Patent Publication Number: US-2010118452-A1

Title: Safety device for prevention of electrical shocks

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Patent Application No. 60/528,067, entitled “Safety Device to Protect Against Shock from Improper Electrical Connections in Electrical Outlets,” and filed Dec. 10, 2003. The subject matter of U.S. Provisional Patent Application No. 60/528,067 is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of electrical devices, and more specifically to safety devices that prevent electrical shock from improperly wired electrical outlets. 
     BACKGROUND 
     An electrical appliance or load receives electrical energy from one terminal of an electrical outlet or source (the so-called high voltage or hot side), electrical current flows to the load through an electrical conductor (a wire), this current passes through the load and is then returned to another terminal of the electrical outlet through another wire called the neutral wire. The two wires that connect source and load may have a coating of rubber or some other electrical insulating material or they may be bare, in which case air, which is a good insulator, functions to inhibit electrical current flow outside of the wire. Since the human body can conduct the flow of electrical current, if a person comes into contact with one electrified object, such as the so-called hot wire in an electrical system, while also making contact with a second object having a substantially different voltage, then an electrical leakage current that is proportional to the voltage difference will flow through the person and may cause injury or death. If the second object that the person comes in contact with is electrically connected to the earth (“ground”) then this is called a ground fault. 
     Electrical current is the flow of electrons. Electrons are neither created nor destroyed so any functioning electrical appliance will require both an entry path for electrons and an exit path for electrons in order for electrical current to flow. For most household appliances that operate from a plug, electrons will sometimes enter path one and exit path two and sometimes enter path two and exit path one. This is known as alternating current or AC operation. 
     Although the two conductors coming out of an AC power source are often designated as “hot” and “neutral”, in an AC system, the hot conductor will cyclically have a more positive voltage than the neutral for half the time and will cyclically have a more negative voltage than neutral for half the time, having a momentary value of zero (the so-called “zero crossing”) each time the voltage passes from positive to negative or from negative to positive. 
     Any electrical appliance that plugs into a wall outlet can be the source of electrical shock, electrical burns, and possibly electrocution. Worn or damaged appliances or appliance cords may come in contact with a conducting surface that is accessible to the user, thus presenting a hazardous condition. For example, if the rubber insulation on a wire within an appliance is worn, then the exposed metal wire strands might touch the appliance housing. If the appliance housing were made out of metal or a similar electrical conducting material then a shock hazard could exist. 
     In order to minimize the potential hazards intrinsic to electrical appliances, government and industry standards serve to regulate the way in which appliances are built and used. One outcome is that many classes of electrical appliances are required to have a grounded connection over and above the standard two electrically conductive wires that supply electrical power to the appliance. A grounded electrical cord is recognizable because it has a plug with three prongs. In the U.S., in a 120 volt (sometime referred to nominally as 110 volts, 115 volts, or 120 volts) single phase system, two of the prongs on a grounded plug are flat and the third, ground prong, is generally rounded. 
     To achieve electrical protection, the grounding line will connect to ground at the plug where it is plugged into a grounded outlet. The other end of the grounding line, within the cordset, will connect to the appliance housing or other exposed or potentially exposable metal parts. If an electrically hot (that is, having a voltage potential that is significantly different from a ground potential) conductor comes into contact with a grounded conductor, the grounded conductor will present a low resistance path to ground, causing the hot conductor to maintain a voltage at or near a ground potential. If the current drawn from an electrically hot source, through ground, exceeds the “trip” current of an in-line circuit breaker, the circuit breaker will open, removing power from the outlet that the appliance is plugged into, thereby protecting the electrical distribution system from over-current damage and protecting the user from electrical shocks. 
     If the ground connection on a grounded appliance is removed, the safety of the appliance is compromised. This removal of ground condition can occur through abuse or damage but more commonly occurs when the appliance is plugged into an outlet in which the ground has not been connected or is improperly wired. The removal of ground may also occur if the grounding prong is cut off of the plug or is otherwise disconnected. An open ground condition often goes unnoticed by the casual user. This is because most appliances will operate normally without a ground connection. 
