Abstract:
An apparatus and method for providing redundant protection to a fault detection/interruption circuit, thereby ensuring safe operation even in the case of a failure of the primary fault detection/interruption means. Upon the occurrence of a failure in the primary circuit interruption means, a secondary circuit breaker, or in some embodiments, a redundant primary circuit breaker release mechanism, serves to remove power from a protected outlet or output conductors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing of U.S. Provisional Patent Application Ser. No. 60/322,368, entitled “Fail Safe Interrupter Using Secondary Breaker”, filed on Sep. 9, 2001, and the specification thereof is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus and method for providing redundant protection to a fault detection/interruption circuit, thereby ensuring safe operation even in the case of a failure of the primary fault detection/interruption means. Upon the occurrence of a failure in the primary circuit interruption means, a secondary circuit breaker, or in some embodiments, a redundant primary circuit breaker release mechanism, serves to remove power from a protected outlet or output conductors. 
     2. Background of the Invention 
     A common source of electrical injuries occurs when an accidental electrical leakage from one electrified object to a second object having a substantially different voltage potential occurs, with the electrical leakage passing through a human. When one of the two electrified objects is at the same potential as the earth (or so-called ground), this is called a ground fault. A circuit to protect against injury due to ground faults is called a ground fault circuit interrupter or GFCI. These devices are built into the electrical outlets of many homes and businesses and, in particular, are required by code in the U.S. for bathrooms and outdoor outlets in new construction. 
     GFCI&#39;s are not immune from failure. In the U.S., the 2001 GFCI Field Test Survey Report by the National Electrical Manufacturers Association found that an estimated 14% of circuit breaker GFCI&#39;s and 8% of receptacle GFCI&#39;s in the field are not operational. As the installed base of GFCI circuits ages, this percentage will increase. The finding of such a large percentage of non operational GFCI&#39;s has led to a great deal of concern about unprotected power. Clearly, any circuit improvements that can enhance the robustness of GFCI devices will serve to reduce the potential for electrical injury. 
     The present invention combines a secondary circuit breaker with a standard fault interrupter. This standard fault interrupter can be a ground fault circuit interrupt (GFCI) or one of the derivative fault interrupters including, but not limited to, arc fault circuit interrupt (AFCI), immersion detection circuit interrupt (IDCI), leakage current detect and interrupt (LCDI) or appliance leakage circuit interrupt (ALCI). The secondary circuit breaker is triggered some interval after certain events such as a sensed fault or a manual test. This secondary circuit breaker receives its power from a point that should have no power if the fault detection and interrupt mechanism is correctly functioning. Consequently, if the fault detection/interruption circuit works satisfactorily, then the secondary circuit breaker is never fired. In its preferred embodiment, the secondary breaker would be a one-shot circuit breaker, serving to permanently remove power from the output and forcing the user to replace the malfunctioning unit. One shot circuit breakers are designed as normally closed switches, which, when activated, open permanently. 
     There are a variety of circuit interruption means that comprise the class of one-shot circuit breakers. The most common example is a thermal fuse, whereby two electrical conductors are in electrical contact through a low melting point linkage that opens when the current flow exceeds a certain threshold. U.S. Pat. No. 3,629,766 (Gould) describes a circuit breaker wherein a fusible wire link holds spring biased conductors in a closed position. When a predetermined electrical current is passed through the fusible link it causes it to break, effecting the snap action release of the spring arms and breaking the electrical connection. Other examples of circuit interruption means include the one-shot breaker described in U.S. Pat. No. 5,394,289 (Yao and Keung) wherein wire fuses connect two sets of two conductors. A current overload is used to break one fuse, whereupon, a cutting element is released to cut through the other fuse. U.S. Pat. No. 4,829,390 (Simon) describes a switch that is held in a normally closed position by a flash bulb. A sensor detects a dangerous condition and actuates the flash bulb, causing it to disintegrate and allowing the switch to open. Bimetallic thermal and thermal magnetic circuit breakers are well known in the art and are the basis for many resettable circuit breakers, although they can be used for one-shot operation. These work by employing a blade made of two metals having different thermal coefficients of expansion. When the blade is heated, it deforms, breaking a circuit. The magnetic breakers use heating to reduce the magnetic attraction of a magnet, thereby causing a spring loaded contact to release and open a circuit. Other designs for circuit breakers include piezoelectric actuators as in U.S. Pat. No. 4,473,859 (Stone et al) and shape memory alloy actuators as in U.S. Pat. No. 3,403,238 (Buehler and Goldstein). 
