Patent Publication Number: US-8125748-B2

Title: Ground fault circuit interrupter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a ground fault interrupter for ground fault protection of persons using an electrical appliance. More particularly, the invention relates to GFCI receptacles utilizing a low friction pivotal latch, an end-of-life indicator and reverse wiring protection. 
     2. Description of Related Art 
     A ground fault occurs when current improperly flows through a ground line. Such a condition may indicate a shock hazard, even when the current flow is insufficient to trip a main breaker in the building in which the GFCI has been installed. Known ground fault circuit interrupters have been mounted in a receptacle housing with a detector to sense the ground fault condition. A ground fault is often detected by determining whether there is an imbalance in current between the two primary power lines. One or more toroidal coils can encircle the primary power lines to detect an imbalance in the currents in those lines. The imbalance can produce an output voltage from the toroidal coil to trigger a semiconductor circuit that energizes a solenoid coil. The solenoid coil drives an armature to release a latch that otherwise holds a pair of movable electrical contacts against a pair of stationary electrical contacts. When the movable contacts are released, power is disconnected from the terminals of the receptacle protected by the ground fault circuit interrupter (GFCI). 
     A GFCI generally includes a housing, a tripping means, a reset button, a test button, a mounting strap with a grounding strap and banding screw, a pair of movable contact holders with electrical contacts, a pair of fixed contact holders with electrical contacts, and a control circuit. 
     GFCIs are widely used to prevent electric shock and fire caused by a ground fault. 
     In the past, a GFCI receptacle generally utilized a mechanical actuator, which limited the performance of such products, especially insofar as these GFCIs did not provide reverse wiring protection. Examples of mechanical GFCIs include those disclosed in U.S. Pat. No. 5,935,063 and in U.S. Pat. No. 4,802,052. 
     The GFCI shown in published U.S. Patent Application No. 2006/0018062 A1 has reverse wiring protection that incorporates an electromagnetic tripping means and a corresponding control circuit. A significant disadvantage of this device is the relatively high mechanical resistance in initiating movement of a movable assembly of the device. 
     In addition, there is no end of life indicator in the above GFCIs which standard UL 943 now requires. 
     Accordingly, there is a need for a GFCI with an end of life indicator, reverse wiring protection, using a solenoid that easily overcomes frictional forces associated with a releasing latch means. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a GFCI having the above discussed needs. 
     In a preferred embodiment of the invention, a novel ground fault circuit interrupter includes a central body portion, an upper cover, a control circuit, latch means, and a mechanism for reverse wiring protection. The latch means has stationary arms with electrical contacts and terminals with electrical contacts. A movable bracket includes electrical contacts and a latch plate, a first solenoid coil for encircling a first armature located in a central body structure, and a reset button. The reset button has a latch pin and a press block engaging a second armature, the pin having return springs, all of which is located in the central body. The latch plate has two opposed cylindrical shafts that seat in two round recesses of the movable bracket that allows the latch plate to rotate pivotally in the round recesses. The upper end of the latch plate engages the latch pin of the reset button while the lower end of the latch plate has a vertical slot. One end of the first armature has a return spring while the other end has a narrow core and an impact step. The axial core of the armature seats in the vertical groove of the latch plate. 
     The mechanism for the reverse wiring protection includes a second solenoid coil encircling the second armature. One end of the armature has a return spring. The press block of the reset button presses against the second armature, while the end face of the armature engages an end wall of a support yoke. 
     The present invention includes an end of life circuit and indicator. If the GFCI fails, an LED is illuminated to tell the user that the GFCI is at or near the end of its life. 
     The present invention is also provided with reverse wiring protection that uses a mechanical means and a corresponding electrical control circuit. The control circuit is connected to the AC supply of the GFCI; it is de-energized when the GFCI is miswired by connecting the AC line to a load terminal (instead of a line terminal) so that the GFCI receptacle cannot be reset. When the GFCI is miswired, the face portion of the cover, particularly at the entry ports and the ground-prong-receiving opening, is without an electrical potential, which provides a safety feature for human use. 
