Patent Publication Number: US-2023155370-A1

Title: Electrical power distribution systems with a bypass unit that couples to a load and electrically engages one of two alternate units for powering the load and related methods

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/911,899, filed Jun. 25, 2020, which claims the benefit of and priority to U.S. Provisional Application No. 62/867,995, filed Jun. 28, 2019, each of which is hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electrical distribution systems and is particularly suitable for motor control centers. 
     BACKGROUND OF THE INVENTION 
     In general, electrical power distribution systems distribute electrical power from power sources, such as private or public power grids, to different loads, such as motors. As a specific example, motor control centers (MCC) distribute electrical power to and control motors in a central location. MCCs can include structures, cabinets or enclosures containing a common power bus and multiple, typically modular, bucket assemblies or units, which generally contain a motor starter of various types, a circuit breaker or fuse(s), and a power disconnect. See, e.g., U.S. Pat. No. 4,024,441, the contents of which are hereby incorporated by reference as if recited in full herein. Eaton Corporation has recently introduced a MCC product line with compact bucket assemblies that conveniently plug into a slot, compartment or space in an MCC structure. The product is sold under the product name, Freedom 2100 MCC. See also, U.S. Pat. No. 8,934,218, the contents of which are hereby incorporated by reference as if recited in full herein. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the invention are directed to enclosed bypass units that can connect to separate primary and secondary units with motor starters, allowing for one to power a load at any one time through the bypass unit while electrically isolating the other thereby providing redundant powering options which can electrically isolate the primary and secondary units from a load and/or line side bus and each other. 
     Embodiments of the invention are directed to electrical power distribution systems that include: a power bus; a bypass unit with a power transfer switch coupled to a load; a first unit having a first disconnect switch configured to couple the power bus to the power transfer switch; and a second unit having a second disconnect switch configured to couple the power bus to the power transfer switch. The power transfer switch has mechanically interlocked first and second contactors configured to selectively couple only one of the first and second units to the load at any one time. The bypass unit includes a circuit breaker configured to couple the first unit to the first contactor. The circuit breaker has an operator mechanism configured to be moved between ON and OFF positions to close and open the circuit breaker, respectively, and configured to allow a user to lock the operator mechanism in the ON or OFF position. When the first disconnect switch is closed and the power transfer switch couples the first unit to the load, the second disconnect switch is open to electrically isolate the second unit from the load and the first unit and when the first disconnect switch is closed and the power transfer switch couples the second unit to the load, the first disconnect switch is open to electrically isolate the first unit from the load and the second unit. At any one point in time during normal operation, the power transfer switch of the bypass unit is configured to electrically connect the power bus to the load using only one of the first unit or the second unit. When the power transfer switch couples the first unit to the load, the first disconnect switch is closed while the second disconnect switch is open to electrically isolate the second unit from the load and the first unit, and when the power transfer switch couples the second unit to the load, the second disconnect switch is closed while the first disconnect switch is open to electrically isolate the first unit electrically from the load and the second unit. 
     The first disconnect switch can include a first circuit breaker and the second disconnect switch can include a second circuit breaker. The first and second circuit breakers and the power transfer switch can be electrically interlocked, such that, when the power transfer switch couples one of the first and second units to the load, only the associated first or second circuit breaker can be closed. 
     The power transfer switch can further include at least one auxiliary switch coupled to the circuit breakers that is configured to transmit a trip signal to the circuit breaker of the first disconnect switch when the power transfer switch couples the second unit to the load. 
     Embodiments of the invention are directed to electrical power distribution systems that include a bypass unit with a power transfer switch. The power transfer switch includes mechanically interlocked first and second contactors configured to electrically couple to a load and define first and second electrical paths where only one of the first and second contactors close at any one time. The bypass unit further includes a circuit breaker in the bypass unit coupled to the first electrical path to thereby inhibit feedback to an isolated unit. The circuit breaker has an externally accessible operator handle that faces a front of the bypass unit and is configured to allow a user to lock the handle in an off position associated with non-conduction in the first electrical path. The electrical distribution device also include a first unit with a first disconnect switch electrically coupled to the first electrical path of the power transfer switch and electrically coupled to a circuit breaker and a second unit with a second disconnect switch electrically coupled to the second electrical path of the power transfer switch. At any one point in time during normal operation, the power transfer switch of the bypass unit is configured to electrically connect a power bus to the load using only one of the first unit or the second unit. When the power transfer switch electrically connects the first unit to the load, the first disconnect switch is electrically on and allows conduction in an electrically active (energized) state while the second disconnect switch is electrically off and in an electrically inactive (non-energized) state with the second unit electrically isolated from the load and the first unit. When the power transfer switch electrically connects the second unit to the load, the second disconnect switch is electrically on and allows conduction in an electrically active (energized) state while the first disconnect switch is electrically off and in an electrically inactive (non-energized) state with the first unit electrically isolated from the load and the second unit. 
     The first and second units can each further comprise a power disconnect assembly with extendable/retractable power stabs that move relative to a rear of the first and second units, respectively, to connect and disconnect from the power bus. Only a single one of the first and second units can be operated to have power stabs that are in an extended state to connect to the power bus at any one time. 
     The bypass unit can have an enclosure with a front wall, side walls and a rear wall and can be devoid of power stabs and does not directly connect to the power bus. 
     Each of the bypass unit, the first unit and the second unit can be held in separate housings and are independently slidably removable from a respective compartment in at least one structure of the electrical distribution device. 
     The first unit can include a first motor starter and the second unit can include a second motor starter. The electrical distribution device can further include at least one structure with defined spaced apart internal compartments that slidably hold the bypass unit, the first unit and the second unit in different ones of the defined spaced apart internal compartments. The first unit can have a first front door with a first disconnect operator handle in communication with the first disconnect switch. The second unit can have a second front door with a second disconnect operator handle in communication with the second disconnect switch. The electrical distribution device can include interlocks that (a) prevent the first door from opening when either the first disconnect switch is on or the first motor starter is electrically coupled to the load and (b) prevent the second door from opening when either the second disconnect switch is on or the second motor starter is electrically coupled to the load. 
     The bypass unit and the first and second units can reside inside spaced part compartments of a structure defining internal compartments. The second electrical path in the bypass unit can be devoid of a circuit breaker. 
     The bypass unit can further include a manually activatable back-up bypass path as a third path in the bypass unit comprising a circuit breaker that connects the load to the second unit without requiring the second path from the second contactor. Optionally the third path and the second path can merge in the bypass unit to a common conductor output from the bypass unit. The common conductor output can be configured to couple to only one unit, i.e., only the second unit. 
     The first and second disconnect switches can be circuit breakers. At least one of the bypass unit or the first and second disconnect switches can be electrically coupled to cause a first one of either the first disconnect switch or the second disconnect switch to trip to an open state when a second one of the first disconnect switch or the second disconnect switch is on to allow conduction whereby only a single one of the first or second disconnect switch is in an on state at any one time. 
     The electrical power distribution device can be a motor control center (MCC). 
     The first disconnect switch and the second disconnect switch can each include a circuit breaker. The power transfer switch can include auxiliary switches that are configured to automatically transmit a trip signal to the first unit prior to or concurrently with engaging the second contactor. 
     The electrical distribution device can further include a relay system coupled to the first unit. The relay system can be configured to detect a malfunction or power off event of the first unit and then (i) direct the power transfer switch to automatically transfer power from the first unit and the first electrical path to the second unit and the second electrical path to the load and (ii) automatically directly or indirectly via an auxiliary switch transmit a control signal to the circuit breaker in the bypass unit to cause the circuit breaker to trip. 
     The first unit and the second unit can be provided as one of options A-L: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Option 
                 first unit 
                 second unit 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 VFD unit 
                 VFD unit 
               
               
                   
                 B 
                 Soft Starter 
                 Soft Starter 
               
               
                   
                   
                 (Reduced Voltage 
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
                 starter) unit 
               
               
                   
                 C 
                 NEMA Starter unit 
                 NEMA Starter unit 
               
               
                   
                 D 
                 IEC Starter unit 
                 IEC Starter unit 
               
               
                   
                 E 
                 VFD unit 
                 Soft Starter 
               
               
                   
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                   
                 starter) unit 
               
               
                   
                 F 
                 Soft Starter 
                 NEMA Starter unit 
               
               
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
               
               
                   
                 G 
                 NEMA Starter unit 
                 IEC Starter unit 
               
               
                   
                 H 
                 VFD unit 
                 NEMA Starter unit 
               
               
                   
                 I 
                 Soft Starter 
                 IEC Starter unit 
               
               
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
               
               
                   
                 J 
                 VFD unit 
                 IEC Starter unit 
               
               
                   
                 K 
                 Feeder Breaker 
                 Feeder Breaker 
               
               
                   
                 L 
                 Feeder Fused 
                 Feeder Fused 
               
               
                   
                   
               
            
           
         
       