     U.S. Pat. No. 6,262,871 B1 (Nemir et al) discloses an electronic test circuit for the self-testing of fault detection devices. This self-test circuit enhances the safety of such devices by periodically and automatically testing the function of the fault detection portion of the device without the need for manual intervention. By using a secondary circuit breaker, power may be safely and automatically removed from a malfunctioning fault detection device. One problem with this device is that the self-test circuit has a complexity that is of a higher order than that of the original fault detection/interruption electronics, thereby adding to overall system complexity and cost. 
     U.S. Pat. Nos. 6,282,070 B1 (Ziegler et al), 6,288,882 B1 (DiSalvo et al), and 6,381,112 B1 (DiSalvo) all disclose a fault detection/interruption device having a so-called “reset lockout”. With a reset lockout, the electrical connections between input and output conductors are said to be prevented from resetting if the circuit interruption mechanism is non-operational or if an open neutral condition exists. However, these inventions have no means for self-test during normal operation. For example, if the fault detection component fails at some time during use, this failure will go undetected until such time as a manual test is implemented. Since there is no way to ensure that a periodic manual test is implemented, this approach can result in unprotected power being furnished at the outlet or over the branch wiring that connects the reset lockout equipped GFCI to an electrical outlet. Furthermore, some failure modes, such as welded circuit breaker contacts, will be undetected and uncorrected by these inventions. 
     3. Objects and Advantages 
     The present invention is designed to be easily added to, or integrated within, an existing technology GFCI circuit and to operate independently of that circuit. The present invention serves as an auxiliary tester that causes the overall device to fail safe in the event of a failure in the GFCI. One major advantage to the proposed invention is that it is inexpensive and can be added to an existing ground fault circuit interrupter, thereby taking advantage of existing technology while improving robustness. A second advantage is that it can automatically detect a malfunctioning electrical current interruption means and can cure that event by firing a secondary circuit breaker, thereby removing power from the system. Alternatively, rather than employing a completely independent secondary circuit breaker, some embodiments may utilize a combined release mechanism on a single, primary circuit breaker, with fail safe protection provided by a redundant, independently controlled, auxiliary circuit breaker release. 
     SUMMARY OF THE INVENTION 
     The present invention is a fail safe fault interrupter that consists of a conventional GFCI with either a second circuit breaker and a second circuit breaker trigger, or an auxiliary circuit breaker release mechanism. When either (a) a fault is sensed; or (b) a manual test is engaged; the second circuit breaker is triggered with a time delayed signal that takes its power from the load side of the fault interrupter. Accordingly, if the power to a fault is satisfactorily interrupted within a designated time after the application of either a manual test or a sensed fault, then there will be no power available to trigger the second circuit breaker (alternatively, the auxiliary circuit breaker release mechanism) and this second circuit breaker will remain in a closed position. Alternatively, if the power is not removed within the designated time interval, the secondary breaker will be opened, thereby removing power from the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 —Block diagram of prior art GFCI circuits 
     FIG.  2 —Block diagram of GFCI with attached test circuit 
     FIG.  3 —One specific embodiment of the test circuit 
     FIG.  4 —GFCI modified to automatically detect certain types of failures 
     FIG.  5 —Primary circuit breaker with redundant release mechanisms 
    
    
     LIST OF REFERENCE NUMERALS 
       22 —Source conductor 
       24 —Source conductor 
       26 —Differential transformer 
       28 —Secondary from current sense transformer 
       30 —Detection electronics 
       32 —Hot side primary circuit breaker contact 
       33 —Neutral side primary circuit breaker contact 
       34 —Load 
       36 —Solenoid 
       38 —Ground fault 
       39 —Ground 
       40 —Ground fault 
       42 —Thyristor 
       44 —Conductor delivering power to detection electronics 
       46 —Conductor delivering power to detection electronics 
       50 —Test button 
       52 —Test fault resistance 
       53 —Test fault resistance 
       56 —Test circuit 
       57 —GFCI circuit 
       58 —Source side conductor 
       60 —Source side conductor 
       66 —Secondary circuit breaker contacts 
       70 —Fault sense signal coming out of detection electronics 
       72 —Test button pressed signal 
       76 —Hot conductor on load side of primary circuit breaker 
       80 —Control for secondary breaker 
       82 —Diode 
       83 —Diode 
       84 —Charging resistor 
       86 —Charging resistor 
       88 —Charge storage capacitor 
       90 —Discharge resistor 
       92 —Charging resistor 
       94 —One shot resistor 
       96 —One shot thyristor 
