Abstract:
A protective arrangement in a device that protects the device and personnel or users of the device when undesirable conditions develop within the device. The protective arrangement de-couples a power source from the device, prevents access to the device that may harm a user or aggravate the undesirable condition, and actuate an alarm when the undesirable condition occurs.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/476,562, filed Jan. 3, 2000, now U.S. Pat. No. 6,525,914, issued Feb. 25, 2003, entitled “Protection System for Devices Connected to an Alternating Current Electrical Power Supply,” by Raymond H. Legatti, and is hereby incorporated by reference herein or all purposes. 
    
    
     BACKGROUND OF THE INVENTION TECHNOLOGY 
     FIELD OF THE INVENTION 
     The present invention relates to a safety system and method in a device for the protection of people and property, and more specifically when a potentially harmful event and/or undesirable conditions occur in an electrical device, e.g., an electrical appliance, the system and method of the present invention will de-energize power, lock-out access and/or alarm on the harmful event. Such undesirable conditions may include among others, undesirable leakage currents from the power source causing electrical shock potential on metal parts of the device, fire within or on the device, too high of a temperature, a pressure deviating too far from normal, the presence of smoke or particulate matter or, the presence or absence of light or of a light opaque member within the device. 
     The safety system and method of the present invention provides additional protection, not provided by prior systems, for electrical appliances and tools. In addition to providing protection from electrical leakage currents, and protecting a user from the hazard of electrical shock, it includes additional sensors for detecting and providing protection from undesirable events such as for example but not limited to flames, smoke, excessively high temperatures, and/or abnormal pressures in the device. Upon detection of the undesirable event, an alarm may be actuated and/or access to an affected area of the device blocked or locked-out so that harm to a device user or exasperation of the undesirable condition does not occur. 
     More particularly it relates to an enhancement of appliance leakage current interrupters (ALCIs) and equipment leakage current interrupters (ELCIs), to provide additional protection to a leakage current protected electrical device. More generally, it relates to an enhancement of any Leakage Current Protection Device as set forth in Underwriters Laboratories, Inc. Reference Standard UL943A, which includes not only ALCIs and ELCIs, but also immersion detection circuit interrupters (IDCIs). Whenever the term GFCI or ground fault circuit interrupter is used herein, it is intended in the broader sense of any leakage current protection device, where applicable. 
     More particularly it relates to protection of personal proximate to and users of the device by preventing access to the area of the device where the undesirable or harmful condition is occurring. It further prevents worsening the undesirable or harmful condition, e.g., allowing more oxygen to feed the fire, spread of the fire to adjacent flammable materials, release of excess pressure that could be harmful to people close to the device, release of smoke and/or toxic fumes to the surrounding area. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with this invention, the protection provided against leakage currents and arcing (generally referred to as arcing currents herein), either separate from or in connection with a GFCI, to an electrical device power delivery cord, is extended to the electrical device which is connected to a power supply through the power delivery cord. The extended protection encompasses not only leakage currents and arcing, but also other undesirable conditions which may occur in the electrical device. Typical of the electrical devices which may be protected in accordance with this invention are electric heaters, air purifiers, room air conditioners, water distillers, and such kitchen appliances as toasters, toaster ovens, bread makers, and coffee makers. The extended protection upon detection of unsafe or undesirable conditions indicated by abnormal light or source of light, temperature or pressure conditions, or the presence of flame or smoke will result in disconnecting the electrical device form its power source. 
     Further in accordance with this invention, protection is provided against potentially dangerous access to the undesirable or harmful event by latching or locking an area, volume or compartment of the device from being inadvertently opened during the occurrence of the undesirable or harmful event. 
     Further in accordance with this invention, an alarm may be activated so as to alert personnel to the occurrence of the undesirable or harmful event. 
     One embodiment of the protection system and method of this invention is for use with two wire electrical devices such as an appliance or tool, which receives electrical power through a power cord, which includes an unshielded power line, and an unshielded neutral line. As described in co-pending application Ser. No. 08/682,957, entitled: “Electrical System With Arc Protection”, which application is hereby incorporated by reference herein for all purposes, a sensing lead, substantially electrically isolated from the power and neutral lines and the electrical load of the appliance, is provided in the power cord. A sensing lead integrity checking device, such as a neon glow tube or a switch, may be connected between the sensing lead and the power line. The integrity checking device connects the power line to the neutral line through the sensing lead. The impedance of the integrity checking device is increased by substantial additional resistance, so that the current flow is very small, well below the predetermined magnitude that would initiate response of the arcing current protection arrangement. Thus, although the integrity checking device does provide a connection between the sensing lead and the power line, the power line and the sensing lead are only connected for a brief integrity checking period through a switch, or the total impedance between the power line and the sensing lead is so high that, for practical purposes of the arcing current protection arrangement, the sensing lead is isolated from the power line and the appliance or tool load. 
     A rectifier, such as a diode rectifier circuit, may be connected between the power line and the neutral line to provide DC power to a circuit opening device, such as a solenoid which actuates normally closed contacts in the power line. In most situations, it is also desirable to open the neutral line. In such situations, a second pair of normally closed contacts in the neutral line are actuated by the solenoid. 
     Energization of the solenoid coil may be controlled by a suitable switching device, such as a silicon controlled rectifier (SCR) having the anode-cathode circuit connected in series with the solenoid coil. The gate of the SCR coil may be connected to the cathode through a suitable resistance that will bias the SCR to a conducting state when current flow of a pre-determined magnitude passes through the resistance, thus providing a current responsive arrangement. The sensing lead is also be connected to the gate of the silicon controlled rectifier, usually through a limiting resistor. Of course, with a different type of circuit opening device, the sensing lead would be connected to the control circuitry therefor. 
     When an arcing current above a predetermined magnitude is produced on the sensing lead, such as by detecting an arc resulting from a breakdown of the insulation separating the power line and the neutral line, the SCR will be gated to a conducting state and the solenoid will be energized to open the power line and, usually, the neutral line as well. Typically the leakage current detection level is set between a range of 250 microamperes and 5 milliamperes. 
     In accordance with this invention the sensing lead is electrically connected to the metal housing or frame of the electrical device. If there is a dielectric or insulation failure between the metal housing or frame and the power or neutral line, and a leakage current, tracking or arcing occurs with respect to the metal housing or frame of the electrical device, it will flow through the sensing lead to cause the SCR or other suitable switching device to be gated to a conducting state. Thus, the solenoid will be actuated by energization or de-energization, depending on the circuit configuration, to open the power line and usually, the neutral line as well. This embodiment of the arcing current protection arrangement may be used independently of a GFCI, although a GFCI would normally be included somewhere within the electrical system to protect against ground fault currents. 
