Abstract:
A system and method employing a fault circuit interrupter to provide enhanced miswiring protection. The fault circuit interrupter is used with an alternating current (AC) receptacle and comprises first and second source terminals, and first and second load terminals. The fault circuit interrupter also comprises a sensing circuit, which is adapted to selectively enter a fault detection state in response to an imbalance of current flow in the AC receptacle. A latching mechanism is also provided and is adapted to break at least one conductive path between the source and load terminals of the AC receptacle in response to entry of the sensing circuit in the fault state. The sensing circuit is further adapted to maintain the latching mechanism in a condition to maintain a break in at least one of the conductive paths when an AC source is connected to the load terminals instead of to the source terminals.

Description:
This application claims benefit under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application Ser. No. 60/311,346 filed on Aug. 13, 2001, the entire contents of said application being expressly incorporated herein by reference. 
   CROSS REFERENCES TO RELATED APPLICATIONS 
   Related subject matter is disclosed in provisional U.S. patent application Ser. No. 10/032,064 filed on Dec. 31, 2001 by Nelson Bonilla and Joseph V. DeBatolo; the entire contents of said application being expressly incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to ground fault circuit interrupter (GFCI) devices. More particularly, the invention relates to a GFCI device having a latching mechanism that internally prevents reset of the device in the event of miswiring. 
   BACKGROUND OF THE INVENTION 
   GFCI devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault condition results when a person comes into contact with the line side of the AC load and an earth ground at the same time, thus creating a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, a mechanically latched circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground. The latter type of connection, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping at the intended threshold level of different current when a line-to-ground fault occurs. 
   GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. A typical receptacle configuration is shown, for example, in U.S. Pat. No. 4,568,997, to Bienwald et al., the entire content of which is incorporated herein by reference. This type of receptacle includes test and reset pushbuttons and a lamp or light-emitting diode (LED) which indicates that the circuit is operating normally. When a ground fault occurs in the protected circuit, or when the test button is depressed, the GFCI device trips and an internal circuit breaker opens both sides of the AC line. The tripping of the circuit breaker causes the reset button to pop out and the LED to be extinguished, providing a visual indication that a ground fault has occurred. In order to reset the GFCI device, the reset button is depressed in order to close and latch the circuit breaker, and this also causes the LED to illuminate once again. 
   In addition to ground fault detection/protection, protection from miswiring is also needed. Specifically, GFCI receptacles of the type described above may be erroneously connected with the incoming AC source conductors being tied directly to the load or feedthrough terminals of the receptacle rather than to the source terminals. Because of the nature of the internal wiring of the GFCI receptacle, this miswiring condition is not easily detected. AC power will still be present at the receptacle outlets, making it appear that the receptacle is operating normally. If the test push button is depressed, the latching mechanism within the GFCI receptacle will be released and the reset push button will pop out, again making it appear that the GFCI receptacle is operating normally and providing the desired ground fault protection. In reality, however, no such protection is being provided because the AC source has been wired directly to the receptacle outlets without passing through the internal circuit breaker of the GFCI device. 
   Additionally, the safety function of GFCI devices depends upon power being prevented from reaching the receptacle when a trip condition occurs. A potentially unsafe condition occurs if the test button is pressed and the GFCI fails to trip. Therefore, the need exists for a GFCI device with a fail safe system to ensure that when the test button is pressed and the GFCI device fails to trip, the failed condition of the GFCI devices is indicated to the user in some manner. 
   Another concern with regard to miswiring relates to the receptacle terminals. Specifically, the conventional GFCI device has a set of load terminals that are shared with the receptacle terminals leading to the face of the receptacle. In the typical miswiring scenario, the AC source is connected to the load terminals while the downstream load devices are connected to the line terminals. Thus, while tripping the latching mechanism in response to a miswiring condition protects the downstream devices, devices plugged into the GFCI receptacle may still be subjected to AC power. It is therefore desirable to provide a latching mechanism that does not share the contacts between the receptacle terminals and the load terminals. 
   SUMMARY OF THE INVENTION 
   The above and other objectives are substantially achieved by a system and method employing a ground fault circuit interrupter (GFCI) in accordance with the principles of the present invention. The method and system include a sensing circuit having an electro-mechanical device, where the sensing circuit selectively places the electro-mechanical device in a ground fault state in response to an imbalance of current flow in an AC receptacle. A latching mechanism is connected to the sensing circuit, where the latching mechanism breaks a plurality of conductive paths between source and load terminals of the AC receptacle when the electro-mechanical device is placed in the ground fault state. The latching mechanism continues the imbalance of current flow when an AC source is improperly connected to the AC receptacle. By continuing the imbalance of current flow, a miswiring lock out is effected and enhanced safety is achieved. 