     The latch plate of the invention is easy to rotate or pivot. The latch plate is easy to disengage by overcoming friction with a latch pin when the first armature strikes the latch plate such that the electrical power required for the second solenoid is relatively low. Thus, the electromagnetic device can be small, occupying less space within the body of the interrupter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the accompanying drawings wherein like reference numerals indicate like elements, and wherein reference numerals sharing the same last two digits identify similar corresponding elements throughout the various disclosed embodiments, and in which: 
         FIG. 1  is a longitudinal section of a GFCI constructed in accordance with principles of the present invention; 
         FIG. 2  is a top plan view of the GFCI of  FIG. 1 , with its cover removed; 
         FIG. 3A  and  FIG. 3B  are exploded views of the GFCI of  FIG. 1 ; 
         FIG. 4  is an exploded view of two solenoids and a portion of a latch means of  FIG. 1 ; 
         FIG. 5  is a sectional view of the solenoids of  FIG. 4  located in close parallel proximity to each other, and the latch means of  FIG. 1  showing an initial position; 
         FIG. 6  is the sectional view of  FIG. 5 , with the latch means in a position that prevents resetting of the GFCI when it is miswired by connecting a power line to a load terminal or is not wired at all; 
         FIG. 7  is yet another sectional view of the solenoids and latch means showing a reset shaft in latched engagement with a pivotal latch plate; 
         FIG. 8  is a side elevation view of stationary and movable electrical contacts in transit, said contacts being associated with stationary terminals and movable arms of the invention; 
         FIG. 9  is a perspective view of the movable bracket of the invention; 
         FIG. 10  is an exploded view of the bracket of  FIG. 9 ; 
         FIG. 11  is a side view of the latch plate of  FIG. 1 ; 
         FIG. 12  is a front elevation view of the latch plate of  FIG. 1 ; 
         FIG. 13  is a side elevation view of the movable bracket of  FIGS. 3B and 4 ; 
         FIG. 14  is a front elevation view of the movable bracket of  FIGS. 3B and 4 ; 
         FIG. 15  is a schematic circuit diagram for controlling the GFCI of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERABLE EMBODIMENT 
     Referring now to  FIG. 1  of the drawings, a sectional view of a duplex GFCI receptacle of the invention shows a central body  1  of the GFCI and an upper cover  2  suitably attached to the upper portion of the central body.  FIG. 2  presents a plan view of the interior of body  1  from the top thereof, with upper cover  2  removed. A mounting strap  2   a  is visible and upper portions of blade receiving terminals  5  are therefore exposed. Stationary arms  3  having electrical contacts  4  are visible in body  1  ( FIG. 2 ). A terminal  5 , as depicted in  FIG. 3A  has a fixed contact  4  similar to that of arm  3 . Terminal  5  is better seen in  FIG. 8 . A bottom cover  2   b  is shown in  FIG. 3B  that closes the lower face of central body  1 . 
     Terminal  5  is one of two such terminals for receiving the male blades of an electrical plug (not shown) of an electrical appliance (not shown). Contacts  4  of the terminals  5  are located near one end of the terminals, as best seen in  FIGS. 2 and 3A . 
       FIGS. 3B ,  9  and  10  show a movable bracket  6  of a latch means  10  of the invention containing movable arms  7  supporting electrical contacts  8 , and a low friction, pivotal latch plate  9 , discussed in detail hereinafter. A solenoid coil  11  with an axially movable armature  12  is located near bracket  6  and latch plate  9 , as seen in  FIG. 4 . 
       FIGS. 4 to 8  show a reset button  13  having a downwardly extending latch pin  14 , and a press block  15  located at the distal end of the latch pin. Return springs  16  are located beneath lateral portions of reset button  13 . 
     Return springs  17  for bracket  6  seat into openings  17   a  provided in the bracket as indicated in  FIG. 4  of the drawings, while movable arms  7  are provided with extension springs  18  that seat beneath arms  7  that support electrical contacts  8 , as best seen in  FIGS. 9 and 10 , and are supported on portions of bracket  6  labeled  18   a  in  FIGS. 4 and 10 . The upper ends of springs  17  are received into hollow portions (not shown) provided in central body  1 . 
     As seen in  FIGS. 3 ,  4 ,  10  and  12  of the drawings, latch plate  9  has short, opposed shafts  19  that seat in opposed openings or recesses  20  provided in the body of bracket  6 . Openings  20  have round surfaces for seating shafts  19  in a low friction manner. The short shafts  19  of latch plate  9  are made of any low friction material to insure easy rotation of the plate, thus requiring a minimum force to rotate the plate. 
     Further, latch plate  9  has a narrow slot  21  that receives a narrow end  23  of armature  12 , while the other end of the armature retains a return spring  22  between a shoulder  12   a  and the rear wall  34  of a first yoke  35 , all of which is best seen in  FIGS. 5 to 7  of the drawings. The armature, in addition, has an integral impact step  24  that is larger than the width of slot  21 , and is located behind slot  21  so that the armature can engage latch plate  9  to rotate it about its shafts  19  when solenoid coil  11  is energized and de-energized. 
     The GFCI of the invention has, in addition, a mechanism for reverse wiring protection in the form of a second solenoid coil  26  having an armature  27  and a return spring  28 . The spring is held between a shoulder  27   a  of the armature and the rear wall  43  of a second yoke  44 . In  FIGS. 5 to 7 , the first and second yokes  35  and  44  are depicted in section to show assembled retention of the two solenoid coils  11  and  26 , return springs  22  and  28 , and armatures  12  and  27 . 