     In a variation of the first exemplary embodiment of this invention, a supplemental coil is placed on the core of the differential transformer of a GFCI. The sensing lead is connected to this supplemental winding, which is in turn connected to the neutral line through a suitable resistance. If an arcing current above a pre-determined magnitude should occur in the sensing lead, the current flow through the supplemental winding creates an imbalance in the differential transformer, which in turn produces a trip signal in the secondary winding of the differential transformer. The trip signal in the secondary winding initiates actuation of a circuit opening device to break the power line and protect against a hazardous condition that might be created by the arcing current. 
     The number of turns in the supplemental winding may be varied to adjust sensitivity to arcing currents. Thus, the magnitude of the arcing current at which the circuit opening device will be actuated may be adjusted in accordance with the desired protection. Typically the current detection level is set in the range of 250 micro amperes to 5 milliamperes. The resistance in series with the supplemental winding may also be adjusted to establish the predetermined magnitude of arcing current at which tripping is desired. Again, the metal housing or frame of the electrical device is connected to the sensing lead. As set forth with respect to the previous embodiment, if a current above a predetermined magnitude occurs in the sensing lead, due to a current flow between the metallic housing and the sensing lead, the trip signal in the secondary winding initiates actuation of a circuit opening device to break the power line and protect against the hazardous conditions that might be created by the arcing current. 
     Further protection is provided to an electrical device in accordance with this invention by de-energizing the device when an undesirable condition occurs in the device. The further protection is provided by causing a circuit to be completed to the sensing lead from the power line or from the neutral line in response to the occurrence of an undesirable condition in the electrical device. Various parameters, indicative of an undesirable or unsafe condition in the electrical device, may be detected, and if detected cause a circuit to be completed between the sensing lead and the power line or neutral line. Amongst the variety of parameters which may be sensed are light, temperature, flame, pressure and smoke. In accordance with this invention, simple low cost sensors may be used as detectors. Various arrangements may be used to complete the circuit between the sensing lead and the power line or neutral line. In one embodiment of this invention a normally closed switch connected in a circuit connecting the sensing lead to the power line or neutral line is held in an open position by a mechanical restraint. Upon the occurrence of an undesirable condition, the mechanical restraint is released. Closing of the normally closed switch contacts causes a current to flow in the sensing lead, which results in disconnecting the electrical device from the power source. Amongst the other low cost sensors which may be used in accordance with this invention are cadmium sulfide light detection cells, a combination of a light emitting diode and light sensitive semiconductor switch, and a pressure sensitive switch. 
     As an example of the exemplary embodiment of the invention, a switch normally biased to a closed position is held in an open position by a force applied through a tape formed of a heat sensitive material. When an undesirably high temperature is reached in the electrical device, the tape will stretch or disintegrate, thereby allowing the mechanical bias on the normally closed switch to move the switch contacts to the closed position. A resulting current flow through the normally closed switch contacts and the sensing lead will cause the electrical device to be disconnected from the power source. 
     A second embodiment of the safety and protection system of this invention is for use with electrical devices such appliances or tools having three lines, that is power, neutral and ground. It may also be use with three wire or line devices having a metal sheath or cover on the power cord. A ground fault current interrupter circuit with arcing protection for such three wire or line devices is set forth in U.S. Pat. No. 4,931,894—Legatti, which patent is assigned to one assignee of this application. U.S. Pat. No. 4,931,894 is hereby incorporated by reference. An appliance or tool having three lines would normally have its metal housing or frame connected to the ground line such that arcing to the metal housing or frame would be detected by the ground fault current interrupter circuit. Thus, a sensing lead need not be connected to the metal housing or frame to detect arcing thereto, as in the first embodiment. However, in accordance with e second embodiment of this invention, a sensing lead is provided for the purpose of providing a signal, representing an undesirable condition in the electrical device, to the leakage current protective device connected to the electrical device to open the power circuit to the electrical device. As in the first embodiment, the sensing lead may be connected to actuate a switching device such as a silicon controlled rectifier, or it may be connected to a supplemental coil on the core of a differential transformer of a GFCI. 
     In the exemplary embodiments of the safety system and method of the present invention, a latching mechanism to prevent, e.g., an oven door from being opened so long as the harmful or undesirable event is present. The latching mechanism may be a solenoid or relay that causes, e.g., a hook, pawl or pin to engage the door of the device so that it cannot be opened until the latching mechanism disengages from the door. 
     Further in the exemplary embodiments of the safety system and method of the present invention, an alarm may be triggered upon detection of the harmful or undesirable event. The alarm may be for example but not limited to a buzzer, tone, flashing light, bell, horn, computer network signal, wireless signal to a security monitoring system, etc. 
     The present invention is not limited to electrical power sources. Devices powered by natural gas, propane, fuel oil and the like may also be protected according to embodiments of the present invention wherein potentially harmful access to the device is prevented by locking out personnel or users and/or alarm actuation to indicate the occurrence of the undesirable condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic circuit diagram of a first preferred embodiment of the protection system of the present invention for devices connected to an alternating current electrical power supply through a leakage current protection device. 
     FIG. 2 is a schematic circuit diagram of a second preferred embodiment of the protection system of the present invention for devices connected to an alternating current electrical power supply through a leakage current protection device. 
     FIG. 3 is a schematic cross-sectional view of a flat appliance cord illustrating a sensing lead between the power line and the neutral line. 
     FIG. 4 is a top plan view of a toaster provided with a protection system in accordance with a third preferred embodiment of the present invention. 
     FIG. 5 is a partial side view of the toaster shown in FIG. 4; 
     FIG. 6 is a schematic representation of a normal closed switch as employed in the third preferred embodiment of the present invention as shown in FIGS. 4 and 5; 
     FIG. 7 is a schematic representation of a normal closed switch responsive to pressure as used a variation of the third preferred embodiment of the present invention: 
     FIG. 8 is a schematic representation of a switch responsive to light as would be used in a variation of the third preferred embodiment of the present invention. 
     FIG. 9 is a schematic representation of still another embodiment of this invention as applied to a pop-up toaster. 