     In the U.S., agencies such as the National Fire Protection Association (NFPA) and the Occupational Safety and Health Association (OSHA) maintain regulations governing the use of grounded appliances. For many situations, the regulations require that a qualified technician periodically test the continuity of the ground connection between appliance and the grounding prong and between grounding prong and actual ground. 
     One drawback to the periodic test approach is that a significant period of time may elapse between inspections. In many situations the inspection schedule may not be adhered to or may be completely ignored. Also, malfunctions, particularly if intermittent, may go undetected. Often a complete inspection of the grounding impedance between actual ground is not made. Accordingly, there are many situations where a user may assume he is using a grounded appliance when he is not. 
     Another potentially hazardous operating condition occurs when the hot and the neutral conductors are switched at an outlet. The typical appliance that is plugged into an outlet with these so-called “transposed” conductors will still operate. However, the design of many appliances is such as to favor having one of the two power carrying conductors designated as the neutral conductor. For example, in a droplight, the socket for an incandescent light bulb will have the outlet shell as neutral with the base connected to the hot conductor. Then if a user accidentally makes contact with the exposed outer shell, electrical shock is less likely to result. The neutral and hot conductors are distinguished by using a polarized plug wherein one of the two blades is designated as neutral and is physically wider. If a droplight is plugged into a polarized outlet but the hot and neutral conductors in that outlet are transposed, the safety associated with using a polarized plug has been compromised. In such situations, it is important to alert the user. 
     In the prior art, U.S. Pat. No. 3,697,808 (Lee) discloses a system for monitoring chassis potential and ground continuity by injecting a radio frequency (RF) signal into the neutral lead and monitoring the signal amount that is capacitively coupled to ground. A major limitation of this approach is that it is electronically complicated and is sensitive to leakages through capacitances in the power cable and elsewhere. 
     U.S. Pat. No. 3,809,961 (Kershaw) discloses an electrical outlet sentinel that detects an open ground condition in an electrical outlet and opens a mechanical circuit breaker thereby removing power from the outlet. A major objection to this design is that power from the outlet is controlled by a circuit breaker whose relay is energized through power running continuously through ground. For any practical relay, this represents a substantial continuous ground current and is likely to disrupt the correct operation of any ground fault circuit interrupters that are upstream in the electrical distribution system. 
     U.S. Pat. No. 3,996,496 (Volk) discloses a ground integrity monitor that relies on the application of short electrical pulses between the neutral and ground terminals. If the resistance between neutral and ground is less than a threshold amount, this pulsing purportedly causes a photo coupler to be activated, providing power to the load. The system is electronically complicated and this compromises the appeal of the design. 
     U.S. Pat. No. 4,598,331 (Legatti) discloses a ground fault interrupt circuit in which open ground conditions create actuation of the interrupter. An open ground produces current flow through a supplemental secondary winding that, in turn, induces a trip signal. When implemented, this approach does not check for a good connection going forward to the appliance, but only going back to the outlet. If, for example, the grounding conductor on the appliance cordset is cut, thereby leaving the appliance ungrounded, this approach will not detect that condition. 
     U.S. Pat. No. 4,649,454 (Winterton, et al.) discloses an open ground detection circuit for appliances that utilizes a special plug with four electrical connections. Two of these are the standard hot and neutral prongs. A third prong, having the location and shape of a conventional ground prong, has two electrical conductors (the so-called grounding blade and sensing blade) that are separated by an insulating member. Four wires connect this special plug to a housing that is located within an appliance. When the special plug is correctly seated in a three-hole grounded outlet, the two blades on the third prong are electrically connected together. If, however, the two conductors on the third prong are not electrically connected together, then an alarm signals that an open ground condition exists. A major problem with this design is that it only checks to see whether grounding blade and sensing blade are shorted together. This can occur even if the plug is connected to an ungrounded outlet and the user would incorrectly assume that the appliance was grounded. Another problem with this design is that it requires a special plug and four wires to connect between the appliance and this special plug. 
