Patent Publication Number: US-RE43177-E

Title: Apparatus and method for preventing an electrical backfeed

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     Embodiments of the invention relate generally to transfer switches. More specifically, at least one embodiment relates to an apparatus and method for preventing an electrical backfeed resulting, for example, from operation of an automatic transfer switch. 
     2. Discussion of Related Art 
     Transfer switches are employed to increase the reliability of an electrical supply to a load by allowing the load to be supplied from two or more sources. For example, a utility electrical supply (e.g., a public utility, a municipal utility, etc.) may provide one electrical supply to a load and a source of backup power (e.g., a standby/emergency generator, uninterruptible power supply, etc.) can provide a second electrical supply to the same load. The transfer switch is used to transfer the load between the two electrical supplies in the event that one of them is unavailable. With some exceptions, applicable electrical codes and national manufacturing standards generally require that the transfer switch maintain electrical isolation between the two electrical supplies for operational and safety reasons. That is, the conductors of the first electrical supply and the conductors of the second electrical supply cannot be connected to one another, even momentarily, e.g., the two electrical supplies cannot be connected to the load in parallel. A backfeed is created when phase conductors of the first electrical supply and phase conductors of the second electrical supply are connected to one another, for example, as a result of a switch failure in a transfer switch. In one failure mode, contacts in the transfer switch weld shut, that is separate contact surfaces fuse together. Contacts do weld shut and fail to operate due to arcing and/or overheating. If for example, a contact connected to the utility fails to open in the transfer switch, the generator supply will backfeed electricity to the utility when the generator is connected to the load. 
     To meet the requirements for electrical isolation of different electrical supplies, transfer switches (in particular, automatic transfer switches) often employ contactors, force guided relays or motorized circuit breakers to perform the switching that transfers the load from one electrical supply to another. The contactors, relays or circuit breakers are mechanically interlocked to prevent a backfeed between the various power sources connected to the transfer switch. These approaches generally result in transfer switches that are more expensive and more complex than practical for residential applications. 
     In another approach, control logic is used with power transfer relays in an automatic transfer switch in a residential installation. Regardless of the status of relay contacts, the control logic initiates a transfer of a load to a generator electrical supply when the logic detects a loss of voltage in a utility electrical supply. 
     Manual transfer switches are sometimes used as an alternative to automatic transfer switches to connect one of two electrical supplies to the load. Manual transfer switches suffer from the obvious drawback that human intervention is required to switch from one electrical supply to another electrical supply. In addition, these switches typically include a mechanical interlock to prevent different electrical supplies from being connected to one another. 
     SUMMARY OF INVENTION 
     In order to prevent a backfeed between two or more electrical supplies used to supply a load, at least one embodiment of the invention detects when a switch in an transfer switch has malfunctioned. 
     In one aspect of the invention, a transfer switch includes a first input to couple a first multiphase low voltage electrical supply to the transfer switch and a second input to couple a second multiphase low voltage electrical supply to the transfer switch. The transfer switch also includes a first set of switches in electrical communication with the first input, a second set of switches in electrical communication with the second input, and a control module. The control module monitors and controls operation of both the first set of switches and the second set of switches to detect a malfunction of any switches included in at least one of the first set of switches and the second set of switches and prevent the first input from being placed in electrical communication with the second input. 
     In one embodiment, the output couples the transfer switch to a load, and a third set of switches selectively couples the first multiphase low voltage electrical supply and the second low voltage electrical supply to the output. The control module monitors and controls operation of the third set of switches to detect a malfunction of any switches included in the third set of switches and prevent the first input from being placed in electrical communication with the second input. In a version of this embodiment, the transfer switch includes a switching module with a switch included in the first set of switches, a switch included in the second set of switches, and a switch included in the third set of switches. 
     In another one embodiment, the first input is adapted to couple to a first split-phase electrical supply and the second input is adapted to couple to a second split-phase electrical supply. 
     In another aspect, the invention provides a method of preventing a backfeed through a transfer switch. The transfer switch includes a first input adapted to receive a first multiphase low voltage power source, a second input adapted to receive a second multiphase low voltage power source and an output. The first input is connected to the output. A transfer is initiated to disconnect the output from the first input and connect the output to the second input. A plurality of switches are operated to complete the transfer. At least one of the plurality of switches is monitored to detect a malfunction of any of the switches, and to prevent the first input from being placed in electrical communication with the second input, the transfer is stopped if a malfunction is detected. 
     In a further aspect of the invention, a transfer switch includes a first input to couple a first multiphase low voltage electrical supply to the transfer switch and a second input to couple a second multiphase low voltage electrical supply to the transfer switch. The transfer switch also includes a first set of switches in electrical communication with the first input, a second set of switches in electrical communication with the second input, and means for detecting a malfunction in any of the first set of switches and the second set of switches. Upon detecting a malfunction, the means for detecting the malfunction in any of the first set of switches and the second set of switches prevents a connection of at least one of the first input and the second input. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, are not intended to be drawn to scale. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  is a block diagram of a transfer switch employed in an electrical system according to one embodiment of the invention; 
         FIG. 2  is schematic diagram of switch connections in a transfer switch according to one embodiment of the invention; 
         FIG. 3  is a flow chart of a process employed with one embodiment of the transfer switch of  FIG. 2  beginning at a point in time at which neither primary nor alternate power is available; 
         FIG. 4  is a flow chart of another process employed with one embodiment of the transfer switch of  FIG. 2  beginning at a point in time when a primary source of power is supplying a load; 
         FIG. 5  is a flow chart of a further process employed with one embodiment of the transfer switch of  FIG. 2  beginning at a point in time when an alternate source of power is supplying a load; 
         FIG. 6  is a schematic diagram of switch connections in another embodiment of a transfer switch according to the invention; 
         FIG. 7  is a schematic diagram of switch connections in a further embodiment of a transfer switch according to the invention; and 
         FIG. 8  is a schematic diagram of switch connections in yet another embodiment of a transfer switch according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing”, “involving”, and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     Generally, a transfer switch used in a residential application isolates separate power sources when transferring the residential load from a primary power source to an alternate power source. In one embodiment, the invention facilitates operation in this manner by sensing whether the switches employed in the transfer switch are operational, for example, by sensing whether the switches are in positions that are consistent with their states as determined by control logic. In a version of this embodiment the transfer switch is an automatic transfer switch. 