     
     The bypass unit can further include a first lead coupled to a first set of switch contacts of the first contactor, a second lead coupled to the second set of switch contacts of the second contactor, and a third lead configured to couple to the load, the first lead can extend from the bypass unit a distance sufficient to couple to the first unit, the second lead can extend from the bypass unit a distance sufficient to couple to the second unit, and the third lead can have a length sufficient to couple to the load. 
     The first and second leads can extend from the bypass unit through a wire way to connect to the respective first and second units. 
     The electrical distribution device can be a motor control center (MCC). The first and second units can each further include a power disconnect assembly with extendable/retractable power stabs that move relative to a rear of the first and second units, respectively, to connect and disconnect from the power bus. Only a single one of the first and second units can extend respective power stabs to connect to the power bus at any one time. The first unit and the second unit can each have a lock that, when deployed, physically (a) locks the power disconnect assembly in at least one defined position associated with one or both (i) a retracted position associated with an electrically isolated state of the respective unit or (ii) an extended position associated with engagement with the power bus and an electrically active state, and/or (b) locks a laterally movable slide from allowing access to an aperture that allows a crank to change a position of the power stabs. 
     The electrical power distribution device can further include at least one interlock that is configured to allow a user to open or slidably remove the first unit from the MCC only when the first unit is in the electrically isolated state and the bypass unit circuit breaker is off while allowing the second unit to be energized and power the load through the bypass unit. 
     Each of the first and second units can be slidably and releasably held in defined separate compartments of a structure of the electrical distribution device and each is serially interchangeable with a different corresponding unit having a common size and shape housing thereby allowing modular repair and replacement. The bypass unit can be held in a housing with a closed front door and has a height that is in a range of about 6-12 inches. 
     The bypass unit, the first unit and the second unit can be held in separate housings. The bypass unit, the first unit and the second unit can be configured to power a load with horsepower in a range of about ¼ horsepower to 200 horsepower. The bypass unit can have a closed front door with a front operating switch handle coupled to the power transfer switch. 
     The power transfer switch of the bypass unit can further include at least one auxiliary switch. The at least one auxiliary switch can be configured to transmit and/or receive control signals to/from at least one of the first or second units. 
     Other embodiments are directed to methods of assembling an electrical power distribution system. The methods include providing a bypass unit in a housing comprising a power transfer switch configured to electrically couple to only first and second separate units held in separate respective housings to power a load using only one of the first and second units at any one time. The power transfer switch has mechanically interlocked first and second contactors configured to electrically couple to a load and define first and second electrical paths where only one of the first and second contactors close at any one time. The bypass unit can further include a circuit breaker in the bypass unit coupled to the first electrical path to thereby inhibit feedback to an isolated unit and the circuit breaker can have an externally accessible operator handle that faces a front of the bypass unit and is configured to allow a user to lock the handle in an off position associated with non-conduction in the first electrical path. The method also includes allowing a user to select only two units as the first and second units to connect to the bypass unit from one of options A-L: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Option 
                 first unit 
                 second unit 
               
               
                   
                   
               
             
            
               
                   
                 A 
                 VFD unit 
                 VFD unit 
               
               
                   
                 B 
                 Soft Starter 
                 Soft Starter 
               
               
                   
                   
                 (Reduced Voltage 
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
                 starter) unit 
               
               
                   
                 C 
                 NEMA Starter unit 
                 NEMA Starter unit 
               
               
                   
                 D 
                 IEC Starter unit 
                 IEC Starter unit 
               
               
                   
                 E 
                 VFD unit 
                 Soft Starter 
               
               
                   
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                   
                 starter) unit 
               
               
                   
                 F 
                 Soft Starter 
                 NEMA Starter unit 
               
               
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
               
               
                   
                 G 
                 NEMA Starter unit 
                 IEC Starter unit 
               
               
                   
                 H 
                 VFD unit 
                 NEMA Starter unit 
               
               
                   
                 I 
                 Soft Starter 
                 IEC Starter unit 
               
               
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
               
               
                   
                 J 
                 VFD unit 
                 IEC Starter unit 
               
               
                   
                 K 
                 Feeder Breaker 
                 Feeder Breaker 
               
               
                   
                 L 
                 Feeder Fused 
                 Feeder Fused 
               
               
                   
                   
               
            
           
         
       