     FIG. 10 is a schematic representation of a further embodiment of this invention as applied to a pop-up toaster, which is similar to that of FIG.  9 . 
     FIG. 11 is a schematic functional block diagram of a sensor assembly, an alarm assembly, latching assembly and power disconnect, according to exemplary embodiments herein of the present invention; 
     FIG. 12 is a schematic diagram of a solenoid actuator and door hook locking mechanism, according to an exemplary embodiment of a latching mechanism of the invention; 
     FIG. 13 is a schematic diagram of a solenoid and pin actuator, and door hook, according to another exemplary embodiment of a latching mechanism of the invention; and 
     FIG. 14 is a schematic diagram of a rotating solenoid and hook or slot assembly in the door of the appliance, according to another exemplary embodiment of a latching mechanism of the invention. 
    
    
     The present invention may be susceptible to various modifications and alternative forms. Specific exemplary embodiments thereof are shown by way of example in the drawing and are described herein in detail. It should be understood, however, that the description set forth herein of specific embodiments is not intended to limit the present invention to the particular forms disclosed. Rather, all modifications, alternatives, and equivalents falling within the spirit and scope of the invention as defined by the appended claims are intended to be covered. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, the details of exemplary embodiments of the present invention are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     Referring to FIG. 11, depicted is a schematic functional block diagram of a sensor assembly, an alarm assembly, latching assembly and power disconnect, according to exemplary embodiments herein of the present invention. A device, generally represented by the numeral  1100 , has a sensor assembly  1102 , a voltage reference  1114 , a comparator  1104 , an alarm assembly  1106 , a latching assembly  1108  and a power disconnect  1110 . 
     The sensor assembly  1102  is preferably a cadmium sulfide sensor for detecting light, though choice of the sensor assembly  1102  depends upon the application to be sensed, e.g., sensing electrical arcing, shock, smoke, flame, heat, pressure, etc. The voltage reference  1114  in combination with the comparator  1104  may be configured so that when a predetermined threshold voltage from the sensor assembly  1102  is detected an output from the comparator  1104  may be used to actuate the alarm assembly  1106 , the latching assembly  1108 , and/or the power disconnect  1110 . The power disconnect  1110  is more fully described herein below. The sensor assembly  1102  may also be self-contained in that its output may be adapted to directly drive the alarm assembly  1106 , the latching assembly  1108 , and/or the power disconnect  1110 , without requiring the comparator  1104  nor the voltage reference  1114 . 
     The latching assembly  1108  may be in any form that prevents access to the problem area of the device or appliance. The latching assembly  1108  may comprise a locking mechanism in combination with an actuator that is controlled by the sensor assembly  1102 . So long as the harmful or undesirable event is present the sensor assembly  1102  may signal the actuator to maintain the locking mechanism in a locked condition. When the event is over, the sensor assembly  1102  may signal the actuator to release the locking mechanism so that the device or appliance may be accessed in a normal fashion for cleaning, repair, etc. 
     Referring to FIG. 12, depicted is a schematic diagram of a solenoid actuator and door hook locking mechanism. A solenoid actuator  1202  has a pin  1206  that is retracted in normal operation of the device  1100 . When a hazardous or other undesirable condition is detected the solenoid actuator  1202  receives an alarm condition detection signal from the output of the comparator  1104  and the in  1206  slidingly engages the door hook  1204  so that the door  1210  cannot be opened. The pin  1206  may remain in the extended locking position until there is no longer an alarm condition. The solenoid actuator  1202  may be a mechanically bi-stable device wherein the pin may remain the retracted unlocked position or remain in the extended locking position. Power may be briefly applied to solenoid actuator to lock the door  1210 , and power may be briefly applied again to unlock the door  1210 . A manual reset (not shown) may also be employed so that the door  1210  may be unlocked by qualified personal after the trouble has passed. The locking mechanisms are not limited to solenoid type devices. Any type of controllable locking device may be used for the intended purposes herein. E.g., bimetallic triggered catches (not shown) calibrated to trip upon excessive heat from a fire may be used to lock the door of the appliance. A manual reset may be employed to unlock the door once the appliance has cooled down after the fire therein has been extinguished. 
     Referring to FIG. 13, depicted is a schematic diagram of a solenoid and pin actuators, and door hook locking mechanism. A solenoid actuator  1302  engages a pin actuator  1303  that is retracted in a locked position during normal operation of the device  1100   b . When a hazardous or other undesirable condition is detected the solenoid actuator  1302  receives an alarm condition detection signal from the output of the comparator  1104  and the pin  1303  slidingly engages the door hook  1304  so that the door  1210  cannot be opened. The pin  1303  may remain in the extended locking position being biased by a spring  1305 . Power may be briefly applied to solenoid actuator to lock the door  1210 . A manual reset (not shown) may be employed so that the door  1210  may be unlocked by qualified personal after the trouble has passed. The locking mechanisms are not limited to solenoid type devices. Any type of controllable locking device may be used for the intended purposes herein. E.g., bimetallic triggered catches (not shown) calibrated to trip upon excessive heat from a fire may be used to trip the pin  1305  so as to lock the door of the appliance. A manual reset may be employed to unlock the door once the appliance has cooled down after the fire therein has been extinguished. 
     Referring to FIG. 14, depicted is a schematic diagram of a rotating solenoid and hook or slot assembly in the door of the appliance. The rotating solenoid  1402  may be a bi-position rotary device having a hook or catch  1406  that may be positioned in a locking or unlocked position. A slot or hook  1404  attached to the door  1210  may be engaged by the hook or catch  1406  so as to effectively lock the door  1210  during the event. The door  1210  may be unlocked either by an electrical signal to the solenoid  1402  or by a mechanical reset (not shown). 
     Other locking, latching and access prevention apparatus and systems may be employed to prevent unintentional or unwanted opening of the door  1210  of the device  1100 , as is well known to those having skill in the art of appliances. 
     The alarm assembly  1106  may be audible, visual or a combination of both. The alarm assembly  1106  may have an output adapted to signal a remote monitoring station and may be connected to a computer network, and/or security monitoring system (not shown) via wires or by wireless radio frequency or infrared signals (not shown) as is well known by those skilled in the art of computer and security monitoring systems. 
     The power disconnect  1110  may disconnect power to electrical components in the problem area of the device, e.g., heating element, agitator, fan, etc. As discussed more fully below, a ground fault, overload or other anomalies in the electrical components may cause disconnection of power therefrom. 