     Referring now to  FIG. 1 , one embodiment of a transfer switch  20  is shown connected in an electrical system  22 . In addition to the transfer switch  20 , the electrical system  22  shown in  FIG. 1  includes a circuit breaker panel  24 , an uninterruptible power supply  26  and an emergency generator  28 . The circuit breaker panel  24  is connected to a primary source of power  30 , for example, a utility supply. Each of the uninterruptible power supply  26  and the emergency generator  28  provide an alternate source of power to the transfer switch  20 . The emergency generator  28  can be driven by any of a variety of power plants including a gas engine, a diesel engine, gas turbine, etc. Although the uninterruptible power supply  26  and the emergency generator  28  are shown, other alternative sources of power could be used to provide backup power including a second utility supply, a fuel cell inverter and alternative energy sources such as wind turbines, and solar panels. 
     In  FIG. 1 , the primary source of power  30  is a multiphase power source. As used herein, the term multiphase describes any power source that includes at least two line conductors. In one embodiment, the power supplied by each line conductor is out of phase relative to the other line conductors. Thus, as used herein, a multiphase power source can include two or more phase conductors from a three phase system. In addition, a multiphase power source can include two line conductors from a split-phase power source. In one embodiment, the primary source of power  30  is a 120/240 volt split-phase power source. 
     The circuit breaker panel  24  includes a main circuit breaker  32 , two-pole circuit breakers  34 , single-pole circuit breakers  36 , a neutral bus  37  and a ground bus  38 . In one embodiment, the main circuit breaker  32  is a 200 ampere circuit breaker, the two-pole circuit breakers are 20 ampere circuit breakers, and the single pole circuit breakers are 15 ampere circuit breakers. In one embodiment, two-pole circuit breakers  34  include two circuit breakers that are mechanically connected to insure that they operate substantially simultaneously. In one embodiment, the circuit breaker panel  24  is installed in a residential electrical service. In a version of this embodiment, each of the single-pole circuit breakers  36  are connected to a single phase input  40  of the transfer switch  20 , and each of the two-pole circuit breakers are connected to multiphase input  42 . In an alternate embodiment, only selected single-pole circuit breakers  36  are connected to a single phase input  40  and only selected two-pole circuit breakers  34  are connected to a multiphase input  42 . A first alternate source of power is connected to a first alternate input  44  and a second alternate source of power is connected to a second alternate input  46 . Where an uninterruptible power supply  26  is used, the transfer switch  20  may include a UPS output  48  to supply power to the uninterruptible power supply  26 , for example, to charge the UPS batteries. 
     Electrical load circuits (e.g., supplying power to receptacles, lighting circuits, etc.) are connected to outputs of the transfer switch. Each multiphase load circuit  49  is connected to a separate multiphase output, for example, multiphase output  50 . Each single phase load circuit  51 ,  51 ′ is connected to a separate single-phase 120 volt output, for example, single-phase outputs  52 ,  52 ′, respectively. In one embodiment, each single-phase output supplies 120 VAC and each multiphase output supplies 240 VAC. 
     The transfer switch  20  includes multiphase switching modules  54 ,  54 ′, and single-phase switching modules  56 ,  56 ′. In the embodiment shown in  FIG. 1 , each multiphase switching module  54 ,  54 ′ switches one line conductor of a multiphase circuit, and each single-phase switching module  56 ,  56 ′ switches the single line conductor of a single-phase circuit. In a version of this embodiment, a single pair of multiphase switching modules dedicated for multiphase operation are included in the transfer switch  20 . In another embodiment, the switching modules are reconfigurable. For example, the transfer switch  20  can be equipped with two switching modules  56 ,  56 ′ that can separately switch a line conductor of a single-phase circuit and can be reconfigured to operate as a multiphase switching module  54 ,  54 ′ to switch a line conductor of a multiphase circuit. In a version of this embodiment, the transfer switch is equipped with a plurality of reconfigurable switching modules that can be reconfigured to operate as either single-phase switching modules  56 ,  56 ′ or multiphase switching modules  54 ,  54 ′. That is, the quantity of each type of switching module can be changed to suit the requirements of a specific installation. 
     The transfer switch  20  also includes a main control module  58 , a multiphase control module  59 , a user interface  60 , a power supply  62 , a communications module  64 , a UPS switch  66  and a neutral bus  68 . Each switching module  54 ,  54 ′,  56  and  56 ′ can include current and voltage sensing with outputs communicated to the main control module  58 , for example, to monitor for energy use, overload conditions, etc. In one embodiment, logic in the multiphase control module  59  senses the position of switches included in the multiphase switching modules  54 ,  54 ′ and controls the operation of the switches in the multiphase switching modules  54 ,  54 ′. In a version of this embodiment, a single multiphase control module  59  controls each multiphase switching module  54 ,  54 ′ employed in the transfer switch  20 . In another embodiment, the main control module  58  includes logic to sense the position of one or more switches included in both the multiphase switching modules  54 ,  54 ′, and the single-phase switching modules  56 ,  56 ′, and logic to control the operation of the switches included in both the multiphase switching modules  54 ,  54 ′ and the single-phase switching modules  56 ,  56 ′. 
     The main control module  58  can also control operation of the UPS switch  66 . For example, in one embodiment, power is supplied to the UPS input  48  from the circuit breaker panel  24  during normal operation, however, when power from the normal supply  30  is lost, the main control module  58  switches the position of UPS switch  66  so that power is supplied to the UPS from an alternate source, i.e., the UPS output  48  is connected to the second alternate input  46 , for example, a backup generator. 
     In one embodiment, the user interface  60  includes a display and an input device (e.g., a keypad, a touch screen, etc.) to communicate with the main control module  58 . The user interface provides a user the opportunity to review the status of the transfer switch  20  including: 1) monitoring the existing electrical parameters (e.g., current, voltage, etc.) for one or more of the load circuits  49 ,  51 ,  51 ′; 2) monitoring stored electrical parameters (e.g., power consumption, peak current, etc.); and 3) adjusting set points, for example, a minimum voltage level that triggers a transfer from primary power to one of the alternate power sources. In addition, the user interface  60  can provide a means of configuring the transfer switch  20  to meet the user&#39;s needs. For example, the user interface  60  can allow the user to establish whether reconfigurable switching modules are deployed as single-phase switching modules  56 ,  56 ′ or multiphase switching modules  54 ,  54 ′. The user may also be able to select the ampacity of the load circuits  49 ,  51 , and  51 ′, for example, by choosing from standard ratings such as 15 ampere and 20 ampere ratings. The user may also employ the user interface  60  to adjust setpoints or delays. As one example, a user may establish a setpoint for initiating a generator start signal to start a backup generator after primary power is lost. 