       97 —Bilateral trigger diode to control secondary circuit breaker 
       100 —Diode bridge 
       102 —Current sense transformer 
       104 —Current sense transformer 
       106 —Neutral conductor 
       108 —Power supply capacitor 
       109 —Gate of thyristor 
       110 —Primary circuitry breaker trigger thyristor 
       112 —Primary circuit breaker solenoid 
       114 —Primary circuit breaker contacts 
       116 —Test capacitor 
       118 —Zener diode 
       120 —Charge resistor 
       122 —Transistor 
       124 —Diode 
       126 —Secondary circuit breaker thyristor 
       128 —Secondary circuit breaker 
       130 —Reset button 
       132 —Spring 
       134 —Fixed plate 
       136 —Fusible element 
       138 —Leaf spring connectors 
       140 —Electrical contacts 
       142 —Arm 
       144 —Moveable member 
       146 —Spring 
       148 —Solenoid 
       150 —Plunger 
       152 —Collar 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 gives a block diagram that functionally describes the majority of present day GFCI circuits. This is the prior art circuit that is the basis for almost any GFCI found in the home or in commercial or industrial construction. The incoming conductors from the source are conductors  22  and  24 . In the U.S., one of these conductors (for this discussion, conductor  24 ) is generally required by code to be grounded at a distribution panel and is known as the “neutral” conductor. In such a system, the ungrounded current carrying conductor is often called the “hot” conductor. Conductors  22  and  24  pass through a differential current sense transformer  26 , thereby acting as the primary for that transformer. The secondary  28  of the current sense transformer  26  connects to the detection electronics  30 , which may filter and/or amplify and/or otherwise process the voltage from the secondary windings  28  of the current sense transformer  26 . The detection electronics  30  derive power from conductors  44  and  46 . In normal operation, electrical current is delivered to the load  34  through circuit breaker contacts  32  and  33 . In some applications, such as in a load distribution panel, there is a single circuit breaker contact  32  for interrupting electrical current on the ungrounded conductor and there is no circuit breaker contact  33  (equivalently, circuit breaker contact  33  is always closed). For a system with two circuit breaker contacts  32  and  33 , circuit breaker contacts  32  and  33  are closed during normal operation but are driven to be in an open position by solenoid  36  if a ground fault condition is sensed. 
     In FIG. 1, ground faults  38  and  40  are depicted with dashed lines to indicate that these are not always present. These represent fault events that a GFCI is designed to sense and to remedy by opening the relay contacts  32  and  33 . Either of ground faults  38  or  40  could represent a human being that has come into electrical contact with a high potential conductor and with ground  39 . 
     In normal operation, in the absence of a ground fault, the same amount of electrical current flows in conductors  22  and  24  but in opposite directions. The net magnetic flux in the differential current sense transformer  26  is then zero and the voltage that is generated in the transformer secondary  28  is zero. When circuit breaker contacts  32 , 33  are closed and a load side electrical leakage path  38  occurs from conductor  22  to ground  39  or an electrical leakage path  40  occurs from within the load  34  to ground  39  then there is a current imbalance between conductors  22  and  24 . This results in a nonzero net magnetic flux being induced in the differential current sense transformer  26 . This results in a nonzero voltage being induced in the secondary  28  of the transformer  26 . The detection electronics  30  then takes this voltage and processes it to determine if a fault of sufficient magnitude and/or duration is taking place. If the detection electronics  30  determines that an objectionable fault is occurring, then it triggers a thyristor  42 , which energizes a solenoid  36  which opens the circuit breaker contacts  32  and  33 . 
     Test button  50  allows a manual test of the proper operation of the fault sensing/interrupting circuitry. When test button  50  is manually engaged, it causes a current flow through test resistors  52  and  53 , resulting in an electrical leakage around the differential current sense transformer  26 . This imbalance results in a voltage across the secondary  28  and is recognized as a fault by the detection electronics  30 . The detection electronics  30  then energize thyristor  42 , causing the circuit breaker contacts  32 , 33  to be opened. A user can thus manually test the GFCI by engaging the test button  50  and then listening for the relay contacts  32 , 33  to open or by observing a visual indication that the circuit breaker contacts  32 , 33  opened. This is the testing feedback that is built into most commercial GFCI circuits, however, an audible or visual indication can be misleading and the user can be left thinking that the GFCI is offering protection when it is not. For example, if one or both of the two circuit breaker contacts  32 , 33  is stuck in a permanently engaged position, then the GFCI may be unable to provide protection even though a “click” might be heard. 