     An electrically energized appliance  10  is schematically illustrated in FIG.  1 . The electrically energized component(s) of appliance  10  are represented by an electrical load  12 . The appliance is of the ungrounded double insulated type. Although referred to as the electrically energized component of an appliance  10 , it should be recognized that the load  12  may be any type of electrical device, such as a tool or an electric heater utilizing a power cord as disclosed herein. 
     Electrical power for the load  12  of appliance  10  is obtained through an unshielded power line  14  and an unshielded neutral line  16 . The power and neutral lines would normally be encompassed in some type of appliance cord, an example of which is illustrated in FIG. 3 hereof. Included in the appliance cord, with the power and neutral lines, is an unshielded sensing lead  18 . Sensing lead  18  is located between the power line  14  and the neutral line  16 . It is electrically isolated from the electrical load  12  and the power and neutral lines  14  and  16  respectively, by insulation  20  surrounding power line  14 , insulation  22  surrounding neutral line  16 , and insulation  24  surrounding sensing lead  18 . All three conductors and their surrounding insulation are enclosed in insulating sheath  26 . 
     In some situations, it may be desirable to provide an arrangement for checking the integrity of the sensing lead. Accordingly, an integrity checking device  28  may be connected between the sensing lead  18  and the power line  14 . Any suitable type of integrity checking device, such as a neon glow tube with a current limiting resistor connected in series may be used. The impedance of the indicating device should be sufficiently large to limit the current flow through the integrity checking device to an amount well below the predetermined magnitude of arcing current flow selected for actuation of the arcing current protection arrangement. Integrity checking device  28  could also be a switch in series with a suitable current limiting resistor, in which case not only the sensing lead integrity but also the operation of the arc protection arrangement could be tested, if the resistance of the current limiting resistor is chosen so as to permit the flow of a current above the predetermined magnitude of arcing current flow selected for actuation of the arcing current protection arrangement. 
     The circuit opening device of the protection system may be located in the appliance cord, such as in the plug connected to the appliance cord. Placing the arcing protection in the locality of the appliance cord being protected, aids in minimizing nuisance tripping, which might otherwise occur, considering the low level of currents to which a response is desired. 
     Although any suitable type of circuit opening device may be utilized in this preferred embodiment, solenoid actuated normally closed contacts, illustrated as the normally closed contacts  30  in power line  14 , are shown. In most applications, it would also be desirable to have the solenoid open normally closed contacts in the neutral line  16 , as represented by the normally closed contacts  32 . 
     Energization of a solenoid coil  34  is controlled by a suitable switching device, such as a silicon controlled rectifier (SCR)  369 . The circuit of anode  38  and cathode  40  of SCR  36  is connected in series with the solenoid coil  34 . Gate  42  of SCR  36  is connected to the sensing lead  18  and also to the cathode  40  through a resistor  44 . A current limiting resistor  46  is connected between gate  42  of SCR  36  and sensing lead  18 . 
     DC current for energizing the solenoid coil  34  is obtained from a rectifier  48 . Any suitable rectifier circuit may be utilized. As shown, a full wave rectifier formed by diodes  50 ,  52 ,  54  and  56  is employed. Rectifier  48  is connected between power line  14  and neutral line  16  to provide direct current to the solenoid coil  34  and the anode-cathode circuit of the SCR  36 . 
     When an arcing current in excess of a predetermined magnitude occurs in sensing lead  18 , due for example, to a breakdown of the insulation between power line  14  and neutral line  16 , the current through resistor  44  causes the gate  42  of SCR  36  to bias the anode-cathode circuit of SCR  36  to a conducting state. The resulting current flow through the solenoid coil  34  causes the normally closed switch  30  in power line  14  to be opened, as well as the normally closed switch  32  in neutral line  16 , if provided. In this way, the existence of arcing currents in excess of the predetermined magnitude removes the power from appliance load  10  to prevent potentially hazardous conditions caused by the arcing currents. 
     A test circuit may be provided to check operation of the circuit opening device. This test circuit may utilize a simple switch and resistance connected in series between the power line  14  and the sensing lead  18 . Although a single resistor could be employed, in this particular arrangement a pair of resistors  58  and  60  is employed, so that a change in the input polarity will not preclude testing. A manually actuatable switch  62  may be connected from the midpoint of resistors  58  and  60  to the gate  42  of SCR  36 . When normally opened switch  62  is closed, a current in excess of the predetermined magnitude will flow through resistors  58  and  44  (or resistors  60  and  44  in the event of polarity reversal), which will bias gate  42  to trigger SCR  36  into conduction. Thus, the test circuit may be utilized to verify correct operation of the current responsive SCR  36  and the circuit opening solenoid coil  34 . 
     In accordance with a first embodiment of this invention, the just described protection system, as set forth in U.S. patent application Ser. No. 08/682,957, which is hereby incorporated by reference, is enhanced to provide further protection by de-energizing the electrical device  10  when an undesirable condition, other than leakage currents in the power cord, arises in the electrical device  10 . The undesirable condition, with respect to which further protection is provided by the first embodiment of this invention, is the presence of a potential on interconnected metal parts  64  of the appliance which are not intended to be connected to either the power line  14  or the neutral line  16 . The presence of a potential on the interconnected metal parts could result in a leakage current flowing through a person touching the metal parts of the appliance. To provide for the de-energization of the appliance load  10 , should a potential develop on the metal parts, represented in FIG. 1 by the enclosure  64 , an electrical connection  66  is provided between the sensing lead  18  and the metal parts  64  of the appliance  10 . Such a potential may develop as a result of failure of electrical insulation within the appliance, or by the inadvertent contact of a conductive object with one of the metal parts  64  and a conductive member in the appliance  10  which is connected to the power line  14  or the neutral line  16 . If the potential is caused by the insertion of a conductive metal object into the appliance  10  so as to touch one of the lines  14  or  16  or a conductive part connected to those lines, and the metal parts  64 , the current flow through the electrical connection  66  could be quite high. Thus, it may be desirable to increase the resistance of current limiting resistor  46 , to reduce the current flow through electrical connection  66 . An example of the potential being caused by contact with a conductive metal member would be the insertion of a metal knife or fork into a toaster, to extract a stuck item which is being toasted. The metal knife or fork could make contact both with the metal parts  64  and the energized heating elements. 
