Patent Publication Number: US-2006007610-A1

Title: Leakage current detector interrupter with reset lockout

Description:
This application claims the benefit of the filing date of a provisional application having Ser. No. 60/558,954 which was filed on Apr. 2, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates generally to leakage current detector interrupters.  
      2. Description of the Prior Art  
      A type of electrical extension cord which can provide ground fault protection can include a ground fault circuit interrupter (GFCI). A GFCI is a device that may use a trip mechanism to disconnect line conductors from load conductors when an electrical fault such as excessive leakage current to ground occurs. GFCI devices are normally resettable, that is, placed in a condition to detect another occurrence of a ground fault, after they are tripped by, for example, the detection of a first ground fault. Trip mechanisms which cause the mechanical breaking of the circuit (i.e., the connection between input and output conductors) include a solenoid (or trip coil). A test button can be used to test the trip mechanism and circuitry used to sense faults and a reset button can be used to reset the electrical connection between input and output conductors.  
      However, instances may arise where an abnormal condition, caused by, for example, a lightening strike, occurs which may result not only in a surge of electricity which causes a tripping of the device, but also may cause a disabling of the trip mechanism and/or circuitry used to sense faults which cause the breaking of the circuit. That is, the device may have become inoperable with respect to breaking the connection between the input and the output conductors when a fault occurs. This may occur without the knowledge of the user. Under such circumstances an unknowing user, faced with a GFCI which has tripped, may press the reset button which, in turn, may cause the device with an inoperative trip mechanism and/or inoperative circuitry to be reset without the ground fault protection being available.  
      Also, an open neutral condition may exist with the electrical wires supplying electrical power to such GFCI devices. If such an open neutral condition exists with the neutral wire on the Line (as opposed to Load) side of the GFCI device, an instance may arise where a current path may be created from the phase (or hot) wire supplying power to the GFCI device through the load side of the device and, possibly, through a person to ground. That is, electrical power that would normally flow through the phase wire and load back to the neutral could, in the fault condition, return to ground through a person. This condition presents a potential shock hazard.  
      Electrical extension cords or corded electrical appliances can have conductors to carry current from an input or Line side to an output or Load side. The input power can have conductors including a Line Phase, Line Neutral and ground. The cord can have corresponding conductors including a Load Phase, Load Neutral and ground. The Load cord also may include a shield surrounding the conductors. Through usage or age, the insulation of electrical cords may degrade or become damaged which can result in leakage currents not only to ground but between the conductors or a conductor and the shield. Degradation and damage of the insulation also may result in arcing between the conductors and/or a conductor and the shield.  
      In an electrical circuit, including in residences and commercial locations, electrical current can flow from the Line Phase through the electrical appliance and return to the Line Neutral. In normal usage, current does not flow from a Load Phase conductor to the shield. Flow of current from the Load Phase to the shield can present a hazardous shock condition. A leakage current detector interrupter (LCDI) is a device that senses leakage current flowing between or from the attached cord conductors and interrupts the circuit at a predetermined level of leakage current. Because a LCDI can detect a current leakage flowing to ground, it can provide ground fault protection in addition to protection from arcing and other problems which may arise due to leakage between conductors, between a conductor and a shield or a conductor and ground.  
     SUMMARY OF THE DISCLOSURE  
      The disclosure includes techniques for a leakage current detector interrupter (LCDI), which may include a reset lockout, that can interrupt the power being supplied to an extension cord or a corded appliance when a leakage current is detected flowing from any of the wires in the load side cord to a shield within the cord. The leakage current may be caused by degradation of the insulation around the wires due to arcing, fire, overheating, or physical or chemical abuse.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of a preferred embodiment of the invention when taken in conjunction with the following drawings wherein like parts are represented by similar reference numbers.  
       FIG. 1  is a schematic drawing of an implementation of the disclosed leakage current detector interrupter in an electrical extension cord.  
       FIG. 2  is a front elevation view of the face of a plug of the extension cord of  FIG. 1 .  
       FIG. 3  is a front elevation view of the face of the receptacle of the extension cord of  FIG. 1 .  
       FIG. 4  is a schematic of the circuit diagram of a leakage current detector. 
