Abstract:
A transfer switch mountable in a standard electrical panel connects either a utility supply or an auxiliary supply to an electrical system. An internal mechanical interlock prevents both the utility supply and the auxiliary supply from simultaneously being connected to the system. The transfer switch may utilize either a rocker-style or a blade style switch. A light-emitting diode provides an indication of whether the utility supply is connected to and a voltage is present at the utility supply terminal. A power meter provides an indication of the magnitude of power drawn from the auxiliary supply when the auxiliary supply is connected to the load.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 61/427,650, filed Dec. 28, 2010, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This application is directed to an electrical transfer switch and, more particularly, to an electrical transfer switch mountable in a standard electrical panel and having an internal interlock. 
     Typically, a transfer switch assembly is used to connect an auxiliary power supply with certain electrical circuits in a building. The transfer switch assembly includes a first input switch engaged with electrical conductors connected to the utility supply and a second input switch engaged with electrical conductors connected to the auxiliary power supply. In order to prevent both input switches from being ON simultaneously, either the housing of the transfer switch assembly or an external interlock device is typically configured to provide a mechanical interference between the two input switches. The mechanical interference may be, for example, a rigid device directly coupled between the switch arms of the switches or a plate slidably mounted to the housing such that only one of the input switches at a time may be in the ON position to connect an electrical power supply to the electrical system. 
     A transfer switch assembly typically includes a separate sub-panel, requiring additional installation cost and space. Optionally, an input switch for the auxiliary power supply may be added to a standard electrical panel; however, due to the many sizes and arrangements of electrical panels, a mechanical interlock must then be specially configured between the utility input switch and the auxiliary input switch. Thus, it would be desirable to provide a single input switch mountable in a standard electrical panel which selectively connects either the utility supply or the auxiliary supply to a building&#39;s electrical load. Further, if use of a sub-panel is advantageous due to the application requirements, the input switch is similarly mountable in any standard electrical panel configured to fit within the sub-panel. 
     SUMMARY OF THE INVENTION 
     The present invention provides a transfer switch mountable in a standard electrical panel. The transfer switch connects either the utility supply or an auxiliary supply to an electrical system. An internal mechanical interlock prevents both the utility supply and the auxiliary supply from simultaneously being connected to the system. A light-emitting diode (LED) provides an indication of whether the utility supply is connected to, and a voltage is present at, the utility supply terminal. A power meter provides an indication of the magnitude of power drawn from the auxiliary supply when the auxiliary supply is connected to the load. 
     According to one embodiment of the invention, the input switch selectively connects either a first power supply or a second power supply to an electrical load via a bus in an electrical panel. The input switch includes a housing, a first connector configured to receive power from the first power supply, and a first contact contained within the housing and electrically connected to the first connector. The input switch further includes a second connector configured to receive power from the second power supply, a second contact contained within the housing and electrically connected to the second connector, and a load contact configured to engage the bus in the electrical panel. A switch arm is operatively mounted to the housing for movement between a first position and a second position and connects the first contact with the load contact in the first position and the second contact with the load contact in the second position. A mechanical interlock within the housing is operatively connected to the first and second contacts to disconnect one of the first or the second contacts from the load contact prior to connecting the other of the first or the second contacts to the load contact when the switch is moved between the first and the second positions. 
     Thus, it is a feature of the present invention to provide a single input switch mountable in the standard electrical panel which selectively connects either the utility supply or the auxiliary supply to an electrical load. 
     According to another aspect of the invention, the switch arm may include a first end extending outside the housing, a second end extending inside the housing, and a pivotal connection proximate to an upper wall of the housing, wherein the second end engages the mechanical interlock. Optionally, the housing may have an upper wall with a first opening and a second opening, and the switch arm may include a handle extending outside the housing, a plate configured to cover both the first opening and the second opening, and at least one plug extending into the housing. The first position is then defined by the plug inserted into the first opening and the second position is defined by the plug inserted into the second opening. 