     U.S. Pat. No. 4,931,893 (Glennon) discloses a circuit that detects a loss of ground condition in an electrical system by using a capacitive circuit that discharges through the ground. The disclosed embodiments are electronically complex and can generate relatively high ground currents which would cause nuisance tripping in distribution systems having ground fault interrupters. 
     U.S. Pat. Nos. 5,844,795 and 5,943,198 and U.S. patent application Ser. No. 08/756,784, all to Hirsch et al., describe a solid state ground and arc fault detection and interruption technology that has two parts, one part which resides in the load and one part which resides in the source. In applying this to an electrical appliance, the load is the appliance and the source is the plug. The basic theory in this technology is that a load conditioning module in the appliance injects a deadzone in the current flow during each half wave AC cycle. A sensing circuit in the plug looks for the presence of that deadzone each half cycle. If there is leakage around the load conditioning module (indicating a ground fault or arcing fault) this is indicative of a potential dangerous condition and current flow is interrupted at the plug. 
     To address these representative deficiencies in the art, what is needed is an improved capability for controlling electrical power to an appliance in a manner that provides safety to people that may come in contact with the appliance. 
     SUMMARY OF THE INVENTION 
     The present invention supports controlling electrical power on a supply line to an electrical appliance, such as a vending machine, tool, machine, equipment, motor, or other device or load that consumes electrical energy, to protect against electrically shocking people that may contact the appliance. 
     In one aspect of the present invention, a safety device that helps prevent electrical shock can comprise voltage sensing circuits that monitor voltage levels among or between a power conductor, a neutral conductor, and a ground conductor. When one or more of the sensed voltages indicates that these conductors are properly wired to an electrical power utility, inline switches can close to allow power to pass through the safety device to the appliance. When one or more of the sensed voltages indicates that the power level is too low to support properly operating the appliance, one or more switches of the safety device can open to block power from passing through to the appliance. When one or more of the sensed voltages indicates that an electrical condition in the electrical appliance poses a shock hazard or threat, one or more switches of the safety device can open to interrupt power from transmitting to the appliance. 
     In another aspect of the present invention, the safety device can provide shock protection for vending machines that operate in public places. Upon occurrence of an event that poses a safety threat or issue, the safety device can isolate the vending machine from the electrical utility while keeping the connection to earth ground intact. Electrically isolating the vending machine can comprise opening the neutral conductor and the power conductor, which may also be referred to as a hot conductor or lead. 
     The discussion of providing protection against electrical shock presented in this summary is for illustrative purposes only. Various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of exemplary embodiments of the present invention. Moreover, in the drawings, reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is an electrical schematic of an exemplary electrical safety device that provides protection against electrical shocks in accordance with an exemplary embodiment of the present invention. 
         FIGS. 2A and 2B  are electrical schematics of exemplary circuits for an electrical safety device in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The present invention supports providing electrical safety protection for an appliance by monitoring voltage levels on conductors that feed electrical power to the appliance and actuating switches when the voltage levels indicate the presence of an electrical safety issue. 
     This invention can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those having ordinary skill in the art. Furthermore, all “examples” given herein are intended to be non-limiting, and among others supported by exemplary embodiments of the present invention. 
     The present invention can comprise a process or method related to providing electrical safety protection. Certain steps in any exemplary processes or methods described herein must naturally precede others for the present invention to function as described. However, the present invention is not limited to the order of the steps described if such order or sequence does not alter the functionality of the present invention. That is, it is recognized that some steps may be performed before or after other steps or in parallel with other steps without departing from the scope and spirit of the present invention. 
     Turning now to  FIGS. 1 and 2 , circuitry for an exemplary electrical safety device in accordance with an embodiment of the present invention will be described with reference to these figures.  FIG. 1  illustrates a schematic of an exemplary safety device  100  that withholds electric power from an appliance, such as a tool or electrical machine, if the electric outlet  108  that it is plugged into is improperly wired.  FIGS. 2A and 2B  illustrate schematics for exemplary circuits of the safety device  100 . 
     The safety device  100  plugs into an electric outlet, typically wall mounted, and an appliance connects to the safety device  100 . In one exemplary embodiment of the present invention, the safety device  100  provides power protection to a vending machine (not illustrated). The safety device  100  can be mounted on or in such a vending machine. That is, the vending machine and the safety device  100  can share a common housing. Alternatively, the safety device  100  can be external to a vending machine. 