     In accordance with a second embodiment of this invention, an arcing current protection arrangement, combined with a GFCI  68 , as illustrated in FIG. 2 of U.S. patent application Ser. No. 08/682,957, is enhanced to provide further protection to an electrical device by de-energizing the electrical device  10  when an undesirable condition, other than leakage current in the power cord, arises in the electrical device  10 . Referring to FIG. 2, a GFCI  68  employs a conventional differential transformer with a transformer core  70 , in which the magnetic fluxes produced by the power line  14 ′ and the neutral line  16 ′ normally balance one another. (Portions of this circuit corresponding to the circuit of FIG. 1 are indicated by primed numerals.) 
     A secondary winding  72  is also located on the transformer core  70 . If a differential in the flux produced by the current flowing in power line  14 ′ and neutral line  16 ′ is detected, the resultant signal produced in secondary winding  72  will cause a circuit opening device  74  to open the normally closed switch  30 ′ in the power line  14 ′, as well as the normally closed switch  32 ′ in the neutral line  16 ′, if used. 
     For use with an arcing current protection arrangement, a supplemental winding  76  is located on the differential transformer core  70 . A pair of resistors  78  and  80  are connected in series between the power line  14 ′ and the neutral line  16 ′. Supplemental winding  76  is connected between sensing lead  18 ′ and the junction between resistors  78  and  80 . 
     If an arcing current in excess of a predetermined magnitude is present in sensing lead  18 ′, supplemental winding  76  will unbalance the flux in the core of the differential transformer and produce a signal in the secondary winding  72  to actuate the circuit opening device  74 . In this way, the protection of the embodiment of FIG. 1 against undesired arcing currents is directly incorporated into the GFCI  68 . The number of turns in supplemental winding  76  may be adjusted to actuate the circuit opening device at a desired level of current in sensing lead  18 ′. The windings on the transformer core may be configured to actuate the circuit opening devices for current flows in the sensing lead  18 ′ of 2.5 milliamperes or less. 
     A test circuit (not shown), such as that illustrated in FIG. 1, may also be incorporated in this embodiment. Also, it should be noted that in some situations the sensing lead  18 ′ could be the ground line (in a three line arrangement), so that a separate sensing lead would not be required. 
     In accordance with the second embodiment of this invention, the just described protection system, as set forth in U.S. patent application Ser. No. 08/682,957 is enhanced to provide further protection by de-energizing the electrical device  10 ′ when an undesirable condition, other than leakage currents in the power cord, arises in the electrical device  10 ′. The undesirable condition with respect to which further protection is provided by the second embodiment of this invention is the presence of a potential on interconnected metal parts of the appliance which would result for example in a leakage current flowing through a person touching the metal parts of the appliance. To provide for the de-energization of the appliance load  12 ′, should a potential develop on the metal parts, represented by the enclosure  64 ′, an electrical connection  66 ′ is provided between the sensing lead  18 ′ and the metal parts  64 ′ of the appliance  10 ′. Such a potential may develop as a result of a failure of electrical insulation within the appliance, or by the inadvertent contact of a conductive object with the metal parts  64 ′ and a conductive member in the appliance  10 ′ connected to the power line  14 ′ or the neutral line  16 ′. If the potential is caused by a conductive metal object, the current flow through the electrical connection  66 ′ could be quite high, such that it may be desirable to provide a current limiting device or resistor  82 . 
     The first two embodiments of the present invention have been described in combination with a leakage current protection system for the power delivery cord of a electrical device which may include a ground fault circuit interrupter. The present invention may provide leakage current and arcing protection to an electrical device separate from a leakage current protection system for the power delivery cord of the electrical device, and without a ground fault circuit interrupter. For instance, the sensing lead  18  or  18 ′ may be a fine wire of considerable small cross-section than the power line  14  or  14 ′ and the neutral line  16  or  16 ′, such that it would not necessarily provide leakage current and arcing protection to the power delivery cord, in accordance with the teachings of previously cited U.S. patent application Ser. No. 08/682,957, but would provide the necessary electrical flow path for leakage current and arcing protection of the electrical device. 
     A third embodiment of this invention is set forth, by way of example, as applied to a toaster, as illustrated in FIGS. 4 and 5. The toaster housing  84 , is shown as having two opening  86  and  88  for receiving items to be toasted by heat produced by heating elements  90 ,  92 , and  94 . To protect the toaster against continued energization in the presence of an excessively high temperature in the toaster, or when flames develop therein, a heat or flame sensing arrangement is provided, which in conjunction with the protection arrangements illustrated in FIGS. 1 and 2, disconnects the toaster from the power supply. Heat or flame or both are detected by a ribbon  96  positioned under the top of the toaster. The ribbon is formed of a material which will disintegrate or stretch and/or break at a temperature which is considered excessively high, or when exposed to an open flame. An example of a material suitable for use as the ribbon  96  is Nomex®, a high temperature resistant synthetic aramid polymer insulating material manufactured by DuPont. The ribbon is secured at one end to a projection  98  extending from the bottom surface of the toaster top. The ribbon passes around guide posts  100 ,  102 ,  104  and  106 , as is secured at the other end  108  to an extension of a movable contact of a normal closed switch  110 . The door of the toaster may also be locked, as described above, to prevent harmful access to the fire or excessive heat. An alarm may also be sounded or sent. 
     The normally closed switch  110  is schematically represented in FIG.  6 . The normally closed switch  110  is shown having a housing  112  which has a fixed contact  114  located at one end and a movable contact  116  supported on a member  118  which extends from the opposite end of the housing. A coil spring  120  surrounding member  118  exerts a force on movable contact  116  which, unless resisted, will cause the movable contact  116  to engage the fixed contact  114 . An attachment device  122  is provided on the member  118 , to which is secured other end  108  of the ribbon  96 . The switch is secured in the toaster housing  84 , such that with other end  108  of the ribbon secured to the attachment device  122 , the force of coil spring  120  is overcome and the movable contact  116  is held away from fixed contact  114 . Upon the occurrence of excess heat in the toaster, such as cause by ignition and resulting flames from an item placed in the toaster, the ribbon  96  will disintegrate and break or stretch such that it will no longer hold the movable contact away from the fixed contact. Rather, the force of the coil spring  120  will close the contacts resulting in a closed circuit between terminals  124  and  126 . 
