Abstract:
A protective device includes a test button for enabling a test signal for testing an operating condition of at least one of the device components, such as the sensor, detector, switch, solenoid and trip mechanism. The test button also enables a current through a resistor body which is affixed to a stationary part of the device. The resistor body keeps a lockout spring under tension. Failure of the test signal to operate the trip mechanism within a predetermined time interval causes the resistor body conducting said current to reach a predetermined temperature, wherein the resistor body ceases to hold a lockout spring, thereby permitting the lockout spring to move to a position which causes the set of interrupting contacts to remain permanently in a disconnected position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 60/326,531 filed Oct. 02, 2001 and entitled SECONDARY TRIPPING MECHANISM, and from co-pending U.S. application Ser. No. 09/827,007 filed Apr. 05, 2001 and entitled LOCKOUT MECHANISM FOR USE WITH GROUND AND ARC FAULT CIRCUIT INTERRUPTERS, both of which are incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to the field of devices for protecting electrical circuits in the event of faults, and more particularly to a device that protects from arc faults and ground faults, which is provided with a manual test feature that permanently denies power to the protected circuit should the test fail.  
         BACKGROUND OF THE INVENTION  
         [0003]    The electrical distribution system is defined to include the circuit breaker, branch circuit conductors, wiring devices, cord sets or extension cords, and electrical conductors within an appliance. A protective device is incorporated in an electrical distribution system for protecting a portion of the system from electrical faults. Ground fault circuit interrupters, also called GFCIs, are one type of protective device that has become quite widely used. They provide a very useful function of disconnecting an electrical power source from the protected portion of the system when a ground fault is detected. Among the more common types of ground faults sensed by known GFCIs are those caused when a person accidentally makes contact with a hot electrical lead and ground. In the absence of a GFCI, life threatening amounts of current could flow through the body of the person.  
           [0004]    Arc fault circuit interrupters, also called AFCIs, are another type of protective device but that has been in use more recently. AFCIs disconnect an electrical power source from a load when an arc fault is detected. Among the more common type of arc faults sensed by known AFCIs are those caused by damaged insulation such as from an overdriven staple. This type of arc fault occurs across two conductors in the electrical distribution system such as between the line and neutral conductors or line and ground conductors. The current through this type of fault is not limited by the impedance of the appliance, otherwise known as a load coupled to the electrical distribution system, but rather by the available current from the source voltage established by the impedance of the conductors and terminals between the source of line voltage and the position of the fault, thus effectively across the line, and has been known as a “parallel arc fault.” Another type of arc fault sensed by known AFCIs are those caused by a break in the line or neutral conductors of the electrical distribution system, or at a loose terminal at a wiring device within the system. The current through this type of fault is limited by the impedance of the load. Since the fault is in series with the load, this type of fault has also been known as a “series arc fault.” In the absence of an AFCI, the sputtering currents associated with an arc fault, whether of the parallel, series or some other type, could heat nearby combustibles and result in fire.  
           [0005]    Protective devices are typically provided with line terminals for coupling to the supply voltage of the electrical distribution system, and load terminals coupled to the protected portion of the system and a circuit interrupter for disconnection of the load terminals from the line terminals. The protective device is provided with a sensor for sensing the fault, a detector for establishing if the sensed signal represents a true hazardous fault, as opposed to electrical noise, and a switch responsive to the detector sensor, wherein the circuit interrupter comprising the contacts of a relay or trip mechanism are operated by a solenoid responsive to the switch to disconnect the load terminals from the line terminals. The disconnection is also known as tripping. A power supply may be required to furnish power to the sensor, detector, switch or solenoid.  