     The transfer switch  20  also includes a power supply module  62  that can include one or more circuits for generating control power and logic levels used in the switch  20 . In one embodiment, an input to the power supply module  62  is supplied from one of the primary source and the alternate source. Generally, in one embodiment, the switching of the power supply input is controlled either manually or by logic that determines the availability of primary and alternate power and the status of at least the multiphase switching modules (e.g., determines which source is supplying the load circuits). In a version of this embodiment, the power supply input is connected to the primary source whenever the primary source is available. When the primary source is unavailable, the power supply input is automatically disconnected from the primary source and is automatically connected to the alternate source if the alternate source is available. Further, in one version the power supply input is automatically disconnected from the alternate source and automatically re-connected to the primary source when the primary source again becomes available, i.e., primary source power is restored. In one embodiment, the power supply module  62  provides +3.3 V, +5 V, +12V and −12V DC outputs. 
     The communications module  64  provides a connection for an external communication network, for example, a connection to one or both of a local area network or a wide area network, e.g., a wireless network. As a result, a remote user can access the user interface  60  via phone, a remote touchpad, or a remote computer. 
     Referring now to  FIG. 2 , one embodiment of multiphase switching modules  70 ,  70 ′, which may be used in the transfer switch  20  in place of the multiphase switching modules  54 ,  54 ′ of  FIG. 1 , are shown. In the embodiment shown in  FIG. 2 , the multiphase switching module  70  provides switching for a first line conductor (e.g., a first phase) of a multiphase supply and multiphase switching module  70 ′ provides switching for a second line conductor of the multiphase supply (e.g., a second phase). Together, the multiphase switching modules  70 ,  70 ′ include three sets of switches: a first set of switches includes switch  72  and switch  74 ; a second set of switches includes switch  76  and switch  78 ; and, a third set of switches includes switch  80  and switch  82 . In the embodiment shown in  FIG. 2 , each switch is a single pole double throw switch, that is, there is a single switch element that can be moved between a first closed position and a second closed position (e.g., a form C relay). In addition each switch  72 ,  74 ,  76 ,  78 ,  80  and  82  is electrically operated such that each switch has a normally open contact position (NO) and a normally closed contact position (NC). Although each switch  72 ,  74 ,  76 ,  78 ,  80 ,  82  is described as being included in a single pole double throw relay in the embodiment of  FIG. 2 , each switch can be any contact capable of carrying current and operable between a first position and a second position. As one example, each switch  72 ,  74 ,  76 ,  78 ,  80 ,  82  can also be replaced by two single pole single throw switches that are operated in a manner that mimics the operation of the corresponding single pole double throw switches. In one embodiment, switches  72 ,  74 ,  76 ,  78 ,  80  and  82  are a miniature relay for printed circuit board mounting, for example, part number 832A-1C-S-24DC manufactured by Song Chuan. In other embodiments, switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  could instead be included in contactors, circuit breakers, or solid state switches. Further, in one embodiment, switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  are included in a single multiphase switching module. 
     In  FIG. 2 , the multiphase switching modules  70 ,  70 ′ provide a primary input  91  including a first terminal  90  for multiphase switching module  70  and a second terminal  92  for multiphase switching module  70 ′. The multiphase switching modules  70 ,  70 ′ also include an alternate input  93  including a first terminal  94  for multiphase switching module  70  and a second terminal  96  for multiphase switching module  70 ′. The multiphase switching modules  70 ,  70 ′ also provide an output  84  including a first terminal  86  for multiphase switching module  70  and a second terminal  88  for multiphase switching module  70 ′. In one embodiment, a multiphase control module  98  (similar to multiphase control module  59  discussed above) communicates with the multiphase switching modules  70 ,  70 ′ via a communication bus  100  which is connected to each switching module  70 ,  70 ′. The multiphase control module controls the operation of the switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  to supply power to the output  84  while preventing a connection between the primary input  91  and the alternate input  93 . In one embodiment, the multiphase control module  98  includes logic that prevents even a momentary connection between the primary input  91  and the alternate input  93  including connections via a load, i.e., it prevents a connection between any of the first terminal  90  and the second terminal  92  of the primary input  91 , and any of the first terminal  94  and the second terminal of the alternate input  93 . 
     The control module  98  can be implemented in hardware, software, firmware or a combination thereof. In one embodiment, the control module  98  is a complex programmable logic device (“CPLD”), for example, from the MAX 7000 family sold by Altera Corporation. In another embodiment, the control module  98  can be implemented in a microprocessor or microcontroller executing embedded software and/or firmware instructions. In an alternative embodiment, the control module  98  is implemented in combinatorial logic and sequential logic. 
     The communication bus  100  can be any single line or multi-line bus capable of transmitting control and sensing signals between the multiphase control module  98  and the multiphase switching modules  70 ,  70 ′. For example, in one embodiment, the communication bus  100  includes a plurality of discrete lines with a separate line dedicated to each input to the multiphase control module  98  (e.g., voltage sensing inputs) and a separate line dedicated to each output. The outputs can, for example, be signals supplied by the multiphase control module  98  to transistors co-located with relays employed in multiphase switching module  70 ,  70 ′, for example, mounted together on a printed circuited board. The signals provided by the multiphase control module  98  can operate a transistor used to switch on and off a coil of a relay employed in multiphase switching module  70 ,  70 ′. In a further embodiment, the communication bus  100  also includes one or more lines that connect the multiphase control module  98  to a main control module, such as main control module  58  of transfer switch  20 . In a version of this embodiment, the communication bus also includes one or more lines that connect the main control module  58  to the multiphase switching modules  70 ,  70 ′. In addition, the transfer switch may include a control power bus (not shown) that is connected to the main control module  58  and the multiphase control module  98 . 
     The multiphase switching modules  70 ,  70 ′ include a plurality of sensing nodes that can each supply an input to the multiphase control module  98  via the communication bus  100 . One or more of the sensing nodes can also supply an input to the main control module  58  via the communication bus  100 . In an alternate embodiment, the main control module  58  receives as inputs signals provided by the multiphase control module  98  instead of or in addition to signals provided from the multiphase switching modules  70 ,  70 ′. In addition, communication bus  100  connects the control module  98  to each switch  72 ,  74 ,  76 ,  78 ,  80 ,  82  to provide switching control logic to the switches. In one embodiment, based on a monitored voltage or current provided by sensing nodes, the multiphase control module  98  senses the availability of normal and alternate power sources and the status of switches  72 ,  74 ,  76 ,  78 ,  80 ,  82 , i.e., whether the switch is in the normally-closed or the normally-open position. In this embodiment, the multiphase control module  98  uses this information to determine whether the switches must be operated to supply power to the load, for example, when it is necessary to switch to an alternate source of power because the primary source of power is unavailable. In addition, the multiphase control module  98  implements control logic to insure that a backfeed, even a momentary backfeed, is not created when one or more of the switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  are operated. 