     In accordance with this embodiment of the invention, one of the terminals  124  or  126  is connected to the sensing lead  18  or  18 ′ and the other terminal is connected to either the power line  14  or  14 ′ or the neutral line  16  or  16 ′. One of the possible connections is illustrated in each of the FIGS. 1 and 2. Thus, the closing of the contacts  114  and  116  causes a potential to appear on the sensing lead  18  or  18 ′, which as previously described results in the opening of normally closed switches  30  and  32  or  30 ′ and  32 ′. By detecting the abnormal high temperature or the presence of flame in the toaster, the toaster may be disconnected from its power source prior to the “pop up” mechanism raising the burning toast and possibly dispersing burning embers into the area surrounding the toaster. 
     The detection of excessive heat or flames in a toaster in accordance with this invention will also reduce or eliminate the risk of causing a fire when the operating mechanism of a “pop-up” toaster is inadvertently depressed to start the toaster with a piece of paper or other readily flammable material in the toast holder. In accordance with this invention, flames from the readily flammable material will result in the de-energizing of the toaster, such that the “pop-up” mechanism will not be actuated to discharge the inflamed material from the toaster. E.g., the “pop-up” mechanism may be locked in the down position as described for the door  1210  above. An alarm may also be activated or sent. 
     While the temperature sensing element has been illustrated as a ribbon, it is contemplated by this invention that the sensing element could take other forms, such as a pellet that melts or disintegrates in the presence of an excessively high temperature. Also, it could be in the form of a temperature responsive bimetal switch, taking the place of the ribbon  96  and the switch  110 . It may also be an electronic fire, smoke and/or heat sensor, as is well know to those skilled in the art of fire and smoke detectors. 
     In accordance with the third embodiment of this invention, other abnormal conditions in an electrical device could also be sensed for the purpose of disconnecting the device from the electrical power supply. For instance, an abnormal gas pressure in an electrical device could be sensed for the purpose of disconnecting the device from the electric power supply. As illustrated in FIG. 7, a normally closed switch  110 ′ is shown having a housing  112 ′ which has a fixed contact  114 ′ located at one end and a movable contact  116 ′ supported on a member  118 ′ extending from the opposite end of the housing. A coil spring  120 ′ surrounding member  118 ′ exerts a force on movable contact  116 ′, which unless resisted will cause the movable contact  116 ′ to engage the fixed contact  114 ′. The member  118 ′ is connected to a diaphragm  128  in a housing  130 . Providing sufficient pressure exists in chamber  132  as applied through an inlet  134 , the force of spring  120 ′ will be overcome and the switch contacts will be maintained in an open position. Should a gas pressure in the electrical device, as provided to the chamber  132  through inlet  134 , fall below a desired level, the pressure on diaphragm  128  will not be sufficient to overcome that of spring  120 ′, and the contacts will close. As previously described, the closing of the contacts will result in the de-energization of the protected electrical device. Also, access to the device may be locked out and/or an alarm actuated and/or sent during the event. 
     Similarly, in accordance with this invention an abnormally high pressure in an electrical device could be sensed for the purpose of disconnecting the device from an electrical power supply. Referring again to FIG. 7, spring  120 ′ would be designed to hold the movable contact  116 ′ in a spaced or open position with respect to contact  114 ′. However, if an abnormally high pressure is applied to a chamber  136  through an inlet  138 , the force applied to diaphragm  128  will overcome the force of spring  120 ′ and movable contact  116 ′ will be moved into engagement with fixed contact  114 ′. Should a gas pressure in the electrical device, as provided to the chamber  136  through inlet  138 , raise above a desired level, the pressure on diaphragm  128  will be sufficient to overcome that of spring  120 ′, and the contacts  114 ′ and  116 ′ will be close. As previously described, the closing of the contacts will result in the de-energization of the protected electrical device. Also, access to the device may be locked out and/or an alarm actuated and/or sent during the event. 
     Another example of the sensing of an abnormal condition in an electrical device in accordance with this invention is set forth in FIG.  8 . The sensing arrangement in FIG. 8 would be used to sense the destruction or removal of a light barrier  140  in the electrical device. A light emitting diode  142  is positioned to emit light on a light sensitive semiconductor switch  144 , in the absence of light barrier  140 . The light barrier is designed to be removed in the presence of an abnormal condition. For instance, the light barrier is formed of a thin sheet of opaque Nomex®, a high temperature resistant synthetic aramid polymer insulating material manufactured by DuPont, which disintegrates or stretches and or breaks at a temperature which is considered excessively high. When the light barrier is removed, light from light emitting diode  142  impinging upon light sensitive semiconductor switch  144  will cause the switch to conduct, thereby completing a circuit between terminals  124 ″ and  126 ″. As previously described, the closing of the circuit between terminals  124 ″ and  126 ″ will result in the de-energization of the protected electrical device, when incorporated in one of the embodiments shown in FIGS. 1 and 2. Also, access to the device may be locked out and/or an alarm actuated and/or sent during the event. 
     Similarly, the circuit including the light sensitive semiconductor switch  144 , could be reconfigured such that when light is impinging on it from light emitting diode  142 , and the circuit between  124 ″ and  126 ″ is closed, it would result in opening the circuit between the sensing lead  18  or  18 ′ and the power line  14  or  14 ′. In accordance with this embodiment of the invention, the development of sufficiently dense smoke within the appliance in the space between the light sensitive semiconductor switch  144  and the light emitting diode  142  would result in the light sensitive semiconductor switch  144  turning off, to open the circuit between the terminals  124 ″ and  126 ″. Thus closing the circuit between the sensing lead  18  or  18 ′ and the power line  14  or  14 ′, will result in the de-energization of the protected electrical device. Also, access to the device may be locked out and/or an alarm actuated and/or sent during the event. 
     In accordance with a modification of the embodiment of this invention shown in FIG. 8, a photoelectric cell, such as a cadmium sulfide cell, is used as a sensor, rather than the combination of the light emitting diode and the light sensitive semiconductor switch. The cadmium sulfide cell like the light sensitive semiconductor switch is positioned to detect the presence of light or flame in the protected electrical device. The current produced by the cadmium sulfide cell in the presence of undesirable light or flame is provided to a circuit arrangement which results in the flow of a current in sensing lead  18  or  18 ′, which as previously described results in the opening of normally closed switches  30  and  32  or  30 ′ and  32 ′. Also, access to the device may be locked out and/or an alarm actuated and/or sent during the event. 
