Abstract:
A method and system for selectively controlling the supply of electrical power to an electrical panel that is capable of receiving electrical power from more than one power source, such as a utility power supply and an auxiliary power supply, e.g., an electrical generator, prevents the neutral conductor of the electrical panel from being connected to one power supply when the hot conductors of the electrical panel are connected to another power supply.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Ser. No. 61/427,655 filed on Dec. 28, 2010, the disclosure of which is incorporated herein in its entirety. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention generally provides a transfer switch for automatically connecting the neutrals of one or more loads to an alternate power source, such as an electric generator, when a transfer to alternate power is made from primary power, such as a utility, and for automatically reconnecting the neutrals to the primary power supply when a transfer back to primary power is made. 
     A transfer switch is an electrical switch that connects an electrical load to a standby or auxiliary power supply when electrical power from a primary power supply, such as a utility power supply, has been lost. Transfer switches can be classified into one of two groups: manual transfer switches and automatic transfer switches. Manual transfer switches require a user, such as a homeowner, to physically throw the switch to connect electrical loads to the standby power supply, which in many instances is an electrical generator. When the primary power supply has been restored, the user must then throw the switch back to its previous position to reconnect the electrical loads back to the primary power supply. 
     An automatic transfer switch, on the other hand, is designed to automatically connect the electrical load to the standby power supply upon the loss or interruption of primary electrical power. Automatic transfer switches are often installed where a backup electrical generator is located so that the electrical generator can provide temporary electrical power if the primary power supply fails. In this regard, as well as transferring the electrical loads to the electrical generator, an automatic transfer switch also commands the electrical generator to start based on a disruption in the primary power supply. The transfer switch also isolates the electrical generator from the primary power supply when the generator is on and is providing temporary power. 
     Thus, for example, in a home equipped with a backup generator and an automatic transfer switch, when an electric utility outage occurs, the automatic transfer switch will signal the backup generator to start. Once the automatic transfer switch sees that the generator is ready to provide electric power, the automatic transfer switch breaks the connection to the electric utility and connects the generator to certain selected electrical circuits in the building, which are typically those circuits that power critical items or equipment such as a sump pump, furnace, refrigerator, medical equipment, etc. If the generator is large enough, it may be connected to all of the loads in the building. The generator supplies electrical power to the electric loads, but is not connected to the electric utility. Isolation of the generator from the electrical distribution system is required to protect the generator from overload, and to prevent accidental energization of the service wiring. When utility power returns for a set time, the automatic transfer switch will transfer back to utility power and command the electrical generator to turn off, after another specified amount of “cool down” time with no load on the generator. 
     In addition to being classified as manual or automatic, transfer switches can also be classified as either separately derived systems or non-separately derived systems. In a separately derived system, there is no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors of the electrical generator. On the other hand, in a non-separately derived system, the electrical generator has a “solidly connected” grounded circuit conductor. In the case of the latter, the grounded conductor, i.e., neutral, of the electrical generator is solidly connected to the neutral conductor of the main electrical panel. In the case of the former, however, the transfer switch must switch the neutrals. That is, the transfer switch must switchably connect the neutral conductor of the electrical panel to the neutral of either the electrical generator or the utility power supply. So, for use in a separately derived system, the automatic transfer switch must also switch the neutrals automatically upon the loss of primary electrical power and the subsequent return of primary electrical power. 
     One proposed automatic switch for switching the neutrals uses a contactor that switches the neutral conductor of the electrical panel to the electrical generator neutral whenever a running generator is connected to the system. More particularly, the contactor has a coil that is energized when electrical current flows from one generator hot conductor to the generator neutral. Energization of the coil causes an armature to move the neutrals switch to connect the panel neutral conductor with the neutral conductor of the electrical generator. When the electrical generator is shut-off or otherwise not supplying electrical power to the load center, the coil will become de-energized and the armature will be released to allow the neutrals switch to reset and reconnect the neutral conductor of the load center with the neutral conductor of the primary power supply. This design, however, allows the transfer switch to reconnect the hot conductors of the electrical panel back with the hot conductors of the primary power supply when primary power is restored but, if the electrical generator is still running and connected to the transfer switch, the neutral of the electrical panel will not be switched back. This makes primary power, e.g., utility power, available to the loads of the electrical panel with no neutral connection. As a result, either a load on the electrical panel may receive electrical power at high, and potentially damaging, voltages or the ground wire may conduct current, creating a potential safety issue. The exposure to undesirable high voltages may also occur during post-installation testing when it is not uncommon for an installer to test operation of the automatic transfer switch and electrical generator with the primary supply on and connected to the electrical panel. 