           [0006]    Protective devices are commonly equipped with a test button which the owner of the protective device is instructed to operate periodically to determine the operating condition of the sensor, the detector, the switch, trip mechanism or relay, or power supply, any of which can fail and which may cause the circuit interrupter to not operate to remove power from the load side of the protective device to interrupt the fault. Since the protective device comprises electronic and mechanical components, failure modes comprise normal aging of electronic components, corrosion of mechanical parts, poor connections, mechanical wear, mechanical or overload abuse of the protective device in the field, electrical disturbances such as from lightning, or the like. Once the test has been manually initiated by operating the test button, the outcome of the test has often been indicated mechanically such as by a popping out of a button, visually through a lamp display or pivoting flag that comes into view, or audibly through an annunciator. As an alternative to a manual test, a self-test feature can be added to the protective device for automatic testing such as is described in U.S. Pat. No. 6,421,214 and U.S. application Ser. No. 09/827,007 filed Apr. 05, 2001 and entitled LOCKOUT MECHANISM FOR USE WITH GROUND AND ARC FAULT CIRCUIT INTERRUPTERS, both of which are incorporated herein by reference. Once the test has been automatically initiated through the self-test feature, the outcome of the test can be indicated by any of the previously described methods or by the permanent disconnection of the load terminals from the line terminals of the protective device, also known as “lock-out.” 
           [0007]    Protective devices have been located in an electrical distribution system in a variety of conventional device housings such as but not limited to circuit breakers typically installed inside a panel at the service entrance having an interrupting contact that disconnects the load in response to sustained overcurrent, receptacle outlets or snap switches typically installed inside a wall box, portable housings typically installed in plugs or connectors or as protective modules within appliances. Constructional requirements for the different device housings differ. Some differences arise from the pertinent UL (Underwriters Laboratories) safety standards, for example, UL standard 943 for GFCIs and UL standard 1699 for AFCIs. Unlike circuit breaker and receptacle devices, portable devices are susceptible to a poor connection between the receptacle and neutral plug blade. Therefore, only portable devices must continue to afford provide protection or interrupt load side power due to neutral supply conductor failure. This requirement for the portable protective device has often been accomplished using a relay with normally open contacts serving as the circuit interrupter, such as disclosed in U.S. Pat. No. 4,574,324, whereas receptacle devices commonly use a circuit breaker and mouse-trap mechanism such as is disclosed in U.S. Pat. No. 4,939,615. Other differences arise from the nature of the housing itself, wherein protective devices that are housed in a circuit breaker and that require a power supply most conveniently derive power for the supply power from the load side of the circuit interrupter.  
           [0008]    The prior art discloses methods for denying power to the load when there is protective failure. U. S. Pat. Nos. 6,040,967 and 6,282,070 deny power to the load side of the device when there is a loss of protective function. The device&#39;s test button is manually operated which causes the interrupting contacts to open via a mechanical linkage. Next, the reset button is manually operated which initiates a test signal. Failure to detect the test signal prevents the interrupting contacts of the device from being connected.  
           [0009]    U.S. Pat. No. 6,262,871 is another example of a self-testing device but that opens a redundant set of mechanical contacts permanently upon detection of failure.  
           [0010]    U.S. Pat. No. 6,324,043 denies power through use of a fusible link that opens when there is a loss of protection.  
           [0011]    International Patent No. 01/97243 discloses the use of a redundant solenoid that operates in the event of device failure.  
           [0012]    Prior art protective devices that afford self-test comprise complicated circuitry that is both expensive and subject to failure. Prior art protective devices that have required manual manipulation of test and reset buttons comprise complicated mechanical linkages. This type of manual lock-out device also requires the power supply for powering the protective circuitry to derive power from the line terminals of the protective device which is not convenient for the protective device housed in a circuit breaker enclosure that derive power typically from the load side terminals of the protective device. Manual lock-out devices have not been suitable for protective devices housed in a portable enclosure. Portable protective devices typically use a relay with normally open contacts comprising the interrupting contacts, a relay not being compatible with prior art manual lock-out devices.  
         SUMMARY OF THE INVENTION  
         [0013]    Briefly stated, a protective device includes a test button for enabling a test signal for testing an operating condition of at least one of the device components, such as the sensor, detector, switch, solenoid and trip mechanism. The test button also enables a current through a resistor body which is affixed to a stationary part of the device. The resistor body keeps a lockout spring under tension. Failure of the test signal to operate the trip mechanism within a predetermined time interval causes the resistor body conducting the current to reach a predetermined temperature, wherein the resistor body ceases to hold a lockout spring, thereby permitting the lockout spring to move to a position which causes the set of interrupting contacts to remain permanently in a disconnected position.  