     
     The first and second units can each include a power disconnect assembly having extendable/retractable power stabs. 
     The methods can further include slidably inserting the bypass unit and the selected first and second units into compartments of a structure of the electrical distribution system and electrically connecting the power transfer switch of the bypass unit to the selected first and second units during, before or after the inserting. 
     The electrical power distribution system can be a motor control center. 
     The selected first and second units and the bypass unit each have a dedicated, respective front door. The bypass unit can have a closed rear panel with a plurality of conductors extending outward therefrom including a conductor that couples to a load, and conductors that couple to the selected first and second units. 
     The power transfer switch of the bypass unit can have at least one auxiliary switch attached thereto and the at least one auxiliary switch can be configured to transmit and/or receive control signals to/from at least one of the first or second units. 
     A first set of three switch contacts of the first contactor can be coupled only to the first unit while a second set of three switch contacts of the second contactor, different from the first set of three switch contacts, can be coupled only to the second unit to thereby serially couple the load to the first unit and the second unit via the bypass unit. Optionally, the bypass unit can further include a manually activatable back-up bypass path with a circuit breaker that connects the load to the second unit. 
     Yet other embodiments are directed to a bypass unit. The bypass unit include: a housing comprising a rear wall, a front panel, opposing side walls, a floor and a ceiling defining an enclosure. The housing holds a bypass circuit with a power transfer switch. The power transfer switch includes mechanically interlocked first and second contactors configured to electrically couple to a load and define first and second electrical paths where only one of the first and second contactors close at any one time. The bypass unit can further include a circuit breaker in the bypass unit coupled to the first electrical path to thereby inhibit feedback to an isolated unit, wherein the circuit breaker has an externally accessible operator handle that faces a front of the bypass unit and is configured to allow a user to lock the handle in an off position associated with non-conduction in the first electrical path. The bypass unit also includes a first conductor coupled to a first set of three switch contacts of the first contactor; a second conductor coupled to a second set of three switch contacts of the second contactor; and a third conductor coupled to a load side of the power transfer switch and extending out of the housing with a length sufficient to couple to a load. The first and second conductors extend a length outside the housing. The first conductor is configured to electrically couple a first unit with a first motor starter with the first set of switch contacts and the second conductor is configured to electrically couple a second unit with a second motor starter to the second set of switch contacts. 
     The housing can have a height in a range of about 6-12 inches. 
     The power transfer switch can be configured to power a load from a power bus to the load using the second unit only when a disconnect switch of the first unit and the circuit breaker of the bypass unit are in an off state associated with non-conduction to thereby provide electrical isolation of the first unit allowing a user to access or remove the first unit from a structure while the second unit is powering the load through the bypass unit. 
     The housing can be rectangular and configured to be slidably and releasably held in a compartment of a structure of an electrical distribution device. 
     The bypass unit can further include at least one auxiliary switch in the bypass unit coupled to the power transfer switch. The at least one auxiliary switch can be configured to transmit and/or receive control signals to/from at least one of the first or second units. 
     The bypass unit can further include a manually activatable back-up bypass path with a circuit breaker that is configured to connect the load to the second unit as a back-up to the second contactor. The circuit breaker of the back-up bypass path can have an operator handle. Only one of the circuit breaker of the back-up bypass path and the circuit breaker coupled to the first contactor can be in an on position associated with conduction at any one time. 
     Yet other aspects of the invention are directed to methods of transferring power from one unit to another unit to power a load using a motor control center (MCC). The methods include: providing an MCC with a bypass unit in a housing comprising a power transfer switch configured to serially electrically couple to a single one of first and second units held in separate respective housings to a load at any one time to thereby provide a redundant, back-up drive capacity; and mechanically interlocking first and second contactors of the power transfer switch to electrically couple either the first or the second contactor to the load at any one time and define first and second electrical paths whereby only one of the first and second contactors close at any one time. The first electrical path is electrically coupled to a circuit breaker in the bypass unit and the first unit and the second electrical path is coupled to the second unit. The methods also include electronically detecting a power failure or malfunction of the first unit; automatically transmitting a trip signal to the circuit breaker in the bypass unit; automatically engaging the second contactor and disengaging the first contactor of the power transfer switch to power the load using the second unit and the second electrical path; automatically tripping a disconnect switch in the first unit; and automatically tripping the circuit breaker in the bypass unit to thereby isolate the first unit from the load. 
     The methods can further include allowing a user to manually engage a back-up bypass path comprising a circuit breaker that connects the load to the second unit. The disconnect switch in the first unit can be interlocked in an off position in response to the manual engagement. 
     Further features, advantages and details of the present invention will be appreciated by those of ordinary skill in the art from a reading of the figures and the detailed description that follow, such description being merely illustrative of the present invention. 
     It is noted that aspects of the invention described with respect to one embodiment, may be incorporated in a different embodiment although not specifically described relative thereto. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination. Applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to be able to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. These and other objects and/or aspects of the present invention are explained in detail in the specification set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of an exemplary electrical power distribution system according to embodiments of the present invention. 
         FIG.  2 A  is a schematic illustration of an exemplary bypass unit according to embodiments of the present invention. 
         FIG.  2 B  is a front, side perspective view of the bypass unit shown in  FIG.  2 A  according to embodiments of the present invention. 
         FIG.  2 C  is a rear, side perspective view of the bypass unit shown in  FIG.  2 A  according to embodiments of the present invention. 
         FIGS.  3 A,  3 B, and  3 C  are box diagrams of electrical distribution systems according to embodiments of the present invention. 
         FIGS.  4 - 6    are schematic illustrations of electrical distribution systems according to embodiments of the present invention. 
         FIG.  7 A  is an enlarged front view of a transfer switch and deadfront switch handle suitable for the bypass unit shown in  FIG.  2 B  according to embodiments of the present invention. 
         FIG.  7 B  is a schematic illustration of an electrical distribution system according to embodiments of the present invention. 
         FIG.  8    is a partial front side perspective view of a motor control center (MCC) according to embodiments of the present invention. 
         FIG.  9    is a top, side perspective view of an example unit with power stabs according to embodiments of the present invention. 
         FIG.  10    is a bottom view of the example unit shown in  FIG.  9    according to embodiments of the present invention. 
         FIG.  11    is a table of actions/conditions an electrical interlocking system for electrical distribution systems according to embodiments of the present invention. 
         FIG.  12    is a table of actions/conditions for a mechanical interlocking system according to embodiments of the present invention. 
         FIG.  13 A  is a partial front view of a front panel segment of an example unit with a keyed interlock according to embodiments of the present invention. 
         FIG.  13 B  is a top view of the device shown in  FIG.  13 A . 
         FIG.  14 A  is an enlarged view of the keyed interlock shown in  FIG.  13 A . 
         FIG.  14 B  is an enlarged view of a mechanically locked slide of a unit of the MCC shown in  FIG.  8   . 
         FIG.  15    is a schematic illustration of an electrical distribution system with a power transfer switch having first and second contactors in combination with a manually engageable back-up bypass path according to embodiments of the present invention. 
         FIG.  16    is a front perspective view of another embodiment of a bypass unit suitable for the electrical distribution system shown in  FIG.  15    according to embodiments of the present invention. 
         FIG.  17    is a partial front view of a bypass unit suitable for the electrical distribution system shown in  FIG.  15    according to embodiments of the present invention. 
         FIG.  18    is a top schematic view of a bypass unit with a mechanical interlock suitable for the electrical distribution system shown in  FIG.  15    according to embodiments of the present invention. 
         FIG.  19 A / 19 B is a flow chart of actions that can be carried out to power a load according to embodiments of the present invention. 
         FIG.  20    is a flow chart of an example build process allowing for end user builds from a defined set of modular unit options according to embodiments of the present invention. 
         FIG.  21    is a flow chart of example actions that can be used to power a load according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. Like numbers refer to like elements and different embodiments of like elements can be designated using a different number of superscript indicator apostrophes (e.g., 10, 10′, 10″, 10′″). 
     In the drawings, the relative sizes of regions or features may be exaggerated for clarity. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures. For example, if the device or system in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device or system may be otherwise oriented (rotated 90° or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The term “about”, when used with a number, refers to numbers in a range of +/−20% of the noted value(s). 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms “includes,” “comprises,” “including” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     The terms “operating mechanism” and “operator mechanism” are used interchangeably and refer to an assembly that is a primary disconnect for a unit and typically has a manually operative lever for opening and closing separable contacts in a circuit breaker and/or for turning power ON and OFF using a switch associated with a fuse (e.g., a fused disconnect). When a disconnect switch (e.g., circuit breaker) of the operator mechanism is ON and connected to a power bus, the unit is energized. 
     The terms “bucket assembly”, “bucket”, “control unit,” and “unit” are used interchangeably and refer to a housing (typically a protective metal shell) that contains a disconnect switch, such as an isolation switch (for a bypass unit), a fused disconnect switch, or a circuit breaker (which can be manually operated by an operator mechanism) for controlling energization/de-energization of the power circuit in the unit. A unit can also include other components such as a power transformer, PLCs (programmable logic controllers), position sensors and the like. 
     The unit can be a motor starter unit, a feeder unit, a unit with a transfer switch or any other unit type. The term “motor starter” is used herein to refer to any starter type. The motor starter can be, for example, a variable frequency drive (VFD) (also known as a variable speed drive), a soft starter (reduced voltage starter), a NEMA starter (NEMA contactor and overload relay, or an IEC starter (IEC contactor and overload relay). The unit can comprise a feeder such as a feeder with a circuit breaker (“feeder breaker”) or a feeder with a disconnect switch with a fuse (“feeder with fused disconnect switch”), a lighting contactor, a resistive contactor or an ATS (automatic transfer switch), by way of further example. 
     The term “disconnect switch” when used with respect to a unit refers to a switch in or on the unit for controlling energization or de-energization of the unit, including a circuit breaker or a switch for opening and closing separable contacts in, e.g., a circuit breaker and/or for turning power ON and OFF using a switch associated with a fuse (e.g., a fused disconnect). 
     The terms “load” and “load device” are used interchangeably and are intended to mean devices that consume electrical power and that are connected to and controlled by the electrical power distribution system (e.g., a motor control center). Load devices are typically motors but may also be pumps or other machinery that may comprise motors or pumps or other miscellaneous critical loads such as hospitals, dam emergency pumps, data centers, back-up generator systems and the like. 
     Referring to  FIG.  1   , one embodiment of an electrical power distribution system  100  (e.g., a motor control center) is shown. The electrical power distribution system  100  includes a bypass unit  10  with a power transfer circuit  20 , a first unit  50   1  and a second unit  50   2 . As shown, the electrical power distribution system  100  includes a structure with compartments  110 , a wire way  100   w  ( FIG.  4   , for example), a power bus  200 , units  50   1 ,  50   2  and a bypass unit  10  for providing power from the power bus  200  of a power bus bar system  1000  ( FIG.  7 B , for example) to an external load  80 . The electrical power distribution system  100  can include more than two units  50   1 ,  50   2  and the bypass unit  10  as shown in  FIG.  8   , for example. 
     Embodiments of the invention can allow for electrical isolation of one of the units  50   1 ,  50   2 , from the other of the units  50   1 ,  50   2 , while the other unit is operational, online, and providing power to the load  80 , thereby providing a continuous operational system while also providing increased safety for a technician. For example, since the primary or first unit  50   1  is electrically isolated from the secondary or second unit  50   2 , a technician can access the first unit  50   1  which is offline and electrically isolated from the second unit  50   2 , the power bus  200  ( FIGS.  1 ,  4   ) and the load  80 , while the second unit  50   2  is operational, online, and providing power to the load  80 , thereby providing a continuous operational system while also providing increased safety for a technician. 
     Embodiments of the invention can also provide for physical separation/isolation of the first and second units  50   1 ,  50   2  and the bypass unit  10  from each other in compartments  110  of the structure  100  using barriers such as partitions, walls, ceilings and floors, for example. 
     Referring to  FIG.  2 A , an example bypass unit  10  is shown. As shown, the bypass unit  10  includes a housing  10   h  that encloses the power transfer circuit  20  which comprises a power transfer (bypass) switch  25 . The power transfer circuit  20  is coupled to a load  80 , such as a motor  80   m , via a conductor  30 , shown as a three pole/three phase lead with three electrical contact connections. The term “conductor” refers to one or more cables, each of which can include one or more wires, leads or other elements that conduct electricity, or one or more wires, leads, traces, lines or other elements that conduct electricity. 
     The power transfer circuit  20  is also electrically coupled to a first unit  50   1  and a second unit  50   2  ( FIGS.  1 ,  3 A,  3 B,  4 - 6   ) via conductors  32 ,  34 , respectively, and is configured to only electrically connect the load  80  to a single one unit of the first unit  50   1  or the second unit  50   2  at any one time during normal operation. The conductors  30 ,  32 ,  34  can be of the same or different lengths. The conductors  32 ,  34  can extend from the bypass unit  10  to the first and second units  50   1 ,  50   2 , inside a wire way  100   w  of a structure  100 , such as an MCC  100 M ( FIGS.  4 - 6   ). 