     According to still another aspect of the invention, a retaining assembly positively connects the input switch to the bus. The retaining assembly may include a threaded member rotatably engaged with the housing; a locking member slidably engaged with the housing; and a lever engaged at a first end by the threaded member, engaged at a second end by the locking member, and pivotally engaged with the housing at a point along the lever between the first and second ends. A power meter may be operably connected between the second terminal and the second contact to indicate the amount of power drawn from the second power supply. A visual indicator may be activated when the first power supply is connected to the first connector and has power available to be supplied to the load. 
     According to yet another aspect of the invention, the input switch may further include a third connector configured to be connected to the first power supply and a third contact contained within the housing and electrically connected to the first connector. A fourth connector is configured to be connected to the second power supply, and a fourth contact is contained within the housing and electrically connected to the second connector. A second load contact is removably engageable with a second bus in the electrical panel. The switch arm connects the third contact with the second load contact in the first position and the fourth contact with the second load contact in the second position. The mechanical interlock disconnects one of the third or the fourth contacts from the second load contact prior to connecting the other of the third or the fourth contacts when the switch arm is moved between the first and the second positions. Optionally, a first neutral connector is interconnected with the first power supply and a second neutral connector is interconnected with the second power supply. The switch arm selectively engages either the first neutral connector or the second neutral connector. 
     According to another embodiment of the invention, an input switch selectively connects either a first power supply or a second power supply to an electrical load via a bus in an electrical panel. The input switch includes a housing having a first end, a second end, a first side, a second side, a lower wall, and an upper wall. A first connector extends through either the first end or the second end and is configured to receive power from the first power supply. A second connector extends through either the first end or the second end and is configured to receive power from the second power supply. A load contact extends through the lower wall and is configured to engage the bus proximate to the lower wall. A switch arm extends through the upper wall and is configured to selectively connect the first connector with the load contact in a first position and the second connector with the load contact in a second position. A retaining assembly has a drive portion and a driven portion, wherein the driven portion is selectively moved between a first position and a second position by the drive portion. The driven portion positively retains the housing to the electrical panel in one of the first position and the second position. 
     According to another aspect of the invention, the driven portion is configured to be rotatable with a separate tool between the first position and the second position. The electrical panel includes a first rib configured to extend generally perpendicular to the first end of the housing, and the driven portion engages the first rib to positively retain the housing to the electrical panel. 
     According to yet another aspect of the invention, a mechanical interlock within the housing is operatively connected between the first connector and the second connector. The mechanical interlock disconnects one of the first or the second connectors from the load contact prior to connecting the other of the first or the second connectors when the switch arm is moved between the first and the second positions. The switch arm may include a first end extending through the upper wall, a second end extending inside the housing, and a pivotal connection proximate to the upper wall of the housing, wherein the second end engages the mechanical interlock. 
     According to another embodiment of the invention, an input switch selectively connects either a first power supply or a second power supply to an electrical load via a bus in an electrical panel. The input switch includes a housing having a first and a second opening extending therethrough, a load contact configured to engage the bus in the electrical panel, a first connector configured to receive power from the first power supply, and a first receptacle aligned with the first opening in the housing. The first receptacle has a first surface and a second surface electrically isolated from each other. The first surface is electrically connected to the first connector and the second surface is electrically connected to the load contact. A second connector is configured to receive power from the second power supply. A second receptacle is aligned with the second opening in the housing and has a first surface and a second surface electrically isolated from each other. The first surface is electrically connected to the second connector and the second surface is electrically connected to the load contact. A movable selection device has a plug portion configured to selectively establish an electrical connection between the first and the second surfaces of either the first or the second receptacles. 
     According to another aspect of the invention, a first spring-biased contact may have a first plunger slidably engaging a second end of the first receptacle. A first spring biases the first spring-biased contact in a normally open position and the movable selection device engages the first plunger to compress the first spring, closing the first spring-biased contact. A second spring-biased contact may have a second plunger slidably engaging a second end of the second receptacle. A second spring biases the second spring-biased contact in a normally open position and the movable selection device engages the second plunger to compress the second spring, closing the second spring-biased contact. 