     In a further embodiment, the main control module  58  determines the availability of the primary and alternate power sources and provides information to the multiphase control module  98  regarding the power source that should be used. The multiphase control module  98  is then responsible for operating the switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  in a manner that prevents a backfeed. In a version of this embodiment, the main control module  58  receives inputs associated with the primary and the alternate sources of power, for example, from one or more sensing nodes or from another voltage-sense signal that may not be provided by the switching module  70 ,  70 ′, e.g., the voltage sensing may be performed elsewhere in the transfer switch  20  or the circuit breaker panel  24 . The main control module  58  uses the information provided by the voltage-sense signal to determine the availability of the primary and the alternate power sources. In this embodiment, the main control module  58  determines the appropriate source of power and provides this information as one or more inputs to the multiphase control module  98 . The multiphase control module  98  then determines whether any switching is required, and if so, whether the switching can be accomplished without creating a backfeed. 
     In one embodiment, the transfer switch  20  complies with Underwriter&#39;s Laboratory Standard 1008 (“UL 1008”). UL 1008 requires that a transfer switch prevent even a momentary backfeed between different sources of power (e.g., prevent a backfeed between a primary and an alternate source of power) even when there is only a single switch failure. In a version of this embodiment, three switches (e.g.,  72 ,  76 ,  80 ) are employed in each of switching modules  70 ,  70 ′. In an alternate embodiment, however, only two switches are employed in each multiphase switching module  70 ,  70 . For example, in  FIG. 2 , switches  76  and  78  are eliminated, or alternatively, switches  72  and  74  are eliminated in this alternate embodiment. 
     In one embodiment, transfer switch  20  is configured for operation with primary and alternate power sources that include 3 or more phases. For example, a transfer switch  20  can include multiphase switching modules capable of switching 3-phase sources in a manner that complies with UL 1008. In a version of this embodiment, a third multiphase switching module corresponding to multiphase switching modules  70 ,  70 ′ is added to the transfer switch  20  to switch the third phase of a multiphase source. 
     In one embodiment, to prevent a connection between a normal source of power and an alternate source of power, the multiphase control module  98  also determines, based on the outputs of the sensing nodes, whether one or more of switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  are operative before generating a control signal to operate a switch. In the embodiment shown in  FIG. 2 , these sensing nodes include a first primary sense node  102 , a second primary sense node  104 , a first alternate sense node  106 , a second alternate sense node  108 , a first primary switched node  110 , a second primary switched node  112 , a first alternate switched node  114 , a second alternate switched node  116  and an output node  118 . In one embodiment, an optoisolator  120  is connected across the output  84  between terminals  86  and  88  to provide the signal at the output node  118 . In one embodiment, for example, the optoisolator corresponds to part number SFH615A-4 manufactured by Vishay. In one embodiment, the sensing nodes  102 ,  104 ,  106 ,  108 ,  110 ,  112 ,  114 ,  116  employ the same optoisolator to generate logic signals. In another embodiment, each pair of sensing nodes (e.g.,  102 / 104 ,  106 / 108 ,  110 / 112 ,  114 / 116 ) can employ a separate optoisolator so that a single logic signal is provided for the primary input, the alternate input, etc. 
     In one embodiment, the multiphase control module  98  of  FIG. 2  include a connection to the system neutral. In a version of this embodiment, the neutral input allows for independent detection of a single-switch failure. For example, such an approach can provide information used to determine whether switch  72  or switch  74  failed. In one or more versions of this embodiment, the neutral conductor is not disconnected when switching is performed. 
     Operation of the multiphase switching modules  70 ,  70 ′ of  FIG. 2  will now be described with reference to the flow charts provided in  FIGS. 3 ,  4  and  5 . In addition, in the interest of clarity, each sense node is associated with a corresponding logic point as follows: the first primary sense node  102  corresponds with logic point PR 1 ; the second primary sense node  104  corresponds to logic point PR 2 ; the first alternate sense node  106  corresponds to logic point ALT 1 ; the second alternate sense node  108  corresponds to logic point ALT 2 ; the first primary switched node  110  corresponds to logic point SWPR 1 ; the second primary switched node  112  corresponds to logic point SWPR 2 ; the first alternate switched node  114  corresponds to logic point SWALT 1 ; the second alternate switched node  116  corresponds to SWALT 2 ; and, the output node  118  corresponds to CKTO. One additional logic point, CPL corresponds to the status of the power supplied to the multiphase control module  98 , i.e., the control power. Each logic point typically provides HI and LO logic levels which depend upon the status of the transfer switch  20 . 
     For purposes of explaining operation of the multiphase switching modules  70 ,  70 ′ in  FIGS. 3 ,  4  and  5  the following logic scheme applies: each of logic points PR 1 , and PR 2  provide a logic HI signal when the corresponding line voltage of the primary source is present at PR 1  and PR 2 , respectively; each of logic points ALT 1  and ALT 2  provide a logic HI signal when the corresponding phase voltage of the alternate source is present at ALT 1  and ALT 2 , respectively; each of logic points SWPR 1 , SWPR 2 , SWALT 1 , and SWALT 2  provide a logic LO signal when a voltage is present at their respective sensing node; the CKTO logic point provides a logic LO signal when voltage is present at the output  84 ; and, the CPL provides a logic HI signal when voltage is present on a control power bus for the multiphase control module  98 . In other embodiments, the logic signals corresponding to the circuit conditions mentioned above differ from the logic states described above. In one embodiment, an opto isolator is used with each logic point (e.g., opto isolator  120 ). In another embodiment, an opto isolator is only used with those logic points that provide a LO signal when voltage is present at the corresponding sensing node. Of course, with the flexibility provided by discrete logic sensing, a variety of configurations may be employed. For example, in one embodiment, each logic point provides a logic HI signal when a voltage is present at the corresponding sensing node. In an alternate embodiment, each logic point provides a logic LO signal when a voltage is present at the corresponding sensing node. 
     The physical location of the sensing nodes may also be changed in various embodiments. In one embodiment, the primary sense nodes  102 ,  104  and the alternate sense nodes  106 ,  108  are not located in multiphase switching modules  70 ,  70 ′. Instead, the primary sense nodes  102 ,  104  can be located on the line side of multiphase switching modules  70 ,  70 ′, for example at primary input  42 . The alternate sense nodes  106 ,  108  can also be located on the line side of switching modules  70 ,  70 ′, for example, at second alternate input  46 . In one embodiment, the sensing nodes  102 ,  104 ,  106 ,  108  are connected to inputs of the main control module  58 . 