           [0014]    The present invention denies power to the protected side of the device when there is a loss of protective function. Manual operation of the device&#39;s test button enables an electrical test signal for testing the device. At the same time, a current is initiated through a resistor body in an embodiment, or fusible component in an alternate embodiment. If the test signal does not cause the interrupting contacts to disconnect within the expected time interval, the ongoing current through the resistor body causes solder connections to melt and the resistor body to physically dislodge to a second position under bias from a spring, the motion of resistor and spring resulting in the interrupting contacts of the protective device remaining permanently in the disconnected position. In an alternate embodiment, a fusible resistor burns open and ceases to conduct electrical current, resulting in the interrupting contacts of the protective device remaining permanently in the disconnected position.  
           [0015]    According to an embodiment of the invention, a protective device for interrupting power upon detection of an electrical fault in an electrical distribution system includes a plurality of line terminals connectable to a source of voltage; a plurality of load terminals connectable to a load; interrupting means that connects or disconnects the plurality of line terminals from the plurality of load terminals; a sensor for sensing the electrical fault; a detector coupled to the sensor for detecting the electrical fault; a switch coupled to the detector responsive to the detected electrical fault; a solenoid coupled to the switch; a trip mechanism coupled to the solenoid which moves the interrupting means to a disconnected position upon occurrence and detection of the electrical fault; a lockout spring; a resistor body which holds the lockout spring in a first position under tension; a test button for enabling a test signal for testing an operating condition of at least one of the sensor, detector, switch, solenoid and trip mechanism, wherein the test button also enables a first current through the resistor body; and wherein failure of the test signal to operate the trip mechanism within a predetermined time interval causes the resistor body conducting the first current to reach a predetermined temperature, wherein the resistor body ceases to hold the lockout spring in the first position, thereby permitting the lockout spring to move to a second position, wherein the lockout spring being in the second position causes the interrupting means to remain permanently in the disconnected position.  
           [0016]    According to an embodiment of the invention, a protective device for interrupting power upon detection of an electrical fault in an electrical distribution system includes a plurality of line terminals connectable to a source of voltage; a plurality of load terminals connectable to a load; a sensor for sensing the electrical fault; a detector coupled to the sensor for detecting the electrical fault; a switch coupled to the detector responsive to the detected fault; a relay, coupled to the switch, including a solenoid and a plurality of interrupting contacts wherein the relay disconnects the line terminals from the load terminals upon occurrence of the electrical fault, thereby moving the interrupting contacts to a disconnected position; a fusing element; a test button for enabling a test signal for testing an operating condition of at least one of the sensor, detector, switch, solenoid and trip mechanism and for providing a first current through the fusing element; wherein failure of the test signal to disconnect the interrupting contacts within a predetermined time interval causes the fusing element conducting the first current to reach a predetermined temperature, wherein the fusing element causes an open circuit, and wherein the interrupting contacts remain permanently in the disconnected position.  
           [0017]    According to an embodiment of the invention, a protective device for interrupting power upon detection of an electrical fault in an electrical distribution system includes a plurality of line terminals connectable to a source of voltage; a plurality of load terminals connectable to a load; a set of bus bars that connect or disconnect the plurality of line terminals from the plurality of load terminals, the set of bus bars including a hot bus bar and a neutral bus bar, wherein the hot bus bar connects a hot line terminal to a hot load terminal, and the neutral bus bar connects a neutral line terminal to a neutral load terminal; a resistor body one-way moveable between a first and second position, wherein when the resistor body is in the second position, the set of bus bars is permanently disconnected from the plurality of line terminals and the plurality of load terminals; a test button for enabling a test signal for testing an operating condition of the device to determine if the device is in good operating condition or bad operating condition, and for providing a current through the resistor body; the resistor body being connected on one end to the test button and on another end to the neutral bus bar; and wherein when the source of voltage is miswired to the load terminals and the test button is pressed, the resistor body does not move to the second position when the protective device is in the good operating condition.  
           [0018]    According to an embodiment of the invention, a method for locking out a reset mechanism of an electrical protective device includes the steps of (a) providing a spring driven lockout for the reset mechanism; (b) providing a resistor body which holds the lockout in a first position, wherein the first position permits resetting the electrical protective device; (c) pressing a test button to check an operating condition of the electrical protective device; (d) sending a current through the resistor body in response to pressing the test button; and (e) moving the lockout to a second position in response to the resistor body reaching a predetermined temperature as a result of the current being sent through the resistor body for a longer period of time than a normal trip time of the electrical protective device, wherein the second position permanently prevents resetting of the electrical protective device.  
