Abstract:
A tamper-proof receptacle is provided wherein the receptacle comprises sliding shutters disposed between phase and neutral terminals of the receptacle and openings in the face of the receptacle. The shutters are connected to circuit interrupting circuitry such that when one of the shutters is displaced through a specific range of motion, the circuit interrupting portion of the receptacle is triggered to disconnect electrical power from the phase and neutral terminals of the receptacle. In an embodiment, a logic circuit is connected to the shutters, and a monitoring circuit monitors supply of power to the face terminals. The logic circuit is configured to detect insertion of an object into only one of the pair of openings. A signal from the logic circuit to the circuit interrupting device is effective to prevent the object from touching any of the face terminals while power is connected to the face terminals.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority pursuant to 35 U.S.C. 119(e) from U.S. Provisional Application having Application No. 60/732,327 filed Oct. 31, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a tamper-proof receptacle for supplying electrical current to a load.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to electrical receptacles of the type having slidable shutter mechanisms arranged behind the receptacle openings that receive the blades or prongs of an electrical plug connected to the receptacle.  
         [0004]     In order to prevent electrical shocks and possible injuries which may result from insertion of an electrically conducting member into the live terminals of an electrical receptacle, electrical receptacles with shutter mechanisms have been developed to provide an additional level of safety to users.  
         [0005]     These mechanisms typically include a plurality of moveable members that are spring-biased to positions wherein the moveable member masks the plug-receiving openings thereby shielding the live terminals within the receptacle. The shutter members are moved laterally by objects inserted through the openings in the receptacle cover. However, the structure is such that the shutter members must be moved in unison, such as when plug blades are inserted in the receptacle openings, in order to achieve physical access to and electrical communication with the terminals. That is, when a single shutter member is moved independently by insertion of a device through a single opening, the other shutter member remains stationary with a portion in a blocking position to prevent advance of the device into contact with the receptacle terminals. Such an arrangement does not, however prevent tampering with the receptacle such as when an object is inserted through a single opening and contacts an energized contact within the receptacle.  
         [0006]     There is a need, therefore, for an improved shutter mechanism to address tampering of an energized receptacle or the inadvertent insertion of an object into one of its openings.  
       SUMMARY OF THE INVENTION  
       [0007]     It is the object of the present invention to provide an electrical receptacle having a novel and improved safety shutter mechanism to overcome a number of deficiencies of prior art mechanisms of this type. The present invention is directed to a tamper-proof receptacle having a circuit-interrupting device. The receptacle is configured so that the circuit-interrupting device will trip and remove power from the face terminal of the receptacle if an object is pushed into a single opening in the face of the receptacle. In contrast, if a two or three-pronged plug is inserted into the face of the receptacle, it will not trip the circuit interrupter.  
         [0008]     According to a first aspect of the invention, a receptacle for providing power to a load includes terminals for connection to an electrical power source, and slidable shutters located between openings in the receptacle and the terminals; the shutters are operatively connected to a circuit interrupter. Displacement of one shutter causes the circuit interrupter to disconnect the terminals from the power source. In an embodiment, each of the shutters has an angled end surface so that insertion of an object through a corresponding opening causes a cam action between the object and the angled end surface, thereby causing the displacement of the shutter; a switch is operatively connected to each of the shutters, so that displacement of a shutter causes the corresponding switch to close and causes the circuit interrupter to disconnect the terminals from the power source. The inserted object therefore does not touch any of the terminals while power is connected to the terminals.  
