2E  shows the waveform for the output of comparators  32  and  34  at microprocessor input  40 ; 
       FIG. 2F  shows the waveform for the positive di/dt logic pulses; 
       FIG. 2G  shows the waveform for the negative di/dt logic pulses; and 
       FIG. 3  shows a schematic circuit diagram of an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Since both types of changing currents, also referred to herein as di/dt, are associated with current initiation, they are designated “start di/dt.” Although the term “di/dt” usually means the first derivative of the current, as used herein the term also encompasses step increases and step decreases of the current. An arc fault detector according to an embodiment of the invention ignores start di/dt associated with phase controllers to prevent false interruption and consequently ignores indistinguishable start di/dt associated with arc faults. Arc faults are detected and interrupted when cessation di/dt is detected. Cessation di/dt occurs at a phase angle typically late in the half cycle of the line voltage where the instantaneous line voltage has diminished to a value where the arcing current is no longer sustainable. The arc fault current abruptly ceases and produces cessation di/dt thereby. By comparison, the switching device in a phase controller turns off at or near the current zero crossing where there is little or no load current, and although the turn-off is abrupt, there is little or no associated cessation di/dt. Thus, the arc fault detector detects the di/dt signal that is unique to arc faults and ignores the di/dt signal that is produced by phase controllers, thereby reducing or eliminating false detections. 
   Another differentiating feature of arc fault cessation di/dt over phase controller start di/dt is that arc cessation di/dt is always of the opposite polarity to phase controller start di/dt in a given half cycle of line current. This allows differentiation of phase controller start di/dt, which can occur near the end of the current half wave when a user selects a low bulb brightness, from arc cessation di/dt which always occurs near the end of the half cycle of current. In another aspect of the invention, the arc fault circuit interrupter ignores a certain band of phase angles in half cycles of the line voltage in order to prevent false interruption by the circuit interrupter. The phase angle band does not contain cessation di/dt expected from arc faults, but may contain cessation di/dt produced by caused by safe loads that produce arcing during normal operation, for example, motors having commutators and brushes. 
   Referring to  FIG. 1 , an arc fault circuit interrupter (AFCI)  100  for protecting a circuit is shown. The circuit includes a hot conductor  52 , a neutral conductor  54 , and a load  60 . Conductors  52  and  54  receive line voltage from an AC power source. The frequency of the AC power source is typically, but not limited to, 50 or 60 Hz. For example, some specialized applications requiring excellent frequency stabilization use 400 Hz electricity. Upon detection of an arc fault condition, a set of interrupting contacts  24  open the circuit to disconnect load  60  from the line voltage. 
   Conductors  52  and  54  may have an electrical discontinuity caused by such things as a loose twist-on wire connector, a loose terminal screw, a loose plug, or a frayed cord set or the like, which can result in a series arc fault when current through load  60  flows through the discontinuity. Discontinuities where series arc faults can occur are shown at locations  62  and  68  on the line side of AFCI  100  and at locations  64  and  66  on the load side of AFCI  100 . Locations where parallel arc faults can occur, for example where electrical insulation is compromised by an overdriven staple, are shown at location  70 , which shows a parallel arc fault across hot conductor  52  and neutral conductor  54 , and a location  72  , which shows a parallel arc fault across hot conductor  52  and a nearby ground. An arc fault may occur at a location  72 ′ if the hot and neutral conductor polarity happens to be reversed. 
   It is the function of an AFCI to discern the signatures unique to arc faults, whether series or parallel, and to interrupt the current flow through these faults before the intense energy associated with the arc current produces an electrical fire. In particular, since parallel arc faults are not limited by a load impedance, parallel faults are of higher current and higher energy, so it may be desirable to interrupt parallel arc faults faster than series arc faults. For purposes of this invention, the interruption of load current by AFCI  100  refers to the interruption of current through load  60  and consequently the series arc fault locations  64 ,  66 ,  62 , and  68 , as well as the interruption of current through the parallel arc fault locations  70 ,  72 , and  72 ′. 
   In this embodiment, a sensor  1  is a transformer, typically ferrite, with two windings. A winding  56  senses electrical current in hot conductor  52 , whereas a winding  58  senses electrical current in neutral conductor  54 . Windings  56  and  58  are preferably series connected for signal adding. The electrical current in conductors  52 ,  54  includes the current through load  60  which is at the frequency of the line voltage. If there is a series arc fault at locations  62 ,  64 ,  66 , or  68 , or a parallel arc fault at locations  70 ,  72  or  72 ′, the electric currents in either conductor  52  or  54 , or both, includes step transitions. A step transition indicative of a series arc fault is an increase or decrease in current exceeding  2  A. A step transition indicative of a parallel arc fault is an increase or decrease in current exceeding  50  A. If load  60  includes a phase controller, there are step transitions in the sensed current as previously described. Sensor  1 , as with all voltage transformers, differentiates the electrical current signal, thereby accentuating the high frequency components associated with step transitions compared to the 60 Hz component, and producing a di/dt signal in the form of a voltage pulse that corresponds with each step transition. 