       FIG. 3  depicts a flow diagram of a process  300  for controlling the multiphase switching modules  70 ,  70 ′ in accordance with one embodiment. Specifically, the process  300  begins at a time when neither the primary nor the alternate power source is available and continues until such time as power first becomes available on either the primary input  91  or the alternate input  93 , shown in  FIG. 2 . The CPL logic point provides a logic LO signal when neither the primary nor the alternate power sources are available (Stage  310 ). At this time, each of the switches  72 ,  74 ,  76 ,  78 ,  80 ,  82  are in their normally closed (NC) position and the control logic is off. When either primary power or alternate power become available, control logic becomes available and CPL transitions to a logic HI state. If primary power becomes available, PR 1  provides a logic HI signal because voltage is present at first primary sense node  102  and PR 2  provides a logic HI signal because voltage is also present at second primary sense node  104  (Stage  312 ). If alternate power becomes available, ALT 1  provides a logic HI signal because voltage is present at first alternate sense node  106  and ALT 2  provides a logic HI signal because voltage is also present at second alternate sense node  108  (Stage  314 ). 
     Because the default state of the switches  72 ,  74  and  80 ,  82  is normally closed (NC) in the embodiment shown in  FIG. 2 , primary power is supplied to the output  84  of the multiphase switching modules  70 ,  70 ′ if primary power is restored before alternate power is available (Stage  313 ). That is, without operating any switches  72 ,  74 ,  76 ,  78 ,  80 ,  82 , the primary input  91  is connected to the output  84  when neither primary power nor alternate power are available. Conversely, the default state of the switches  76 ,  78  and  80 ,  82  disconnects the alternate input  93  from the output  84  when neither primary power nor alternate power are available. Thus, in one embodiment, when the alternate power supply is available at the alternate input  93 , the multiphase control module  98  processes logic to determine whether the alternate input  93  should be connected to the output  84 . If primary power is unavailable, the primary power source is isolated (Stage  315 ). 
       FIG. 4  depicts a process  400  including a sequence of stages employed in one embodiment to transfer the load connected to output  84  from the primary power source to the alternate power source. The process  400  starts at Stage  313  of  FIG. 3  with the primary power source supplying power to the load. In one embodiment, the multiphase control module  98  continuously monitors the state of PR 1  and PR 2  to determine whether primary power is available (Stage  416 ). As described above, PR 1  and PR 2  each provide a logic HI signal if power is present at the primary input  91 . When PR 1  and PR 2  are HI the primary input  91  remains connected to the output  84 . In one embodiment, the availability of alternate power at alternate input  93  allows a user to override the standard logic, disconnect the primary input  91 , and connect the alternate input  93  to the output  84 , for example, to conduct load testing of the alternate source. In one embodiment, a transition of either or both of PR 1  and PR 2  to a LO state provides an indication that primary power is unavailable. When primary power is unavailable, the multiphase control module  98  determines whether alternate power is available. In one embodiment, the multiphase control module  98  continuously monitors the state of ALT 1  and ALT 2  to determine whether alternate power is available (Stage  418 ). The availability of power at alternate input  93  is indicated when ALT 1  and ALT 2  provide a logic HI signal. For example, in one embodiment, each of ALT 1  and ALT 2  provide a logic LO signal until a generator connected to the alternate input  93  starts and begins to supply voltage to the alternate input  93 , at which time ALT 1  and ALT 2  transition to a logic HI. When the primary source is unavailable and alternate power is sensed, the multiphase control module  98  provides logic signals to open switches  72  and  74  in order to disconnect the primary input  91  from the output  84  (Stage  420 ). In one embodiment, the multiphase control module  98  also provides a generator start signal to start a generator when primary power is unavailable. In another embodiment, the main control module  58  provides the generator start signal. 
     Once multiphase control module  98  provides the logic signals to open switches  72 ,  74 , the multiphase control module  98  determines whether the switches  76 ,  78  connected to the alternate input are in the correct position (Stage  422 ), i.e., normally closed (NC). If either switch  76 ,  78  is in the normally open (NO) position, a voltage is present at the corresponding sensing node  114 ,  116 , respectively. If switch  76  has malfunctioned and remains in the normally open position then the presence of voltage at sensing node  114  is indicated by logic point SWALT 1  providing a logic LO signal. Similarly, if switch  78  has malfunctioned and remains in the normally open position the presence of voltage at sensing node  116  is indicated by logic point SWALT 2  providing a logic LO signal. If either SWALT 1  or SWALT 2  is LO, the multiphase control module  98  is not provided a logic signal to operate switches  76  and  78 , but instead generates a signal indicating that there is a fault with either or both of switches  76 ,  78  (Stage  424 ). In one embodiment, a switch-failure indication is provided at the user interface  60  (e.g., an audible alarm, a flashing display, etc.) and an alarm signal is transmitted to a remote location via the communication module  64  to indicate that there is a problem with transfer switch  20 . Further, although the fault sensing described herein is referred to as detecting a switch failure or switch malfunction, the fault sensing detects any failure mode that prevents contacts from operating according to the switching logic. For example, the multiphase control module  98  will detect a failure due to an open relay coil, mechanical binding of a switch operator, and the like. Thus, switch failure and switch malfunction refer to the failure of a contact to be in the state desired by the switching logic regardless of the cause. 
     If each of SWALT 1  and SWALT 2  is HI, the multiphase control module  98  provides logic signals to operate switches  76 ,  78  (Stage  426 ). The multiphase control module  98  then determines whether the switches  76 ,  78  connected to the alternate input have moved to the normally open (NO) position in response to the logic signals provided by the control module  98  (Stage  428 ). At this time, the presence of voltage (supplied from the alternate input  93 ) at sensing nodes  114  and  116  is expected. Therefore, if both SWALT 1  and SWALT 2  are LO, the switches  76 ,  78  are in the correct positions. Conversely, a logic HI signal provided by SWALT 1  indicates that switch  76  remains in the normally closed (NC) position, and a logic HI signal provided by SWALT 2  indicates that switch  78  remains in the normally closed (NC) position. If either SWALT 1  or SWALT 2  is in a logic HI state, the alternate input  93  is not connected to the output  84 . Instead, the multiphase control module  98  generates a signal indicating that there is fault with either or both of switches  76 ,  78  (Stage  424 ). 
     If each of SWALT 1  and SWALT 2  is LO, the multiphase control module  98  provides logic signals to operate switches  80 ,  82  (Stage  430 ). In the embodiment shown in  FIG. 2 , switches  80 ,  82  are source-selector switches that when in their normally closed (NC) position connect the output  84  to the switches  72 ,  74  connected to the primary input  91 . When in their normally open (NO) position, switches  80 ,  82  connect the output  84  to the switches  76 ,  78  connected to the alternate input  93 . Thus, provided that switches  76 ,  78  are in their normally open (NO) position, the alternate input  93  is connected to the output  84  when switches  80 ,  82  move to their normally open (NO) position in response to logic signals provided by the multiphase control module  98  at Stage  430 . In addition, the multiphase control module  98  confirms whether the switches  80 ,  82  have in fact moved to their normally open (NO) position in response to the logic signals. In the embodiment shown in  FIG. 4 , sensing node  118  provides an indication of the position of switches  80 ,  82  (Stage  432 ). If the CKTO logic point provides a logic HI signal the multiphase control module  98  generates a signal indicating that there is fault with either or both of switches  80 ,  82  (Stage  424 ). In one embodiment, in response to a fault associated with switches  80 ,  82 , the multiphase control module generates logic signals to operate switches  76 ,  78  in order to isolate the alternate input  93  from the faulty switch or switches  80 ,  82 . If the CKTO provides a logic LO signal, the alternate input  93  remains connected to the output  84  until the primary power becomes available again at primary input  91  (Stage  433 ). 