         [0009]     According to another aspect of the invention, a receptacle includes a face plate having a pair of openings; face terminals for connection to an electrical power source; a pair of shutters with each shutter located between an opening and a corresponding face terminal; a logic circuit connected to the shutters; a monitoring circuit, connected to the logic circuit, for monitoring supply of power to the face terminals; and a circuit interrupting device, connected to the logic circuit, for disconnecting power from the face terminals in accordance with a signal from the logic circuit. In an embodiment, the receptacle also includes a switch coupled to each of the shutters and providing an input to the logic circuit, so that displacement of a shutter causes the corresponding switch to close. The logic circuit is configured to detect insertion of an object into only one of the pair of openings. In an embodiment, the logic circuit includes an XOR gate having a pair of inputs coupled respectively to the switches, and also includes an AND gate having a first input connected to the monitoring circuit and a second input connected to the output of the XOR gate. The output of the AND gate therefore indicates insertion of an object into only one of the pair of openings while power is supplied to the face terminals.  
         [0010]     According to a further aspect of the invention, the face plate of the receptacle has a plurality of pairs of openings and the logic circuit has a plurality of XOR gates; each of the pairs of openings has a pair of shutters with switches and face terminals corresponding thereto. Each pair of shutters is coupled to one of the XOR gates by the switches. The logic circuit further includes a plurality of AND gates and an OR gate. Each of the AND gates has a first input connected to the monitoring circuit and a second input connected to the output of one of the XOR gates. The OR gate has a plurality of inputs each connected to the output of a respective AND gate; the output of the OR gate provides the signal to the circuit interrupting device.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0012]      FIG. 1  depicts a receptacle according to the present invention.  
         [0013]      FIG. 2  depicts a detailed top sectional view of a receptacle according to the present invention with shutters closed.  
         [0014]      FIG. 3  depicts a detailed top sectional view of a receptacle according to the present invention with shutters open.  
         [0015]      FIG. 4  details a circuit diagram depicting the trip circuitry of the present invention.  
         [0016]      FIG. 5  details a circuit diagram depicting tamperproof circuitry of the present invention for a receptacle with multiple outlets. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]     An embodiment of the invention comprises tamper-detection and tamper-proofing circuitry implemented in a circuit-interrupting receptacle. The device is designed to trip, and remove power from the face terminals of the receptacle, if an object is pushed into a single opening in the face. Two- and three-pronged plugs inserted into the face of the receptacle will not trip the circuit interrupter.  
         [0018]     The following description is directed to tamper-proof circuitry implemented in a Ground Fault Circuit Interrupter (GFCI) such as described in commonly owned U.S. Pat. No. 6,040,967, the disclosure of which is incorporated herein by reference. It will be understood, however, that an embodiment of the invention may be implemented with any kind of circuit interrupting receptacle (an AFCI for example).  
         [0019]     Referring to  FIG. 1 , depicted therein is a receptacle  100  in accordance with an embodiment of the present invention. Receptacle  100  includes a faceplate  102 ; the faceplate has openings  101  for phase and neutral plug blades, an opening  104  for a ground prong of a plug, and TEST and RESET buttons  103 ,  105 . Referring now to  FIG. 2 , depicted therein is a top sectional view of the receptacle  100  of  FIG. 1 . As shown in  FIG. 2 , the two openings  101  in receptacle  100  are each blocked by a shutter mechanism including a shutter  201 . The shutters  201  are placed between openings  101  and terminals  203  and adapted to slidably open and close switches  202  when an object is inserted into one of the openings  101 .  
         [0020]     Each shutter  201  has an angled end  201   a  and a switch end  201   s . The angled end of the shutter  201   a  is completely covers the opening  101 . If an object  205  is pushed into an opening  101 , the angled end of the corresponding shutter is pushed sideways due to a cam action between the object and the angled end surface of the shutter (see  FIG. 3 ). Sideways movement of the shutter  201  greater than a relatively small threshold distance (approximately 0.050″) causes switch  202  to close. The shutter switch  202  remains closed throughout the range of movement of the shutters; that is, from the point where the threshold is reached to a point where the shutter  201  is completely out of the way of the opening. It should be noted that as object  205  is inserted, switch  202  is closed before the object touches terminal  203 . Accordingly, power may be disconnected from terminal  203  before tampering with the receptacle can create a shock hazard.  