   In another aspect of the invention, a method and latching system include a first switch and a second switch. The first switch selectively breaks a first conductive path between a first source terminal and a first load terminal. The second switch selectively breaks a second conductive path between a second source terminal and a second load terminal. The second switch provides an alternative current path between the second load terminal and the first source terminal when the first conductive path has been restored by the first switch and an AC source is connected to the load terminals. 
   Further in accordance with the present invention, a system and method for preventing improper wiring of an AC source to an AC receptacle as provided. The method includes the step of selectively placing an electro-mechanical device in a ground fault state in response to an imbalance of current flow in the AC receptacle. A plurality of conductive paths between source and load terminals of the AC receptacle are broken when the electro-mechanical device is placed in the ground fault state. The method further provides for continuing the imbalance of current flow when the AC source is improperly connected to the AC receptacle. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a circuit schematic of an example of a GFCI in accordance with an embodiment of the present invention; 
       FIG. 2  is a circuit schematic of an example of a GFCI in accordance with a first alternative embodiment of the present invention; and 
       FIG. 3  is a circuit schematic of an example of a GFCI in accordance with a second alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In accordance with the present invention, a ground fault circuit interrupter (GFCI) device  10  is provided with a latching mechanism  12 , which enables “lockout” of the GFCI  10  in the event that the GFCI  10  is miswired.  FIGS. 1-3  illustrate, respectively, three embodiments of the present invention that each employ a lockout enabling latching mechanism. Generally, it can be seen in  FIG. 1  that the GFCI  10  has a sensing circuit  14  and the latching mechanism  12 . The sensing circuit  14  has an electro-mechanical device such as solenoid  16  and selectively places the solenoid  16  in a ground fault state in response to an imbalance of current flow in an AC receptacle. While the electro-mechanical device is shown here as being a solenoid, other devices such as piezoelectric components and micro electro-mechanical systems (MEMS) may be used. It can also be seen that the latching mechanism  12  is connected to the sensing circuit  14  and is placed in series with a plurality of conductive paths between source and load terminals of the receptacle. Specifically, the latching mechanism  12  breaks a plurality of conductive paths leading from source terminals  18  to load terminals  20  of the AC receptacle when the solenoid  16  is placed in the ground fault state. As will be discussed in greater detail below, the latching mechanism  12  has an internal structure that continues the imbalance of current flow when an AC source (not shown) is improperly connected to the AC receptacle. By continuing the imbalance of current flow, the latching mechanism  12  provides improved safety while maintaining a relatively low level of complexity with regard to conventional approaches. 
   It should also be noted that the sensing circuit  14  effectively defines an imbalance of current flow as any difference in the amount of current flowing in the candidate paths that rises above a predetermined threshold. Thus, the “imbalance” resulting from the miswiring lockout feature is the same as an imbalance resulting from manually testing the GFCI although the amount by which the predetermined threshold is exceeded may differ. 
   It can further be seen that the latching mechanism  12  preferably includes a first switch  22  which, when closed, connects the load neutral terminal  20   a  to first contact  32 , and is capable of selectively breaking the first conductive path. A second switch  26 , when closed, connects the load hot terminal  20   b  to second contact  34 , and selectively breaks the second conductive path. To better demonstrate the operation of latching mechanism  12 , the sensing circuit  14  will now be described in greater detail. Generally, it can be seen that the sensing circuit  14  has a transformer arrangement  40 , a control circuit  42  and a primary test switch  44 . The transformer arrangement  40  generates control signals in response to the imbalance of current flow, while the control circuit  42  is connected to the transformer arrangement  40  and selectively generates a switching signal based on the control signals. The primary test switch  44  is connected between the second load terminal  20   b  and the first source terminal  18   a  such that the primary test switch  44  enables manual generation of the imbalance of current flow. 
   Specifically, when the primary test switch  44  is closed (for example, manually, by an installer of the device), a circuit path is created from the second load terminal  20   b  to the first source terminal  18   a , which creates an imbalance that is detected by a first (or sense) transformer  46 . The first transformer  46  detects imbalances in the net flux on the load hot and load neutral lines of the device, and operates in conjunction with the control circuit  42  to energize the solenoid  16 . Detection of the imbalance condition by the first transformer  46  and the control circuit  42  causes activation of the solenoid  16  such that the first and second switches  22 ,  26  to the load are open as shown in FIG.  1 . It can be further be seen that a second (grounded neutral) transformer  48  is also provided to allow the transformer arrangement  40  to measure the change in net flux between the first conductive path and the second conductive path. 
   It can be seen that the control circuit  42  preferably includes an amplifier and trip circuit  50 , a full-wave bridge rectifier  52  and a silicon controlled rectifier (SCR)  54 . The amplifier and trip circuit  50  generates the switching signal, where the bridge rectifier  52  is connected to the first source terminal  18   a  and the second source terminal  18   b . It can be seen that the bridge rectifier  52  provides power to the amplifier and trip circuit  50  and that the SCR  54  selectively energizes the solenoid  16  based on the switching signal. The control circuit  42  preferably includes the components listed in Table 1 below: 
   