     While the third embodiment of the present invention has been described in combination with a leakage current protection system for the power delivery cord of a electrical device which may include a ground fault circuit interrupter, the present invention may provide leakage current and arcing protection to an electrical device separate from a leakage current protection system for the power delivery cord of the electrical device, and without a ground fault circuit interrupter. For instance, the sensing lead  18  or  18 ′ may be a fine wire of considerable small cross-section than the power line  14  or  14 ′ and the neutral line  16  or  16 ′, such that it would not necessarily provide leakage current and arcing protection to the power delivery cord. However, the sensing lead  18  would provide the necessary electrical flow path for both leakage current and arcing protection of the electrical device, and for the sensing of other abnormal conditions in accordance with the third embodiment of the present invention. 
     In still another embodiment of this invention, even further protection is provided by coupling a timing circuit in the appliance or tool with an appliance or tool ground fault circuit interrupter. Many appliances and tools are provided with timers for interrupting the electrical supply to the appliance or tool after a predetermined time interval. For instance, in the case of a pop-up toaster, after a predetermined time interval, the pop-up mechanism is actuated to raise the toast from the toaster, and to de-energize the toaster heating elements. During this time period the pop-up mechanism is locked in the down position and an alarm may be actuated. 
     Referring now to FIG. 9, a circuit diagram for providing additional protection to a pop-up toaster in accordance with this invention will be described. As shown in FIG. 9, the additional protection in accordance with this invention is provided with a GFCI similar to that shown in FIG. 1 of co-pending U.S. patent application Ser. No. 08/631,553, which application is assigned to one assignee of this application, and which application is hereby incorporated by reference. FIG. 1 illustrates a GFCI with a power line lead  208 , and a neutral lead  210  supplying power to a toaster represented by load  212 . A circuit opening contact  214  is located in power line lead  208 , and a similar circuit opening contact  216  is located in the neutral lead  210 . A metal oxide varistor  218  is connected between power line lead  208  and the neutral  210  to provide transient voltage protection. 
     The GFCI has a differential transformer  220 . Differential transformer  220  has a magnetic core  222 , with portions  224  and  226  of leads  208  and  210 , respectively, passing through core  222  forming the primary windings of the differential transformer. The primary windings  224  and  226  are connected to produce opposing or “bucking” fluxes. Accordingly, if all of the load current returns through the neutral lead, the fluxes of the primary windings will cancel one another. 
     A secondary winding  228  is also located on the transformer core  222 . If a ground current in excess of a predetermined size (e.g., 6 milliamperes) occurs, the fluxes produced by the primary windings  224  and  226  will not cancel and there will be a resulting flux flow in the core  222 . This flux flow will induce a signal in the secondary winding  228 , which is then conveyed to the GFCI control circuitry to open the power line and neutral lead. 
     A capacitor  230 , connected in series with a diode  232  and a current limiting resistor  234  between power line lead  28  and neutral line  210  provides a direct current power source for the GFCI. 
     The signal induced in secondary winding  228  is conveyed to terminals  1  and  3  of an integrated circuit  236  that linearly amplifies and provides a stable output control or trip signal under varying conditions. A phase adjusting resistor  238  and a DC blocking capacitor  240  are connected in series from secondary winding  228  to terminal  1  of the integrated circuit  236 . 
     A positive voltage supply is provided to terminal  6  of the integrated circuit  236  from capacitor  230  through resistor  242 . Resistor  244  provides a feedback loop from terminals  7  and  8  of the integrated circuit  236  to terminal  1 . Resistor  244  provides feedback gain control. The resistance of resistor  244  may be varied to adjust the ground current tripping level for the GFCI. This may be achieved by making the resistor  244  replaceable so that different magnitude resistors may be selectively utilized, or, alternatively, by using a variable resistor with selectable discrete settings. At the present time, settings for trip levels of 6 milliamperes, 10 milliamperes and 30 milliamperes are probably all that are required, but the number of discrete settings may be altered as needed or desired. 
     The contacts  214  and  216  of the GFCI are closed by enegization of a solenoid  246  from the capacitor  230  through lead  248 , and the collector to emitter circuit of a transistor  250 . The closing of a switch  252  will cause the transistor  250  to be forward biased by current flow through resistor  254  and zener diode  256 , thus causing current flow through solenoid  246  and closing of contacts  214  and  216 . Upon the detection of a current above the predetermined ground current tripping level, a signal from terminal  5  of the integrated circuit  236  will be applied to gate  258  of an SCR  260 . The anode  262  to cathode  264  circuit of SCR  260  is connected between the junction of zener diode  256  and resistor  254 . The signal from terminal  5  of the integrated circuit  236  supplied to the gate  258  of SCR  260  is also applied to capacitor  266 , which provides a time delay in the triggering of conduction of SCR  260  by the application of a signal to gate  258 . Conduction between anode  262  and cathode  264  of SCR  260  will remove the forward bias from transistor  250 , such that collector to emitter current flow will be interrupted, as will the current flow through solenoid  246 . Thus, the contacts  214  and  216  will be opened to de-energize the toaster represented by load  212 . In addition, a locking mechanism may be actuated as described above. An alarm may be actuated as described more fully above. 
     The toaster is provided with flame detection by a circuit arrangement similar to that shown in FIG. 2. A supplemental winding  268  is located on differential transformer core  222 . A pair of resistors  270  and  272  are connected in series between the power line  208  and the neutral line  210 , in parallel with the metal oxide varistor  218 . Supplemental winding  268  connected in series with a light sensitive element, such as a cadmium sulfide cell  274 , are connected in parallel with resistor  272 . The detection of a flame in the toaster by the cadmium sulfide cell  274  will cause the flow of a current in supplemental winding  268  which will be detected by secondary winding  228 . As previously described, the current flow in secondary winding  228  will result in the opening of contacts  214  and  216 . Thus, the toaster is provided with both ground fault and flame protection. In addition, either or both of these events may lock access to the toaster, and/or actuate an alarm. 