     Referring to  FIGS.  2 A and  4 - 6   , the power transfer circuit  20  of the bypass unit includes a power transfer switch  25 . The power transfer switch  25  comprises a mechanical interlock  125  ( FIG.  7 A ) that mechanically interlocks the first and second contactors  26   1 ,  26   2 , respectively, so that the power transfer switch  25  is configured to electrically couple to the load  80  and define a first electrical path P 1  and a second electrical path P 2  where only one of the first and second contactors  26   1 ,  26   2  close at any one time. 
     As shown, the bypass unit  10  can also include a circuit breaker  29  in the bypass unit  10  that is coupled to a load side of the power transfer switch  25  and also coupled to the first electrical path P 1  inside the bypass unit  10  between the line side of the power transfer switch  25  and the first unit  50   1  to thereby inhibit (electrical) feedback to an electrically isolated unit, i.e., the second unit  50   2  when the first electrical path P 1  is active. 
     As shown in  FIG.  2 B , the circuit breaker  29  typically has an externally accessible operator handle  230  that faces a front  10   f  of the bypass unit  10  and is configured to be externally accessible to allow a user to lock the operator handle  230  in a desired operational state, using a lock such as a padlock or scissors lock. For example, a user can lock the operator handle  230  in an OFF position associated with non-conduction in the first electrical path P 1  while the bypass unit  10  provides power to the load  80  from the power bus  200  to the second unit  50   2  through the second path P 2 , then the second contactor  26   2  of the switch  25 , then to the load  80 . 
     As shown in  FIGS.  2 B and  2 C , the bypass unit  10  has a front panel  10   f , sidewalls  10   s , a ceiling  10   c , a floor  10   b  and a back wall  10   r  that defines an enclosed compartment  11  and does not require, and typically does not have, power stabs (for engaging a power bus) that extend out from the back wall  10   r . A plurality of conductors  30 ,  32 ,  34  extend out of the housing  10   h , typically one of the side walls  10   s , shown as the right side wall  10   e  ( FIG.  2 C ). The term “right side” refers to the orientation when held in a structure for normal operation with the front  10   f  facing forward. The incoming wires (e.g., nine wires, three associated with each conductor  30 ,  32 ,  34 ) can be fed into the bypass unit  10  through one side of the unit and into a wire way  100   w  ( FIG.  4   ) and the rear  10   r  of the housing  10   h  can be a closed panel. The bypass unit  10  can electrically isolate each of the first unit  50   1  and the second unit  50   2  from the load  80  and/or from each other while respective front panels  50   f  ( FIGS.  1 ,  3 A,  3 B ) remain closed and typically locked in the closed position. 
     In some embodiments, the housing  10   h  of the bypass unit  10  can define an enclosure with a solid back wall  10   r  with the conductors  30 ,  32 ,  34  extending from a common portion or different portions of the housing  10   h  via a ceiling, floor, sidewall or back wall. The housing  10   h  can be a 1X size housing (about 6 inches tall) or a 2X size housing (about 12 inches tall) in some embodiments. 
     As shown in  FIG.  2 B , the bypass unit  10  can include a (pilot) control switch  15  to electrically activate an automatic bypass mode of the unit  10  whereby the power transfer switch  25  engages the second electrical path P 2  and the second contactor  26   2  of the switch  25  and second unit  50   2 . The control switch  15  allows a user to manually activate the automatic bypass switch  25  of the power transfer circuit  20  to automatically electronically transfer to a bypass mode transferring power from the power bus  200  to the primary or first unit  50   1  through the first path P 1  of the bypass switch  25  to the load  80  is switched from the power bus  200  to the bypass or second unit  50   2  through the second path P 2  and the second contactor  26   2  of the bypass switch  25  to the load  80 . Example relay logic protocols are discussed below that can automatically initiate the power transfer to the second path P 2  if the main/primary/first unit fails. A controller and/or relay system  1510  ( FIG.  7 B ) can, upon detection of a defined condition associated with a malfunction or failure of the first unit  50   1 , direct the power transfer circuit  20  to engage the second contactor  26   2  and disengage the first contactor  26   1  to power the load  80  through the second unit  50   2 . 
     It is contemplated that embodiments of the invention can comply with the recommendation of IEEE P1814 with respect to a reduced hazard requirement which recommends a drive unit, such as a VFD unit, be isolated. Embodiments of the invention can provide a bypass unit that couples to both primary and secondary (redundant) units with respective motor starters of the same or different motor starter types. Embodiments of the invention provide modular build options without requiring expensive, complex and larger cumbersome customizations for providing different build configurations while providing the bypass power transfer function. 
     Embodiments of the invention can allow for electrical isolation of one of the units  50   1 ,  50   2 , from the other of the units  50   1 ,  50   2 , while the other unit is operational, online, and providing power to the load  80 , thereby providing a continuous operational system while also providing increased safety for a technician. For example, since the primary or first unit  50   1  is electrically isolated from the secondary or second unit  50   2 , a technician can access the first unit  50   1  which is offline and electrically isolated from the second unit  50   2 , the power bus  200  ( FIGS.  1 ,  4   ) and the load  80 , while the second unit  50   2  is operational, online, and providing power to the load  80 , thereby providing a continuous operational system while also providing increased safety for a technician. 
     Referring to  FIG.  2 B , the handle  230  can be a lever or handle of an operator mechanism that can be physically moved and configured to engage a lock so as to be able to be physically locked (e.g., padlocked) in an OFF or ON position, ON for when unit one  50   1  is energized, connected to the power bus  200 / 1000  and to the first contactor  26   1  to power the load  80  through the first unit  50   1  and OFF for when the second unit  50   2  is energized, connected to the power bus  200 / 1000  and to the second contactor  26   2  to power the load through the second unit  50   2 . The handle  230  can include, e.g., a rotary lever or an up-down lever. 
     Still referring to  FIG.  2 B , the bypass unit  10  can also optionally include a lock  13  that locks the front panel  10   f  (which can be a front door that pivots open from a side) closed so that a technician cannot open the front panel or door  10   f  when the lock is deployed. The lock  13  can be a physical/mechanical interlock that locks the front panel or door  10   f  in a closed position. 
     One or all of the units  10 ,  50   1 ,  50   2  ( FIGS.  1 ,  3 A,  3 B ) can be configured as a modular unit to allow the internal components to be assembled as a unit  10 ,  50   1 ,  50   2  that can be easily, typically slidably and replaceably, installed into a compartment  110  of a structure  100  ( FIGS.  1 ,  3 A,  3 B ) such as an enclosure, a cabinet, and/or a motor control center (MCC)  100 M ( FIGS.  4 - 6   ). Embodiments of the invention provide modular (plug and play) configurations which can provide economic advantages to known conventional custom bypass designs that are cumbersome and may not provide the additional electrical isolation allowed by embodiments of the present invention. To be clear, the term “modular” refers to units that have defined standardized dimensions so that one unit of one type is replaceably interchangeable with another unit of that type. 
     Referring again to  FIG.  2 B , the bypass unit  10  can be provided in package or frame sizes of about 6 inches to about 72 inches (tall) in a height dimension “H” with substantially common depth and width dimensions, known as 1X (6 inches) to 12X (72 inches) sizes. The sizes can be in single X increments, from 1X, 2X, 3X, 4X, 5X, 6X, 7X, 8X, 9X, 10X, 11X and 12X. Thus, a 5X unit  10  can be about 30 inches tall. In some embodiments, the bypass unit  10  can be a 1X unit with a height “H” of about 6 inches or a 2X unit with a height H of about 12 inches. The frame sizes can be provided for target operational amperages, including a plurality of:  125 A,  150 A,  225 A,  250 A,  400 A,  600 A,  1200 A and  2000 A, for example. 
       FIGS.  1 ,  3 A,  3 B  illustrate that the bypass unit  10  can be placed in a compartment  110  of a structure of the power distribution system  100  and electrically coupled to the first unit  50   1  and the second unit  50   2 . The first and second units  50   1 ,  50   2  can each include a respective motor starter  50   m  ( FIGS.  4 - 6   ) and a respective disconnect switch  60 . Each unit  50   1 ,  50   2  can include a front panel or door  50   f.    
     The front door  50   f  of each unit  50   1 ,  50   2  may be configured to engage at least one lock  53  that, when deployed, can lock the door  50   f  in the closed position. The lock  53  can be a physical mechanical interlock. 
     As shown in  FIGS.  1 ,  3 A,  3 B , a conductor  50   2  can couple the first and second units  50   1 ,  50   2  and can allow the first unit  50   1  can transmit a trip signal to the disconnect switch  60  (e.g., circuit breaker) of the second unit  50   2  when the first unit  50   1  is selected by the power transfer circuit  20  via the power transfer switch  25  to create the electrical path to power the load  80  via the bypass unit  10 . Likewise, the second unit  50   2  can transmit a trip signal to the disconnect switch  60  (e.g., circuit breaker) of the first unit  50   1  when the second unit  50   2  is selected by the power transfer circuit  20  via the bypass switch  25  to create the electrical path to power the load  80  via the bypass unit  10 . This can provide an extra degree of safety via an electrical interlock system. 
     In some embodiments, each disconnect switch  60  of the first and second units  50   1 ,  50   2  includes a circuit breakers  60   b  with a shunt trip  60   s  as shown schematically in  FIG.  7 B . The terms “shunt trip” and “shunt trip coil” are used interchangeably herein and are well known components of circuit breakers. 
     The power transfer switch  25  of the bypass unit  10  can be configured with open and close switch states of each switch contact  26   c  ( FIG.  2 A ) so that only one set of three switch contacts/poles is electrically active and connected to one of the first and second units  50   1 ,  50   2  at any one time to serially provide a first electrical path P 1  between a power bus  200  ( FIG.  4   ), the first unit  50   1 , the bypass unit  10 , and the load  80  and a second electrical path P 2  between the power bus  200 , the second unit  50   2 , the bypass unit  10 , and the load  80 . 
     The bypass unit  10  can be configured to provide automatic power transfer (bypass) using a reverse contactor without an overload that is mechanically coupled. 
     Referring to  FIGS.  3 A,  3 B and  7 B , the disconnect switch  60  of the first unit  50   1  can electrically couple to the disconnect switch  60  of the second unit  50   2 . In some embodiments, the disconnect switch  60  of each of the units  50   1 ,  50   2  comprises a circuit breaker  60   b . In certain defined conditions, the first unit  50   1  can send a trip signal to the circuit breaker  60   b  of the second unit  50   2  to cause the breaker shunt trip coil  60   s  to trip the breaker  60   b  in the second unit  50   2 . In certain defined conditions, the second unit  50   2  can send a trip signal to the circuit breaker  60  of the first unit  50   1  to cause the breaker shunt trip coil  60   s  to trip the breaker  60   b  in the first unit  50   1 . 
     Referring to  FIGS.  1 ,  2 A,  2 B,  3 A,  3 B , a conductor  50   2  can be coupled to the first and second units  50   1 ,  50   2  to permit signal communication therebetween, to thereby provide electrical interlocking functionality. For example, when a user selects the first unit  50   1  to power the load by switching the user interface member  130  (e.g., operator handle of an operator mechanism) to an ON position and switching the isolation bypass switch  25  to Unit 1-ON position, the first unit  50   1  (either or both the auxiliary switch of the circuit breaker  60   b  or the auxiliary relay  150   r  of the motor starter  50   m ) can transmit a trip signal to the circuit breaker  60   b  of the second unit  50   2 , typically via the shunt trip  60   s  ( FIG.  7 B ). Likewise, the second unit  50   2  can transmit a trip signal to the circuit breaker  60   b  of the first unit  50   1 , typically the shunt trip  60   s  ( FIG.  7 B ), when the second unit  50   2  is selected by the power transfer circuit  20  via the isolation bypass switch  25  to create the electrical path to power the load  80  via the bypass unit  10 . This can provide an extra degree of safety via an electrical interlock system. 
     Referring to  FIGS.  3 A,  3 B and  7 B , the disconnect switch  60  of the first unit  50   1  can electrically couple to the disconnect switch  60  of the second unit  50   2 . In some embodiments, the disconnect switch  60  of each of the units  50   1 ,  50   2  comprises or is a circuit breaker  60   b . In certain defined conditions, the first unit  50   1  can send a trip signal to the circuit breaker  60  of the second unit  50   2  to cause the breaker shunt trip coil  60   s  to trip the breaker in the second unit  50   2 . In certain defined conditions, the second unit  50   2  can send a trip signal to the circuit breaker  60   b  of the first unit  50   1  to cause the breaker shunt trip coil  60   s  to trip the breaker in the first unit  50   1 . 
     The first unit  50   1 , for ease of discussion can be referred to as a “primary unit”, and a second unit  50   2 , for ease of discussion can be referred to as a “secondary unit” that can be coupled to the bypass unit  10  for selectively powering a load  80  through the power transfer circuit of the bypass unit  10 . 
     In some embodiments, an electrical power distribution system  100  (e.g., MCC) can include a plurality of different electrical interlocks to ensure that only one unit of units  50   1 ,  50   2  is energized and capable of providing power to the load  80  through the bypass unit  10  at any one time.  FIG.  11    lists exemplary electrical interlocks. These defined conditions and associated actions can provide an electrical interlock system.  FIG.  11    lists example actions of the first (primary) unit  50   1 , the second (redundant) unit  50   2 , and the bypass unit  10  providing an electrical interlocking system based on defined operative conditions or states of the first and second units  50   1 ,  50   2 . 
     Example interlocks associated with a primary mode (when the first unit  50   1  is powering the load  80  through the bypass unit  10 ) or a bypass mode (when the second unit  50   2  is powering the load  80  through the bypass unit  10 ) are listed. The defined conditions can include positions of power stabs  554  optionally provided as retractable/extendable power stabs  546 ,  548 ,  550  of a power disconnect assembly  500  ( FIGS.  9  and  10   ) whether the power stabs  546  are fully extended and connected to a power bus  1000  or retracted ( FIGS.  4 - 6   ) based on position sensors  582 ,  594  such as one or more microswitches ( FIG.  10   ) in the respective units  50   1 ,  50   2 . Other conditions can be based on an energized status of a respective primary or secondary unit based on an auxiliary switch and/or auxiliary relay in each of the first and second units which can send a trip signal to the other unit. The bypass unit  10  can also send a trip signal to the primary  50   1  or secondary unit  50   2  depending on whether the bypass unit  10  is in the primary mode (trip signal to the secondary unit) or the bypass mode (trip signal to the primary unit). 
     However, while the power disconnect assembly  500  is believed to be desired for certain end applications/uses, it is not required. For further description of position sensors using auxiliary switches such as microswitches in a unit with a power disconnect assembly  500 , see U.S. 2008/0022673 (labeled as features  82  and/or  94  in  FIG.  17    of this document), the contents of which are hereby incorporated by reference as if recited in full herein. For further descriptions of example power disconnect assemblies and interlocks, see also, U.S. patent application Ser. No. 15/848,103, the contents of which are hereby incorporated by reference as if recited in full herein. 
     Table 1 below provides a list of example configurations of the first unit  50   1 , for ease of discussion referred to as a “primary unit”, and a second unit  50   2 , for ease of discussion referred to as a “secondary unit”, that can be coupled to the bypass unit  10  for serially (selectively) powering a load  80  through the power transfer circuit of the bypass unit  10 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 UNIT COMBINATION OPTIONS 
               