     According to yet another aspect of the invention, the housing may have a third and a fourth opening extending therethrough. The input switch further includes a second load contact configured to engage a second bus in the electrical panel, a third connector configured to connect to the first power supply, and a third receptacle aligned with the third opening in the housing. The third opening has a first surface and a second surface electrically isolated from each other. The first surface is electrically connected to the third connector and the second surface is electrically connected to the second load contact. A fourth connector is configured to connect to the second power supply, and a fourth receptacle is aligned with the fourth opening in the housing. A first surface and a second surface of the fourth receptacle are electrically isolated from each other. The first surface is electrically connected to the fourth connector and the second surface is electrically connected to the second load contact. The plug portion of the movable selection device is configured to selectively establish an electrical connection between the first and the second surfaces of either the first and the third receptacles or the second and the fourth receptacles. 
     According to still another aspect of the invention, the housing may have a third and a fourth opening extending therethrough. The input switch further includes a neutral conductor extending through the housing configured to be connected to a neutral connection within the electrical panel. A third connector is configured to receive a neutral connection from the first power supply, and a third receptacle is aligned with the third opening in the housing. The third receptacle has a first surface and a second surface electrically isolated from each other. The first surface is electrically connected to the third connector and the second surface is electrically connected to the neutral conductor. A fourth connector is configured to receive a neutral connection from the second power supply, and a fourth receptacle is aligned with the fourth opening in the housing. The fourth receptacle has a first surface and a second surface electrically isolated from each other. The first surface is electrically connected to the fourth connector and the second surface is electrically connected to the neutral conductor. The plug portion of the movable selection device may be configured to selectively establish an electrical connection between the first and the second surfaces of the first and the third receptacles in a first position and between the first and the second surfaces of the second and the fourth receptacles in a second position. When the input switch is in the first position, the movable selection device establishes the electrical connection between the first and the second surface of the third receptacle prior to establishing the electrical connection between the first and the second surface of the first receptacle and breaks the electrical connection between the first and the second surface of the first receptacle prior to breaking the electrical connection between the first and the second surface of the third receptacle. When the input switch is in the second position, the movable selection device establishes the electrical connection between the first and the second surface of the fourth receptacle prior to establishing the electrical connection between the first and the second surface of the second receptacle, and breaks the electrical connection between the first and the second surface of the second receptacle prior to breaking the electrical connection between the first and the second surface of the fourth receptacle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the best mode presently contemplated of carrying out the invention. 
       In the drawings: 
         FIG. 1  is a schematic representation of a transfer switch including an internal connection between a terminal and a bus according to one embodiment of the present invention; 
         FIG. 2  is a perspective view of a single pole transfer switch as in  FIG. 1 , incorporating the internal interlock of the present invention; 
         FIG. 3  is a perspective view of a double pole transfer switch in the form of two side-by-side transfer switches as in  FIG. 1 , which incorporate the internal interlock of the present invention; 
         FIG. 4  is a schematic representation of a transfer switch including an internal connection between a terminal and an external conductor according to one embodiment of the present invention; 
         FIG. 5  is a perspective view of a three pole transfer switch incorporating the internal interlock of the present invention; 
         FIG. 6  is a schematic representation of an internal connection between a terminal and a bus according to a second embodiment of the present invention; 
         FIG. 7  is a schematic representation of an internal connection between a terminal and an external conductor according to the second embodiment of the present invention; 
         FIG. 8  is a side elevation view of a switching member illustrated in  FIG. 7 ; 
         FIG. 9  is a front elevation view of a switching member illustrated in  FIG. 7 ; 
         FIG. 10  is a perspective view of a three pole transfer switch incorporating the internal interlock according to the second embodiment of the present invention; 
         FIG. 11  is a schematic representation of an internal connection between a terminal and a bus according to a third embodiment of the present invention; and 
         FIG. 12  is a schematic representation of an internal connection between a terminal and a bus according to a fourth embodiment of the present invention. 
     
    
    
     In describing the representative embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
     DETAILED DESCRIPTION 
     Specific embodiments of the present invention will now be further described by the following, non-limiting examples which will serve to illustrate various features of the invention. With reference to the drawing figures, like reference numerals designate like parts throughout the disclosure. 