         [0021]      FIG. 3  shows the shutter mechanism in accordance with an embodiment of the invention with one of the shutter switches  202  closed. The 0.050″ of travel referred to above (i.e. the movement threshold) serves to prevent tripping one or both of the switches  202  as a result of inserting a plug into the receptacle  100  at a shallow angle. Insertion at greater angles is prevented by the size of the openings  101  themselves. It will be appreciated that in this embodiment, sliding shutters  201  and switches  202  require only a minimal increase in the depth of receptacle  100 , in contrast to conventional tamperproof receptacles having cantilever or locking movements which add considerable depth.  
         [0022]      FIG. 4  is a schematic diagram of an embodiment of the invention, showing circuitry for rendering receptacle  100  tamperproof In this embodiment, tamper-detection and tamper-proofing circuitry is integrated with a typical GFCI as shown in  FIG. 4 . It should be noted that the circuit in this embodiment includes the circuit elements contained in logic circuit  200 , operating in conjunction with power monitoring circuit  250 . The operation and interaction of these circuits is described below. The tamper-detection detection circuit of the present invention is powered from the power supply from the GFCI via circuit  290 .  
         [0023]     Other elements shown in  FIG. 4  comprise a typical GFCI, the structure and operation of which will be described here only briefly. The GFCI comprises a sensing circuit including a differential transformer  417 ; a Ground/Neutral (G/N) transformer  418 ; an integrated circuit  419  for detecting current and outputting a voltage once it detects a current; a full wave bridge rectifier comprising diodes  420 ,  421 ,  422  and  423 ; metal-oxide varistors  424  and  447  as surge suppressors; various filtering coupling capacitors and other capacitors  425 - 433 , a gated semiconductor device  451 ; a relay coil assembly  448 ; rectifying diode  449 ; various current limiting resistors  434 - 439 ; and a voltage limiting Zener diode  440 .  
         [0024]     Mechanical switch  106  (coupled to TEST button  103 ) is shown connected to the conductors of the line terminals. Movable bridge contacts are shown as switches  441 - 444 , connecting line terminals  445  to face terminals  203  and load terminals  446 . The line terminals  445 , load terminals  446  and face terminals  203  are electrically isolated from each other unless connected by the movable bridge contacts  441 - 444 .  
         [0025]     When a predetermined condition occurs (e.g. a ground fault), a difference in current amplitude appears between the two line terminals  445 . This current difference is manifested as a net current which is detected by the differential transformer  417 . A resulting voltage signal is provided to integrated circuit  419 , which then generates a voltage on pin  411 , connected to the gate of gated semiconductor device  451 . Semiconductor device  451  is typically implemented using a Silicon Controlled Rectifier. The full wave bridge rectifier has a DC side connected to the anode of semiconductor device  451 . The voltage signal from pin  411  turns device  451  on, shorting the DC side of the bridge rectifier and thereby energizing relay  448 , which engages the movable bridges  441 - 444  causing them to remove power from the face terminals  203  and load terminals  446 . Relay  448  is also energized when mechanical switch  106  is closed, causing a current imbalance on the line terminal conductors that is detected by the differential transformer. The G/N transformer  418  detects a remote ground voltage that may be present on one of the load terminal conductors and provides a current to integrated circuit  419  upon detection of this remote ground which also energizes relay  448 .  
         [0026]     The tamper-detection and tamper-proofing circuitry in this embodiment of the invention will now be described. In the receptacle  100 , each pair of phase and neutral shutter switches  202  is connected to a 5V DC supply on one side and the inputs of an XOR (exclusive OR) gate  401  on the other side. Resistors  402  and  403  independently hold the inputs of the XOR gate  401  to ground unless the shutter switches  202  are closed. In accordance with XOR logic, if both shutter switches  202  are open then the inputs  401   a ,  401   b  to the XOR  401  gate are both 0 and the output of the gate 0. If both shutter switches  201  are closed, the inputs to the XOR gate  401  are both 1 and the output is 0. However, if only one of the shutter switches  202  is closed and the other one is open then the inputs to the XOR  401  are 01 or 10, and the output is 1 or logic high.  