            
           
           
               
               
               
               
            
               
                   
                 Option 
                 Primary Unit 
                 Secondary unit 
               
               
                   
                   
               
               
                   
                 A 
                 VFD unit 
                 VFD unit 
               
               
                   
                 B 
                 Soft Starter 
                 Soft Starter 
               
               
                   
                   
                 (Reduced Voltage 
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
                 starter) unit 
               
               
                   
                 C 
                 NEMA Starter unit 
                 NEMA Starter unit 
               
               
                   
                 D 
                 IEC Starter unit 
                 IEC Starter unit 
               
               
                   
                 E 
                 VFD unit 
                 Soft Starter 
               
               
                   
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                   
                 starter) unit 
               
               
                   
                 F 
                 Soft Starter 
                 NEMA Starter unit 
               
               
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
               
               
                   
                 G 
                 NEMA Starter unit 
                 IEC Starter unit 
               
               
                   
                 H 
                 VFD unit 
                 NEMA Starter unit 
               
               
                   
                 I 
                 Soft Starter 
                 IEC Starter unit 
               
               
                   
                   
                 (Reduced Voltage 
               
               
                   
                   
                 starter) unit 
               
               
                   
                 J 
                 VFD unit 
                 IEC Starter unit 
               
               
                   
                 K 
                 Feeder Breaker 
                 Feeder Breaker 
               
               
                   
                 L 
                 Feeder Fused 
                 Feeder Fused 
               
               
                   
                   
               
            
           
         
       