     Referring to  FIG. 1 , an input switch  10  for mounting in an electrical panel  24  is disclosed. The input switch  10  includes a housing  50  which may be configured to mount in a standard electrical panel  24 . The input switch  10  includes a first terminal  12  on a first end  51  of the housing  50  and a second terminal  16  on a second end  52  of the housing  50 . Optionally, the first and second terminals,  12  and  16  respectively, may be located in any suitable position on the housing  50  for connecting an electrical conductor, such as a wire. Each terminal,  12  or  16 , may be any suitable connector which positively retains an electrical conductor, for example, with a screw-type or a spring-loaded clamping member. A conductor from a first power supply, such as a utility grid, is connected to the first terminal  12 , and a conductor from a second power supply, such as a backup generator, is connected to the second terminal  16 . 
     The input switch  10  also includes a load contact  20  removably connected to a bus bar  22  in the electrical panel  24 . The load contact  20  may be constructed of either a unitary conductive member or, optionally, be constructed of multiple conductive members or a combination of conductive and non-conductive members. The load contact  20  is configured to make a first electrical connection internal to the housing  50  and extends through the lower surface  55  of the housing  50  to slide, snap, hook, or otherwise connect to the bus  22 , according to the configuration of the bus  22 , to make a second electrical connection. 
     The first and second terminals,  12  and  16  respectively, are selectively connected to the load contact  20  within the housing  50 . The first terminal  12  is electrically connected, for example by a wire, to a first contact  14 , and the second terminal  16  is electrically connected, for example by a wire, to a second contact  18 . The first and second contacts,  14  and  18  respectively, are coupled to a switch arm  26  by a mechanical interlock  40 . The switch arm  26  extends through an opening  57  in the upper wall  56  of the housing  50 . The switch arm  26  is selectively positioned between a first and a second position. 
     The input switch  10  may further include a retaining assembly  30  configured to positively retain the input switch  10  in engagement with the bus  22  in the electrical panel  24 . The retaining assembly  30  may include a drive portion and a driven portion. The drive portion includes a protrusion or a recess complementary to a tool used to move the drive portion. The drive portion engages the driven portion, causing the driven portion to alternately engage or disengage the input switch  10  with the electrical panel  24 . According to one embodiment, the retaining assembly  30  may include a threaded member  32 , such as a screw, extending through the housing  50 . The end or head of the threaded member  32  external to the housing  50  includes a recess configured to receive a tool, for example, a screwdriver, alien wrench, torx or box head driver, or any other suitable tool, for rotating the threaded member  32 . Rotation of the threaded member  32  engages the wall of the housing  50  to drive the threaded member  32  further into or out of the housing  50 . The end of the threaded member  32  internal to the housing  50  engages a first end of a pivotable lever  36 . A second end of the lever  36  engages a locking member  34 . The lever  36  is pivotally connected to the housing  50  at a point between the first and second ends. The locking member  34  slidably extends from and retracts toward the housing  50  to engage and disengage a hook, lip, rib, or other retaining member provided on the electrical panel  24 . Although one embodiment of the retaining assembly  30  is described, it is contemplated that other suitable assemblies may similarly engage the electrical panel  24  to retain the input switch  10  in engagement with the bus  22 . 
     The input switch  10  may also include a power meter  45  mounted on the housing  50 . The power meter  45  may be mounted on the upper wall  56  of the housing for ease of viewing. The power meter  45  is connected between the second terminal  16  and the second contact  18 . The power meter  45  is configured to read the magnitude of the voltage present at and/or the current flowing from the auxiliary power supply. The power being delivered from the auxiliary power supply may subsequently be determined and displayed on the power meter  45 . Alternatively, the power meter  45  may measure current in the wire between second terminal  16  and second contact  18  (or in any other satisfactory conductor) using a current transformer in order to provide a power reading. 
     The input switch  10  may further include a visual indicator  60 , such as an LED, mounted on the housing  50 . A first terminal  62  of the visual indicator  60  is connected to the first terminal  12 , and a second terminal  64  of the visual indicator  60  is connected to the neutral bus (not shown) in the electrical panel  24 . When the primary supply is connected to and a voltage is present at the first terminal  12 , the visual indicator  60  will be energized to indicate the presence of voltage at the first terminal  12 . 