   The polarity of the di/dt signal is indicative of whether the step transition is due to a step increase or a step decrease in the sensed electrical current. A capacitor  2  is used to lower and set the resonant frequency of sensor  1 , while a resistor  3  is used to dampen ringing. Diodes  4  &amp;  6  are di/dt clamps used to suppress signals which may damage comparators  8  and  10 , with comparators  8  and  10  being set to produce output signal if the magnitude of either the plus or minus di/dt signals is greater than a predetermined magnitude. The outputs of comparators  8  and  10  are inputs  14  and  16 , respectively, of microprocessor  12 . A line voltage monitor  50  monitors the line voltage to AFCI  100  and provides signal to an input  15  of microprocessor  12 . The signal at input  15  is used by microprocessor  12  to establish whether a particular di/dt signal is occurring in a positive or negative half cycle of the AC line voltage. Thus, line voltage monitor  50  is a polarity detector. 
   It is desirable to distinguish arc faults from phase control loads having a triac, since both arc faults and triacs produce a start di/dt within the half cycle of the line voltage. However, the phase control load produces a minimal cessation di/dt as compared to the arc fault. Arc cessation di/dt occurs during a particular time interval of the half wave of the 50 or 60 Hz line frequency, i.e., about 5 milliseconds after the zero crossing. Arc-mimicking cessation pulses such as from a variable speed electric power drill may occur within 0.5 msec of the zero crossing. A predetermined detection time interval (DTI) for evaluating di/dt pulses may encompass the entire half cycle, or for improved discrimination of arc fault cessation pulses, may cover a portion of each half wave of the AC line frequency, such as between 5 msec and 7.8 msec after the zero crossing. The predetermined DTI is preferably set by microprocessor  12 , which, within each DTI during the positive half cycle of current, counts negative di/dt pulses arriving at input  16  of microprocessor  12  while ignoring any positive di/dt pulses arriving at input  14  of microprocessor  12 . 
   Likewise, any positive di/dt arriving at input  14  of microprocessor  12  during a predetermined DTI of each negative half wave of the line voltage is counted by microprocessor  12  while any negative di/dt pulses arriving at input  16  of microprocessor  12  are ignored. Thus di/dt pulses are identified as cessation di/dt pulses on the basis of their polarity with respect to the polarity of the line voltage. A negative di/dt pulse near the end of the positive half cycle of the line voltage and a positive di/dt pulse near the end of the negative half cycle of the line voltage are referred to herein as being of “proper” polarity. “Proper” also has the same meaning when applied to step transitions in the current. If M cessation di/dt pulses are counted during either the positive or negative half cycle polarities of the line voltage within a detection window of N number of half cycles, then the microprocessor  12  issues a trip command to SCR  18 , which in turn activates trip solenoid  20  releasing interrupting contacts  24  and interrupting the load side current. M and N are experimentally predetermined constants chosen such that the arc fault circuit interrupter complies with the maximum permissible interruption times in Underwriters Laboratories standard for arc fault circuit interrupters, UL 1699, and also so that the arc fault circuit interrupter does not interrupt safe arcs, such as from toggling a snap switch under load, or from having a light bulb burn out. 
   The detection window may be opened when a detected proper polarity di/dt occurs in the DTI interval, or when more than one proper polarity di/dt occurs in one or more half cycles, or may be opened when a low frequency event in the load side current occurs, such as a step up or down in load current or a random variation of the current, or a similar low frequency event in the line voltage, or a low frequency event simultaneously in the load current and the line voltage. Events may be singular or in predetermined patterns, whatever suggests the presence of an arc fault. Predetermined patterns include low frequency arc fault patterns detectable from such methods as (a) PWM (pulse width modulation) wherein voltage or current is present for a portion of a half cycle of the line voltage and there is a fluctuation in the portion from half cycle to half cycle, (b) PPM (pulse position modulation) wherein there is a step change in load current or line voltage at a measured time interval from a line voltage or load current zero crossing and there is a fluctuation in the measured time interval from half cycle to half cycle, or (c) PAM (pulse amplitude modulation) wherein there is a fluctuation of the load current or line voltage from half cycle to half cycle. 