     Referring now to  FIG. 5 , a flow diagram depicts a process  500  employed in one embodiment to transfer the load connected to the output  84  from the alternate power source to the primary power source. Initially, the alternate input  93  is connected to the output  84  in order to supply the load during periods when the primary power source is unavailable (Stage  534 ). In one embodiment, during the period that the primary power source is unavailable, the multiphase control module  98  continuously monitors for the return of the primary power at sensing nodes  102 ,  104  (Stage  536 ). When primary power is again available, both PR 1  and PR 2  provide a logic HI signal. In response, the multiphase control module  98  performs the switching necessary to isolate the alternate input  93  from the output  84  and to connect the primary input  91  to output  84 . The multiphase control module  98  generates logic signals to operate switches  76 ,  78  so that each switch  76 ,  78  moves to the normally closed (NC) position (Stage  538 ). The multiphase control module  98  senses the status of each switch  76 ,  78  based on the state of logic points SWALT 1  and SWALT 2  corresponding to sensing nodes  114 ,  116 , respectively (Stage  540 ). A voltage present on sensing nodes  114 ,  116  indicates that the corresponding switch ( 76 ,  78 , respectively) has malfunctioned and remains in the normally open (NO) position. Thus, if either or both of SWALT 1  and SWALT 2  are LO the multiphase control module  98  senses a malfunction of the corresponding switch ( 76 ,  78 , respectively) and generates a signal (e.g., a fault signal, trouble signal, etc.) indicating that there is fault with either or both of switches  76 ,  78  (Stage  424 ). In one embodiment, a generator supplying the alternate source is shutdown regardless of the status of switches  76 ,  78  when the primary source is again present at the primary input  91 . In one embodiment, once the multiphase control module  98  generates a fault signal none of switches  72 ,  74 ,  76 ,  78 ,  80  and  82  are operational (e.g., the switches are electrically locked out) until the multiphase control module  98  receives an input indicating that the problem was cleared, for example, a user provides such an indication via the user interface  60 . Such an approach assures that a backfeed will not occur as a result of a switch malfunction by preventing, for example, a connection between the first terminal  90  of the primary input  91  and the second terminal  108  of the alternate input  93  via the load connected to the output  84 . 
     The multiphase control module  98  generates a logic signal to operate switches  72 ,  74  from their normally open (NO) position to their normally closed (NC) position if switches  76 ,  78  have successfully switched to their normally closed (NC) position (Stage  542 ). After generating the logic signal to operate switches  72 ,  74 , the multiphase control module  98  senses the status of switches  72 ,  74  based on the state of logic points SWPR 1  and SWPR 2  (Stage  544 ). A voltage present at sensing nodes  110 ,  112  indicates that the switches ( 72 ,  74 , respectively) have moved to their normally closed (NC) position. Thus, if both SWPR 1  and SWPR 2  are LO then switches  110 ,  112  have in fact moved to their normally closed (NC) position, and the multiphase control module  98  will generate a logic signal to operate switches  80 ,  82  to move them from their normally open (NO) position to their normally closed (NC) position (Stage  546 ). If, however, either SWPR 1  or SWPR 2  are HI then the corresponding switch ( 72 ,  74 , respectively) has malfunctioned and therefore failed to move to the normally closed (NC) position. As a result, the multiphase control module  98  senses a switch malfunction and generates a signal (e.g., a fault signal, trouble signal, etc.) indicating that there is fault with either or both of switches  72 ,  74  (Stage  424 ). 
     The multiphase control module  98  generates logic signals to operate switches  80 ,  82  from their normally open (NO) position to their normally closed (NC) position if switches  72 ,  74  have successfully switched to their normally closed (NC) position (Stage  546 ). After generating the logic signal to operate switches  80 ,  82 , the multiphase control module  98  senses the status of switches  80 ,  82  based on the state of logic point CKTO (Stage  548 ). When voltage is present at the first terminal  86  of the output  84  and the second terminal of the output  88 , CKTO will provide a LO signal indicating that switches  80 ,  82  have successfully moved to their normally closed (NC) position. Conversely, with CKTO HI the multiphase control module  98  senses a malfunction of at least one of the switches  80 ,  82  and generates a signal (e.g., a fault signal, trouble signal, etc.) indicating that there is fault with either or both of switches  80 ,  82  (Stage  424 ). The primary input  92  remains connected to the output  84  provided that switches  80 ,  82  successfully switched to their normally closed (NC) position and provided that multiphase control module  98  does not initiate a transfer to an alternate source, for example, if the primary power source fails. 
     The embodiment described with reference to  FIGS. 3 ,  4  and  5  employs the multiphase control module  98  to detect the availability of the primary and the alternate sources, i.e., via PR 1 /PR 2  and ALT 1 /ALT 2 , respectively. In another embodiment, however, the main control module  58  is employed to monitor logic points PR 1 , PR 2 , ALT 1 , and ALT 2 . In a version of this embodiment, the multiphase control module  98  waits to receive information from the main control module  58  concerning the status of the primary and the alternate sources of power before performing any switching. 
     In one embodiment, the logic described with reference to  FIGS. 3 ,  4  and  5  is implemented in a state machine implemented by the multiphase control module  98 . In a version of this embodiment, a CPLD includes the state machine. Further, the state machine can be embodied in an algorithm or a plurality of algorithms stored in the memory and executed by a processor or processors located in the multiphase control module  98 . In one embodiment, the memory is included in the multiphase control module  98 . In a version of the preceding embodiments, a portion of the logic described with reference to  FIGS. 3 ,  4  and  5  is implemented in the multiphase control module  98  and another portion of the logic is implemented in the main control module  58 . 