     The basic safety device  100  can be self-contained and designed to plug into an electric outlet with the appliance receiving its electrical power through the safety device  100 . The safety device  100  can be hard-wired to the appliance or to a receptacle that mates with a power cable. The safety device  100  can provide an essentially equivalent level of protection against shock independent of its mounting configuration. 
     If the safety device  100  is configured in a plug-in format, the safety protection can be bypassed by simply unplugging the appliance from the safety device  100  and plugging the appliance directly into the electric outlet. Thus, when used on large electrical appliances, the safety device  100  is preferably hard-wired in the electric cable between the electric outlet and the operating mechanism of the machine. For example, the device can be mounted inside the machine so that users can not be tamper with it. A flashing light and remote audio alarm  120  can provide an indication of the state of the safety device  100 . That is, an indicator can provide a visible or audible signal that indicates whether power is properly or improperly wired to the appliance. 
     In a nonpermanent installation, the safety device  100  can also be used as the outlet box on an extension cord. That is, the safety device  100  can comprise an outlet that mates with a power cord of a tool or other appliance. For example, hand-held power tools can plug into the safety device  100 , thereby protecting users from shock. Again, if the electric outlet that the safety box plugs into has been improperly wired, the tools will not receive any power. That is, the safety device  110  prevents electrical power from transferring to the tool if the electrical wiring that feeds the tool is miswired. 
     The safety device can comprise two or more indicator lights  128 . One of the indicator lights  128  can emit red light as an indication that the power feed lines  102 ,  104 ,  106  to the appliance are properly wired. Another indicator light  128  can emit green light to indicate that the appliance is properly wired or improperly configured. 
     If the voltages on the electric receptacle are the proper values when the appliance is plugged in, the safety device  100  will allow power to transmit to the appliance. If the voltages are incorrect, the safety device  100  blocks power from transmitting to the appliance. A user or operator of an appliance connected to the safety device  100  will notice the power interruption as a motivation to troubleshoot the problem and correct the wiring error. That is, the electrical wiring problem will need to be corrected before a person or member of the public can operate the appliance. 
     In addition to providing protection for connecting power to an appliance, the safety device  100  continuously monitors the voltages on the power feed lines  102 ,  104 ,  106 . That is, the safety device  100  provides power protection after the appliance tool begins operating. If any of the measured voltages become incorrect, the safety device  100  will remove power from the appliance. Thus, in one exemplary embodiment, the safety device  100  is not limited to checking power when the appliance is initially plugged in to an outlet. The safety device&#39;s continuous monitoring capabilities can protect against a wiring change of the outlet that occurs subsequent to initially connecting electrical service to the appliance. Thus, the safety device  100  can provide on-line, continuous monitoring of the electrical service to an appliance. This feature is useful for vending machines that operate in public places, for example. 
     In one exemplary embodiment, the safety device  100  comprises a differential current detector. If the hot lead  106 , which may also be referred to as the power conductor  106 , into the appliance comes into electrical contact with the metal frame of the appliance, a separate safety switch opens up, removing the power. The voltage remains off until the differential switch is reset. This feature is also continuously monitoring the circuit. 
     These features reduce the chance that a person will be shocked or injured by electricity on an appliance equipped with the safety device  100 . 
     As discussed above, in one exemplary embodiment of the present invention, the safety device  100  provides electrical safety protection for a vending machine, such as a soft drink vending machine that dispenses cold drinks in aluminum cans. Operation of the safety device  100  in an exemplary vending machine application will now be discussed in further detail below. Those skilled in the art should be able to apply electrical safety protection in accordance with the present invention to a wide variety of tools, equipment, appliances, and machinery applications following this discussion, the drawings, and the remaining disclosure. For example, the safety device  100  can provide electrical protection to a domestic refrigerator or freezer. 
