Patent Publication Number: US-2009231767-A1

Title: Electronic device with electricity leakage protection

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Taiwanese application no. 97204308, filed on Mar. 13, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an electronic device, more particularly to an electronic device with electricity leakage protection. 
     2. Description of the Related Art 
     Differences in ground potential between electrically connected electronic devices can create a resistive path between grounds through which hazardous leakage current can flow. Such differences in ground potential often emerge as a result of characteristics and conditions unique to each ground path, and frequently manifest between a power ground of the electronic device and a signal ground of a peripheral device, such as a power ground of a computer and a signal ground of a modem. Under ground fault conditions, electrical current can flow through the power ground into the signal ground, overheating wiring of the signal ground and presenting a shock hazard. 
       FIG. 1  illustrates one prior art method for mitigating the aforementioned hazards associated with differences in ground potential. In this method, a ground wire  82  of an electronic device  81  and a signal ground wire  83  of a peripheral device (not shown) are isolated from one another by means of an isolating component  84  (such as rubber, plastic, an isolating capacitor, etc.). Although a broken circuit state between the ground wire  82  and the signal ground wire  83  is effectively achieved, electrical current flow into the ground wire  82  may still pose a safety threat. 
       FIG. 2  illustrates another prior art method for mitigating the aforementioned hazards associated with differences in ground potential, in which the ground wire inside a conventional plug for an electronic device is replaced with a floating ground  91 . Although this method prevents electrical connection between grounds, the ground wire  91  may still carry a charge, and moreover lacks an outer covering. Consequently, this method also poses safety hazards. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the present invention is to provide an electronic device with electricity leakage protection. 
     According to this invention, the electronic device comprises a ground wire, a system ground unit, and a protective element connected electrically at one end to the ground wire and at another end to the system ground unit. The protective element limits flow of electrical current through the ground wire into the system ground unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic diagram of a conventional method for preventing electricity leakage; 
         FIG. 2  is a schematic diagram of another conventional method for preventing electricity leakage; 
         FIG. 3  is a schematic diagram of a first preferred embodiment of an electronic device with electricity leakage protection according to the present invention; 
         FIG. 4  is a schematic diagram of a second preferred embodiment of an electronic device with electricity leakage protection according to the present invention; and 
         FIG. 5  is a schematic diagram of a third preferred embodiment of an electronic device with electricity leakage protection according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure. 
     Referring to  FIG. 3 , a first preferred embodiment of an electronic device  1  with electricity leakage protection according to this invention is shown to comprise a ground wire  11 , a plug  12 , a system ground unit having terminals  13 , and a protective element  14 . In this embodiment, the plug  12  is provided with a positive polarity prong  121 , a negative polarity prong  122 , and a ground prong  123 , so as to correspond to a positive polarity insertion aperture, a negative polarity insertion aperture, and a ground wire insertion aperture of an outlet having a three-wire power distribution scheme (not shown). One end of the ground wire  11  is connected to the ground prong  123  of the plug  12 , and another end is connected to the protective element  14 , which in this embodiment is a safety fuse. The protective element  14  is further connected to one terminal  13  of the system ground unit. 
     Preferably, the system ground unit of the electronic device  1  with electricity leakage protection is adapted for electrical connection to a signal ground wire  21  of an external device (not shown). The electronic device  1  with electricity leakage protection of this embodiment could therefore be an LCD monitor with a system ground unit adapted for electrical connection to a coaxial cable, or a computer with a system ground unit adapted for electrical connection to a network cable. In this embodiment, the signal ground wire  21  of the external device is connected to one terminal  13  of the system ground unit. 
     Under ground fault conditions, in which electrical current flows through the ground wire  11  and the protective element  14  into the system ground unit, the protective element  14  burns away rapidly, creating a broken circuit state between the ground wire  11  and the system ground unit. Since electrical current flow into the system ground unit is limited, electrical potential of the system ground unit remains stable and equal to an electrical potential of the signal ground wire  21  of the external device. Therefore, electricity leakage through the ground wire  11  and the system ground unit into the signal ground wire  21  is averted, and accompanying risks of overheating and electrical shock are eliminated. 
       FIG. 4  illustrates a second preferred embodiment of the electronic device  1  with electricity leakage protection according to this invention. This embodiment differs from the first embodiment only in that the protective element  15  that is connected electrically at one end to the ground wire  11  and at the other end to one terminal  13  of the system ground unit is a temperature-sensitive resistor with a positive thermal coefficient (PTC). Under ground fault conditions, heat generated by the flow of electrical current causes the protective element  15  to increase its resistance value, thereby limiting the flow of electrical current into the system ground unit. 
       FIG. 5  illustrates a third preferred embodiment of an electronic device  1  with electricity leakage protection according to this invention. This embodiment differs from the preceding embodiments only in that the protective element  16  is an isolation capacitor. Under ground fault conditions, the isolation capacitor buffers the electrical signal flowing through the ground wire  11  into the system ground unit. 
     In sum, the electronic device  1  with electricity leakage protection of the present invention employs the protective element  14 , 15 , 16  to isolate the ground wire  11  from the system ground unit in the event of a ground fault, thereby limiting electricity leakage to the system ground unit and the signal ground wire  21 , and mitigating fire and shock hazards. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.