Abstract:
A low cost, low parts count electronic circuit that is built into an appliance automatically and continuously checks for an open ground condition or a condition where the power conductors are transposed, at which time, power is interrupted in the appliance. By combining this with an electrical leakage detection circuit and power interrupter at the plug, the electrical safety of the appliance is greatly enhanced.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing of US. Provisional Patent Application Ser. No. 60/100,577 entitled “Ground Loss Detection for Electrical Appliances”, filed on Sept. 16, 1998, and the specification thereof is incorporated herein by reference. 
     This application is a continuation in part application of U.S. patent application Ser. No. 08/756,784, entitled “Shock and Arc Protection Device for an Electrical Distribution System”, to Hirsh et al, filed on. Nov. 26, 1996 now U.S. Pat. No. 5,973,896, which is a continuation in part application of Ser. No. 08/653,943 now U.S. Pat. No. 5,844,759, entitled “Electrical Fault Interrupter” to Hirsh et al, that was filed on May 22, 1996 and issued on Dec. 1, 1998, and also 08/799,919 now of U.S. Pat. No. 5,943,198 entitled “Electrical Fault Interrupt Circuits” to Hirsh et al, that issued on Aug. 24, 1999 and was filed on Feb. 13, 1997 as a continuation of Ser. No. 08/453,664 filed on May 26, 1995 now abandoned, the teachings of all of which are incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 Block diagram of ground detection circuitry 
     FIG. 2 Specific embodiment of ground detect circuit 
     FIG. 3 Ground detect circuit using sensitive gate triac 
     FIG. 4 Ground detect circuit with low current on/off switch 
     FIG. 5 SCR based ground detect circuit with low current, low voltage on/off switch 
     FIG. 6 G round detect circuit with indicator 
     FIG. 7 Ground detect circuit that can be used with shock prevention circuit 
     FIG. 8 Deadzone in the AC current during which a fault can be detected 
     FIG. 9 Alternative ground detect circuit that can be used with shock prevention circuit 
     FIG. 10 Shock protection combined with open ground detection 
    
    
     LIST OF REFERENCE NUMERALS 
       20 —Plug 
       21 —Ground prong on plug 
       22 —Electrical outlet 
       23 —Neutral prong on plug 
       24 —Hot conductor 
       25 —Hot prong on plug 
       26 —Neutral conductor 
       28 —Ground conductor 
       29 —Earth ground 
       30 —Load 
       32 —Appliance housing 
       34 —Control circuit 
       36 —Triac 
       37 —Sensitive gate triac 
       38 —Triac gate 
       39 —SCR gate 
       40 —Resistor 
       41 —Desensitizing resistor 
       42 —MT 1  terminal of triac 
       43 —Cathode of SCR 
       44 —MT 2  terminal of triac 
       45 —Anode of SCR 
       46 —Low current switch 
       47 —Silicon controlled rectifier 
       48 —Limiting resistor 
       49 —NPN transistor 
       50 —Neon indicator lamp 
       51 —PNP transistor 
       52 —Low current/low voltage switch 
       53 —Steering diode 
       54 —Back to back zener diodes 
       55 —Steering diode 
       56 —Current dead zone 
       57 —Ground current limiting resistor 
       58 —Diac 
       60 —Hot prong 
       62 —Neutral prong 
       64 —Ground prong 
       66 —Hot side triac 
       68 —Diac 
       70 —Charge capacitor 
       72 —Steering diodes 
       74 —NPN Darlington 
       76 —PNP Darlington 
       78 —Resistor 
       80 —Voltage divider 
       82 —Conductor 
       84 —Conductor 
       86 —Hot to ground fault 
       88 —Hot to neutral fault 
       90 —Load to ground fault 
       92 —Node 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 portrays a block diagram of the ground detection circuitry. A plug  20  has three prongs  21 , 23 , 25  on one side that insert into an electrical outlet  22 . By convention, these prongs are configured to correspond to either the ungrounded conductor  24  (also known as the “hot” conductor), the grounded conductor  26  (also known as the “neutral” conductor) or the ground conductor  28 , which, when plug  20  is inserted into outlet  22 , should be electrically connected to earth ground  29 . This connection to earth ground  29  may be at the outlet  22  or at a remotely located distribution panel, transformer, or other location. The load  30  represents an appliance load, for example, the heater coil in an electric heater, the light bulb filament in an electric light, or the motor in a pump. The load  30  may be optionally surrounded by a grounded appliance housing  32 . A control circuit  34  is connected in electrical series between the load  30  and the neutral conductor  26 . The control circuit  34  also makes a connection to the ground conductor  28 . The control circuit  34  serves to monitor the connection to ground through the ground conductor  28 . When this connection is broken, the control circuit responds by inhibiting the flow of electrical current out of the load  30  and to the neutral conductor  26  thereby forcing the appliance into an off condition. 