     Although FIG. 1 is a representative embodiment of a GFCI, there are many possible permutations. For example, the detection electronics  30  in FIG. 1 may be simply a pass through connection to the gate of thyristor  42 , in which case the secondary  28  of differential transformer  26  generates sufficient energy to trigger thyristor  42 . Alternatively, the detection electronics  30  may consist of transistors and other components to amplify and/or filter the voltage developed on secondary  28 . The solenoid  36  and/or the test button  50  may receive power from the load side of circuit breaker contact  32  as shown, or may be connected on the source side of circuit breaker contact  32 . The solenoid  36  may be energized by a thyristor  42  as depicted, or may use a transistor or other type of switch. The solenoid  36  may be replaced by an alternative type of trip mechanism such as a bimetallic element or a fusible link. 
     FIG. 2 portrays a block diagram showing the present invention. The design builds upon the GFCI circuit of FIG. 1 which is inside the dashed box  57 . Test circuit  56  monitors the fault detection signal  70  and the manual test signal  72 . Test circuit  56  delays these signals for a specified time interval and then uses them to trigger a secondary circuit breaker using energy derived from line  76  which is connected to the load side of circuit breaker  32 . Accordingly, if circuit breaker contact  32  opens within a time that is less than the test circuit delay, this represents a successfull functioning of the fault detection/interruption and no triggering of the secondary circuit breaker  66  will occur. A test circuit  56  takes signals  70  and  72  and combines these, delays their effect and then applies them to fire a secondary circuit breaker  66 . Secondary circuit breaker  66  represents a separate, completely functional circuit breaker that has the ability to interrupt electrical current flow into the GFCI circuit and subsequent load. The secondary circuit breaker may be built as a so-called one shot circuit breaker. That is, it can be a nonresettable circuit breaker that is designed to open only once and then to stay open thereafter. Such one-shot circuit breakers can be built to be relatively inexpensive because they do not require an intricate firing and reset mechanism. In its simplest form, each contact of the secondary circuit breaker  66  may be built as two spring contacts that want to come apart but that are held together by a fusible element. When the fusible element is blown, it releases the contacts and the contacts separate, thereby breaking the current flow. 
     FIG. 3 depicts a preferred embodiment of the test circuit  56  of FIG.  2 . There are  2  inputs, either of which can activate the test circuit. Signal  72  is nonzero whenever the test button  50  in FIG. 2 is pressed and the load ( 34  in FIG. 2) is receiving power. Signal  70  is nonzero whenever the detection electronics in FIG. 2 detects a fault and tries to fire the thyristor  42 . If either signal  70  or signal  72  develops a positive potential then it charges capacitor  88  through the series combination of resistor  84  and diode  82 , and/or the series combination of resistor  86  and diode  83 . Diodes  82  and  83  serve a dual role of rectification and steering. Resistors  84  and  86  are sized so as to give equivalent capacitor  88  charging rate regardless of the source ( 70  or  72 ). When either of the optocouplers  82  are fired, this causes a capacitor  88  to be charged through one or both of charging resistors  92 . When the capacitor  88  is sufficiently charged, it fires the transistor  96 , causing the oneshot resistor  94  to disintegrate, thereby causing the secondary circuit breaker contacts ( 66  in FIG. 2) to open. Discharge resistor  90  serves to discharge the capacitor  88  when no fault or manual test is in progress. Accordingly, discharge resistor  90  would be chosen to have a relatively high resistance value with respect to charging resistors  84  and  86 . Bilateral trigger diode  97  serves to maintain a standoff voltage that must be overcome before firing transistor  96 . This gives a level of noise immunity to the circuit, preventing nuisance tripping. 
     Charging resistors  92 , capacitor  88  and discharge resistor  90  are sized so that the oneshot thyristor  96  will be fired after a delay time that is in excess of a reasonable opening time for the primary circuit breaker. For example, Underwriters Laboratories, in its 943 standard, mandates that all Class A GFCI&#39;s will open within 25 milliseconds of the application of a fault. So, a reasonable delay time might be 50 milliseconds. That is, the size of the capacitor  88  is chosen so that it charges up to a sufficiently high voltage to trip the oneshot thyristor  96  in a time that is 50 milliseconds after it begins to receive charge in response to a press of the test button, or in response to a sensed fault from the detection electronics ( 30  in FIG.  2 ). 
     If the GFCI is functioning correctly, the oneshot thyristor  96  will never be energized. This is because the circuit breaker contacts  32  and  33  should open within 25 milliseconds after a fault is sensed, thereby removing the effect of the fault and also removing power from the test button  50 . Since the fault will be removed (either a real fault like  38  or  40  in FIG. 1 or a simulated fault such as imposed via the test button  50 ), the detection electronics will no longer be sensing a fault and there will be no power available from the output  80  from the detection electronics  30 . If, however, circuit breaker contact  32  does not open, then capacitor  88  will charge high enough to trip oneshot thyristor  96  and power will be removed from the entire circuit by the opening of secondary circuit breaker contacts  66 . 