    
    
     DETAILED DESCRIPTION  
       FIGS. 1, 2  and  3  illustrate an implementation of an electrical extension cord constructed in accordance with the principles of the invention. Plug  10  has a housing  12  from which project flat plug blades  14  and  16  and a curved ground blade  18 . The phase blade  16  is smaller than the neutral blade  14  as is the usual industry practice. Further, within the housing  12  a solenoid operated relay  20  may be disposed and coupled to movable contact  22  for the line phase and to movable contact  24  for the line neutral. The position of movable contacts  22 ,  24 , respectively, in  FIG. 1  are in the open position which opens both the phase and neutral conductors. When the solenoid operated relay  20  no longer receives an operating signal, the movable contacts  22  and  24  can engage fixed contacts  26  and  28  and complete the phase and neutral conductors and current can flow to the receptacle  32  or load side of the electrical cord. A control device  30  may be coupled to solenoid operated relay  20  to operate it in accordance with the detection of leakage current in the electrical cord such as may result from arcing or degradation of the cord.  
      On the load side of the electrical cord can be a receptacle  32  including a housing  34  having a front face  36  in which are placed blade passageways. Passageway  38  can receive the neutral conductor, passageway  40  can receive the phase conductor and passageway  42  can receive the ground conductor. Behind the blade passageways through the front face  36  may be chambers in which the contacts are placed. The contacts (not shown) can engage the flat plug blades  14  and  16  and the curved ground blade  18  and make electrical and mechanical contact between the conductors of the extension cord and the load (not shown) plugged into receptacle  32 .  
      The electrical cord  48 , which joins plug  10  to receptacle  32 , can include a phase conductor  50  to connect movable contact  22  and plug blade  16  to the contact (not shown) in passageway  40 ; a neutral conductor  52  to connect plug blade  14  and movable contact  24  through fixed contact  26  to the contact (not shown) in passageway  38 ; and a ground conductor  54  to connect the curved ground blade  18  to the contact (not shown) in passageway  42 . In some implementations, the electrical cord may include a conductive shield  56  that may surround one or more of the phase, neutral and ground wires. In another implementation, the disclosed electrical cord protection may be used where the cord terminates in an electrical appliance rather than a receptacle  32 .  
      The control device  30  may be coupled to phase, neutral and shield on the load side of the electrical cord. The control device  30  can monitor leakage currents among the phase, neutral and shield. The control device  30  can actuate the solenoid controlled relay  20  to open the connection between the line phase  16  and line neutral  14  and the corresponding load side conductors when the leakage current exceed a predetermined level. In an implementation, the control device can be a leakage current detector that can detect arcing currents in the electrical cord.  
       FIG. 4  illustrates a schematic of an implementation of the leakage current detector interrupter (LCDI) that can be contained within a plug that can be plugged into a receptacle, which provides electrical power. In an implementation, the electrical power is 120 volts alternating current (VAC), which is household line current. The electrical power may have two conductors indicated as Line Phase and Line Neutral. The LCDI can provide electrical power to an electrical cord having conductors Load Phase and Load Neutral corresponding to the Line Phase and Line Neutral, respectively. A double-poled relay  20 , capable of disconnecting power to Load Phase and Load Neutral upon the detection of leakage current. The relay  20  can be latched mechanically and tripped by a solenoid ( 20 , pins  1  and  2 ) The solenoid may be of the discontinuous type so that the solenoid is actuated only when required to move the catchment that can moves the relay contacts RL 1   a , RL 1   b.    
      A ground wire also may be present with a hardwired connection from Line to Load side. The ground may not disconnected upon detection of leakage current. An indicator LD 1  on the load side of the device can be included to indicate whether power is connected to the load. When the contacts RL 1   a , Rl 1   b  are closed, Line power is connected to the load. The load side power can be rectified by diode D 5  and used to illuminate indicator LD 1 . A resistor R 10  can be included to limit current through the indicator LD 1 . When the contacts RL 1   a , RL 1   b  are open, such as after the LCDI is tripped, the Line power is disconnected from the load power and the indicator LD 1  is extinguished.  
      With the device plugged in input power is connected to Line Phase and Load Phase, the LCDI may be in the tripped state. Relay coil  200  is de-energized with the contacts RL 1   a , RL 1   b  open) and Line Phase and Line Neutral are disconnected from Load Phase and Load Neutral, respectively. Upon pressing reset button (not shown) to mechanically reset the relay, the catchment, which closes the contacts, may be blocked and the lockout switch SW 1  ( 20 , pins  3  and  4 ) is closed instead. With SW 1  closed, Line power can be provided to the solenoid coil  200  ( 20 , pins  1  and  2 ) of the relay  20  through diode D 3 . Simultaneously, voltage is applied to a gate of a silicon-controlled rectifier (SCR) SC 1  through current limiting resistor R 1  and rectifying diode D 1 . Resistors R 1 , R 2  form a voltage divider to establish a voltage on the first gate of the SCR SC 1 . The voltage at the gate is sufficient to trigger the SCR SC 1 , which starts to conduct current from anode to cathode. Thus, a current path is established from the input power, through the path switch SW 1 , diode D 3 , relay coil  200 , SCR SC 1  and then return to Line Neutral. The reset button can engage the catchment and close the relay contacts. However, if the solenoid cannot be energized, then the reset button is locked out. The button can not engage the catchment and power is not supplied to the load. Thus, if the LCDI is functional, then the device can be reset to having relay contacts RL 1   a , RL 1   b  closed because the trigger circuit of SCR SC 1  is working.  