     FIG. 2 depicts a specific embodiment of the ground detect circuit. In its simplest embodiment, the control  34  consists of a triac  36  electrically connected at the MT 1  terminal  42  to the load  30  and from the MT 2  terminal  44  to the neutral conductor  26 . A triac is a type of thyristor which may be thought of as a latching electrical switch. When the magnitude of the voltage potential at the triac gate  38  exceeds the magnitude of the voltage potential at the MT 1  terminal  42  by more than some characteristic gate turn-on voltage, the triac  36  turns on, allowing electrical current to flow from the load  30  to the neutral conductor  26 . Once the triac  36  is triggered into a conducting state, it continues to conduct electrical current as long as current magnitude is above some minimum threshhold known as the holding current. In this case, the triac  36  is said to be latched and continues to conduct even in the absence of a gate stimulus. If the triac  36  is in a nonconducting state, and the voltage potential at the triac gate  38  remains below the gate turn-on voltage, then the triac  36  will remain in a nonconducting state and little or no electrical current will flow from the load  30  into the neutral conductor  26 . A resistor  40  may be used to connect between the ground conductor  28  and the triac gate  38 . This serves to limit current flow to ground to that amount sufficient to turn on the triac  36  without drawing excessive currents that might damage the triac  36  or exceed accepted standards for ground current. 
     In a properly wired electrical distribution system, the neutral conductor  26  will have a voltage potential very close to that of the earth ground potential. Accordingly, as the voltage potential of the hot conductor  24  cyclically rises above that of the neutral conductor  26  and then falls below that of the neutral conductor  26 , the triac  36  is turned on to allow current flow in first one direction and then the other. The load  30  then receives essentially full power for each half cycle of alternating current excitation and the appliance operates normally. 
     If the electrical connection from the triac gate  38  to the earth ground  29  through resistor  40  and through ground conductor  28  is nonexistent or becomes broken, this corresponds to a loss-of-ground condition. A loss-of-ground condition can occur because the appliance is plugged into an ungrounded electrical outlet  22 , or because the ground conductor  28  is broken or missing, or because the grounding prong on the plug  20  has been cut off or does not seat correctly in the electrical outlet  22 . Under a loss-of-ground condition, the triac gate  38  will maintain a voltage potential very close to that of the potential of the MT 1  terminal  42 , and the triac  36  will remain in a nonconducting state. For an AC excitation, upon the occurrence of a loss-of-ground condition, the triac  36  will allow electrical current to flow for the balance of the AC half cycle. Upon the first subsequent zero crossing of the AC excitation, as the current drops to zero, the triac  36  will turn off and stay off until a subsequent time when the ground connection is restored. Accordingly, the maximum amount of time that will elapse between the occurrence of a loss-of-ground connection and the removal of electrical current from the load is one half cycle. For a 60 Hz electrical system this corresponds to 8.33 milliseconds. The user of the appliance will note the loss-of-ground condition because the appliance will not function. 
     The circuit in FIG. 2 does more than just detect a loss-of-ground condition and interrupt current upon the detection of that event. It also serves to detect the occurrence of transposed hot and neutral conductors. In FIG. 2, if the hot and neutral conductors are transposed, then the conductor  24  that is supposed to be the ungrounded conductor becomes grounded. In this situation, the triac MT 1  terminal  42  will have the same potential as the triac gate terminal  38 . The triac  36  will therefore not conduct and the load  30  will not be. powered. All of the following discussions regarding the detection of a loss-of-ground condition can apply equally well to the detection of transposed power conductors. Because gate resistance  40  limits the electrical current that can be drawn through the triac gate  38 , the load  30  does not receive significant power if there is an open neutral connection. Accordingly, the load  30  will not operate in the case of an open neutral. 