     Still further protection is provided with respect to operation of the pop-up mechanism of the toaster. Switch  252  is closed upon lowering the toast carriage in a pop-up toaster to the toasting position. Closing the switch  252  will cause transistor  250  to be forward biased by the direct current voltage on capacitor  230  being applied to the base through lead  248  and resistor  254 , thereby energizing solenoid  246  and closing of contacts  214  and  216 . The closing of contacts  214  and  216  results in the energization of the toaster represented by load  212 . The toast carriage is mechanically restrained in the lowered position until a carriage return solenoid  276  is energized. The energization of the carriage return solenoid is controlled by a timing circuit  278 . The timing circuit is energized from the power line  208  through a diode  280  and a resistor  282  which supply direct current to charge a capacitor  284  to a voltage limited by a zener diode  286 . The anode of a programmable unijunction transistor  288  is connected to a voltage bridge comprising resistors  290 ,  292 , and potentiometer  294 , which is connected across capacitor  284 . Gate  296  of programmable unijunction transistor  288  is connected to the junction of resistors  298  and  300  which are also connected across the capacitor  284 . A capacitor  302  is connected in parallel with resistor  300 . The setting of potentiometer  294  may be adjusted to determine the time delay between the energization of the toaster and the conduction of programmable unijunction transistor  288 . Conduction of programmable unijunction transistor  288  will cause the application of a potential to gate  304  of an SCR  306  and to resistor  308  and capacitor  310 . Conduction of SCR  306  will provide a path for current flow through a diode  312 , solenoid  276 , and diodes  314  and  316 . The energization of solenoid  276  should result in the actuation of the toast carriage to the raised position, and the opening of switch  252 . However, failure of the mechanism to raise the toast carriage to the raised position would result in the switch remaining closed, and therefore the continued undesirable energization of the toaster. Even though the solenoid  276  is energized, and the mechanism to raise the toast carriage is operating properly, the toast carriage may be jammed in the lowered position by some object inserted in the toaster. 
     In accordance with this embodiment of this invention, further protection is provided to the toaster by causing the GFCI contacts  214  and  216  to be opened if the toaster carriage is jammed in the lower position, thereby preventing the opening of switch  252 . With switch  252  in the closed position, voltage is supplied to the anode of SCR  306  through diode  318  and resistor  320 . The voltage appearing across diodes  314  and  316  is applied to base  322  of a transistor  324  through a resistor  326 . After a time delay caused by the charging of a capacitor  328  connected to the base  322 , transistor  324  will be caused to conduct, thereby removing the forward bias from transistor  250 , and causing solenoid  246  to be de-energized and therefor opening contacts  214  and  216 . 
     Wherein the pop-up mechanism of the toaster is actuated by the timing circuit  278 , should a flame be detected by the cadmium sulfide cell  274 , or a leakage current be detected, so as to cause a current flow in secondary winding  228  which results in the opening of contacts  214  and  216 , the timing circuit  278  will be de-energized, such that the carriage return solenoid  276  will not be energized. Thus, detection of a flame, excessive heat, or of a leakage current in the toaster prior to actuation of the pop-up mechanism by the carriage return solenoid  276 , will prevent actuation of the pop-up mechanism. Thus, a shock to a person reaching for toast raised from the toaster in the case of a leakage current will be prevented. Similarly, in the case of detected flame or excessive heat, an excessively hot or flaming object in the carriage will not be discharged from the toaster, with a resultant greater likelihood of starting a fire near the toaster or causing heat damage to the material on which the excessively hot or flaming object lands. Rather, the excessively hot or flaming object will be retained in the toaster which has been de-energized. An alarm may further be actuated upon detection of a fire or excessively hot object. 
     Referring to FIG. 10, a circuit diagram of a further embodiment of this invention for providing additional protection to a pop-up toaster will be described. Major portions of the circuit of this embodiment are similar to those of FIG. 9, such that similar components of the circuit are identified by the same numerals in FIGS. 9 and 10. One difference between FIGS. 9 and 10, is that the differential transformer  220  is located on the load side of the circuit opening contacts  214  and  216 , rather than on the source side as shown in FIG.  9 . While the differential transformer  220  might be placed in either position in either of the embodiments shown in FIGS. 9 and 10, the usually preferred location is that shown in FIG.  10 . Additional protection provided by the embodiment of the invention shown in FIG. 10, compared with that shown in FIG. 9, is that neutral line connection to the carriage return solenoid  276  is isolated from the neutral line connection of the remainder of the circuit, which is on the source side of the circuit opening contacts  214  and  216 . Using this circuit arrangement, opening of the contacts  214  and  216  isolates the toaster carriage return solenoid  276  from both the power line  208  and the neutral line  210 . In the circuit arrangement of FIG. 9, a connection of the neutral line  210  to the toaster carriage return solenoid  276  is maintained even after the circuit opening contacts  214  and  216  are opened. The elimination of such a connection may be a requirement of standards such as those set by Underwriters Laboratories, Inc. 
     To provide the desired isolation of the energization of toaster carriage return solenoid  276 , it is connected between the power line  208  and the neutral line  210  on the load side of the contacts  214  and  216  in series with diode  312  and the anode to cathode circuit of SCR  304 . Connected in parallel with the anode to cathode circuit of SCR  304  are series connected resistors  330  and  332 , with a capacitor  334  being connected in parallel with resistor  332 . The junction of the series connected resistors  330  and  332  is connected to the collector of a light sensitive transistor  336 , the emitter of which is connected the gate of SCR  304 . A resistor  338  is connected between the gate of SCR  304  and neutral line  210 . 
     Conduction of SCR  260  causes a current flow through a light emitting diode  340 . The light sensitive transistor  336  and light emitting diode  340  are positioned such that light emitted by the diode  340  impinges on the light sensitive transistor  336  to cause it to conduct. Conduction of light sensitive transistor  336  causes a signal to be applied to the gate of SCR  304 , causing in turn the conduction of SCR  304  and of the energization of carriage return solenoid  276 . Thus, the same protective features are offered by the circuit of FIG. 10, as offered by the circuit of FIG. 9, but with the additional benefit of isolating the carriage return solenoid energization circuit from the remainder of the protection circuit, the neutral of which is supplied from the source side of the circuit opening contact  216 . 
     Several embodiments of the present invention have been described, wherein the power delivery cord is set forth as having unshielded power  14  and neutral line  16  and a sensing line  19  and shown in FIG.  3 . The previously described embodiments of the present invention are also applicable to an electrical device provided with shielded power and neutral lines. Each of the power line  14  and the neutral line  16  could be separately shielded, or a single shield could surround both lines. In either case, both of the shields surrounding power line  14  and neutral line  16 , or the single shield surround both lines could be utilized as the sensing lead  18  in practicing the previously described embodiments of this invention. 
     The invention, therefore, is well adapted to carry out the objects and to attain the ends and advantages mentioned, as well as others inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the invention, such references do not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.