     
       
             
           
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               EXEMPLARY COMPONENT LIST 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                CAPACITOR C1 
               10 MIC OF AND, 16 VDC ALUM, ELECTROLYTIC 
             
             
               CAPACITOR C2 
               3.3 MIC, 16 VDC ALUM, ELECTROLYTIC 
             
             
               CAPACITOR C3 
               .01 MIC, 50 VDC CERAMIC 
             
             
               CAPACITOR C4 
               .033 MIC, 25 VDC CERAMIC 
             
             
               CAPACITOR C5 
               .01 MIC, 500 VDC CERAMIC 
             
             
               CAPACITOR C6 
               .01 MIC, 50 VDC CERAMIC 
             
             
               CAPACITOR C7 
               470 PIC, 50 VDC CERAMIC 
             
             
               DIODE D1 
               IN4004 
             
             
               DIODE D2 
               IN4004 
             
             
               DIODE D3 
               IN4004 
             
             
               DIODE D4 
               IN4004 
             
             
               DIODE D5 
               IN4004 
             
             
               RESISTOR R1 
               15K OHM, ¼ W CARBON FILM 
             
             
               RESISTOR R2 
               1.5 MED OHM, ¼ W METAL FILM 
             
             
               RESISTOR R3 
               24K OHM, ½ W CARBON FILM 
             
             
               RESISTOR R4 
               200 OHM, ¼ W CARBON FILM 
             
             
               IC 
               RV4145 
             
             
                 
             
           
        
       
     
   