     Although FIG. 2 utilizes a triac  36  to implement ground loss sensing, it would be apparent to one skilled in the art that another type of thyristor such as a silicon controlled rectifier (SCR) might be used or that another type of electronic switch such as a transistor might be used. 
     FIG. 2 portrays the simplest configuration that yields ground loss detection. With only two, readily available components (triac  36  and resistor  40 ), appliances which require proper grounding can benefit from this enhanced safety at a minimal cost to the end user. 
     FIG. 3 shows a ground detect circuit utilizing a sensitive gate triac boost. In this embodiment, an auxiliary sensitive gate triac  37  is added to the circuit in order to obtain minimal current draw from the ground conductor  28 . When a ground conductor  28  is grounded, a very low level current at the gate  39  of the sensitive gate triac  37  will result in a higher level of gate current at the gate  38  of the triac  36 . In this embodiment, the gate current at the triac gate  38  will not be sourced through the ground conductor  28  but will be sourced from the hot conductor  24  through the load  30 , resistor  40  and sensitive gate triac  37 : As in the discussion pertaining to FIG. 2, this circuit will respond in the same way to transposed conductors as it does to a loss-of-ground condition. In FIG. 3, the sensitive gate triac  37  serves as an amplifier. In a similar way, any electronic amplifier might be used to allow the control of the triac  36  by the momentary application of a Very low level ground current. 
     FIG. 4 shows a ground loss detection circuit that uses a transistor based amplifier to control the triac  36  based upon the state of the ground connection. When a ground connection is valid, a low level electrical current is drawn through ground current limiting resistor  57  to turn on one of the transistors  49  or  51 . During negative half cycles of the applied AC source, the NPN transistor  49  is turned on if a ground connection is present, in turn triggering the triac  36 . During positive half cycles of the applied AC source, the PNP transistor  51  is turned on if a ground connection is present, causing triac  36  to fire. Steering diodes  53  and  55  serve to block current flow in a reverse direction on whichever is the inactive transistor during each half cycle. At the beginning of each half cycle, once the triac  36  is turned on, the voltage at the triac gate  38  becomes very small in magnitude relative to the MT 2  terminal  44 . This causes the active transistor, either  49  or  51  to turn off, the result being that extremely low levels of electrical current are drawn through ground using this approach. 
     FIG. 5 shows a ground detection circuit in which a low current switch  46  has been added. Even though the appliance load  30  may require many amperes of electrical current, the on/off control of the current flow may be controlled by the triac  36  by using a low current switch  46 . When the low current switch  46  is in the closed position, the circuit acts as the circuit of FIG. 2, and allows power to go the load as long as a ground connection is present. If the low current switch  46  is in an open position, this serves to remove the connection of the triac gate  38  from a ground potential. The triac  36  will go to a nonconducting state and no power will be furnished to the load. Desensitizing resistor  41  serves to prevent false firings of triac  36  when the switch  46  is in an open position. It does this by effectively forcing the gate  38  voltage to be at the same potential as the MT 1  terminal  42 . The advantage to using the low current switch  46  is that it can be designed for relatively low currents even though it controls (through the triac  36 ) relatively high currents to be sourced to the load  30 . In this way it can replace a more expensive high current switch that might be located in electrical series with the load. 
     Also shown in FIG. 5 is an indicator that can be added to the loss-of-ground detector to indicate when a loss-of-ground condition has occurred. The indicator consists of a neon indicator lamp  50  in electrical series with a limiting resistor  48  and connecting across the MT 1   42  and MT 2   44  terminals of the triac  36 . When the triac is placed into a nonconducting state, as happens with a loss-of-ground condition, then when power is applied to the appliance, the potential difference between the hot conductor  24  and the neutral conductor  26  will appear across the resistor/neon combination, causing the neon  50  to turn on. This neon indicator lamp.  50  serves to notify the user of the appliance of a loss-of-ground condition. When a ground connection is present, the triac  36  conducts, essentially “shorting out” the neon  50  and causing the neon indicator lamp  50  to be in an off condition. 