     Referring to  FIG. 1 , the safety device  100  connects to a wall outlet or power socket (not shown) via a polarized plug  108 . The plug  108  can have three prongs, one for the power line  106 , one for the neutral line  104 , and one for the ground line  102 . These three lines or power conductors  102 ,  104 ,  106  supply electrical power to the vending machine (not shown). The electrical power, which is typically 120 volts, single phase alternating current, passes through the safety device  100  on its path to the vending machine. 
     The safety device  100  blocks, interrupts, or does not allow power to pass through to the vending machine if the electric outlet is miswired. Further, the safety device can isolate the vending machine from the electrical utility while maintaining a connection to earth ground. The vending machine simply will not turn on or operate if the electric supply source is not properly wired. 
     The safety device  100  comprises three voltage detection circuits  116 ,  114 ,  130  one current differential circuit  108 , and one alarm circuit  120 . The three voltage detection circuits  114 ,  116 ,  130  monitor, detect, or look for levels of voltage difference among the power or hot conductor  106 , the neutral conductor  104 , and the ground conductor  102 . Each of these detection circuits  114 ,  116 ,  130  can monitor the respective voltage levels as compared to a threshold voltage, which can be a minimal, trickle, or essentially zero voltage. 
     The first voltage detection circuit  116  monitors the voltage difference between the power conductor  106  and the neutral conductor  104 . The second voltage detection circuit  114  monitors the voltage difference between the power conductor and the ground conductor  102 . The ground conductor  102  typically comprises an equipment ground and is electrically connected to or comprises the frame of the vending machine. The third voltage detection circuit  130  comprises a low voltage detector  132  that monitors the magnitude of the voltage on power and neutral conductors  106 ,  104 . 
     If the proper voltages are present on the electrical service lines  102 ,  104 ,  106  when the vending machine is first plugged into the outlet, the control switches  118 ,  120  close and power is allowed to pass through to the current differential circuit  108  and on to the operating mechanisms of the vending machine. In response to this power, the vending machine&#39;s lights will come on and its cooling compressor will start cooling. 
     If an improper voltage appears at any of the detection circuits  114 ,  116 ,  130  when the vending machine is first plugged in to the outlet, the control switches  118 ,  120  remain open and the safety device blocks voltage from passing through to the vending machine. Thus, if the wiring configuration is backwards or otherwise wrong, the vending machine does not receive power and does not comes on. Having a vending machine that is in a state of non-operation typically motivates a technician or other service personnel to inspect the electric outlet and address the problem. 
     Once the vending machine is plugged in and is operating properly, the safety circuit continuously monitors the supply voltages. If any supply voltage changes, the control switches  122 ,  128  open, thus removing electrical power from the vending machine. 
     If the hot line  106  in the electric outlet becomes switched to the safety ground contact, then one of the voltage detectors  1 ,  2  will trigger an alarm circuit  120 . A strobe light mounted on the exterior of the vending machine will start flashing. In addition, a wailing audible alarm will sound off. This notifies people not to touch the vending machine. 
     In one exemplary embodiment of the present invention, the safety device  100  outputs an audible warning message, for example “Do not touch this vending machine,” via a digital recording coupled to a speech synthesis chip or similar device. 
     While a flashing red light may provide an adequate warning of a safety hazard to an adult, young children may not understand the significance of the light. A young child might even be drawn to a siren or flashing light. However, even a young child can understand a warning sentence, phrase, or word delivered in a verbal or spoken format. Such digitally synthesized speech can have a harsh tone or other inflection that children can recognize and appreciate. 
     In addition, the current differential detector  108  continuously monitors the current of both the power conductor  106  and the neutral conductor  104 , once the vending machine is functional. When the power current equals to, or is within a threshold of, the neutral current, the vending machine operates in a normal manner. However, if the current through the one of these conductors  104 ,  106  becomes a certain amount more than the current on other conductor  104 ,  106 , the current differential monitor  108  opens up the power conductor  106  to the vending machine. The line remains open until the differential detector  108  is reset. 