   The state of switches  22  and  26  shown in  FIG. 1  indicates that the solenoid  16  has entered the ground fault state, due to depression of the test button  44  or due to an actual ground fault. However, when the solenoid  16  is not in the ground fault state and the latching mechanism has been properly reset so that first switch  22  is closed to first contact (S 1 )  32  and second switch  26  is closed to second contact (S 2 )  34 , the first conductive path includes the first source terminal  18   a , first source conductor  24 , first switch  22 , first load conductor  25  and first load terminal  20   a . Similarly, the second conductive path includes second source terminal  18   b , a second source conductor  28 , second switch  26 , second load conductor  27  and second load terminal  20   b . While the first and second conductive paths are shown as corresponding to the neutral and hot connections respectively, it will be appreciated that these assignments can readily be reversed without parting from the spirit and scope of the invention. 
   It is also important to note that when in the ground fault (open) state, as shown in  FIG. 1 , the second switch  26  provides an alternative current path between the second load terminal  20   b  and the first source terminal  18   a . Thus, if the AC source is connected to load terminals  20  (i.e. miswired to the receptacle), second switch  26  being closed to third contact (S 3 )  36  enables current to flow through current limiting resistor R 1   30 . It can further be seen that the latching mechanism  12  is structured such that, in response to a reset button (not shown) being pressed on the AC receptacle, switch  26  and third contact (S 3 )  36  continue to complete the alternative path through conductor  38  after switch  22  closes on first contact (S 1 )  32 . Thus, when the AC source is improperly connected to the load terminals  20 , this current path will create an imbalance in the transformer arrangement  40  which will prevent sensing circuit  14  from being reset. That is, switch  26  remains in an open state. 
   Turning now to  FIG. 2 , an alternative latching circuit  12 ′ is provided in which additional protection is provided. Specifically, it will be appreciated that if the receptacle face terminals are shared with the load terminals  20 , the receptacle face terminals may receive power in the event that the AC source is connected to the load terminals  20 . In fact, it is quite common for the receptacle face terminals to be tied directly to the load terminals without any isolation mechanism. If protection from such a condition is desired, the latching mechanism  12 ′ can be equipped with a dedicated set of receptacle contacts  56   a ,  56   b , where the receptacle contacts  56   a ,  56   b  are electrically isolated from the load terminals  20  when switches  22 ,  26  are open. Thus, power is removed from the receptacle contacts  56   a ,  56   b  when there is an imbalance in current flow and/or when the GFCI  10  is locked-out due to miswiring. On the other hand, when the switches  22 ,  26  are allowed to close during reset if the receptacle is wired properly, the unique alignment of the receptacle contacts  56   a ,  56   b  enables the receptacle face to receive power from the source conductors  28  and  24 , respectively. 
   Turning now to  FIG. 3 , an alternative GFCI  10 ″ is provided with a notification system  58  for enabling diagnosis of the GFCI  10 ″. As will be discussed in greater detail below, the notification system  58  provides the installer/user of the receptacle with valuable information regarding both ground fault sensing and miswiring. Thus, the installer/user of the receptacle is able to determine whether the GFCI  10 ″ is correctly sensing ground faults and whether the AC source has been properly wired to the receptacle. 
   It can generally be seen that the notification system  58  includes a first indicator such as a green LED  60  and a second indicator such as red LED  62 . It should be noted that while the illustrated embodiment uses LEDs to provide visual notification, audible notification can also be achieved with the use of buzzers, or the like. As indicated, the green LED  60  is connected between the second load terminal  20   b  and the first source terminal  18   a , while the red LED  62  is connected between the second load terminal  20   b  and the first load terminal  20   a . It can further be seen that a fusing mechanism such as fuse  64  is connected in series with the green LED  60 . Similarly, a current limiting resistor  66  is connected in series with the red LED  62 . A secondary test switch  68  selectively short-circuits the green LED  60  based on operation of the primary test switch  44 , where the primary test switch  44  enables manual generation of the imbalance of current flow. The secondary test switch  68  completes the short-circuit after the primary test switch  44  completes the imbalance of the current flow such that the short-circuit blows the fuse  64  when the primary test switch  44  fails to place the solenoid  16  in the ground fault state. Thus, when the primary test switch  44  is closed, the green LED  60  is extinguished if the GFCI  10 ″ is not detecting ground faults correctly. The various potential indication scenarios will now be discussed in detail. 
   It will be appreciated that when the GFCI  10 ″ is initially installed, it is intended that the AC source (not shown) be connected to the source terminals  18 . Under these conditions, the switches  22 ,  26  are in the closed position with the green LED  60  and the red LED  62  effectively being connected in parallel between diode  70  and the first conductive path. It can further be seen that the green LED  60  is shorted through fuse  64 , whereas the red LED  62  is connected in series with the resistor  66 . Thus, the current flowing through resistor  72  and diode  70  will take the path of the least resistance through the fuse  64  to enable the green LED  60  to illuminate. There is no danger of blowing the fuse  64  because the 20k resistor  72  limits the current flowing between the conductive paths. When the primary test switch  44  is depressed and the GFCI  10 ″ is operating properly, the resulting imbalance of current flow will cause the latching mechanism  12  to move into the position shown in FIG.  3 . Since both current paths are broken and the AC source is connected to the source terminals  18 , no current is available to flow through either of the LEDs  60 ,  62 . Both of the LED&#39;s are therefore extinguished. Furthermore, the closing of secondary test switch  68  has no effect because the conductive paths have already been broken. When the GFCI  10 ″ is reset (by depressing an appropriately designed reset button, for example), the green LED  60  re-illuminates and the red LED  62  remains extinguished because the current flow through the green LED  60  and the fuse  64  has been restored. 
   When the AC source is connected to the load terminals  20 , on the other hand, a different notification sequence takes place. For example, when the GFCI  10 ″ is operating properly, depressing the primary test switch  44  causes the latching mechanism  12  to open the conductive paths but the red LED  62  will illuminate. This is because the red LED  62  is connected on the load side of the latching mechanism  12 . Thus, current is still able to flow through resistor  72 , diode  70 , red LED  62  and-resistor  66 . Furthermore, the latching mechanism  12  will not reset due to the lock-out feature discussed above. Thus, when the GFCI  10 ″ is operating properly and wired incorrectly, the green LED  60  will extinguish, the red LED  62  will remain illuminated and the latching mechanism  12  will be locked-out. 
   When the GFCI  10 ″ is wired correctly and operating improperly, yet another sequence of events takes place. In this case, when the GFCI  10 ″ is tested the imbalance of current flow resulting from the primary test switch  44  fails to trip the latching mechanism  12 . The secondary test switch  68  shorts the two current paths directly together through fuse  64 . This short blows the fuse  64  and permanently extinguishes the green LED  60 . When the GFCI  10 ″ is reset, the red LED  62  illuminates because the current path through the fuse  64  is no longer available. It is important to note, however, that the latching mechanism  12 ′ will not be locked-out because the AC source is connected to the source terminals  18 . 
   It will further be appreciated that it is possible for the GFCI  10 ″ to be both reverse wired and operating improperly. In this case, when the GFCI  10 ″ is tested the green LED  60  will extinguish due to the short circuit of fuse  64 . Furthermore, the red LED  62  will illuminate due to the reverse wiring condition. It will further be appreciated that the above-described lock-out of latching mechanism  12  will be in effect such that the reset feature is disabled and the red LED  62  remains illuminated. 
   Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention can be described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.