   A shunt resistor  42  in series with neutral conductor  54  produces a signal proportional to the load current, so shunt resistor  42  is also known as a current view resistor. The signal from shunt  42  is sent to a low pass filter  44  to strip off high frequency broadband noise components. The signal from low pass filter  44  is amplified by an amplifier  46  whose output is connected to an ADC input  17  of microprocessor  12  for conversion to digital information by microprocessor  12 . Likewise, the low frequency line voltage signal is taken from an amplifier  49  and converted to digital information by an ADC input  19  of microprocessor  12 . Each ADC conversion of current and voltage is used to identify the low frequency arc fault patterns. 
   While the detection window is open, having been opened using one of the methods previously described, microprocessor  12  may simply search for and count proper polarity di/dt&#39;s, or search for random proper polarity di/dt&#39;s, randomness interpreted either in missing proper polarity di/dt&#39;s in some half cycles or in varying counts of proper polarity di/dt&#39;s during each half cycle, including zero, which counts vary from a given half cycle to a subsequent half cycle, or in varying counts of proper polarity di/dt&#39;s during non-subsequent half cycles which are also during the detection window. Microprocessor  12  may search for proper polarity di/dt&#39;s outside of the DTI and proper polarity di/dt&#39;s within the DTI during the detection window, and in particular for one or more proper polarity di/dt&#39;s in the DTI during one or more half cycles, followed by a predetermined quiet interval without proper polarity di/dt&#39;s in any half cycle, with the quiet period then followed by one or more cessation pulse di/dt&#39;s in the DTI during one or more half cycles during the remaining detection window. If a sufficient proper polarity di/dt count is not attained, or a sufficient proper polarity di/dt randomness is not discerned at the end of the Nth half cycle of the detection window, microprocessor  12  resets a counter, denoted M of N, and waits for another current or voltage event for re-opening the detection window. 
   For high current arc faults where the arc fault circuit interrupter must interrupt quickly, including parallel arc faults, an additional circuit is preferably added to AFCI  100 . This additional circuit includes a passive low pass filter  26 , a plurality of active low pass filters  28  and  30 , a positive going signal detecting comparator  32 , a negative going signal detecting comparator  34 , a plurality of OR diodes  36  and  38 , and a current zero cross comparator  48 . This circuit is used for an arc fault detection speed up. A polarity detector such as a current zero cross comparator  48 , which sends signal to an input  15   a  of microprocessor  12 , can be used as an alternative to line voltage monitor  50  and input  15  of microprocessor  12 , having the advantage of accurately identifying the proper polarity even when load  60  is inductive, the load current is phase shifted, and the voltage zero crossing is not coincident with the current zero crossing. During an arc fault, the output signal from sensor  1  includes larger di/dt signal associated with arcing that is imposed on a comparatively lesser 60 Hz signal associated with the load current. The 60 Hz signal is stripped of di/dt by filters  26 ,  28 , and  30  and applied to comparators  32  and  34 . The comparators are set to respond to a 60 Hz current of a predetermined high level above the circuit rating, suggestive of a parallel arc fault current, and whereupon the OR&#39;d comparator  32  or  34  output signal is applied to an input  40  of microprocessor  12 . 
   When a high current occurs that is sustained and thereby indicative of a parallel arc fault, comparators  32 ,  34  output a signal through OR diodes  36 ,  38  to microprocessor  12  for a duration sufficient to cause microprocessor  12  to enter a sped-up mode. The sped-up mode assures that microprocessor  12  acts on arriving di/dt of the proper polarity, and in the correct interval of the appropriate polarity half wave as described above, and outputs a signal to SCR  18  to engage trip solenoid  20  to open interrupting contacts  24  within approximately eight arcing half cycles after the arc fault has started, which is required by Underwriters Laboratories standard for Arc Fault Circuit Interrupters, UL1699. The speed-up may be accomplished by reducing the number M of proper polarity di/dt&#39;s during the detection window necessary for interruption, or by a simpler requirement for opening the window, such as opening upon the first detected proper polarity di/dt, or opening upon the first detected proper polarity di/dt residing within the DTI. When a high current is not sustained, such as when load  60  includes a motor or tungsten filament load having a momentarily high startup (inrush) current, the signals from comparators  32 ,  34  are not of sufficient duration to cause microprocessor  12  to enter the sped-up mode. 
   Alternately the 60 Hz fundamental signal can be taken from low resistance shunt  42 , passed through low pass filter  44 , and amplified by amplifier  46  before being applied to comparators  32 ,  34 . The advantage of deriving signal from shunt resistor  42  instead of from sensor  1  is that large di/dt pulses are not produced that need to be stripped off by added circuitry, and the current zero cross signal from amplifier  46  is in phase with the line current. 
   Referring to  FIGS. 2A–2G , when the arc fault starts, a step in current such as  301  occurs causing a positive di/dt pulse  500  in  FIG. 2D  at the sensor  1  output. The positive di/dt pulse also causes a positive di/dt logic pulse  700  in  FIG. 2F  in the comparator  8  output signal to input  14  of microprocessor  12 . When the arc fault current ceases, or extinguishes, near the end of the given half cycle, a step down in current such as  302  in  FIG. 2B  occurs, causing negative di/dt pulse  502  in  FIG. 2D  and negative di/dt logic pulse  800  in  FIG. 2G  in the output signal of comparator  10  which is input  16  of microprocessor  12 . The DTI acceptance band  420  in  FIG. 2C  is set by microprocessor  12  in a region near to or including the zero cross transition near the proper end of the half cycle, and preferably referenced to the voltage zero cross, in which a negative di/dt cessation pulse from sensor  1 , which causes a negative di/dt logic pulse at input  16  of microprocessor  12  and that resides within the acceptance band, is accepted and counted by microprocessor  12 . 
   Likewise, another acceptance band  440  in  FIG. 2C  occurring during the negative half cycle of the AC line voltage is set by microprocessor  12  for accepting and counting the positive di/dt arc cessation pulse  504  in  FIG. 2D  which causes a logic positive di/dt pulse  702  in  FIG. 2F  at input  14  of microprocessor  12 . Arc fault pulse  306  in  FIG. 2B  shows a high current pulse caused by a circuit current above the circuit rating which passes through a pre-determined level  308  corresponding to a  50 A circuit current. This causes a pulse  600  in  FIG. 2E  at input  40  of microprocessor  12 , indicating a high circuit current, in this case greater than  50 A. If pulse  600  is sufficiently wide or repetitive, representing sustained overcurrent, this causes microprocessor  12  to decrease the M of N requirement for speedy arc interruption. 
   The zero cross signal is preferably derived from the line voltage via the line voltage monitor circuit  50 . Alternately, the current zero cross at input  15   a  of microprocessor  12 , taken from comparator  48 , can be compared to the voltage zero cross for determining phase shift. In this way, the acceptance windows  420  and  440  can be offset through the voltage zero cross in order to follow and cover the interval of expected arc cessation di/dt during a phase shifted current. The phase shift may be determined, and held, by microprocessor  12  in a constantly updated phase shift memory before an arc fault occurs, so that during an arc fault, when the current zero cross is affected by the arc fault, the di/dt acceptance window  420  and  440  locations in the half wave can be determined from the phase shift memory, with the locations held in the expected arc cessation interval. 
     FIG. 3  is circuit schematic of an alternative embodiment showing an arc fault circuit interrupter  900 , wherein electrical components bearing like functions to those in  FIG. 1  bear like designations. Sensor  1  in  FIG. 1  senses and differentiates current step transitions occurring in conductors  52 ,  54 , or both, providing di/dt pulse signal to comparators  8  and  10 . The same functions are accomplished by shunt  42  which also detects step transitions in the load current, but a differentiator  902  is required to convert the step transitions into di/dt pulse signals. Differentiator  902  is in turn coupled to the inputs of comparators  8  and  10 . The polarity of the di/dt pulse is indicative of whether the step transition in load-side current is a step increase or a step decrease. In another alternative embodiment, the output of differentiator  902  is coupled to a second differentiator  904  which in turn provides the input signal to the inputs of comparators  8  and  10 . Second differentiator  904  transforms the di/dt pulse at its input into a pulse doublet at its output. The polarity of the leading edge of the pulse doublet is indicative of whether the particular step transition in sensed current is a step increase or a step decrease. The pulse doublet amplitude, if greater than the thresholds of comparators  8  and  10 , produces signals at both inputs  14  and  16 , respectively, of microprocessor  12 . Microprocessor  12  continuously strobes inputs  14  and  16  in order to determine leading edge polarity. The benefit of second differentiator  904  is to further emphasize the high frequency signal associated with the transition step to aid discernment of step transitions from line frequency components in the sensed current. 
   In yet another embodiment, the signal from shunt  42  is amplified by an amplifier  906  which is connected to an input  908  of microprocessor  12  and converted by microprocessor  12  to digital information, whereby microprocessor  12  is able to directly discern step transitions. Amplifier  906  can be connected to a high pass filter  907  which in turn is connected to input  908  of microprocessor  12  to reduce the line frequency component of the load side current sensed by resistor  42  to improve step transition discernment. Although the embodiment is shown with shunt  42  located in neutral conductor  54 , the shunt could also be included in the hot conductor. 
   The aforementioned embodiments are powered by a positive DC supply  910  and a negative DC supply  912 , but negative DC supply  910  can be omitted from all or a portion of components in arc fault circuit interrupter  900  in order that the device be active during only one polarity of the AC power line and active only for sensing the polarity of the load step associated with arc fault cessation. Load steps associated with phase controllers produce the opposite load step polarity and are ignored. 
   While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.