     A properly wired electric outlet that provides  120  volt (nominal) single phase alternating current (AC) will typically provide approximately 125 volts AC between the power conductor  106  and the neutral conductor  104 . The voltage detector  1  of the circuit  116  will detect this voltage and close the switches  118  and  120  (switches S- 1 ) upon determining that the voltage indicates a proper wiring configuration. In a properly wired electric outlet, there will also be approximately 125 volts AC between the power conductor  106  and the ground conductor  102 . The voltage detector  2  of the circuit  114  will detect this voltage and will close the switch S- 2   126 . 
     The closure of the switch  116 , the switch  120 , and the switch S- 2   126  puts a voltage on relay K- 4   116 , which energizes and closes both switches S- 4 - 1   118  and S- 4 - 2   118 . This action allows power to pass through the safety device  100  to the vending machine and its internal circuits. In operation, voltage detector  1  supplies  12  VDC to operate relay K- 4  and voltage detector  2  closes switch  126 , thereby energizing relay K- 4  to pull in the switch  118  and the switch  120 . That is, the twelve volts from voltage detector  1  and switch  126  must be in the “on” or closed state at the same time to allow electrical power to pass to the vending machine. 
     Thus, the safety device  100  prevents operation of the vending machine if the equipment ground wire is open or otherwise provides an inadequate level of grounding. In many circumstances, an improperly grounded vending machine (not shown) might continue to operate in a dangerous state. Since many vending machines installations predate government regulations that mandate electrical grounding, the safety device  100  can help prevent electrical injury associated with these older machines. 
     The equipment ground conductor  102  is connected directly to the metal frame of the vending machine. Its purpose is to keep the frame of the machine at “earth ground” potential to prevent electrical shock. It does this by tying the frame all the way back to a ground buss on the main breaker panel with a conductive path that may be referred to as one “continuous wire.” 
     Thus, if the power conductor  106  accidentally comes into electrical contact with the metal frame inside of the vending machine, the ground circuit provides an electrical path back to the electrical system ground. This ground fault current should trigger the circuit breaker to trip, if and when that current reaches the breaker rating. Such arrangement is specified by the National Electric Code (NEC). 
     The NEC specification are generally viewed as providing an adequate level of protection for the electrical conditions that may damage electrical wires. However, an added level of protection, as provided by the safety device  100 , can help prevent electrical shock to people that may inadvertently come in electrical contact with a vending machine having improper wiring or a fault or other potentially hazardous condition. 
     The safety device  100  can immediately detect an open equipment ground, issue a warning, and interrupt electrical service to a vending machine. When the equipment ground opens, the voltage detection circuit  114  does not close the switch S- 2   114 . When the switch S- 2   114  opens, relay K- 4   116  drops out and power is removed from the vending machine. Power remains disconnected until the electric outlet is rewired correctly. 
     Detection circuit  114  and detection circuit  116  provide a level of protection against shock that covers the majority of the wiring problems that are associated with electrical outlets. However, in one exemplary embodiment, the safety device  100  comprises two additional circuits  130   108  that provide an additional level of protection against shock hazards. 
     The low voltage detector  132  and its associated circuit  130  provide low-voltage protection. That is, the low voltage circuit  130  interrupts power to the vending machine if the voltage drops to an unacceptably low level. Under such a low voltage condition, electric motors can draw excessive current, heat up, and burn out. The voltage detector  132  opens the switches  118 ,  120  of the control circuit, relay K- 4   116 , when the hot-to-neutral voltage falls below a predetermined value or threshold. The voltage detector circuit  116  can be an optional feature of the safety device  100  for protecting electric equipment with motors and/or compressors that are sensitive to low-voltage damage. 
     The second extra protective device is the differential current monitor  108 , which can be a built-in ground fault detector. If an un-insulated section of the power conductor  106  accidentally touches the metal frame of the machine, some current will flow through the equipment ground back to the ground buss. In this situation, the power conductor  106  will carry essentially all of the incoming current and the return current will be divided between the neutral conductor  104  and the ground conductor  102 . The differential current detector  108 , which can be referred to as a ground fault indicator (GFI), senses this difference in current and trips, thereby opening up the hot lead  106  and the neutral lead  104  to the vending machine. In comparison to opening only the hot lead  106 , opening both the hot lead  106  and the neutral lead  104  offers an added level of safety protection. Thus, the safety device  100  can electrically isolate a vending machine from the electrical utility, while keeping the earth ground intact, when a ground fault occurs. In the ground fault situation, switch  122  and switch  124  remain open until reset and the ground fault is cleared. If the problem is not corrected, the differential current detector  108  will trip again as soon as it is reset. That is, the ground fault switches  122 ,  125  will continue tripping until the leakage path to ground is removed. 