     
     Referring to  FIG.  7 B , the trip signal for the electrical interlock system can be generated/transmitted via at least one interlock circuit  1500  that can include auxiliary switch inputs from one or more auxiliary switches  27  in the bypass unit  10  via conductor lead  127  and disconnect switch inputs from the first and second units  50   1 ,  50   2  via conductor lead  50   2 , for example. 
     Referring to  FIG.  7 B , the trip signal for the electrical interlock system can be generated/transmitted via at least one interlock circuit  1500  that can include (a) one or more inputs for receiving a signal from an auxiliary switch  27  ( FIG.  7 A ) of the bypass unit  10  via conductor  127  and (b) one or more input(s) for receiving a signal from the first and second units  50   1 ,  50   2  such as from an auxiliary switch or contact associated with the circuit breaker  60   b  of the disconnect switch  60  and/or an auxiliary relay  150   r  coupled to or in the motor starter  50   m . As discussed above, the first and second units  50   1 ,  50   2  can be coupled via a conductor  50   2 , for example. One or more of the outputs/signals from the auxiliary switch  27  and/or the interlock circuit  1500  can provide a voltage signal as a trip signal to a shunt trip  60   s  in a circuit breaker  60   b  of one of the first and second units  50   1 ,  50   2  when the other unit of the first and second units  50   1 ,  50   2  has a circuit breaker  60   b  that is energized. 
     Still referring to  FIG.  7 B , each motor starter  50   m  can be coupled to an auxiliary relay  150   r  that can transmit a trip signal  150   s  to the interlock control circuit  1500 . For example, when a motor starter  50   m  of one of the first and second units first and second units  50   1 ,  50   2  is on and/or operating, an auxiliary relay  150   r  can transmit a voltage trip signal  150   s  to the circuit breaker  60   b  of the other unit to make sure that the other unit is off, contacts open, as a safety interlock feature. 
     In some embodiments, the auxiliary relay  150   r  of the motor starter (e.g., soft starter)  50   m , the auxiliary switch  27  of the bypass unit  10  and an auxiliary switch  60   s  in the breaker  60   b  of first unit  50   1  can be synchronized and/or transmit in parallel, trip signals to the second unit  50   2  when the first unit  50   1  is energized. The auxiliary relay  150   r  of the motor starter (e.g., soft starter)  50   m , the auxiliary switch  27  of the bypass unit  10  and the auxiliary switch  60   s  in the breaker  60   b  of the second unit  50   2  can be synchronized and/or transmit in parallel trip signals to the first unit  50   1  when the second unit  50   2  is energized. 
     The term “auxiliary switch” for the primary unit and the secondary unit in  FIG.  11    that can send the trip signal(s) to the appropriate breaker can include one or more auxiliary switches including, for example, an auxiliary switch  582 ,  594  ( FIG.  10   ) identifying a position of the retractable/extendable power stabs, an auxiliary switch of a circuit breaker  60   b  ( FIG.  7 B ) associated with a contacts closed condition, or an auxiliary relay  150   r  ( FIG.  7 B ) of a motor starter  50   m  indicating an motor on condition. 
     The structure  100  can be designed to slidably receive multiple units  10 ,  50   1 ,  50   2  in various defined sizes. For example, the first and second units  50   1 ,  50   2  can each have housings  50   h  of the same or different modular heights (i.e., 1X-12X frame sizes as discussed above). Each housing  10   h ,  50   h  can include a front door  10   f ,  50   f  that can remain closed when a respective unit is energized. Only one of the two units  50   1 ,  50   2  can be energized when connected to the power bus bars  200  and connected to the load  80  via the bypass unit  10  at any one time. When in a de-energized state, the front door  50   f  of only that de-energized unit, one of the two units  50   1 ,  50   2 , can be opened to allow a technician access to replace or repair that unit while the other of those two units  50   1 ,  50   2  is energized, connected to the power bus and load while the de-energized unit is electrically isolated from the energized unit and the bypass unit  10 . 
       FIG.  3 A  illustrates that the bypass unit  10  can be placed in a first structure  1001  while the first unit  50   1  and the second unit  50   2  are in a separate, but typically adjacent, structure  1002 . Each structure  100  can optionally have a front door  100   f  (shown by the shading over the respective units) that closes and releasably locks via at least one mechanical lock  114  in the closed position over one or more of the units  50   1 ,  50   2  and  10 , respectively. 
       FIGS.  3 B and  1    illustrate that the bypass unit  10  and the first and second units  50   1 ,  50   2  are held in the same structure  100 .  FIG.  3 B  illustrates that the units  50   1 ,  50   2  can be held in side by side, laterally adjacent compartments  110 .  FIG.  1    illustrates that the bypass unit and the first and second units  50   1 ,  50   2  can be held in a vertically stacked arrangement of compartment  110 . 
     Referring to  FIG.  1   , each compartment  110  can have a closed floor  110   f  and/or ceiling and/or rails  112  coupled to internal sidewalls  1101  that slidably receive and support the units  10 ,  50   1 ,  50   2 . 
     Referring to  FIGS.  4 - 6   , the first and second units  50   1 ,  50   2  can have power stabs  554  extending rearward from the back  50   r  that connect to one or more (typically vertically oriented) power bus bars  200  that are part of a power bus system  1000  that carries power (current) to the compartments of a vertical section in the structure such as the MCC  100 M. These power stabs  554  may optionally be provided by a power disconnect assembly  500  as discussed above. As is well known, the bus bars  200  can be connected to larger (typically horizontal bus bars) that bring power to the vertical sections. The larger (typically horizontal) bus bars are usually in the top, but some structures may have them in the center or bottom. The structures of MCCs  100 M usually have at least one wire way  100   w  for conductors  30 ,  32 ,  34  (i.e., wires) to the load(s) and control wires. 
       FIG.  4    illustrates an MCC  100 M with the bypass unit  10  and a first unit  50   1  that can comprise a VFD  50   VFD  as the motor starter  50   m  and can also comprise power stabs  554 , shown as provided by a power disconnect assembly  500 .  FIG.  4    also illustrates that the second unit  50   2  can comprise a NEMA starter  50   Nm  as the motor starter  50   m  and can also comprise power stabs  554 , shown as provided by a power disconnect assembly  500 . The microswitches  582  and/or  594  ( FIG.  10   ) can identify when the stabs  554  are fully engaged to the (vertical) power bus  200  and when the stabs  554  are fully withdrawn and isolated from the (vertical) bus  200 . 
       FIG.  5    illustrates an MCC  100 M with the bypass unit  10  and a first unit  50   1  that can comprise a soft starter  50   ss  as the motor starter  50   m  and can also comprise power stabs  554 , shown as provided by a power disconnect assembly  500 .  FIG.  5    also illustrates that the second unit  50   2  can comprise a NEMA starter  50   Nm  as the motor starter  50   m  and can also comprise power stabs  554 , shown as provided by a power disconnect assembly  500 . The position sensors  582  and/or  594  ( FIG.  10   ) can identify when the stabs  554  are fully engaged to the (vertical) power bus  200  and when the stabs  554  are fully withdrawn and isolated from the (vertical) power bus  200 . 
       FIG.  6    illustrates an MCC  100 M with the bypass unit  10  and a first unit  50   1  that can comprise a VFD  50   VFD  as the motor starter  50   m  and can also comprise power stabs  554 , shown as provided by a power disconnect assembly  500 .  FIG.  6    illustrates that the second unit  50   2  can comprise a soft starter  50   ss  as the motor starter  50   m  and can also comprise power stabs  554 , shown as provided by a power disconnect assembly  500 . The position sensors (e.g., microswitches)  582  and/or  594  ( FIG.  10   ) can identify when the stabs  554  are fully engaged to the (vertical) power bus  200  and when the stabs  554  are fully withdrawn and isolated from the (vertical) bus  200 . 
     Thus, as shown by the examples of  FIGS.  4 - 6   , modular units  50   1 ,  50   2  of different motor starter configurations can allow a number of different MCC build selections and/or configurations without requiring unique more expensive custom units. 
     Referring to  FIG.  7 A , the power transfer switch  25  of the bypass unit  10  can comprise a first contactor  26   1  that is mechanically interlocked with a mechanical interlock  125  to the second contactor  26   2 . A first set of three contacts  26   c  coupled to the first unit  50   1  and a second set of three contacts  26   c  electrically coupled to the second unit  50   2 . The contactor can be a double throw contactor. The switch  25  can include a main switch body  25   m  and one or more auxiliary switches  27 , typically at least one auxiliary switch  27  on each opposing end or side of the main switch body  25   m . The auxiliary switches  27  can be configured to receive and/or transmit signals from and/or to the first unit  50   1  and the second unit  50   2 . 
     Referring to  FIG.  7 B , a relay system  1510  in an MCC  100  can be coupled to one or more of the bypass unit  10 , the first unit  50   1 , and/or the second unit  50   2  can be used to identify when the first unit  50   1  powers down, malfunctions or turns off, then automatically directly or indirectly sends a trip signal to direct or cause a shunt trip coil in the breaker  29  to trip the breaker  29  and automatically switch the power transfer switch  25  to electrically connect the second unit  50   2 . This automatic trip and transfer operation may be particularly suitable for critical loads. Tripping the circuit breaker  29  will electrically isolate the first unit  50   1  as a standard contactor is not an isolation device. 
     One or more of the at least one auxiliary switch  27  can transmit a trip signal to the disconnect switch  60  of one of the first unit  50   1  or the second unit  50   2  when the other of the first unit  50   1  or the second unit  50   2  is energized with the stabs  554  contacting the power bus  200 . A respective auxiliary switch  27  can be coupled to one or both units  50   1 ,  50   2  via a conductor  127  (e.g., wire(s)) ( FIG.  7 B ). A trip signal can be transmitted to the circuit breaker  29  to automatically direct the circuit breaker  29  to trip to electrically isolate the first unit  50   1  from the load and the second unit  50   2  when the first unit  50   1  fails or powers down. 
     One or more of the at least one auxiliary switch  27  can transmit the trip signal to the circuit breaker  29  in the bypass unit  10  when the first unit  50   1  fails or malfunctions, for example. 
     As also shown in  FIG.  7 B , the first and second units  50   1 ,  50   2  can be electrically coupled via one or more conductors  50   2  so that the first unit  50   1  can send a trip signal to the second unit  50   2  when the first unit  50   1  is connected to the power bus bars  200 . 
     The second unit  50   2  can send a trip signal to the first unit  50   1  and the circuit breaker  29  when the second unit  50   2  is connected to the power bus bar  200 . In some embodiments, the trip signal can be generated by an auxiliary switch in a respective unit  50   1 ,  50   2  such as a microswitch assembly associated with a position sensor  594  or  582  ( FIG.  10   ) associated with one of the units with a power disconnect assembly  500  when the stabs of one of the units  50   1 ,  50   2  are fully extended and the stabs of the other unit are not extended (i.e., are retracted). 
       FIG.  7 B  illustrates that the structure  100 , optionally an MCC  100 M, can comprise the bypass unit  10  coupled to the first unit  50   1  and the second unit  50   2 . Each unit  10 ,  50   1 ,  50   2 , can be in separate housings  10   h ,  50   h  and placed in separate compartments of the structure  100 . Each housing  10   h ,  50   h  can have a closed front panel or door  10   f ,  50   f.    
       FIG.  8    illustrates an example MCC  100 M with the first and second units  50   1 ,  50   2  and the bypass unit  10 . The first and second units  50   1 ,  50   2  include operator handles  51  that are coupled to internal disconnect switches  60 , such as circuit breakers, as is well known to those of skill in the art. The bypass unit  10  includes the switch handle  230  and a control switch  15  operable through a closed door. The (op-mech) switch handle  230  may be a rotary or up-down handle that is coupled to the power transfer circuit  20  ( FIG.  1   ). 
