Abstract:
A three-prong electrical plug includes a replaceable ground prong and circuitry inside the plug to preclude its functioning without the ground prong.

Description:
This invention relates to electric three-prong connectors, and more specifically, to a three-prong connector that will not transmit electricity if the ground prong is removed or damaged. 
   BACKGROUND OF THE INVENTION 
   The use of three-prong plug electrical connectors for appliances, extension cords, electric tools, etc. has become widespread. The use of three prongs, hot, neutral and ground, makes the operation of any electric appliance, tool, etc. safer. If there is a short, the ground wire provides a safe path for current. 
   The standards for a three-prong plug include, hot and common or neutral prongs that are elongate and flat (usually sheet metal, backfolded at the distal end of the prong). In contrast, the ground prong is cylindrical in shape, i.e., usually a piece of sheet metal bent to form a hollow cylindrical shape or “U” shape with a rounded distal end. In a typical three-prong plug, the hot and neutral prongs are somewhat ductile being flat and may be bent from side to side when being pulled from a socket and then manually straightened again. However, the hollow cylindrical or U shape ground prong has normally been heavily worked by being bent into its hollow shape and has become work hardened or brittle, even before it is put in use. Also, it is not ductile from side to side as are the hot and common prongs. Further, the ground prong being made from sheet stock, a tongue or unbent central portion that extends from the hollow cylinder into the plug housing becomes a weak point in the ground prong design, if it is repeatedly bent (as are the hot and common prongs) when being pulled from an electrical socket. 
   The differences in ductility and brittleness between the hot and neutral prongs and the ground prong, results in differences in their fracture rates on the same three prong plug. In use, the brittle stiff ground prong often severely bends or fractures at its base with the rubber insulation cover of the plug. If thus deformed, an operator may then break off the ground prong. The problem with conventional three-prong plugs is that the plug will continue to work as a two-prong plug, although it will not provide the safety of a ground connection in case of a short. 
   Many municipal codes require or mandate discarding or non-use of electrical plugs or extension cords lacking a useable ground prong. However, in use, especially at construction sites, workmen encountering a damaged ground prong on such a connector will break off the ground prong and use the connector in an ungrounded condition, in violation of such codes. 
   In an effort to overcome the deficiencies of a standard three-prong plug, U.S. Pat. No. 5,622,509 provides a three-prong electrical connector having a flexible resilient grounding prong. However, the spiral wound grounding prong may be too resilient for its application as disclosed in the patent. 
   U.S. Pat. No. 3,914,565 discloses a three-prong plug invented in the 1970&#39;s when three-prong sockets were new in use. The solid ground prong is biased in a non-working position and must be moved into the body to close a pair of switches to provide a usable plug. The device is more complex than necessary and the added components provide more chance for failure in use. This patent depends upon the ground prong bottoming out in the socket which is not possible in all socket designs. 
   Other patents, U.S. Pat. Nos. 3,308,415 and 6,419,504 disclose retractable ground prongs. U.S. Pat. No. 3,205,486 discloses an extendable socket-like prong in connection with standard prongs. U.S. Pat. No. 4,318,578 discloses an electrical connector with a ground fault connector. Other electrical connections are shown at U.S. Pat. Nos. 5,772,447; 6,910,911; 1,944,296, and 1,605,904. 
   A need has developed for a simple three-prong electrical connector (plug) that provides the added safety feature of not functioning when the ground prong is disabled. Additionally, a need has developed for a three-prong male connector or plug that has the provision of easy repairability or replacement of the ground prong should the same become disabled. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention, generally stated, to provide a new and improved three-prong male electrical connection plug. Another object of the present invention is the provision of a three-prong male electrical connector plug that ceases to function if the ground prong should become disabled. 
   Further, the ground prong is designed to break away or dislocate from its socket, prior to the breaking point of the prong itself simultaneously rendering the plug usable in an unsafe condition. 
   A further object of the present invention is the provision of an easily replaceable ground prong in a three-prong male electrical connector. 
   The invention is directed to a three-prong grounded electrical connector for operatively engaging a three-conductor electrical cord. The connector comprises a body of electrically-insulated materials including a first electrically-conductive lead secured in the body and extending therefrom, a second electrically-conductive lead secured in the body and extending therefrom, and an electrically conductive grounding prong selectively releasably secured in the body and extending therefrom. A switch in the body is operatively connectable to a hot conductor of the three conductor cord. A spring between the grounding prong in its operative position and the switch maintain the switch in a closed position. The switch is biased in an open position when the prong is released from securement in the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention may best be understood from the following detailed description of currently preferred embodiments thereof taken in conjunction with the accompanying drawings wherein like numerals refer to like parts, and in which: 
       FIG. 1  is a three quarter top perspective view of a three-prong male electrical connection constructed in accordance with the present invention; 
       FIG. 2  is a three quarter top perspective view similar to that shown in  FIG. 1  with the three-prong connection therein exploded outwardly; 
       FIG. 3  is a top plan diagrammatic view of the connector shown in  FIG. 1  showing the inner workings of the positive and ground prongs therein; 
       FIG. 4  is an elevational diagrammatic view of the three prong connector shown in  FIG. 1  showing the inner workings of the three prongs therein; 
       FIG. 5  is a side elevational diagrammatic view similar to  FIG. 4  of the three-prong male connector of the present invention showing the disabling of the positive circuitry when the ground prong is damaged. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a three-prong male electrical connector or plug, generally indicated at  10 , and constructed in accordance with the present invention, includes an insulated housing, generally indicated at  11 , preferably made of molded rubber, plastic or other non-conductive material, and further includes sticking outwardly therefrom on the distal end  16  thereof, a hot electrical prong  12 , a neutral or common prong  13 , and a ground prong  14 . At the rear of the housing  11 , a three wire electrical cord, generally indicated at  15 , extends therefrom. 