     Referring next to  FIGS. 2 and 3 , the input switch according to one embodiment of the present invention may be incorporated, for example, as a single pole or a double pole switch. According to one embodiment of the invention, additional poles are added by adding segments to the housing that reproduce the internal electrical connections and utilize the same switch arm  26  of a single pole switch. The housing  50  on each input switch  10  again may be configured to mount in a standard electrical panel  24 . The input switch  10  is removably connected to the electrical panel  24 , for example, by pressing or sliding the load contact  20  onto the bus  22 . The lower wall  55  of the housing  50  is generally parallel with the electrical panel  24 . The housing  50  is elongated with the first end  51  and the second end  52  extending away from the electrical panel  24 . A first side  53  and a second side  54  extend between the first end  51  and the second end  53 . For each pole of the switch, the height is typically greater than the width of the ends,  51  and  52 , and the length is typically greater than the height of the sides,  53  and  54 . An upper wall  56  joins the ends,  51  and  52 , and sides,  53  and  54 , opposite the lower wall  55 . The upper wall  56  includes at least one opening  57  for the switch arm  26  to extend therethrough. Additional openings may be formed in the upper wall  56  as required such as the openings,  58  and  59 , for the power meter  45  or the visual indicator  60 , respectively, to extend therethrough. A single pole switch is configured to connect to a single bus  22  in the electrical panel  24 . A double pole switch is configured to connect to two busses  22 , typically offset from and extending parallel to each other, in the electrical panel  24 . 
     According to another embodiment of the present invention, the input switch may be incorporated into a three-pole switch  70  as shown in  FIG. 5 . The three-pole switch  70  uses a two-pole switch and incorporates a neutral segment, as shown in  FIG. 4 . The three-pole switch  70  functions generally similarly to switch  10  as shown in  FIGS. 1-3  and described above, incorporating an internal interlock that connects either a primary or auxiliary power supply to the electrical load. 
     The three-pole switch  70 , in addition to connecting either the first or second power supplies to the two buses  22 , includes a neutral switching feature, as shown in  FIG. 4 . A first side of the three-pole switch  70  includes three terminals  112 , two of which are adapted for connection between energized conductors and one of the buses  20  and one of which is adapted for connecting the neutral conductor to a neutral terminal on the panel  24 . The opposite side of switch  70  from that shown in  FIG. 5  also includes three terminals  116 , similar to terminals  112 , which likewise are arranged such that two of the terminals are adapted for connection to energized conductors and one of the terminals is adapted for connection to a neutral conductor. On one side of switch  70 , the three terminals  112  are connected to two energized conductors and a neutral conductor from a first power supply, such as a utility. On the other side of switch  70 , the three terminals  116  are connected to two energized conductors and a neutral conductor from an auxiliary power supply, such as a standby generator. 
     Similar to the powered connections, the neutral conductor of the first and second terminals,  112  and  116  respectively, is selectively connected to a neutral load contact  120 . The first terminal  112  is electrically connected, for example by a wire, to a first contact  114 , and the second terminal  116  is electrically connected, for example by a wire, to a second contact  118 . The first and second contacts,  114  and  118  respectively, are coupled to the switch arm  26  by a mechanical interlock  40 . The switch arm  26  extends through an opening  57  in the upper wall  56  of the housing  50 . A single switch handle  74  is utilized to actuate the internal switching and interlock components as described above, as opposed to the tied-together switch handles as shown in the two-pole version of  FIG. 3 . 