     As will be appreciated by those skilled in the art, the level of safety protection that an individual GFI circuit provides can be inadequate in certain circumstances. However, including the GFI circuit in the safety device  100  can provide an enhanced level of shock protection. Without the safety device, a GFI circuit can protect against a hot wire contact to the frame with the equipment ground open. However, such an individual GFI circuit can fail to provide adequate protection in a scenario in which the safety wire was open and the hot lead touches the metal frame. In this situation, the individual GFI will not trip open as there is no return path for the current. If a person touches the machine under these conditions and completes the circuit to the earth ground, the GFI should trip within one half of a cycle. In contrast, the safety device  100  can recognize this situation and open switch  118  and switch  120 , thereby removing power from the vending machine entirely without needing a conductive path through the person. 
     The safety device  100  can further comprise a surge suppression device (not illustrated) that benefits the operation of the GFI  108 . Some GFI&#39;s are prone to tripping on conditions unrelated to local ground faults. When an electrical transient occurs on a power distribution network, power spikes can propagate to the safety device  100  and cause the GFI  108  to trip. For example, a ground fault at a remote location on the power grid can send a voltage spike on a hot line that can travel to the safety device&#39;s GFI. The GFI  108  can misinterpret this spike as a ground fault. A surge suppressor, either integral to the safety device  100  or externally connected to the power utility in the vicinity of the safety device  100 , can suppress such voltage or current spikes or transients. The surge suppressor can operate by shunting the spike to ground, for example. A silicon avalanche photodiode (SAD) is one exemplary type of surge suppressor. Metal oxide varistors are another exemplary form of surge suppressor that can be used with the safety device  100 . 
     The safety device  100  has protection against faulty wiring in the electric outlet, against an open equipment ground between the receptacle and the main circuit breaker panel, and against a high leakage path to the frame within the vending machine. Further, the safety device  100  can be installed without modifying the electrical receptacle or the vending machine. In one exemplary embodiment, vending machines can be retrofitted with safety devices  100 , for example external to the vending machine frame or case. As discussed above, the safety device  100  can be used with a variety of household devices as a plug-in safety box. 
     Components of the safety device will now be discussed. In an exemplary embodiment, the safety device  100  comprises a supply cable with a nominal plug  108 , a container box  110 , and internal parts. When used with vending machines, an external flashing light and audible alarm  120  can be wired into the circuit but be mounted directly on the vending machine. That is, the safety device  100  can comprise a tether, extension, or umbilical cord that leads to an alarm  120  that can be situated on a vending machine or another prominent location. 
     The power cable can have a standard 3-prong male plug. The larger flat prong connects to the neutral conductor  104 . The smaller flat prong connects to the hot conductor  106 . The rounded longer prong connects to the equipment ground conductor  102 . The cable can comprise three stranded wires or leads, individually insulated, encased in an insulating cover and capable of carrying the current load of the machine. The cable can be sized for compatibility with a circuit breaker, for example a 20-amp breaker. 
     The cable enters one end of the safety box  110  and is secured to the box&#39;s frame with a clamp (not shown). The individual leads connect to the appropriate points on a control board (not shown) that holds the safety device&#39;s various electrical components. The hot lead  106  connects to terminal H 1   134 . The neutral lead connects to terminal N 1   136 . The equipment ground lead connects to terminal G 1   138 . The control board circuits connect to these points  134 ,  136 ,  138 . 
     The voltage detection circuit  116  between H 1   134  and N 1   136  resides on the control board. The detection circuit  116  can comprise a simple 120/12 volts alternating current (VAC) transformer, with a bridge rectifier to convert the low AC voltage to a direct current (DC) voltage. The DC voltage energizes K- 1  relay  116  and closes switch S- 1   118 ,  120  when sensing voltages indicative of correct wiring.  FIG. 2A  illustrates an exemplary circuit diagram or schematic of the circuitry associated with the voltage detector  1 . 