           [0019]    According to an embodiment of the invention, a method for locking out a reset mechanism of an electrical protective device includes the steps of (a) providing a tripping mechanism which includes a normally open relay; (b) providing a fusing element which permits power to said relay so that said relay remains closed; (c) pressing a test button to check an operating condition of said electrical protective device; (d) sending a current through said fusing element in response to pressing said test button; and (e) blowing said fusing element in response to said fusing element reaching a predetermined temperature as a result of said current being sent through said fusing element for a longer period of time than a normal trip time of said electrical protective device, wherein said blowing of said fusing element creates an open circuit to said relay, thereby permanently preventing resetting of said electrical protective device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 shows a circuit diagram for a ground fault circuit interrupter (GFCI) according to the prior art;  
         [0021]    [0021]FIG. 2 shows a partial sectional view of a prior art mechanical implementation of the schematic of FIG. 1;  
         [0022]    [0022]FIG. 3 shows the mechanical implementation of FIG. 2 in the tripped state;  
         [0023]    [0023]FIG. 4 shows a partial sectional view of a mechanical implementation of an embodiment of the invention;  
         [0024]    [0024]FIG. 5 shows a partial sectional view of the mechanical implementation of FIG. 4 is shown in the lock-out position;  
         [0025]    [0025]FIG. 6 shows a three-dimensional view of some of the components of the embodiment of FIG. 4;  
         [0026]    [0026]FIG. 7 shows a protective device according to an embodiment of the invention;  
         [0027]    [0027]FIG. 8 shows a protective device according to an embodiment of the invention;  
         [0028]    [0028]FIG. 9 shows a protective device according to an embodiment of the invention;  
         [0029]    [0029]FIG. 10 shows a protective device according to an embodiment of the invention;  
         [0030]    [0030]FIG. 11 shows a protective device according to an embodiment of the invention;  
         [0031]    [0031]FIG. 12 shows a protective device according to an embodiment of the invention; and  
         [0032]    [0032]FIG. 13 shows a protective device according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]    Referring to FIG. 1, a prior art GFCI  2  includes line terminals  3  and  5  for coupling to a power source of the electrical distribution system and load terminals  37  and  39  appropriate to the installed location, whether a circuit breaker, receptacle, plug, module, or the like. A ground fault represented by resistor  41  produces an additional current in conductor  4  that is not present in conductor  6 . Sensor  12  senses the difference current between conductors  4  and  6  which is then detected by a ground fault detector  14 . Detector  14  issues a trip command to an SCR  22  which in turn activates a solenoid  24 , which activates a trip mechanism  26  releasing contact armatures  34  and  32 , thereby disconnecting power to the load by breaking the circuit from a line hot  4  to a load hot  36  and from a line neutral  6  to a load neutral  38 . A contact  10  along with a resistor  8  form a test circuit which introduces a simulated ground fault. When contact  10  is depressed, the additional current on conductor  4  is sensed by sensor  12  as a difference current causing the device to trip. Current flows through resistor  8  for the interval between depression of the contact  10  and the release of contact armatures  34  and  32 , which is nominally 25 milliseconds. The device is reset by pressing a reset button  40  which mechanically resets trip mechanism  26 . A resistor  20 , a Zener  18 , and a capacitor  19  form a power supply for GFCI  2 .  
         [0034]    Referring to FIG. 2, the mechanical layout for the circuit diagram of FIG. 1 is shown in which like elements are like numbered. Trip mechanism  26  is shown in the set state, meaning that contacts  37  and  35  are closed. Contacts  35  and  37  are held closed by action of a trapped make-force spring  46  acting on an escapement  55  on a rest stem  54  to lift a reset latch spring  52  and by interference, an armature  32 . Reset latch spring  52  includes a hole  53  and armature  32  includes a hole  33 , which holes  33  and  53  permit entry of a tip  58  of reset stem  54 . Reset stem  54  is held in place by a block  60 . Armature  32  and a printed circuit board (PCB)  56  are mechanically referenced to a housing  48  so that the force in spring  46  is concentrated into armature  32 .  