     The switch arm  26  is selectively positioned between a first and a second position to alternately connect both the powered connections and the neutral connection of either the primary or the auxiliary power source to the buses  22  or panel neutral, respectively. A neutral conductor  122  extends through an opening  124  in the housing to establish an electrical connection between the load contact  120  and the neutral bar of the electrical panel  24 , without connection to the powered bus  22  of the electrical panel  24 . Thus, when the switch is in the first position, the neutral of the first power source is connected to the neutral conductor  122  and when the switch is in the second position, the utility from the second power source is connected to the neutral conductor  122 . The second terminal  64  of the visual indicator  60  is internally connected to the neutral connection from the utility supply rather than the neutral bus of the electrical panel  24  as described above in connection with the two-pole version. It is further contemplated that, when switching between the first position and the second position, the connections between the neutral contacts  114  and  118  and the neutral load contact  120  are configured to be broken after the connections between the contacts,  14  and  18 , for the energized connectors and the load contact  20  to the busses  22  of the electrical panel  24 . Similarly, the new connection between the other of the neutral contacts  114  and  118  and the neutral load contact  120  are configured to be made prior to the new connection between the other of the contacts,  14  and  18 , for the energized connectors and the load contact  20  to the buses  22  of the electrical panel  24 . 
     According to still another embodiment of the present invention, the internal interlock may be incorporated into the switching member  220 . Referring next to  FIG. 6 , a switch  210  is configured to mount to a panel  24  and connect to either a first power supply or a second power supply. The switch  210  operates in a similar manner as the input switches,  10  and  70 , described above. However, the switch arm  26  and mechanical interlock  40  are replaced by a switching member  220 . The switching member  220  includes a plate  224  having a handle  222  extending away from a first surface of the plate  224  and a prong or plug  226  extending away from a second surface of the plate  224  in generally the opposite direction from the handle  222 . The handle  222  and plate  224  are constructed of an electrically non-conductive material and the plug  226  is constructed of an electrically conductive material. 
     A first opening  232  and a second opening  234  extend through a wall of the housing  50 , such as the upper wall  56  of the housing  50 , and the plug  226  slidably extends through either the first opening  232  or the second opening  234 . The first and second openings,  232  and  234  respectively, are positioned close enough to each other such that the plate  224  extends over the opening not occupied by the plug  226 . As the plug  226  is inserted into either the first or second opening,  232  or  234  respectively, the plug  226  will engage either a first receptacle  228  aligned with the first opening  232  or a second receptacle  230  aligned with the second opening  234 . 
     As the plug  226  engages one of the receptacles,  228  or  230 , an electrical connection is established between either the primary or auxiliary power supply and the panel bus  22 . Each receptacle,  228  and  230 , may include a pair of plates spaced apart from each other. One of the plates is connected to one of the terminals,  240  or  242 , which is, in turn, connected to either the utility supply or the auxiliary supply. The other plate is connected to the load contact  20 , which is connected to the panel bus  22  as previously described. Because the plates are spaced apart from each other, there is initially no electrical conduction between the plates. The distance between the plates is generally equal to or slightly less than the thickness of the plug  226 , such that when the plug  226  is inserted into the receptacle, the plug  226  establishes an electrical connection between the two plates, connecting either the primary or auxiliary power supply to the bus  22 . It is understood, however, that the use of spaced-apart plates in the receptacles  228 ,  230  is representative of any other satisfactory female engagement arrangement that may be employed. 
     Referring next to  FIG. 7-9 , the switching member  220  may also selectively establish an electrical connection between the neutral conductor from either the primary or auxiliary supply power and the neutral bus on the panel  24 . In this regard, the switching member  220  may have multiple prongs or plugs  226 , and each plug  226  may have a different length. The longest plug  226  is configured to engage one of the receptacles,  228  or  230 , connected to the neutral conductor from either the primary or auxiliary power supply. As a result, when the hot receptacles are configured and positioned similarly to the neutral receptacles  228 ,  230 , the neutral connection is made first and broken last during switching between power supplies during removal and engagement of the switching member  220 . The neutral conductor from the power supply is wired to one of the terminals,  240  or  242 , which is, in turn, connected to one of the plates on the receptacles,  228  or  230 . The other plate on each of the receptacles,  228  or  230 , is connected to an electrical conductor  122  which passes through an opening  124  in the housing  50  and is connected to the neutral bus in the panel  24 . Consequently, the switching member  220  may selectively connect both neutral and power conducting leads from either the primary or auxiliary supply to either a neutral bus or a power bus  22  within the panel  24 . 