     A second set of contacts  30  and  32  are shown in  FIGS. 3B ,  9  and  10 . Contacts  30  are mounted on an arm  29  which is held in place in bracket  6  by two springs  31 , see  FIG. 9 . 
     Latch pin  14  of reset button  13  is provided with a ledge  36  on a surface of the pin facing latch plate  9 , see  FIGS. 5 to 7 . Similarly, the latch plate has a ledge  37  facing ledge  36  of latch pin  14 . This structure of the latch system in cooperation with solenoid coil  26  and armature  27  provides reverse wiring protection as follows: 
     When the GFCI is connected to the AC power leads, the control circuit of  FIG. 15  in the GFCI is energized. However, if the GFCI is reverse wired by connecting an AC power lead to a load terminal of the GFCI, or if the GFCI is not electrically connected, the control circuit is not energized since the power lead is not properly connected. Obviously, if a load lead is connected to a power terminal, no electrical power is available for the GFCI and its control circuit. With no power for solenoid  26 , there is no translating force for its armature  27 . The armature moves to the right in  FIG. 1  under force of its return spring  28 , and firmly seats in a recess  40  of the pin, as seen in  FIG. 6 . Latch pin  14  has a lower face or ledge  40   a  that prevents downward movement of the reset button so that the GFCI cannot be reset until the reverse wiring connection is corrected, thereby achieving reverse wiring protection. When the GFCI is electrically connected correctly, solenoid coil  26  is energized to translate its armature  27  from latch pin  14 , so that reset button  13  and latch pin  14  can move downwardly a substantial distance. This allows latch plate  9  to seat over ledge  36  of latch pin  14  (see  FIG. 7 ). Reset button  13  is free to move upwardly under the pressure of its return springs  16 , and, as it moves upwardly, it moves bracket  6  upwardly. Electrical contacts  4  and  8  and  30  and  32  close to energize the GFCI. 
     With reset button  13  in a released position, return springs  16  maintain latch pin  14  in an upward position. This allows upward movement of latch plate  9 , bracket  6  and contact arm  29  a distance sufficient to bring contacts  30  into electrical contact with stationary contacts  32  ( FIG. 3B ). After the engagement of contacts  30  and  32 , contacts  8  on movable arms  7  come into contact with fixed contacts  4  on arms  3  and terminals  5  ( FIGS. 3A  and B). 
       FIG. 15  illustrates the control circuit. DB 1  is a bridge rectifier that provides DC voltage to an integrated circuit amplifier IC 1  and solenoids K 2 , K 1  ( 26  and  11 ). CT and NT are current sensing transformers. If the current flowing in the input line L is not equal to the input neutral N, residual magnetic flux flows in the cores of the current sensing transformers (generally indicated by numeral  45  in  FIGS. 3B and 15 ). An induced voltage appears at the secondary of the CT and is sent to a terminal  1  of the IC 1  via C 3  and R 3 . Solenoid K 2  ( 26 ), D 9 , R 19 , R 20 , C 13  and D 8  are used for anti-reversing the line and load terminals of the GFCI. When applying voltage correctly at the input line and neutral terminals, the solenoid K 2  ( 26 ) is energized by the current that flows through D 9 , R 19  and R 20 . Now the RESET button  13  can be pushed down and power is transferred to the load and the receptacle terminals and to R 16  and R 17 . LED 2  and D 7  are in series and connect between lines L and N. When the ground fault circuit interrupter is reset successfully, LED 2  lights in green. If power is connected to the load L and N terminals, solenoid K 2  ( 26 ) cannot be energized because no current flows through K 2 . The GFCI cannot be reset successfully so no power will be transferred to the input L and N terminals. 
     Connected between terminals  1  and  7  of the IC 1  circuit is a resistive-capacitive circuit consisting of capacitor C 5  and resistor R 2 . These components set the gain of the IC 1  amplifier. 
     MOV 1  is a metal oxide varistor. It is connected between the input Line and Neutral, and can absorb inrush current coming from the power supply. 
     C 1  and C 2  are two capacitors that are connected in parallel with the CT and NT. They oscillate, respectively, with the inductance of the CT and NT to preserve loop gain for oscillation. 
     C 4  is a coupling capacitor; it transfers ground neutral fault signals to the IC 1  from the CT. 
     Capacitors C 6 , C 7  and C 8  are filters that clean noise for pin 1 , pin 6  and pin 3  of the IC 1 . 
     R 4  is a resistor employed for detecting a ground neutral fault with the NT, capacitor C 2  and the IC 1 . 