     Still referring to  FIG.  8   , a typical MCC structure  100 M is an enclosure with a number of small doors arranged in rows and columns along the front and flat, mostly featureless, back and sides. The units  10 ,  50  can be provided in varying sizes. For starter units  50   1 ,  50   2 , the size can be based on the size of the load  80 , e.g., motor  80   m  ( FIG.  1   ) they are controlling. The units  10 ,  50   1 ,  50   2  can be configured to be relatively easily removable for repair, service or replacement. MCCs  100 M can have different sets of units, for example, regular starters, reversing starters, soft start, and variable frequency drives. MCCs  100 M can be configured so that sections can be added for expansion if needed. 
     MCCs  100 M can be configured in many ways. Each compartment  110  can have a different height to accept different frame sizes of respective bucket assemblies or units  10 , typically in about 6-inch increments. The vertical bus can be omitted or not run through the full height of the section to accommodate deeper buckets for larger items like variable frequency drives. The MCC can be a modular cabinet system for powering and controlling motors and/or feeder circuits. Several may be powered from main switchgear which, in turn, gets its power from a transformer attached to the incoming line from a public or private grid, e.g., a power company. 
     Referring to  FIG.  8   , a partial perspective view of a motor control center structure  100 M is shown. Units  10 ,  53 ,  50   1 ,  50   2  are shown fully installed into a motor control center compartment  110  such that its respective front panel  10   f ,  53   f ,  50   f  is seated securely against the periphery of the compartment enclosure and flush with the front panel  54   f  of unit  54  and with the front panel  50   f  of units  50   1 ,  50   2 . 
     In some embodiments, some or all units, e.g., units  53 ,  50   1 ,  50   2  can include a number of latching mechanisms  22  on front panels thereof so that an operator may lock a unit into place once installed. In some embodiments, the front panel  50   f ,  53   f  may be a deadfront door having a set of hinges  19  in order to permit access to motor control components within a unit while the unit is installed in a compartment  111  of the MCC  100 M. However, even when closed or sealed, front panel or door  50   f  still permits access to the disconnect switch  60  which can comprise a circuit breaker, stab indicator  24 , shutter indicator  26 , and line contact actuator  31 . Line contact actuator  31  is a mechanism of the power disconnect assembly  500  ( FIG.  9   ) for selectively engaging a power bus  200  to engage power stabs  554  defining line contacts ( FIG.  4   ) with line power from the MCC  100 M. Thus, even when a unit  10 ,  50  is fully installed in a compartment  110  and latches  22  have been secured, an operator may still use respective disconnect switch handles  51 ,  230 . 
     As shown in  FIG.  8   , the first unit  50   1  can have the handle  51  in an ON position associated with conduction and an energized state while the second unit  50   2  has the handle  51  in an OFF position associated with non-conduction and a de-energized state and the handle  51  can engage a lock  151  such as a padlock to lock the unit  50   2  in this state. 
     For units with the power disconnect assembly  500 , a user can also open slide  132  to insert crank  134  to move one or more line contacts (not shown) of the unit. When slide  132  is moved laterally aside to permit access to actuating mechanism  131 , door  50   f  is prevented from opening, thereby closing off access to components inside the unit  50   1 ,  50   2 . Additionally, a user may desire to padlock  232  the slide  132  in the closed position ( FIG.  14 B ), to further regulate who may operate actuating mechanism  131  and when. 
     When slide  132  is moved aside, an aperture  136  ( FIG.  9   ) is exposed. Opening  36  accepts a crank  134 . Additionally, when slide  132  is moved aside as shown, slide  132  may optionally extend over a portion of front panel  50   f . Thus, in embodiments in which front panel  50   f  is a hinged door, moving slide  132  to expose aperture  136  will inhibit a user from opening front panel  50   f . Accordingly, so long as an operator has a crank  134  inserted into aperture  136  aligned with the internal actuator  131 , the operator cannot open the door of the unit  50   1 ,  50   2 . 
       FIGS.  9  and  10    show an example unit  50   1 ,  50   2  with an operator handle  51  coupled to the disconnect switch  60  and a power disconnect assembly  500  with retractable/extendable power stabs  554 . During the extension of the power disconnect assembly  500  with the stabs  554  an automatic latch  60  can be triggered to engage the compartment  110  into which unit  50   1 ,  50   2  has been installed. Also due to the extension of the power disconnect assembly  500 , a rod  78  is pulled by a stab bracket  59  such that a cam  80  is rotated away from a microswitch  582 . Microswitch  582  is thus actuated to permit control voltage from a control power contact  44  to a motor control component, such as a contactor or overload relay (not shown). It is appreciated, however, that the position sensor assembly using the microswitch  582 , cam  80  and rod  78  can be provided in other manners. 
     Also shown in  FIG.  10    a second microswitch  594  can be connected to activate and deactivate a disconnect switch  60  such as a circuit breaker. When stabs  546 ,  548 ,  550  reach the fully engaged position with bus bars  200 , stab bracket  59  actuates microswitch  594 . When actuated, microswitch  594  permits closure of the disconnect switch  60 , completing the circuit between bus bars  200  and the line side of motor control components (not shown) in unit  50   1 ,  50   2 . Otherwise, microswitch  594  can prevent closure of its disconnect switch  60 , e.g., circuit breaker. The microswitch  594  of the first unit  51  can send a trip signal to the disconnect switch  60  of the second unit  50   2  when the microswitch  594  permits closure of its disconnect switch to trip the disconnect switch  60  in the second unit  50   2  (and vice versa). This can be the signal transmitted via conductor lead  50   2  ( FIGS.  3 A,  3 B,  3 C,  7 B ). 
     Control power stab  44  can be un-shielded and connected to a control power once a respective unit  50   1 ,  50   2  is installed into a motor control center. However, microswitch  582  is in an activated state, due to the pressure thereon by cam  80 . When microswitch  582  is in the activated state, as shown, microswitch  582  is interrupting control power from contact  44 . Thus, the motor control components (not shown) housed in the unit housing  50   h  cannot initially be operated. Cam  80  will be moved by rod  78  via advancement of stab bracket  59 , deactivating microswitch  582  and thereby permitting the flow of control power to motor control components (not shown) of the unit. Cam  80  also acts to display a location status of the stabs  546 ,  548 ,  550  to an operator. Cam  80  can have a number of colors thereon which can be displayed through front door  50   f  of the unit via stab indicator  24  ( FIG.  8   ). 
     In the embodiment shown in  FIG.  10   , a disconnect switch (e.g., circuit breaker) interlock  316  includes microswitch  594 , which gates the operation of the disconnect switch  60 .  FIG.  10    shows microswitch  594  in a deactivated state, in which button  334  thereof is not depressed. Arm  330  of microswitch  594  is positioned to abut a ledge  332  of interlock  316 . Thus, when disconnect switch interlock  316  moves, due to the motion of stab assembly  58 , the arm  330  of microswitch  594  will pivot, depressing button  334 . When button  334  is depressed, microswitch  594  will be activated and will electrically allow operation of the disconnect switch ( FIGS.  4 - 6   ). 
       FIG.  9    also has height “H”, width “W” and depth “D” dimensions which may be provided in modular frame sizes of 1X-12X as discussed above with respect to the bypass unit  10 . In some embodiments, the bypass unit  10  has a height that is less than the height of the units  50   1 ,  50   2 . The bypass unit  10  and the units  50   1 ,  50   2  can have common depth and width dimensions. In other embodiments, the units  50   1 ,  50   2  can have common depth and width dimensions and the bypass unit  10  can have a different depth and/or width dimension. 
     Also shown in  FIG.  10    is a shutter arm  336 , having a sloped end  338 . As stab  546  is advanced, stab  546  will engage the sloped end  338  and slide past shutter arm  336 , thereby shifting shutter arm  336  to the left, as depicted in  FIG.  10   . When shutter arm  336  is shifted, it will strike a tab  340  of a rod  76  extending in a front to back direction of the unit. When tab  340  is struck, rod  76  will rotate, changing the color showing on shutter indicator  26  through door  50   f  of a respective unit  50   1 ,  50   2 . 
     The electrical distribution devices contemplated by embodiments of the invention can include electrical and mechanical interlocks.  FIG.  11    lists example actions of the first (primary) unit  50   1 , the second (redundant) unit  50   2 , and the bypass unit  10  providing an electrical interlocking system based on defined conditions or states of the first and second units  50   1 ,  50   2 . 
       FIG.  12    lists example mechanical locks that can be used to provide a mechanical interlocking system for an electrical distribution device  100  with the first (primary) unit  50   1 , the second (redundant) unit  50   2 , and the bypass unit  10 . That is, when units  50   1 ,  50   2  comprising a power disconnect assembly  500  is used ( FIGS.  4 - 6 ,  9 ,  10   ), when one unit is energized, the other unit can be padlocked via padlock  132  ( FIG.  14 B ) to lock the unit that is not being used to power the load to lock the power disconnect assembly  500  with its stabs in a retracted state with the slide  32  ( FIGS.  8 ,  9   ) misaligned from portal  36 . 
     At the same time the bypass unit  10  can be padlocked or otherwise locked into a primary mode associated with powering the load through the first unit  50   1  or the bypass mode associated with powering the load through the second unit  50   2 . 
     A third interlock may be used where a trapped-key interlock device  250  of a unit  50   1 ,  50   2  that allows a disconnect switch  51  (e.g., breaker) to be operated only when the key is turned to allow the handle  51  to be turned to the ON position ( FIGS.  13 A,  13 B,  14 A ). As is known to those of skill in the art, a trapped-key interlock typically has a lock cylinder which operates a sliding bolt through a cam. The sliding bolt, when extended, mechanically prevents operation of a switch, valve, gate, or other device. Many variations exist, with different shapes of interlock bolt and multiple lock cylinders on an interlock. The key is held or trapped in one position of the lock; releasing the key indicates that the interlocked device has been made safe; the interlocked device cannot be re-energized until the key has been returned and operated to retract the bolt. An example of a trapped-key interlock device  250  is a Kirk® key safety interlock from Kirk Key Interlock Company, North Canton, Ohio. 
     A first standard unit  50   1  with a first motor starter (e.g., a VFD, Soft starter, NEMA starter, IEC starter or the like) can be referred to as unit “A” and a second standard unit  50   2  as a redundant unit with a motor starter of the same or different type (e.g., a VFD, Soft starter, NEMA starter or IEC starter of the like) can be referred to as unit “B” and can be placed in an adjacent location next to unit A. A third (compact) unit “C” provided as the bypass unit  10  comprises the power transfer circuit  20  with a transfer switch  25  which controls the connection to the load  80 , and which switches motor control from the unit “A” to unit “B”. Both unit A and unit B can comprise power disconnect assemblies  500  allowing for (FlashGard™ isolator features providing arc flash safety, e.g., a stab racking mechanism with bus isolation and stab position indicators) power bus isolated unit configurations. The disconnect switches  60  in the two units can be configured as main circuit breakers that can be mechanically interlocked with a mechanical lock such as a Kirk® Key interlock and can also be electrically interlocked with shunts trip accessories controlled by position sensors such as microswitches and auxiliary switches and/or relays in those units  50   1 ,  50   2  associated with, for example, FlashGard™ isolators and interlocks. As each unit  50   1 ,  50   2  has a separate unit door  50   f , each of these units can be electrically isolated and completely disconnected from the power bus  200 , i.e., a 480V/600V system, providing a safe working environment. 