     Each of the switching modules described thus far can also include integral overcurrent protection. For example, in embodiments of each of the switching modules described herein (e.g.,  52 ,  54 ,  70 , etc.), a fuse may be included in each line conductor. In a version of this embodiment, the fuse is connected to the output of the switching module with which it is associated (e.g., each line of output  84  of  FIG. 2 ). In addition, embodiments of each of the switching modules described herein (e.g.,  52 ,  54 ,  70 , etc.) may include current and voltage sensing. For example, in an embodiment of multiphase switching modules  70 ,  70 ′ of  FIG. 2 , a first current sensor senses current flow at a point between switch  80  and first terminal  86 , a second current sensor senses current flow at a point between switch  82  and second terminal  88 . In a version of this embodiment, a primary of a first potential transformer is connected to first terminal  86  and second terminal  88  to decrease the voltage level supplied to the multiphase control module  222 . The outputs of the current sensors and the voltage sensors can be supplied to the main control module  58  for processing, display at the user interface  60 , and communication to remote systems via the communication module  64  shown in  FIG. 1 . 
       FIGS. 6 ,  7  and  8  depict alternative embodiments of multiphase switching modules. Each of the switching modules shown in  FIGS. 6 ,  7  and  8  can be employed in a transfer switch, for example, the transfer switch  20  shown in  FIG. 1 , in place of multiphase switching modules  54 ,  54 ′. 
     In  FIG. 6 , switching modules  121 ,  121 ′ are employed to control a two phase electric supply to a load  123 . A primary source of power is connected to a primary input  130  and an alternate source of power is connected to an alternate input  132 . The switching modules  121 ,  121 ′ also provide an output  133 . The primary input  130  includes a first terminal  122  and a second terminal  124  to connect a first phase and a second phase of the primary source of power to the multiphase switching modules  121 ,  121 ′, respectively. The alternate input  132  includes a first terminal  126  and a second terminal  128  to connect a first phase and a second phase of the alternate source of power to the multiphase switching modules  121 ,  121 ′. In the embodiment shown in  FIG. 6 , switches  134 ,  136 ,  138 ,  140 ,  142 ,  144  are included in the multiphase switching modules  121 ,  121 ′. In another embodiment, however, the switches  134 ,  136 ,  138 ,  140 ,  142 ,  144  are included in a single multiphase switching module. The approach shown in  FIG. 6  is scalable, so that in yet another embodiment a multiphase switching module is included to control a third phase of a three phase electric supply. In one embodiment, switches  134  and  136  are employed to isolate the alternate input  132  from the remainder of the circuitry. The switches of one or more switching modules form sets of switches where each set is used to switch all phases of a specific input or output. In  FIG. 6 , for example, switches  134 ,  136  form a first set of switches that switch the alternate input  132 , switches  138 ,  140  form a second set of switches that switch the primary input  130 , and switches  142 ,  144  form a third set switches that switch the output  133 . Each set of switches in  FIG. 6  include a pair of switches, however, each set may include three or more switches depending upon the requirements of the application, e.g., the quantity of line conductors provided by the power sources. 
     The embodiment of  FIG. 6  also includes a multiphase control module  146 . The multiphase control module  146  includes a first module  148  for sensing and control of a first phase and a second module  150  for sensing and control of a second phase. In alternative embodiments, the multiphase control module  146  may be a single module as shown for the embodiment in  FIG. 1 . A neutral conductor  152  is also connected to multiphase control module  146 . As described above concerning the multiphase control modules  70 ,  70 ′ in  FIG. 2 , the multiphase control module  146  of  FIG. 6  includes sensing inputs and switching outputs to ensure that a backfeed between the primary source and the alternate source does not occur at any time, even if one or more of the switches  134 ,  136 ,  138 ,  140 ,  142  and  144  malfunction. For example, the sensing and control logic of the multiphase control module  146  prevent a backfeed that can result when a switch becomes inoperative as a result of a welded contact. In one embodiment, the approach employed in the control of the multiphase switching modules  121 ,  121 ′ is generally the same as that previously described for switching modules  70 ,  70 ′. Switching modules  121 ,  121 ′ include sensing nodes that provide signals to the multiphase control module  146  that allow the control module  146  to detect one or more inoperative switches among switches  134 ,  136 ,  138 ,  140 ,  142  and  144  and control switch operation to prevent a connection between the primary input  130  and the secondary input  132 . 
     The configuration of switches  134 ,  136 ,  138 ,  140 ,  142  and  144  in the embodiment shown in  FIG. 6  does differ from the configuration of switches  72 ,  74 ,  76 ,  78 ,  80  and  82  in the embodiment shown in  FIG. 2 . For example, in  FIG. 6 , the switches  138 ,  140  used to switch the primary input  130  have a terminal connected to the primary input  130  and a terminal connected to the switches  134 ,  136  used to switch the alternate input  132 , i.e., switch  134  is connected to switch  138  and switch  136  is connected to switch  140 . Conversely, in the embodiment shown in  FIG. 2 , the switches  72 ,  74  used to switch the primary input  91  are not connected to the switches  76 ,  78  used to switch the alternate input  93 . Instead, in the embodiment shown in  FIG. 2 , switches  80 ,  82  selectively connect the output  84  to either switches  72 ,  74 , or switches  76 ,  78 . 
     In  FIG. 7 , a further embodiment of multiphase switching modules  154 ,  154 ′, which can be used in place of the multiphase switching modules  54 ,  54 ′ of  FIG. 1  in the transfer switch  20 , is shown. The multiphase switching modules  154 ,  154 ′ include a primary input  157 , an alternate input  158  and an output  159 . The multiphase switching modules  154 ,  154 ′ include a set of switches  160 ,  162  to switch the primary input  157 , a set of switches  164 ,  166  to switch the alternate input  158 , and a set of switches  168 ,  170  to switch the output  159 . Switches  160 ,  164  and  168  are included in a first multiphase switching module  154 , and switches  162 ,  166  and  170  are included in a second multiphase switching module  154 ′. The switch configuration of the embodiment shown in  FIG. 7  is the same as the switch configuration shown in  FIG. 6 , however, the embodiment of  FIG. 7  differs from the embodiment shown in  FIG. 6 . As one example, the multiphase switching modules  154 ,  154 ′ may be connected to a test load  172 . In one version, the test load  172  is located external to the transfer switch  20 . In another version, the test load  172  is included within the transfer switch  20 . In addition, multiphase switching modules  154 ,  154 ′ employ current sensors  174 ,  174 ′ to detect current flow resulting from a malfunction of any of the switches  160 ,  162 ,  164 ,  166 ,  168 ,  170  employed to control the two phase electric supply to a load  176 . In one embodiment, the current sensors  174 ,  174 ′ are a torodial style current transformer, for example, part number L12003 manufactured by Falco Electronics. 