         [0027]     When the output of the XOR  401  gate is logic high or 1, capacitor  404  will begin to charge through resistor  405  with time constant T=RC. When a plug is properly inserted into the receptacle, both shutters  201  will be moved aside, so that both shutter switches  202  will close. It is highly unlikely that the two shutter switches  202  will close at exactly the same time; this offset in closure will produce a short pulse at the output of XOR gate  401 . The RC network of capacitor  404  and resistor  405  allows the device to ignore these short pulses, because the pulse is not on long enough to charge capacitor  404  up to the logic level 1. However, if an object is pushed into only one opening  101 , the output of XOR gate  401  remains high for enough time to charge capacitor  404 . This in turn causes input  406   a  of AND gate  406  to be 1 (logic high).  
         [0028]     The output of AND gate  406  is high when both inputs  406   a ,  406   b  are high. Input  406   b  is supplied by inverter  409 , which is connected to circuit  250  monitoring power at the face terminals  203  of the receptacle. Power monitoring circuit  250  includes an optocoupler  407  and current limiting resistor  408 . When power is supplied to face terminals  203 , the transistor in the optocoupler  407  conducts, thereby providing a logic low signal to the input to inverter  409 . Resistor  410  normally holds the input to the inverter  409  high when the transistor is off (not conducting). A logic high input  406   b  thus indicates that power is present at the face terminals  203 . Accordingly, if power is supplied to the face terminals  203 , and only a single shutter switch  202  is closed (for a time long compared to T, then the output of AND gate  406  goes high.  
         [0029]     When the output of AND gate  406  goes high, current flows into the gate of the Silicon Controlled Rectifier (SCR)  451  through resistor  412  and diode  413 . This causes the SCR to conduct, energizing coil  448  and causing the GFCI to trip, thus removing power from the face terminals  203  and load terminals  446  of the device. When power is removed from the face terminals  203  the output of inverter  409  goes low again, so that the output of AND gate  406  goes low again and SCR  451  is turned off. If the user attempts to reset the circuit-interrupting device with a foreign object still present (see  FIG. 3 ), the device will trip instantly as soon as power to the face terminals  203  is detected.  
         [0030]     The tamper-detection and tamper-proofing circuitry in this embodiment is powered from the power supply from the GFCI via resistor  416 , Zener  414  and capacitor  415 . Otherwise, the additional circuitry is independent of the GFCI. Diode  413  prevents the normally low output from the AND gate  406  from interfering with the GFCI signals to the gate of the SCR  451 .  
         [0031]      FIG. 5  shows how additional pairs of shutters  201  on the phase and neutral receptacle openings  101  may be added to the circuit. In  FIG. 5 , two AND gates  502 ,  503 , each receiving input from a respective pair of shutter switches via XOR gates  512 ,  513 , are provided in place of the single AND gate  406  of  FIG. 4 . The output of each AND gate  502 ,  503  becomes an input to an OR gate  501 . The output of OR gate  501  is connected to resistor  412  in series with diode  413  (compare  FIG. 4 ). In accordance with OR logic, if either of the outputs of AND gates  502 ,  503  goes high, then the output of OR gate  501  goes high and device is caused to trip. More phase and neutral shutter pairs can be added by adding more inputs to the OR gate  501 .  
         [0032]     The above described implementation of the tamper-proof circuit of the present invention (circuit, sliding shutters and shutter switches) can be applied to any two or three hole receptacle design and is not limited to implementation in a GFCI receptacle.  
         [0033]     While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiment, as it presently contemplated for carrying them out, it will be understood that various omissions and substitutions and changes of the form and details of the device illustrated and in its operation may be made by those skilled in the art, without departing from the spirit of the invention.