         [0035]    Referring to FIG. 3, the mechanism of FIG. 2 is shown in the tripped state. The tripped state occurs when SCR  22  activates a magnetic field in solenoid  24 , which in turn pulls in plunger  23  to displace reset latch spring  52 . Displacing reset latch spring  52  allows a flat portion  55  to clear the latch spring  53  interference, which then releases the interference between latch spring  52  and armature  32 . Armature  32  has a memory which returns armature  32  to a resting position against solenoid  24 , opening contacts  35  and  37  and disconnecting power to the load. The principles shown in FIGS.  1 - 3  are adaptable to any number of mechanical configurations including U.S. Pat. No. 5,510,760 which is incorporated herein by reference.  
         [0036]    Referring to FIG. 4, a partial sectional view of a mechanical implementation of an embodiment of the invention is shown. A resistor  8 ′, shown schematically in FIG. 1 as resistor  8 , is designed to withstand self-heating that results from each depression of contact  10 , which causes current to flow through resistor  8 ′ for the expected trip time of the GFCI. For example, resistor  8 ′ for a 6 mA GFCI coupled to a 120 VAC supply is required by UL to be 15 KOhms, which dissipates nominally 0.96 Watts during each trip time interval. In particular, resistor  8 ′ must survive several thousand trip time intervals accomplished by depressing contact  10  and reset button  40  alternately. During normal operation of GFCI  2 , resistor  8 ′ is physically positioned to restrain lockout spring  400 . Resistor  8 ′ is preferably mounted and soldered so that the body of resistor  8 ′ impedes movement of lockout spring  400 .  
         [0037]    Referring to FIG. 5, a partial sectional view of the mechanical implementation of FIG. 4 is shown in the lock-out position. The GFCI  2  has failed in some manner such that the trip time in response to depressing contact  10  is greater than the expected interval including failure of GFCI  2  to trip altogether. Examples of failure modes include a defective sensor  12 , and for a sensor  12  comprising a transformer, open or shorted turns. The detector  14 , typically composed of electronic components, may have poor solder connections or components that have reached end of life. The SCR  22  may short circuit either due to reaching end of life or due to a voltage surge from a lightning storm, thereby causing continuous current through solenoid  24  which burns open through over activation, or, alternatively, SCR  22  may open circuit. The mechanical components associated with trip mechanism  26  may become immobilized from wear or corrosion. The power supply, if provided, may fail to deliver power in accordance with the design such that sensor  12 , detector  14 , SCR  22 , or solenoid  24  are non-operative.  
         [0038]    When failure of GFCI  2  occurs, the current through resistor  8 ′ flows for the time that contact  10  is manually depressed, on the order of at least seconds, which is two orders of magnitude longer than if the trip mechanism  26  were to operate in response to depressing contact  10 . Resistor  8 ′, which is preferably coupled electrically to GFCI  2  through solder, heats from the current and melts the solder. Resistor  8 ′, no longer restrained by the solder, or in an alternative embodiment by an adhesive, is physically dislodged by the bias of lockout spring  400 . Force is then applied by an end  404  of lock-out spring  400  against a feature on the reset latch spring  52 , for example, a tab  402 . The force in lockout spring  400  is greater than the force in reset latch spring  52 . As previously described, reset latch spring  52  is displaced allowing a flat portion  55  to clear the latch spring  53  interference, which then releases the interference between reset latch spring  52  and armature  32 . Armature  32  has a memory which returns armature  32  to a resting position against solenoid  24 , opening contacts  35  and  37  and disconnecting power to the load. Thus when the GFCI  2  is operational, the tripping mechanism  26  is able to operate, and the armatures  32  and  34  disconnect when plunger  23  applies force to reset latch spring  52 . If GFCI  2  is not operative, lockout spring  400  applies force to reset latch spring  52 , likewise causing armatures  32  and  34  to disconnect. When GFCI  2  is tripped under the influence of lockout spring  400 , armatures  32  and  34  are permanently disconnected irrespective of depressing contact  10  or reset button  40  or any further movement in plunger  23 . Thus resistor  8 ′ is integral to the lock-out feature of the present invention.  
         [0039]    Referring to FIG. 6, components of the embodiment of FIG. 4 are shown in a three-dimensional view including lockout spring  400 , end  404 , resistor  8 ′, and latch spring  52 . Spring  404  is preferably affixed to the same structure as resistor  8 ′.  