     The control board further carries the voltage detection circuit  114  between H 1   134  and G 1   138 . The detection circuit  114  can comprise a simple 120/12 VAC transformer, with a bridge rectifier to convert the low AC voltage to DC voltage. The DC voltage energizes K- 2  relay  114  and closes switch S- 2  when sensing voltages indicative of correct wiring.  FIG. 2B  illustrates an exemplary circuit diagram or schematic of the circuitry associated with the voltage detector  2 . 
     The voltage measuring circuit  130  determines the magnitude of the AC voltage between H 1   134  and N 1   136 . The detection circuit  130  measures the rectified DC voltage on the voltage detector  1 . This DC voltage is proportional to and thus indicative of the AC voltage between H 1   134  and N 1   136 . This voltage detection circuit  130  can comprise software or other digital control logic that avoids causing switch  118  and switch  120  to open for momentary low voltage due to initial start up. This capability can also provide automatic restarts when the voltage returns to normal. 
     The GFI  108  also typically mounts on the control board. The lead from the switch S- 4 - 1   118  connects to the hot contact on the supply side of the GFI  108 . The lead from the switch S- 4 - 2   120  connects to the neutral contact on the supply side of the GFI  108 . Switch S- 5   122 ,  124  is located in the GFI unit between the hot supply contact and the hot load contact. The GFI switches  122 ,  124  are normally closed contacts. 
     The hot lead from the load side of the GFI  108  connects to the H 2  terminal  140  on the control board. The neutral lead from the load side of the GFI  108  connects to the N 2  terminal  142  on the control board. A lead runs from the ground contact on the GFI  180  to the N 1  terminal  136  on the control board. A continuous lead runs from the G 1  terminal  138  to the G 2  terminal  146  on the control board, without interruption. 
     An electric cable  112  carries power from the safety device to the vending machine. This cable  112  is brought into the safety device box  110  and is clamped at the entry port. The leads in this cable  112  are hardwired onto the control board. The hot lead  106  for the vending machine cable connects to the H 2  terminal  140  on the control board. The neutral lead in this cable  112  connects to N 2  terminal  142  on the control board. The ground lead in this cable connects to G 2  terminal  146  on the control board. 
     As discussed above, the strobe and the alarm  120  are mounted on the vending machine where they can be seen and heard. The alarm leads, from the strobe and the wailing siren, connected to circuitry on the control board as illustrated in  FIGS. 1 and 2A . Also as discussed above, the box or housing  110  of the safety device  100  can be mounted to the frame of the vending machine. 
     The construction on the safety device can be straightforward. The power cable to the wall receptacle enters the box or housing  110  via a port or hole on one end and is secured with a clamp. A control board containing the electronic components is mounted inside the box. The power cable leads  106 ,  104 ,  102  are separated and connected to terminals H 1   134 , N 1   136 , and G 1   138 , as discussed above. 
     The control board is sized to hold the components selected, plus the terminal lugs for the leads. The components are mounted on the control board and wired according to the schematics. The control board is mounted in the safety box  110 . 
     A power cable  112  from the vending machine equipment enters the box  110  from the end of the safety box opposite the power supply end and is secured to the box  110  via a cable clamp or other restraining device. The leads of the vending machine cable  112  are separated and connected to terminals H 2   140 , N 2   142 , and G 2   146  on the control board. 
     The safety box  110  is sized to hold the control board and to allow for cable termination. The safety box  110  should be made from a non-conducting plastic material if it is to be used as part of an extension cord. 
     Although a system in accordance with the present invention can comprise a circuit that provides electrical safety protection to a vending machine or other appliance, those skilled in the art will appreciate that the present invention is not limited to a specific application and that the embodiments described herein are illustrative and not restrictive. Furthermore, it should be understood that various other alternatives to the embodiments of the invention described herein may be employed in practicing the invention. The scope of the invention is intended to be limited only by the claims below.