     It is contemplated that the switch  210  may have multiple configurations without deviating from the scope of the invention. The switch  210  may have, for example, a single pole or multiple poles, connecting power conductors to their respective bus  22 . Further, the switch  210  may optionally provide a neutral connection. The switching member  220  includes a plug  226  for each of the power or neutral conductors to be switched between the utility or auxiliary supply. As shown in  FIG. 9 , the length of the plugs  226  may vary, allowing electrical connection to be established between each plug  226  and the corresponding receptacle,  228  or  230 , in a predetermined order. For example, the plug  226  used to switch the neutral conductor may be longer than plugs used to switch powered conductors, such that a neutral connection is established prior to connecting either powered conductor. Establishing the neutral connection first and breaking the neutral connection last is desirable to avoid an open neutral condition and/or prevent undesirable current conducted via the equipment ground wiring. 
     Referring next to  FIG. 10 , the housing  50  of the switch  210  is similar to the housing previously described with respect to switch  10  and shown in  FIGS. 2 ,  3 , and  5 . The switch  210  may include a visual indicator  60 , such as an LED, or a power meter  45  mounted on the housing  50 . In addition, a pair of tabs  225  may extend away from the upper wall of the housing  50 . The tabs  225  may be configured to engage opposite edges of the plate  224  and to positively retain the switching member  220  in the housing  50 . In order to move the switching member  220  between the first opening  232  and the second opening  234 , pressure is applied to each of the tabs  225  to flex the tab  225  away from the plate  224 , allowing the switching member  220  to be removed from housing  50 . As the switching member  220  is reinserted, the plate  224  engages a beveled surface  227  on each of the tabs  225 , displacing the tabs  225  outward and allowing the switching member  220  to enter the housing  50 . When the switching member  220  is reinserted into the housing  50 , the plate  224  is located between a shoulder defined by the beveled surface  227  and the upper wall  56  of the housing, allowing the tabs  225  to return to their original position which again positively retains the switching member  220  to the housing  50 . 
     Referring now to  FIG. 11 , a switch  270  may further include a pair of spring-biased contacts,  241  and  243 , to connect the powered conductors to the load contact  20 . The switch  270  operates in a similar manner as switch  210  described above; however, a spring-biased contact,  240  or  242 , is included between the receptacle,  228  or  230  respectively, and the load contact  20 . The second plate of each receptacle,  228  or  230 , is electrically connected to a first terminal of the spring-biased contact,  241  and  243 . The second terminal of each of the spring-biased contacts,  241  and  243 , is connected to the load contact  20 . A spring,  244  or  246 , biases each contact,  241  and  243  respectively, to a normally open position. Each plug  226  on the switching member  220  engages a plunger,  248  or  250 , which compresses the spring,  244  or  246 , and closes the contact,  241  and  243 . Each plunger  248 ,  250  has a portion normally positioned between the plates of its associated receptacle  228 ,  230 , which is formed of a non-conductive material so as to maintain electrical isolation between the receptacle plates. When the contact,  241  and  243 , is closed, an electrical connection from the respective terminal,  240  or  242 , to the load contact  20  is established. The tabs  225  positively retain the switching member  220  and hold the spring,  244  or  246 , in the compressed state. 
     Referring next to  FIG. 12 , a toggle switch  300  may selectively establish an electrical connection between either a powered conductor or a neutral conductor and the powered bus  22  or the neutral bus, respectively. A conductor from the primary supply is connected to one of the first terminals  290 , and a conductor from the auxiliary power supply is connected to one of the second terminals  292 . The first terminal  290  is electrically connected, for example by a wire, to a first contact  316 , and the second terminal  292  is electrically connected, for example by a wire, to a second contact  318 . The first and second contacts,  316  and  318  respectively, are coupled to the toggle switch  300  by a mechanical interlock  310 . The toggle switch  300  extends through an opening  303  in the upper wall  56  of the housing  50  and is selectively positioned between a first and a second position. In the first position, the toggle switch  300  establishes an electrical connection between each of the first terminals  290  and either the powered bus  22  or the neutral bus. In the second position, the toggle switch  300  establishes an electrical connection between each of the second terminals  292  and either the powered bus  22  or the neutral bus. 