         [0040]    Referring to FIG. 7, a protective device  710  shows a resistor  700  which is then used as the resistor body which constrains spring  400 . There are other ground fault circuit interrupters whose trip thresholds are greater than 6 milliamperes intended for a variety of supply voltages or phase configurations, and intended for personnel protection or fire prevention. Alternate trip levels typically include 30 milliamperes in the U.S. or Europe, or 300 or 500 milliamperes in Europe, to which the invention as described applies. For devices where the current through resistor  8  may produce insufficient heat during the anticipated duration that contact  10  is manually depressed to melt the solder, resistor  8  can be supplemented by a resistor  700  in parallel with resistor  8  which connects to line  6  on the other side of sensor  12  from where resistor  8  connects to line  6 . Currents through resistors  8  and  700  are enabled by depressing contact  10 . Resistor  8  generates a simulated test signal comprising a difference current to test GFCI  2  as previously described. Resistor  700  is coupled so as to conduct common mode current but no difference current. Since the current through resistor  700  does not influence the amount of simulated test current required by UL, which is set by the value of resistor  8 , the value of resistor  700  can be whatever value is convenient for producing sufficient heat in resistor  700  when contact  10  is manually depressed to release lockout spring  400  when GFCI  2  is not operational. FIG. 7 also shows how the lockout function is unaffected by whether the power supply for the GFCI comprising resistor  20 , Zener  18 , and capacitor  19  are coupled to the load side of armatures  32  and  34 . Load side power derivation may be convenient for GFCIs or protective devices housed in a circuit breaker. FIG. 7 also shows how SCR  22  can be replaced by a transistor  22 ′, with either device comprising a switch for controlling solenoid  24 .  
         [0041]    Referring to FIG. 8, a protective device  810  which is an alternate embodiment to FIG. 7 shows a resistor  800  which serves the same function as resistor  700  in FIG. 7 but is coupled to the load side of the interrupting contacts, i.e., contact armatures  32 ,  34 . This may be important for 6 milliampere GFCI receptacles and portables where the hot and neutral supply conductors are inadvertently transposed by the installer, wherein the hot side of the supply voltage from the electrical distribution system is connected to line terminal  5 . If the armatures  32  and  34  in FIG. 7 are disconnected in response to a fault current, a hazardous current may yet flow through resistors  8  and  700  through ground fault  702  when contact  10  is depressed. However, if armatures  32  and  34  in FIG. 8 are disconnected, current flows through resistor  8  but not through resistor  800 , which is not a problem because the current flow through resistor  8  alone has already been determined to be non-hazardous.  
         [0042]    Referring to FIG. 9, a protective device  910  which is an alternative embodiment to FIG. 8 is shown in which the trip mechanism comprises one or more bus bars as disclosed in U.S. Pat. No. 5,510,760, incorporated herein by reference, instead of contact armatures. Resistor  900  serves the same function as resistor  800  in FIG. 8 except that resistor  900  is coupled to moveable bus bar  902 ′. For receptacle housings it is possible for the installer to miswire a GFCI such that the supply voltage is connected to load terminals  37  and  39 , which would cause resistor  800  (FIG. 8) to melt solder when contact  10  is depressed, even when device  810  is in good working condition, i.e., operational. The problem is alleviated in the embodiment of FIG. 9 whereby resistor  900  melts solder only when bus bar  902 ′ remains connected when contact  10  is depressed, that is, when device  910  is non-operational. Miswiring thus does not cause a permanent lock-out of device  910 .  
         [0043]    Referring to FIG. 10, a protective device  1010  which is an alternate embodiment to FIG. 7 is shown, wherein contact  10  enables a current through resistor  8 , as previously described, and a second current through a resistor  1000  in which the second current is preferably less than a tenth of the current through resistor  8 . The second current depends on an interface circuit such as a transistor switch  1002 . Transistor switch  1002  causes current to flow through a resistor  1004  of identical function to resistor  700  described in FIG. 7, i.e., resistor  1004  is normally in such a position as to leave spring  400  (FIG. 6) under tension, but when resistor  1004  heats up from the current through it sufficient to dislodge the solder affixing resistor  1004  to a fixed reference surface, the dislodgement of resistor  1004  releases spring  400 .  