   Referring to  FIG. 2 , the ground prong  14  is shown exploded outwardly from its socket type receptacle, generally indicated at  20 , to be described in more detail below. Ground prong  14  is selectably releasably retained in its socket receptacle  20  for two purposes. The first is so that it may be replaced if it becomes damaged, as often happens with electrical connectors or plugs. The second is that the rear of the ground prong  14  includes a spring  31  for biasing a contact or switch ( 26 ,  27 ) in the preferred embodiment, in the wiring of the hot prong  12 , such that the entire plug will not conduct electricity if the ground prong is disabled. 
   Referring to  FIGS. 3 ,  4  and  5 , diagrams of the internal parts of the male three-prong connector or plug  10  are shown. The inside of the housing  11  receives at its back end, the hot, common and ground conductors  22 ,  23  and  24 , respectively, that extend from the three conductor cable  15 . While hot prong  12  is fixedly embedded in the molded housing  11 , a pair of electrical contacts,  26 ,  27  are retained on the ends of prong  12  and conductor  22 , respectively, so as to provide current from the hot conductor  22  to the hot prong  12  when the contacts  26 ,  27  are closed. In this embodiment, contact  27  is stationary in the housing  11  while the other of the contacts  26  is mounted on an arm  28  connected at a bendable bight portion  29  to the interior end of positive prong  12 . With the arm  28  biased in an open direction (shown by the arrow in  FIGS. 3 and 4 ) the contacts  26  and  27  would tend to be open if not for a spring  31 , made of insulative material or at least insulated from contact  26 . While an internal end of spring  31  is in biased contact with the back end of contact  26 , an outward end of spring  31  is received on the inner end of ground prong  14  where, if not made of insulative material, spring  31  is insulated from the remainder of ground prong  14 . The spring may also be affixed at its end with contact  26  and be loosely insulatively received in a pocket at the ball end  14   a  of ground prong  14 . 
   In one aspect of the present invention, the preferably solid ground prong  14 , at its internal expanded or ball end  14   a  is received, in this preferred embodiment, in a ball shape socket  32  in housing  11  that is conductively connected to the ground lead  24 . Outwardly adjacent the ball shape socket  32  is a conical hollow area, generally indicated at  33  which has an annular shaped resilient retaining ring  34  mounted thereacross. While the preferred embodiment is ball shaped at its inner end  14   a , it will be understood that other shapes that provide a known resistance to their removal may be utilized within the scope of the invention. The inner end of ball shaped socket  32  where it joins conical area  33  is of a constricted diameter that retains the ball end  14   a  of prong  14  in its mounted position, up to a point. The size of the constriction may be varied to assure that a predetermined minimum force is necessary to remove the prong from the socket. 
   Referring to  FIG. 5 , the purpose of the resilient annular retaining ring  33  in the preferred embodiment is shown in more detail. When the ground prong  14  is dislodged from the connective conducting socket  32 , either purposefully or by accident, it extends outwardly from the socket to the conical area  33  adjacent the front wall  11   a  of the housing  11 . 
   The resilient annular retaining ring  33  deforms outwardly a sufficient amount to restrain the ball end  14   a  of ground prong  14  from fully exiting the front end of the plug housing  11 . With the ground prong  14  pulled out of its socket  32 , the spring  31  releases its bias against contact  26 . The bias built in the bight portion  29  and arm  28  of contact  26  opens the contacts  26  and  27  stopping current flow from hot lead  22  into hot prong  12 , thus rendering the connector temporarily inoperable. With the ground prong being solid, it is difficult for a user to cut the prong in two in an attempt to provide operability to the connector. Generally less force is necessary to pull the ground prong out through the retaining ring  34  than through the socket constricted area. 
   However, spare or replacement ground prongs  14  are available to replace the damaged ground prong and may be inserted through the resilient annular ring and conical section  33  into the socket  32  at which time the spring  31  positioned in the inner end of the ball portion  14   a  again biases contact  26  against contact  27  to provide a workable three prong plug. 
   When the ground prong  14  is deformed such that it cannot be inserted into a conventional three-prong socket, the connector of the invention will not operate as a two prong connector, if the ground prong  14  is pulled out of its socket. However, identical spare ground prongs may be utilized similarly to bits in a drill. They may be stored away from the connector or, if consistent with UL policies and CPSC rules, a spare prong may be stored by removably embedding same in a slot  11   b  in the top of the insulative housing  11 . When the ball end  14   a  of the spare ground prong  14  is inserted in the socket  32 , the connector will again function properly and safely. 
   While one embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the true spirit and scope of the present invention. It is the intent of the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.