     FIG. 6 shows a ground detect circuit that uses a low current/low voltage switch  52 . This circuit also uses a type of thyristor called a silicon controlled rectifier or SCR  47  to demonstrate a different approach to the ground loss detection. In this embodiment, the silicon controlled rectifier  37  conducts in one direction only and always prevents electrical current flow from the cathode  43  to the anode  45 . When the low current/low voltage switch  52  is in an open position, the operation of the circuit is similar to that described in conjunction with FIG. 2 except that when the system is correctly wired the load receives power only during the negative going half cycle when the ungrounded conductor  24  has a potential that is less than the grounded conductor  26 . When the low current/low voltage switch  52  is in a closed position, it forces the SCR gate  39  to have the same potential as the SCR cathode  43 . This forces the SCR  47  to be in a nonconducting mode. Accordingly, the role of the switch  52  is to control power to the load if a proper ground connection is present. One advantage to this configuration for power control is that the switch  52  may be a low cost switch designed for low current and low voltage operation. It is low voltage because internal to all SCR&#39;s there is a low impedance connection between the gate  39  and the cathode  43 . Note that in this configuration the load  30  is on when the switch  52  is open and off when the switch  52  is closed. 
     FIG. 7 depicts a loss-of-ground detection circuit that imposes a dead zone in the load current at each zero crossing. In this circuit, back to back zener diodes  54  that are in electrical series with the triac gate  38 , serve to induce the dead zone. The dead zone occurs because the back to back zeners  54  will block any current flow while the voltage magnitude across the back to back zeners  54  is less than the zener voltage. Gate current is allowed to flow when the voltage magnitude exceeds the zener voltage plus a diode drop. When the gate current is allowed to flow through the back to back zener diodes  54 , it triggers the triac  36 , thus providing power to the load. Once the triac  36  turns on or “fires”, the voltage between the triac gate terminal  38  and the ground conductor  28  will drop to less than about 2 volts thus causing the back to back zener diodes  54  to turn off and cease conducting. As in the discussion relating to FIG. 2, if a loss-of-ground condition occurs, the triac  36  will cease to be fired, thus turning off the appliance. Accordingly, the circuit in FIG. 7 gives loss-of-ground detection and can detect transposed hot and neutral conductors in addition to imposing a dead zone at zero crossings. 
     The circuit in FIG. 7 may be desireable because in some electrical systems, the ground conductor  28  and neutral conductor  26  may have a potential difference of a few volts. Since the back to back zener diodes  54  allow current flow only so long as their characteristic zener voltage is exceeded, after the triac  36  is fired, there is no ground current drawn through ground conductor  28  until the next half cycle. Accordingly, only brief pulses of electrical current are drawn from ground to fire the triac  36 . 
     The FIG. 7 implementation is particularly helpful if the appliance with ground assurance is to be used in systems having a ground fault interrupt or GFCI. Such systems monitor ground currents and trip a circuit breaker if those currents exceed a threshhold level, typically 5 milliamperes. With the FIG. 7 implementation, the brief pulses that are used to fire triac  36  are not of sufficient duration or overall energy content to impact most GFCI circuits. 
     FIG. 8 depicts the current flow vs. time, through the load  30 , using the circuit in FIG.  7 . In FIG. 8 the load current is seen to follow an essentially sinusoidal AC profile except for a brief dead zone  56  just after each zero crossing. 
     Using the circuit in FIG. 7, during the times of a dead zone (when current flow through the triac  36  is essentially zero), any current flow from the hot conductor  24  will be indicative of an unintentional path around the triac  36 . This unintentional path may represent a potentially dangerous situation such as a ground fault. This information can be used by a shock protection sensing circuitry in the plug to give shock protection in addition to open ground detection and transposed conductor detection. This is discussed in conjunction with FIG.  10 . 
     FIG. 9 depicts an alternative loss-of-ground detection circuit in which the dead zone for detecting an electrical leakage is implemented by adding a voltage triggered bilateral silicon device (diac)  58  in series with the triac gate  38 . When there are low magnitude voltages across it, a diac will block current flow. When the applied voltage exceeds a characteristic voltage magnitude, in either polarity, the diac will act like a short and will allow gate current to flow between ground and triac gate  38 . Once the triac “fires”, the voltage between the triac gate terminal  38  and the ground conductor  28  becomes small and the diac  58  turns off. The circuits in FIG.  7  and FIG. 9 operate identically. 