         [0044]    [0044]FIG. 10 shows an alternative to FIG. 8 wherein a hazardous current does not occur when the hot and neutral supply conductors are inadvertently transposed as described in FIG. 8. In addition, FIG. 10 shows another remedy for the issue described in the FIG. 9 embodiment wherein resistor  1004  melts solder only if protective device  1010  is non-operational and not when protective device  1010  is miswired.  
         [0045]    Referring to FIG. 11, a protective device such as GFCI  1110  according to an alternate embodiment is shown, wherein the so called mouse trap mechanism, i.e., the tripping mechanism of the GFCI of FIGS.  1 - 5 , is replaced by a relay  1100  having normally open contacts  1102  that connect or disconnect line terminals  3  and  5  from load terminals  37  and  39  respectively, and a solenoid  1104 , which is designed to carry current when contacts  1102  of GFCI  1110  are connected, a construction that is common to, but not limited to, portable GFCI devices. Solenoid  1104  is designed to conduct current for the unlimited duration that GFCI  1110  is in use, wherein solenoid  1104  is not susceptible to burn out caused by over-activation as previously described with respect to solenoid  24 . A fusible element  1106  is in series with the solenoid and is designed to carry the continuous current through solenoid  1104  when transistor  22 ′ is closed. Contact  10  enables current through resistor  8  which produces a difference current as previously described, and a common mode current, which, if the device is non-operational, enables a lock-out feature. The common mode current, which is greater than the solenoid current, is conducted through fusible element  1106 .  
         [0046]    If GFCI  1110  is operational, the load side is disconnected from the line side, causing the device to trip and resistor  8  and common mode currents to stop flowing even if contact  10  continues to be manually depressed. Fusible resistor  1106  must survive several thousand cycles of common mode current exposures from alternately depressing contact  10  to trip GFCI  1110  and switch  1108  to electronically reset GFCI  1110 . The duration of each common mode current exposure is the expected time that GFCI  1110  requires for tripping after contact  10  has been depressed. If GFCI  1110  fails in some manner such that the trip time in response to depressing contact  10  is greater than the expected interval including the failure of GFCI  1110  to trip altogether, fusible element  1106  burns to an open circuit, permanently eliminating current through solenoid  1104  and rendering interrupting contacts  1102  in a permanently disconnected position. Fusible element  1106  can include a resistor.  
         [0047]    Referring to FIG. 12, elements of the circuit diagram of FIG. 11 are combined with elements of the circuit diagram of FIG. 8 in a protective device  1210 , wherein components having like functions bear like numbers. The concept shown in FIG. 11 is thus combined with the embodiment of FIG. 8 to protect against the inadvertent transposing of the hot and neutral supply conductors to terminals  3  and  5  from the electrical distribution system. For protective devices not equipped with a resistor  8 , the value of resistor  1000  can be chosen so that current passing therethrough is less than 0.5 mA, which limit has been identified to be the perception level for humans.  
         [0048]    Referring to FIG. 13, an alternate embodiment is shown in which the preceding concepts are applied to a general protective device  1310  representative of the class of general protective devices including AFCIs that require a contact  10  but that are not necessarily equipped with a GFCI or a sensor capable of sensing difference current. Such devices are disclosed in U.S. Pat. No. 6,421,214 which is incorporated herein by reference. Components having like functions bear like numbers. Sensor  1300  is similar to sensor  12  but may be a current sensor or shunt for sensing load current through either conductor  6  or through conductor  4 . A detector  1302  is similar to detector  14  (FIG. 1) but senses particular signatures in the load current as has been demonstrated in other patent applications as a method of identifying arc faults. A contact  1304  is similar to contact  10  (FIG. 1), which initiates a test of protective device  1310  when depressed. The test signal can be controlled by detector  1302  to test sensor  1300 , detector  1302 , switch  22 , and trip mechanism  26 . A resistor  1306  is similar to resistor  700  (FIG. 7) which is affixed to a fixed reference surface. If armatures  32  and  34  fail to operate due to a malfunction of protective device  1310 , the longer duration of current through resistor  1306  causes sufficient self-heating of resistor  1306  to melt the solder affixing resistor  1306  to the fixed reference surface, wherein resistor  1306  is dislodged due to force exerted by lockout spring  400  (FIG. 4), wherein lockout spring  400  causes armatures  32  and  34  to be permanently disconnected.  
         [0049]    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.