     The unit  10 ,  50   1 ,  50   2  can be configured for DC (direct current) and/or AC (alternating current) operation. 
     In some embodiments, the circuit breaker  29  and/or the disconnect switch  60  of the units  50   1 ,  50   2  can comprise a molded case circuit breaker. Molded case circuit breakers are well known to those of skill in the art, as exemplified by U.S. Pat. Nos. 4,503,408 and 5,910,760, the contents of which are incorporated herein by reference as if recited in full herein. In other embodiments, the disconnect switch  60  can comprise a fused disconnect switch to turn power on and off. 
     Exemplary fuses are FUSETRON™ 600V Class RK5 fuses (BU-SB13729) available from Cooper Bussmann Company, St. Louis, Mo. However, the design is flexible and can accommodate other fuses including those in different classes. 
       FIG.  15    illustrates that the electrical distribution system  100 ′, such as, for example, an MCC  100 M′, can be configured with a bypass unit  10 ′ comprising a manual back-up bypass configuration that provides an additional bypass path P 3  that can be engaged if the auto bypass switch  25  with the first and second contactors  26   1 ,  26   2  is not operating properly. The electrical distribution system  100 ′ as discussed herein can include any or all of the features described herein (i.e., the auxiliary inputs, interlocks and control circuits). The manual bypass path P 3  can be provided as an override system that once mechanically engaged by a user operated switch  130  ( FIG.  16   ), typically with an operator handle  231  ( FIG.  17   ) can lock out the automatic bypass switch  25  and provide the power to the load  80  from the second unit  50   2 , through path P 3  to the load  80 . The manually engageable alternate or back-up bypass electrical path P 3  includes a circuit breaker  129  that is normally open so that the alternate back-up bypass electrical path P 3  is isolated until the manual handle  231  ( FIG.  17   ) closes the contacts of the circuit breaker  129  and electrically and/or mechanically locks out the power transfer switch  25  and primary path P 1  and bypass path P 2 . The first unit  50   1  will be electrically isolated when the second unit  50   2  is powering the load  80  as discussed above. 
     The second and third paths P 2 , P 3 , can merge to the conductor  34  inside the bypass unit  10 ′ and be configured to connect with the second unit  50   2  along a common conductor length/segment. 
       FIG.  16   , which is similar to  FIG.  2 A , illustrates the bypass unit  10 ′ can have a front panel  10   f  with the operator handle  230 , the automatic bypass control switch  15  and the manual back-up bypass switch  130 . 
     The operator handle  230  and manual switch  130  with an operator handle  231  can be configured as rotary or up-down operating handles. The electronic control switch  15  can be a push-button or a rotary button, in some embodiments. However, the electronic control switch  15  can have other configurations. 
       FIG.  17    illustrates an example of the bypass unit  10 ′ with the automatic bypass switch  25  and manual bypass switch  130  shown as comprising an operator mechanism handle  231  coupled to the circuit breaker  129  which is adjacent the operator handle  230  coupled to the circuit breaker  29  of the bypass path P 1  coupled to the first contactor  26   1  of the power transfer circuit  25  and the first unit  50   1  ( FIG.  15   ). 
       FIG.  18    illustrates that the bypass unit  10 ′ can comprise a walking beam  229  coupled to the first and second breakers  29 ,  129  which defines an interlock that allows only one of the first breaker  29  in the primary path P 1  or the second breaker  129  of the bypass path P 3  to close at any one time. Other mechanical and/or electrical interlocks may be used. 
       FIGS.  19 A / 19 B illustrate example actions that can be carried out according to embodiments of the present invention. A bypass unit with a power transfer circuit is the power transfer circuit is provided. The power transfer switch comprises mechanically interlocked first and second contactors configured to electrically couple to a load and define first and second electrical paths where only one of the first and second contactors close at any one time. The bypass unit further comprises a circuit breaker in the bypass unit coupled to a load side of the power transfer switch and also coupled to the first electrical path to thereby inhibit feedback to an isolated unit. The circuit breaker has an externally accessible operator handle that faces a front of the bypass unit and is configured to allow a user to lock the handle in an off position associated with non-conduction in the first electrical path (block  800 ). 
     Automatically transferring power from a power bus to a load from the first unit to the second unit using mechanically interlocked contactors while automatically tripping the circuit breaker in the bypass unit to thereby allow power transfer from one unit to one other while electrically isolating the other from the load and the one unit (block  810 ). 
     When the first unit is powering the motor and has a failure, the first disconnect switch is turned off to prevent electrical conduction in the first unit and a trip signal is sent to the circuit breaker in the bypass unit to automatically isolate the first unit (block  820 ). 
     The first disconnect switch is locked in the off position and the circuit breaker in the bypass unit can also be locked in the off position (block  830 ). 
     The second disconnect switch is turned on to allow electrical conduction in the second unit. Power from the power bus, through the second unit and the power transfer circuit is provided to the load (block  840 ). 
     A user is allowed to slidably withdraw or otherwise access the first unit while the second unit is operative (electrically active), with the first unit electrically isolated from the bypass unit and the second unit to thereby allow safe repair, servicing or replacement of the first unit while powering the load through the second unit and providing electrical isolation from the power transfer circuit in the bypass unit and the second unit (block  850 ) 
     The method can include automatically detecting a power failure or malfunction of the first unit, and automatically sending a trip signal to the first disconnect switch and the circuit breaker in the bypass unit to isolate the first unit from the load and the second unit (block  802 ). 
     The bypass unit with the power transfer switch cooperates with the first and second disconnect switches to force a shunt trip coil to trip the first disconnect switch (primary breaker) in the first unit (e.g., opens the primary breaker) and trip the circuit breaker in the bypass unit itself (block  804 ). 
     The bypass unit and the first and second units have a common width and modular housings and are slidably mountable in compartments of a structure of an MCC, each unit with respective front doors that can be independently locked (block  808 ). 
     The bypass unit is in an enclosed housing having a rear wall and first, second and third conductors that extend out of the bypass unit, the first conductor (only) coupled to the first unit and the second conductor (only) coupled to the second unit to electrically couple the first and second units to the load via the bypass unit (block  806 ). 
     Extending retractable/extendable power stabs of a power disconnect assembly of the first unit to electrically engage the power bus while a front door of the first unit remains closed (block  812 ). 
     The first and second units comprise electrical and mechanical interlocks configured to allow only one of the first and second disconnect switches (e.g., circuit breakers) to be ON at any one time (block  814 ). 
     Retracting the retractable/extendable power stabs to disengage from the power bus while a front door of the first unit remains closed (block  822 ). 
     The bypass unit can have an external user switch input that accepts user input to close one of the first set or the second set of switch contacts of one of the contactors. When transferring power from the first unit to the second unit, the first set of switch contacts are opened and the second set of switch contacts are closed (block  824 ). 
     Extending retractable/extendable power stabs of a power disconnect assembly of the second unit to electrically engage the power bus while a front door of the second unit remains closed (block  842 ). 
       FIG.  20    is a flow chart of example actions of a modular build assembly allowing for standardized builds from defined sets of different units avoiding unique single build customization according to embodiments of the present invention. 
     A bypass unit is provided. The bypass unit comprising a power transfer switch with mechanically interlocked first and second contactors configured to electrically couple to a load and define first and second electrical paths where only one of the first and second contactors close at any one time, wherein the bypass unit further comprises a circuit breaker in the bypass unit coupled to a load side of the power transfer switch and also coupled to the first electrical path to thereby inhibit feedback to an isolated unit, wherein the circuit breaker has an externally accessible operator handle that faces a front of the bypass unit and is configured to allow a user to lock the handle in an off position associated with non-conduction in the first electrical path (block  900 ). Two units are selected to connect to the bypass unit from at least three different modular units, optionally two of the same or two different ones from the following at least three types (block  910 ). 
     A first unit comprising a variable frequency drive (block  912 ). A second unit comprising a NEMA starter (block  914 ), and a third unit comprising a soft starter (block  916 ). 
     The first, second and third units can each comprise a power disconnect assembly having extendable/retractable power stabs (block  920 ). 
     The bypass unit and the selected two units can be slidably inserted into compartments of a structure, such as a structure of an MCC and the power transfer switch of the bypass unit is electrically coupled to the selected two units during, before or after the inserting (block  925 ). 
     It is contemplated that both “new” builds and field retrofit structures of electrical distribution devices  100  such as MCCs  100 M can benefit from the new bypass unit  10  and primary and secondary units  50   1 ,  50   2  discussed above. 
       FIG.  21    is a flow chart of actions that can be used to automatically transfer power to a load according embodiments of the present invention. 
     An electrical distribution system such as an MCC is provided. The MCC has a bypass unit in a housing comprising a power transfer switch configured to serially electrically couple to a single one of first and second units held in separate respective housings to a load at any one time to thereby provide a redundant, back-up drive capacity (block  950 ). 
     First and second contactors of the power transfer switch are mechanically interlocked to electrically couple either the first or the second contactor to the load at any one time and define first and second electrical paths whereby only one of the first and second contactors close at any one time, wherein the first electrical path is electrically coupled to a circuit breaker in the bypass unit and the first unit and the second electrical path is coupled to the second unit (block  960 ). 
     A power failure or malfunction of the first unit is electronically detected (typically using a relay system) (block  970 ). 
     If a malfunction of power failure is detected, then automatically transmitting a trip signal to the circuit breaker in the bypass unit, automatically engaging the second contactor and disengaging the first contactor of the power transfer switch to power the load using the second unit and the second electrical path; automatically tripping a disconnect switch in the first unit; and automatically tripping the circuit breaker in the bypass unit to thereby isolate the first unit from the load (blocks  975 - 979 ). 
     Optionally, the bypass unit can include a back-up bypass path with a circuit breaker and the methods can include allowing a user to manually engage the back-up bypass path while concurrently interlocking a circuit breaker of the first primary path from being in an operative position (block  981 ). 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the invention.