     When used with the plug mounted fault interruption device depicted in FIG. 10, either of the circuits in FIG.  7  and FIG. 9 can serve as part of an electrical safety system that not only detects an open ground condition or a miswired appliance but also protects against injury due to ground faults. In FIG. 10, the hot prong  25 , neutral prong  23  and ground prong  21  represent the three prongs that would be plugged into an outlet. The plug  20  has a hot side triac  66  that controls power out of the plug  20  and to the load  30 . A hot to neutral fault  88 , hot to ground fault  86  and load to ground fault  90  are depicted with dashed lines to indicate that these connections are not made by design, but, represent different potentially dangerous occurrences that can lead to hazardous electrical leakages (faults). In FIG. 10, any or all of these conditions can be recognized by the electronics encased within.the plug  20 . 
     In FIG. 10, on each half cycle, in the absence the faults  86 ,  88 , or  90 , the resistor  78  serves to charge capacitor  70  until a voltage in excess of the turn-on voltage for the diac  68  is obtained. At that time, the diac fires, causing the hot side triac  66  to turn on and supplying power to the load  30  through conductor  82  for the balance of that half cycle. At the beginning of each half cycle, the hot side triac  66  turns off as the current through it makes a zero crossing. This triac  66  remains in an off condition until the capacitor  70  charges sufficiently. If, however, the hot to ground fault  86  is present, then during this off time, electrical current will flow from the hot prong  60 , through the voltage divider resistors  80 , through the hot to ground fault  86  to ground. If the hot to ground fault  86  is sufficiently low in resistance value, the current flow will be sufficient so that the voltage at node  92  will be high enough to trigger one of the Darlington transistors  74  or  76 . This will cause the Darlington transistors to conduct, thereby discharging the capacitor  70  and preventing it from firing the hot side triac  66 . The result is that for the balance of the half cycle, current will be inhibited from flowing to the load. A similar analysis can be used to show that a hot to neutral fault  88  or a load to ground fault  90  would result in the hot side triac  66  being turned off. 
     In practice, the resistors  80  would be chosen to be of sufficiently high. resistance that current flow through them would be at a very low level. Two Darlington transistors are necessary because of the symmetry of the circuit. Alternating half cycles will be positive and then negative. For positive half cycles, the Darlington PNP transistor  76  can be activated to inhibit triac  66  tun-on. For negative half cycles, the Darlington NPN transistor  74  can be activated. The steering diodes  72  serve to select the active transistor during each half cycle. The dead zone provided by the control  34  is necessary to the workings of the fault detection circuitry in the plug  20 . At the beginning of every half cycle, the electronic circuit in the plug  20  is looking for a significant leakage before it turns on the triac  66 . If the back to back zener diodes  54  were replaced by a resistor, there would be no dead zone in current flow through the load  30  and to the electronics in the plug  20 , the load  30  would look like a fault. 
     Adding the ground assurance circuitry of the present invention to fault detection circuitry can enhance appliance safety. In particular, the fact that the ground assurance circuitry also can recognize transposed conductors gives a measure of safety. For example, if the hot and neutral conductors are transposed so that conductor  84  has a hot potential referenced to ground  28 , then the triac  36  will not turn on. Accordingly, fault  88  will not be dangerous because the triac  36  will not conduct. Faults  86  and  90  will not be dangerous because in a transposed system they are located between neutral and ground and there is little voltage between neutral and ground potentials. 
     It should be recognized that although the above discussion has been directed at having ground detection circuitry in an appliance and optionally, ground fault protection at the plug, the technology can be applied to any combination of source and load within an electrical distribution system. For example, the ground detection circuitry could be at an electrical outlet, in which case, the outlet and whatever was plugged into it would serve as the load. The fault detection/interruption portion of the system could be located at a remotely located distribution panel. One of the key advantages of the present invention is that it uses conventional wiring schemes without the need for adding additional wiring, either between plug and appliance load, or between an arbitrary source location and load location. 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. 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 teachings of all applications, articles, patents and other references mentioned above are herein incorporated by reference.