     Although the above discussion assumed a oneshot resistor, the element that releases the circuit breaker could be a fusible wire. Although the above discussion centered upon a oneshot breaker, it is easily seen that a solenoidal circuit breaker or other style of circuit breaker could be used for the secondary breaker in an identical configuration to the solenoid  36  driven primary breaker of FIG. 1, and, in fact, this secondary breaker could be resettable. 
     FIG. 4 depicts an embodiment of the present invention wherein certain critical components in the fault detection/interruption circuit are continuously monitored for a failure. When a failure is detected, a secondary circuit breaker is triggered, thereby assuring fail safe operation. In FIG. 4, diode bridge  100  performs full wave rectification to generate a D.C. power supply for the fault detection electronics  30 . The fault detection electronics  30  sense the output of two differential current transformers  102  and  104 . Transformer  102  is used to detect a relatively high resistance leakage path to ground, while transformer  104  is used to detect a relatively low resistance connection between the grounded (neutral) conductor  106  and ground  39 . During normal operation, power supply capacitor  108  holds a constant voltage Vcc. When the fault detection electronics fires the gate  109  of thyristor  110 , thyristor  110  draws current through diode bridge  100  to fire solenoid  112 , thereby causing primary circuit breaker contacts  114  to open. A variety of failure modes in the circuit of FIG. 4 can be detected by monitoring the voltage of the power supply capacitor  108 . For example, if the wires in solenoid  112  are open circuited, or if thyristor  110  fails in a shorted condition, or if capacitor  108  acquires an internal short, then capacitor  108  will acquire a voltage substantially less than the design voltage of Vcc. When this happens, it can cause a failure in the fault detection electronics that would go unnoticed until (possibly) a manual test was performed at some later date. However, with the low voltage detection circuit provided by resistor  120 , transistor  122 , capacitor  116 , zener diode  118  and diode  124 , a secondary thyristor  126  can be triggered, causing a secondary circuit breaker  128  to open. 
     Capacitor  116  is charged by the series combination of diode  124  and resistor  120 . The charge level is limited by zener diode  118  to a value of something less than the desired charge value (Vcc) of capacitor  108 . If, for some reason, the value of voltage across capacitor  108  falls to something less than the breakover voltage of zener diode  118 , then transistor  122  will be turned on, and this, in turn, will cause the firing of thyristor  126  and the opening of the secondary circuit breaker  128 . Accordingly, the embodiment in FIG. 4 provides a level of protection against a failure of the most problematic components in most GFCI circuits. When combined with the test circuit of FIG. 3, this yields a high degree of redundant protection. 
     FIG. 5 depicts one embodiment for a redundant circuit breaker release mechanism. The reset button  130  is rigidly connected to an arm  142  that serves to pull moveable member  144  in a direction so that electrical contacts  140  make electrical connection with leaf spring connectors  138 , thereby closing both sides of a circuit breaker switch. Spring  132  pushes against fixed plate  134  (shown in cut-away) to exert a force on the arm  142  to cause it to pull upward in FIG.  5 . Solenoid  148  is in a fixed position relative to the fixed plate  134 . A spring  146  is attached to solenoid housing  148  on one side and is attached to the collar  152  on the other side. In normal operation, the spring  146  serves to exert a pressure against collar  152 , causing the arm  142  to remain latched beneath moveable member  144 . A plunger  150  within the solenoid housing  148  can move freely within the solenoid housing  148  but is attached on one end to the collar  152 . When the solenoid  148  is energized, it pulls on the plunger  150 , causing the arm to become unlatched and allowing the contacts  140  to separate from  138 , thereby opening the switch. This action describes the latch and release mechanism for the majority of existing fault interrupters. 
     In FIG. 5, a second release mechanism is depicted by the fusible element  136 . This could be, for example, a carbon composition resistor. This fusible element  136  acts as a part of the rigid linkage between the reset button  130  and the arm  142 . If a high electrical current is applied to fusible element  136 , it will break apart. If the fusible element  136  is caused to break apart, it no longer can provide the linkage between the reset button  130  and the arm  142 , in which case, the arm  142  will no longer provide a latching force holding the circuit breaker contacts  140  and  138  together. Accordingly, fusible element  136  represents a one-shot breaker means to opening the circuit breaker contacts and this means is independent of the primary means which employs the solenoid  148 . Even though the fusible element  136  does not control a second independent circuit breaker, it represents a second, independent means to release a primary circuit breaker. 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. While discussion has been directed to providing robustness in a ground fault current interrupt device, the invention can be applied equally well to arc fault current interrupt devices or other types of electrical safety devices. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.