     The mechanical interlock  310  cooperates with the toggle switch  300  to permit only one of the supplies from being connected at a time. The mechanical interlock  310  includes a first segment  312  engageable with the first contact  316  and a second segment  314  engageable with the second contact  318 . Each of the first and second segments,  312  and  314 , extend from the first and second contacts,  316  and  318 , respectively, to a central contact  320  positioned between the first and second contacts,  316  and  318 . The first and second segments,  312  and  314 , are joined at an angle and pivotable about an axis adjacent to the central contact  320 . The first and second segments,  312  and  314 , may be integrally formed from a single member having a bend in the middle, but alternately may be formed from multiple members connected at an angle to each other. The toggle switch  300  includes a handle  302  extending out of the housing  50  and an arm  304  extending into the housing  50  to contact the mechanical interlock  310 . The distal end  306  of the arm  304  slidably engages the mechanical interlock  310 . With the toggle switch  300  in the first position, the distal end  306  of the arm positively engages the first segment  312 , closing the first contact  316 , and the mechanical interlock  310  rotates about the pivot axis, causing the second segment  314  to open the second contact  318 . With the toggle switch  300  in the second position, the distal end  306  of the arm positively engages the second segment  314 , closing the second contact  318 , and the mechanical interlock  310  rotates about the pivot axis, causing the first segment  312  to open the first contact  312 . As the mechanical interlock  310  rotates about the pivot axis, the first or second contact,  316  or  318 , that is closed opens before the other of the contacts closes, ensuring that the two supplies are not simultaneously connected to the load. 
     In operation, the switch  10  is installed in the electrical panel  24  by connecting the load contact  20  to the bus  22 . The threaded member  32  of the retaining assembly  30  is rotated into the housing  50 , which, in turn, rotates the first end of the lever  36  away from the housing wall. In response, the second end of the lever  36  rotates toward the housing wall, causing the locking member  34  to extend away from the wall. The locking member  34  engages a hook or lip connected to the electrical panel  24  to positively retain the input switch  10  in engagement with the bus  22 . An electrical conductor, such as a wire or cable, from the primary supply is connected to the first terminal  12 , and an electrical conductor from the auxiliary supply is connected to the second terminal  16 . 
     The switch arm  26  is used to selectively connect either the first power supply or the second power supply to the load. The switch arm  26  is movable between a first and second position, each position connecting one of the power supplies. The mechanical interlock  40  cooperates with the switch arm  26  to permit only one of the supplies from being connected at a time. The switch arm  26  may be connected to the mechanical interlock  40  to similarly move it from a first position to a second position. In the first position, the mechanical interlock  40  operably connects the first contact  14  to the load contact  20 . In the second position, the mechanical interlock  40  operably connects the second contact  18  to the load contact  20 . As the mechanical interlock  40  moves between the first and second positions, the first or second contact,  14  or  18 , which was engaged with the load contact  20  first disengages from the load contact  20  and, subsequently, the other of the first or second contact,  14  or  18 , engages the load contact  20 . Consequently, at an intermediate position between the first and the second positions neither the first nor second contact,  14  or  18  respectively, engages the load contact  20 . Thus, the switch  10  selectively connects either the primary or auxiliary supply to the electrical load while ensuring that the two supplies are not simultaneously connected to the load. 
     In the alternate embodiment, the switching member  220  may be used to connect either the primary supply or an auxiliary supply to the load. The switching member  220  may be inserted into either the first opening  232  or the second opening  234  to engage either the first receptacle  228  or the second receptacle  230 . Each receptacle,  228  and  230 , is connected to one of the primary or auxiliary supplies. In the first opening  232 , the switching member  220  operably connects the first terminal  240  to the load contact  20 . In the second position, the switching member  220  operably connects the second terminal  242  to the load contact  20 . In order to change which opening,  232  or  234 , the switching member  220  engages, the switching member  220  must be removed from the opening it presently engages and inserted into the other opening. When the switching member  220  is removed from either opening,  232  or  234 , neither the first or second terminal,  240  or  242  respectively, engages the load contact  20 . Thus, the switch  210  selectively connects either the primary or auxiliary supply to the electrical load while ensuring that the two supplies are not simultaneously connected to the load. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.