     In the embodiment shown in  FIG. 7 , a multiphase control module  178  employs logic to prevent a backfeed between any phase of the primary input  157  and any phase of the alternate input  158 , however, the logic differs from that described regarding  FIG. 6  because in addition to teachings described above, current sensing may be used with the embodiment of  FIG. 7  to detect a malfunction of the switches  168 ,  170 . For example, when the connection to the output  159  is transferred from the primary input  157  to the alternate input  158 , switches  168 ,  170  operate to disconnect the load and connect the test load  172  to the primary input  157 . Switches  160 ,  162 ,  164 ,  166  are then switched to connect the alternate input  158  to the test load  172 . Provided that there are no inoperative switches, there is no current flow in the current transformer secondaries  180 ,  182  during these switching operations. More specifically, in one embodiment, switches  160 ,  162  are switched to the position that connects them to switches  164 ,  166 , respectively. The switches  164 ,  166  are switched to the position that connects alternate input  158  to switches  160 ,  162 . If the primary input  157  is connected to the alternate input  158  during the preceding switching operations, for example, as a result of a switch malfunction, any current flowing from an input  157 ,  158  does not have a return path that is sensed by the corresponding current transformer. As a result, provided that at least one of the primary input  157  and the alternate input  158  are energized, current flows in the corresponding current transformer secondary  180  or  182 . The corresponding current transformer output  181 ,  183  then provides a signal to drive an input of the multiphase control module  178 . The multiphase control module  178  provides a fault indication and, in one embodiment, prevents any subsequent switch operation until a user resets the fault indication (e.g., after investigating and clearing the fault condition, for example, replacing a failed switch). If there are no switch malfunctions, however, switches  168 ,  170  are operated to connect the output  159  to the alternate input  158 . Again, provided that there are no inoperative switches, there should be no current flow in the current transformer secondaries  180 ,  182 . 
     In one embodiment, the current drawn by the test load  172  is sufficiently small that it is not considered a hazard if a backfeed between the primary input  157  and the alternate input  158  is created through the test load  172 . In one embodiment, a current is considered non-hazardous if it is less than 3.5 milliamperes. In another embodiment, a lower voltage source (e.g., a DC source or an AC source supplied from a step-down transformer) is connected to the unused terminal of each of switches  164 ,  166  to provide a means of providing a low power test current. 
       FIG. 8  shows a single multiphase switching module  185  which can be employed, for example, in the transfer switch  20  shown in  FIG. 1  in place of both the multiphase switching modules  54 ,  54 ′. The embodiment of  FIG. 8  includes a primary input  184 , an alternate input  186 , an output  188 , switches  190 ,  192 ,  194 ,  196 ,  198 ,  200 , and sensing nodes  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218 . The switch configuration of the embodiment shown in  FIG. 8  is similar to the switch configuration show in  FIG. 7 , however, in  FIG. 8  the test load  172  of  FIG. 7  is replaced with a current source  220 . In addition, current sensors are not included at the primary input and the secondary input in the embodiment shown in  FIG. 8 . In one embodiment (described below), however, current sensing is employed to determine the status of switches  198 ,  200 , in particular, sensing current flow from the current source  220 . 
     In one embodiment, each of the sensing nodes  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  is connected to a multiphase control module  222  over communication bus  224 . As previously described with reference to  FIG. 2 , switch control signals (not shown) control operation of the switches  190 ,  192 ,  194 ,  196 ,  198 ,  200 . In one embodiment, the switch control signals are provided to the multiphase switching module  185  by the multiphase control module  222  over the communication bus  224 . Logic for the switching module  185  is implemented in a fashion very similar to that described for one embodiment of the multiphase switching modules  70 ,  70 ′ of  FIG. 2 . For example, in one embodiment, the status of switches  190 ,  192 ,  194  and  196  is determined by voltage sensing. The status of switches  198 ,  200  connected to the output  188 , however, is determined by the current flow in the circuit created by the current source  220 , switches  198 ,  200  in their normally closed (NC) position, and a load  226  connected to the output  188 . Thus, during normal operation, current from the current source  220  flows through the load  226  and the switches  198 ,  200  when the output  188  is isolated from the switches  190 ,  192 . 
     In the embodiment shown in  FIG. 8 , with the switches  198 ,  200  in their normally open (NO) position, the transfer from the first input  184  to the second input  186  begins when the switches  198 ,  200  are disconnected from the load  226  and connected to the current source  220 . If the switches  198 ,  200  operate properly, current flows from the current source  220  through the load  226 . The presence of current is determined when logic associated with sensing node  214  provides a HI signal indicating that switches  198 ,  200  are in their normally closed position. The multiphase control module  222  detects a malfunction of at least one of switches  198  and  200  when sensing node  214  provides a logic LO signal indicating that current is not flowing as expected. The multiphase control module  222  provides a fault indication and, in one embodiment, prevents any subsequent switch operation until a user resets the fault indication (e.g., after investigating and clearing the fault condition, for example, replacing a failed switch). If switches  198 ,  200  operate as expected, the switches  190 ,  192  are switched to their normally open (NO) position. The switches  194 ,  196  are switched to their normally open (NO) position. Voltage sensing at sensing nodes  210 ,  212  is used to determine whether switches  194  and  196  have operated properly. In one embodiment, the presence of voltage at sensing nodes  210 ,  212  indicates that he switches  194 ,  196  operated properly. The switches  198 ,  200  are then switched to their normally open (NO) position to connect the alternate input  186  to the output  188 . If the switches  198 ,  200  operated properly, the logic associated with sensing node  214  transitions to a LO signal indicating that current from current source  220  is no longer flowing through switches  198 ,  200 . 
     In one embodiment, multiphase switching module  185  includes two multiphase switching modules (as shown in the previous embodiments) with the current source  220  located external to each switching module. 
     Several aspects of at least one embodiment of this invention have been described herein with reference to 240 volt systems. The embodiments described herein, however, may also be employed with a wide range of systems including three phase systems and systems operating at voltages such as, for example, 480 volt systems. Further, the embodiments of the invention may be employed in AC and DC systems including AC systems operating at 50 Hz, 60 Hz or other frequencies. 
     Embodiments of transfer switch  20  described herein can also be configured to selectively switch between three or more multiphase sources of power, for example, a primary power source, a first alternate power source, and a second alternate power source. In these embodiments, the multiphase switching modules (e.g.,  70 ,  70 ′) are configured to switch between the multiple sources, i.e., the embodiments described herein are scalable. In versions of these embodiments, the transfer switch complies with UL 1008 and prevents a backfeed even when only a single switch fails. 
     Further, embodiments of the invention including those described herein may be employed in a variety of transfer switches. For example, embodiments of the invention may be employed in an automatic transfer switch that switches between a primary source and an alternate source without user intervention. Versions of these embodiments may also provide a start signal to automatically start a generator. Embodiments of the invention may also be employed in transfer switches that switch between a primary power source and an alternate power source when instructed to do so by a user. In these embodiments, the multiphase control module can employ logic as described herein to prevent a backfeed between two or more sources of power. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.