      Once the contacts RL 1   a , RL 1   b  are closed, switch SW 1  may be opened. Power for the circuit can be supplied from the Load Phase through a blocking diode D 4  and the solenoid coil  200 . Diode D 4  and C 1  provide a half-wave rectified voltage supply to power the electronics. A metal-oxide varistor (MOV) MV 1  can be included to provide protection from destructive voltage spikes on the line side of the cord.  
      When in operation, the circuit may be triggered to remove power to the load side when a current fault is detected. One or more SCR&#39;s, depending on the fault type, may be triggered and energize the relay coil  200 . The illustrated implementation includes two SCRs (SC 1  and SC 2 ) either of which may be triggered and, in turn, energize the coil  200  and cause actuation of the solenoid and open the contacts RL 1   a , RL 1   b  to remove power from the load side of the electrical cord.  
      SCR SC 1  can be triggered when current leakage is detected from Load Phase to a Shield incorporated into the load side of the cord. Leakage current that flows from the Load Phase through the Shield can flow through resistors R 8 , R 4  and R 2  to Load Neutral. The gate of SC 1  is connected between resistors R 4  and R 2 . SCR SC 1  can trigger, thus conduting current from cathode to anode, when a threshold voltage and current is provided to the gate of SCR SC 1 . The threshold may be exceeded when the leakage current to the Shield increases. When SCR SC 1  triggers, the relay coil  200  can be energized by current that can now flow through the relay coil  200  and through the SCR SC 1  to Line Neutral. Energizing relay coil  200 , in turn, opens relay contacts RL 1   a , RL 1   b , which removes power from the load side. Capacitors C 2 , C 4  may be added to reduce false tripping by filtering electrical noise on the shield. A diode D 2  can be added to discharge the capacitors C 2 , C 4  during the half cycle when line neutral is positive with respect to the line phase to reduce electrical charge from accumulating on the capacitors and, thus, reduce false triggering of the SCR SC 1 . In an embodiment, the resistors R 8 , R 4  and R 2  and the gate sensitivity of SCR SC 1  are selected to trigger the SCR SC 1  at or above a leakage current of approximately 4 milliamps (mA)±1 mA over a large temperature range.  
      SCR SC 2  can be triggered when current leakage current is detected from Load Neutral or Ground to the Shield. The circuit operation does not depend on a Grounded conductor in the cord. Ground may connected to Line Neutral at the service entrance panel. In the illustrated implementation, resistors R 3 , R 5 , R 4  and R 2  create a voltage divider from the Load Phase power, which is rectified by diode D 4  and smoothed by capacitor C 1 . As described above, the voltage level between resistors R 4  and R 2  may not be great enough to trigger SCR SC 1 . However, the voltage between R 5  and R 4  can be sufficient to turn on transistor Q 1  through resistor R 7 . When transistor Q 1  is on, it pulls down the voltage level at the junction between resistors R 6  and R 9  towards the level of Line Neutral. This voltage level can be further divided by resistors R 9 , R 11 , which may be selected to prevent SCR SC 2  from being triggered. However, when leakage current flows between the Shield and Load Neutral or the Shield and Ground, then the voltage level at the connection between resistors R 5  and R 4  is pulled down towards the level of Line Neutral because the leakage current causes an increase in the voltage drop across resistor R 8 . Thus, the voltage level between resistors R 4  and R 5  can depend on the level of leakage current that may be flowing between Shield and Load Neutral and/or between the Shield and Ground. When the leakage current between Shield and Load Neutral and/or between the Shield and Ground is sufficiently high, the voltage level at the junction of resistors R 4  and R 5  can be reduced to a level where transistor Q 1  turns off. When transistor Q 1  is off the voltage level at the junction between resistors R 6  and R 9  can rise and trigger SCR SC 2 , which, in turns, energizes the relay coil  200  and opens the relay contacts RL 1   a  and RL 1   b  to disconnect the load side of the cord. In the implementation illustrated, capacitors C 5  and C 3  (and C 4 ) may be included to provide noise filtering for transistor Q 1  and SCR SC 2 .  
      While there have been shown and described and pointed out the fundamental features of the invention as applied to the preferred embodiment, as is presently contemplated for carrying them out, it will be understood that various omissions and substitutions and changes in the form and details of the device described and illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention.