Abstract:
A protection device connected between hot and neutral conductors of an AC power line includes a fault detection circuit which controls a breaker coil operatively associated with a set of interrupting contacts. A capacitance circuit connected between the hot conductor and the neutral conductor stores a charge of a polarity indicative of a line-load miswire. This stored charge independently activates the set of interrupting contacts when the interrupting contacts are closed and the device is miswired, thus preventing the miswired device from providing power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application Ser. No. 60/187,265 filed on Mar. 6, 2000 and entitled “GFCI OR AFCI WITH LINE-LOAD MIS-WIRE PROTECTION”, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to the field of electrical fault detection devices connected to AC power lines, and in particular to a fault detection device with lineload miswire protection. 
     BACKGROUND OF THE INVENTION 
     Ground fault circuit interrupters (GFCIs) are well known in the art. Their intent is and always has been to protect the electrical power user from electrocution when hazardous ground fault currents are present. Arc fault circuit interrupters (AFCIs) are a more recent development. Their intent is to interrupt power when either a series arc fault or a parallel arc fault is present in the wiring. 
     Historical problems with these devices include the possibility of line/load miswiring in the field by an installer, which causes the interrupter device to become inoperable while electrical power is still present, even under hazardous ground fault or arc fault conditions. A variety of methods are used to prevent or attempt to prevent miswiring with varying levels of success. Labels and installation instruction sheets have been used to prevent miswiring, but can be ignored by the installer. 
     U.S. Pat. No. 5,363,269 discloses revealing reversal of the line and load wires of a receptacle-type GFCI by connecting the test button circuit between load hot and ground. 
     U.S. Pat. No. 5,541,800 discloses avoiding line/load miswiring by placing a Mylar insulator in one power contact and instructing the user to remove the insulator only after the GFCI trips in response to the t-button. 
     U.S. Pat. No. 5,600,524 discloses an intelligent GFCI with an audible alarm to alert the user to periodically test the GFCI, circuitry to test the electronics on a periodic basis, and circuitry that determines if the line and load are miswired and that prevents the GFCI from resetting if the miswired condition is identified. 
     U.S. Pat. No. 5,706,155 discloses an intelligent GFCI with an audible alarm that indicates miswiring. 
     U.S. Pat. No. 6,040,967 discloses a device that guards against miswiring. Although not fully disclosed in the patent, the device is tripped out when sold. If supply voltage is not connected to the line terminals, (i.e., if the device is line/load miswired), it cannot be reset because the solenoid needs line side power to move a mechanical stop out of the way which otherwise prevents resetting. 
     There exists a need for a low cost, effective device which prevents line-load miswiring. 
     SUMMARY OF THE INVENTION 
     Briefly stated, a protection device connected between hot and neutral conductors of an AC power line includes a fault detection circuit which controls a breaker coil operatively associated with a set of interrupting contacts. A capacitance circuit connected between the hot conductor and the neutral conductor stores a charge of a polarity indicative of a line-load miswire. This stored charge independently activates the set of interrupting contacts when the interrupting contacts are closed and the device is miswired, thus preventing the miswired device from providing power. 
     According to an embodiment of the invention, a protection device connected between hot and neutral conductors of an AC power line includes a fault detection circuit including a first switch controlling a breaker coil operatively associated with a set of interrupting contacts; a capacitance circuit connected between the hot conductor and the neutral conductor; a switching circuit connected to the capacitance circuit and to the first switch; wherein connecting AC power to load terminals of the protection device causes the capacitance circuit to store a first charge of a first polarity indicative of a line-load miswire; and wherein a presence of the first charge causes the switching circuit to activate the first switch and open the set of interrupting contacts when the set of interrupting contacts is closed. 
     According to an embodiment of the invention, a protection device connected between hot and neutral conductors of an AC power line includes means for detecting a connection of AC supply voltage to improper terminals of the device; means for tripping an interrupting mechanism upon the detection; and means for preventing the protection device from being reset while the AC supply voltage is connected to the improper terminals of the device. 
     According to an embodiment of the invention, a protection device connected between hot and neutral conductors of an AC power line includes detection means for detecting a fault in the AC power line, the detection means including first switching means for controlling a breaker coil operatively associated with a set of interrupting contacts; capacitance means connected between the hot conductor and the neutral conductor for storing a charge of a polarity indicative of a line-load miswire between the AC power line and the protection device; and second switching means, connected to the capacitance means and to the first switching means, for activating the first switching means when the charge is present in the capacitance means. 
     According to an embodiment of the invention, a method for preventing a lineload miswire of a protection device connected between hot and neutral conductors of an AC power line includes the steps of storing a charge of a polarity indicative of the line-load miswire between the AC power line and the protection device; and interrupting power within the protection device when the stored charge is present. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an embodiment of the invention as applied to a GFCI protection device. 
     FIG. 2 shows a waveform used in explaining the functioning of the invention. 
     FIG. 3 shows a waveform used in explaining the functioning of the invention. 
     FIG. 4 shows a waveform used in explaining the functioning of the invention. 
     FIG. 5 shows a waveform used in explaining the functioning of the invention. 
     FIG. 6 shows a waveform used in explaining the functioning of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a fault detector such as a GFCI  100  is shown. A differential ground fault sensor transformer  1  senses any difference in the currents flowing in a line hot wire  56  and a line neutral wire  52 . A grounded neutral transformer transmitter  2  reacts to downstream grounding of line neutral wire  52  and communicates with sensor transformer  1 . When contactor  12  is closed, the dangerous conditions associated with current flow in a load hot wire  28  and a load neutral wire  26  are sensed. A detector  4 , which receives input from transformer  11  and outputs to transmitter  2 , sends a signal to a switch such as an SCR  6  when a ground fault is sensed. SCR  6  is activated when the signal from detector  4  reaches a gate  50  of SCR  6 , which in turn activates a solenoid  8 . The activation of solenoid  8  in turn activates a trip mechanism  10  which activates contactor  12 , interrupting current through load hot sire  28  and load neutral wire  26 , which may also be connected to a load  60 . 
     When contactor  12  is open and GFCI  100  is miswired with the AC power source connected to the load terminals instead of the line terminals, current flows from load neutral wire  26  in a series string through a diode  16 , a limit resistor  18 , a capacitor  24 , a resistor  20 , and a resistor  22  to return to load hot wire  28 . This current flow charges capacitor  24  to a positive voltage above a GFCI reference voltage  30 . The voltage across capacitor  24  also appears across a gate  33  of an electronic switch such as a FET  32 , which would cause FET  32  to turn ON in the presence of voltage V+ from a GFCI DC power supply  14 . However, because contactor  12  is open and the AC power is miswired to the load terminals, no voltage V+ is present from power supply  14 . 
     When contactor  12  is closed while GFCI  100  is miswired, AC power appears across the series combination of SCR  6  and solenoid  8 , and also activates power supply  14 . The voltage held across charged capacitor  24  now causes FET  32  to turn ON, which conducts current from voltage supply V+ of power supply  14  through a limit resistor  34  to gate  50 , causing SCR  6  to activate solenoid  8  and trip out GFCI  100  through trip mechanism  10  and contactor  12 . Thus, once contactor  12  is open, either by testing GFCI  100  on installation or by installing GFCI  100  in the tripped state, with the line-load terminals miswired, GFCI  100  cannot be reset without an instant trip. 
     When the AC power is correctly connected to the line terminals of GFCI  100 , and when contactor  12  is either open or closed, a charge current flows from line neutral wire  52  through the series string consisting of capacitor  24 , a diode  36 , and a resistor  54  to return to line hot wire  56 . This current charges capacitor  24  in the opposite direction from when GFCI  100  is miswired, causing a negative voltage to appear across capacitor  24  with respect to reference voltage  30 . This negative voltage prevents FET  32  from conducting, and thus prevents SCR  6  from activating via FET  32 . A Zener diode  58  is a voltage clamp which preferably protects FET  32  from excessive voltage. A transistor could be used instead of FET  32 . 
     Referring also to FIGS. 2 and 3, a voltage waveform  202  is shown across gate  33  of FET  32 , shown with (FIG. 2) and without (FIG. 3) a miswired load connected to the line side of GFCI  100 , along with the AC power supply miswired to the load side of GFCI  100 . Contactor  12  is open. Power is applied at time  200 , and waveform  202  is a positive charging waveform across capacitor  24 . In either case, i.e., load or no load, a positive charge occurs in capacitor  24 . The positive charge activates FET  32  and SCR  6  when contactor  12  is closed, thus tripping out GFCI  100 . 
     Referring also to FIGS. 4 and 5, the state of correct GFCI wiring, voltage waveforms  402  and  502  are shown across gate  33 , shown with (FIG. 4) and without (FIG. 5) a load connected to the load side of GFCI  100 . The line side of GFCI  100  is correctly connected to the AC power supply, and contactor  12  is in the closed state. Waveform  502  also shows the voltage waveform when contactor  12  is open. Power is applied to waveform  402  at a time  400  (FIG. 4) and to waveform  502  at a time  500  (FIG.  5 ). Both FIGS. 4 and 5 show the negative charging voltage waveforms  402  and  502 , respectively, across capacitor  24 . This stored negative charge holds FET  32  OFF regardless of the state of contactor  12  so that SCR  6  is not activated by FET  32 . Of course, SCR  6  can still be activated by detector  4  if a ground fault is detected. The negative charging waveforms  402  and  502  also occur when GFCI  100  is miswired with contactor  12  closed, and therefore a positive trip causing charge accumulation in capacitor  24  can only occur when GFCI  100  is miswired and contactor  12  is open. 
     Waveform  402  is different from waveform  502  because of a second charge path from line hot wire  56  through resistor  22 , through load  60 , diode  16 , resistor  18 , and capacitor  24  before returning to line neutral wire  52 . Although this path causes positive charging of capacitor  24 , it is overwhelmed by the negative charge path through diode  36  and resistor  54 , which path is designed with less resistance. 
     Referring also to FIG. 6, a voltage waveform  602  is shown across capacitor  24  when contactor  12  is closed. Waveform  602  appears when GFCI  100  is powered from either the line side or the load side when contactor  12  is closed, with the negative voltage keeping FET  32  out of conduction. 
     The present invention allows resistors  22  and  18  to be of high value, above the Meg-ohm range, which prevents any shock hazard via these paths when GFCI  100  is in the tripped and contactor open state. 
     Although the waveforms show an imbalance between the negative and positive voltages appearing across capacitor  24  during correct wiring and miswiring respectively, the magnitude of the negative charge across capacitor  24  can be increased by decreasing the resistance of resistor  54 . 
     Although the present invention is described using a GFCI, an embodiment using an AFCI will work equally well and application of the invention to an AFCI embodiment is considered to be within the knowledge of one skilled in the art. 
     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.