     R 5 , R 18 , R 6 , R 7  and R 7 ′ are voltage dividing resistors; they can produce an approximately 26VDC voltage to the IC 1 . 
     R 8  and R 9  are upper bias resistors for a transistor T 1  connected between an LED 1  and a diode D 6 . 
     R 10  and R 11  are two current limiting resistors for LED 1 . When the LED 1  is switched on by transistor T 1 , current flows through R 10 , R 11  and LED 1 . Resistors R 10  and R 11  provide an appropriate current for LED 1 . Otherwise too large a current would damage LED 1 . 
     Resistor R 12  and diode D 6  provide a reference voltage for the emitter of T 1 . 
     Resistor R 13  is in series with the base of T 1  and produces driving current for T 1 . 
     Resistor R 14  is the load for a transistor T 2 . When T 2  is switched on, an imitated leakage current flows through it. 
     Resistor R 15  and capacitor C 12  are in series with and are connected between the cathode and anode of an SCR 1 ; R 15  and C 12  can absorb surge voltages appearing on SCR 1 . 
     Diode D 2  is connected in series with the solenoid K 2  so that current can only flow in the positive half cycle of the AC power. In every negative half cycle of the power, an imitation leakage current flows through the power neutral, D 1 , T 2 , R 14  and the power line; the SCR 1  is switched on by a trip signal sent out from the IC 1 , but now the diode D 2  is anti-biased so that no current can flow through the solenoid K 1  at that time. In the positive half cycles of the power, no limitation ground fault occurs, so the GFCI cannot be tripped and stays in its “Reset” state. But if an actual ground fault occurs, the SCR 1  can be tripped to “on” condition in both half cycles of the AC power. At the positive half cycle, current can flow through solenoid K 1 , and the GFCI would trip immediately. 
     Diode D 4  transfers a DC voltage from resistor R 9  to the base of transistor T 2  through diode D 3  and resistor R 13 . Diode D 4  prohibits the flow of current in the reverse direction. 
     Diode D 3  is connected in series with resistor R 13  and the base of transistor T 2 , it can protect T 2  from damage by providing a high collector voltage. 
     Pin  3  of the IC 1  provides an output 13V reference potential. Pin  2  is a positive input of an internal operational amplifier, and is connected to pin  3  of an internal  10 K ohm resistor to produce the 13V reference. Pin  6  of the IC 1  is its supply (26VDC) input pin. 
     During each half cycle of the power supply, transistor T 2  turns on. But only in the negative half cycle, a simulated leakage current can flow from input neutral to input line via D 1 , T 2 , R 14  and DB 1 . The line and neutral wires pass through the center of the current sensing transformer CT and the flow of different currents are now assumed to be a fault current. So an inducting voltage is produced at the secondary of the sensing transformer and fed to IC 1 , and IC 1  produces a trip pulse at its pin 5  to turn SCR 1  on via resistor R 21 . A capacitor C 10  now has a discharge path through D 5  and SCR 1  so that the base of transistor T 1  remains at a low level and the end of life indicator LED 1  remains off. 
     When components of the GFCI lose proper functioning or are at the end of their life, especially the sensing transformer CT, the integrated amplifier IC 1  or the SCR 1 , as the imitated leakage current occurs through D 1 , T 2  and R 14  at any negative half cycle of the power supply, SCR 1  cannot be tripped in its “on” state, capacitor C 10  has no discharge path, the base voltage level of transistor T 1  rises to a high level and remains that way so that transistor T 1  turns on. The end of life indicator  42  (LED 1 ) now gives off a red light, telling the user that the GFCI is at the end of its life and should be replaced by a new one. 
     LED 1  and LED 2  (numbered  42  and  38 ) are shown visible on the front face of the GFCI in  FIG. 2  for easy viewing. 
     Since the lower end (press block  15 ) of the latch  14  presses against one end of armature  27  while the other end of the armature is disposed against the end structure  43  of yolk  44 , only a small electromotive force is needed to prevent the armature from moving to the right (in  FIGS. 1 ,  5 ,  6  and  7 ); thus a minimum amount of current needs to be utilized by solenoid  26  in returning the armature against the force of return spring  28 . 
     When the GFCI is working properly, pressing of test button  41  provides a simulated leakage voltage from load point L ( FIG. 15 ) across resistor R 1  to the input point N. The IC 1  circuit will sense a trip pulse and switch on SCR 1 , which will energize solenoid K 1  ( 11 ) to disconnect contacts  4  and  8 , and  30  and  32 . 
     While the subject invention has been described in terms of a preferred embodiment, the claims appended hereto are intended to encompass all embodiments which fall within the spirit and scope of the invention.