     The present invention is directed to a method and system of selectively controlling the supply of electrical power to an electrical panel that is capable of receiving electrical power from more than one power source, such as a utility power supply and an auxiliary power supply, e.g., electrical generator. More particularly, the invention prevents the neutral conductor of the electrical panel from being connected to one power supply when the hot conductors of the electrical panel are connected to another power supply. In this regard, the present invention prevents the neutral conductor of the electrical panel from remaining electrically connected to the neutral conductor of the electrical generator when primary power has been restored and the hot conductors of the electrical panel have been reconnected to the hot conductors of the primary power supply. In a similar fashion, the invention prevents the neutrals from being switched to the electrical generator when primary power is feeding electrical power to the electrical panel. 
     The present invention uses the voltage supplied to the transfer switch, i.e., the transfer switch bus, to determine the appropriate position for a neutrals transfer switch. When the voltage potential or drop between the transfer switch bus and the primary power supply is at or near the rated voltage of the power supply, e.g., 240V, the neutrals transfer switch connects the neutral conductor of the electrical panel to the neutral conductor of the primary power supply. On the other hand, when the voltage potential between the transfer switch bus and the auxiliary power supply, e.g., electrical generator, is at or near the rated voltage of the auxiliary power supply, e.g., 240V, the neutrals transfer switch connects the neutral conductor of the electrical panel to the neutral conductor of the auxiliary power supply. In this regard, the switching of the neutral conductor of the electrical panel is based on which power supply is feeding the transfer switch bus. 
     In one embodiment of the invention, a contactor having a coil and movable armature is used to detect the voltage potential between the transfer switch bus and the primary power supply or auxiliary power supply. In another embodiment, a microcontroller and associated sensors may be used to measure the aforementioned voltage drop and control relays or switches to make the neutrals connections. 
     One object of the invention is to provide a stand-alone switching apparatus for use with a transfer switch that automatically and independently switches the neutrals of a separately derived transfer system. 
     It is another object of the invention to provide a neutrals switch for use with a transfer switch in which the neutrals switch is integrally formed with the transfer switch yet independently connects the neutral of an electrical panel to one of a pair of power supplies based on which power supply is energized to provide electrical power to the transfer switch. 
     It is a further object of the invention to provide a neutral switching apparatus that prevents the neutrals of a transfer switch system from being electrically connected to a power supply that is different from the power supply that is feeding electrical power to the transfer switch system. 
     Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The drawings illustrate the best mode presently contemplated of carrying out the invention. 
       In the drawings: 
         FIG. 1  is a schematic representation showing a transfer switch arrangement for interconnecting an auxiliary power source, such as a portable electrical generator, with a load center or electrical panel associated with a building and having a neutrals switching apparatus according to a first embodiment of the present invention; and 
         FIG. 2  is a schematic representation showing a transfer switch arrangement for interconnecting an auxiliary power source, such as a portable electrical generator, with a load center or electrical panel associated with a building and having a neutrals switching apparatus according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to  FIG. 1 , a transfer switch arrangement incorporating a neutrals switching apparatus according to a first embodiment of the present invention is shown. This embodiment of the present invention is usable with a conventional power transfer arrangement  10  such as that having a transfer switch panel  12  which is interconnected between an electrical panel or load center  14  and an electrical generator  16 . The transfer switch panel  12  has electrical conductors  18 ,  19  (L 1 , L 2  to the load) that are switchably connected to receive electrical power from a utility power supply (not shown) via electrical conductors  20 ,  22  (L 1 , L 2  from the utility supply) of the electrical panel  14  or the electric generator  16  via electrical conductors  24 ,  26  (L 1 , L 2  from the generator  16 ). The electrical conductors  18 ,  19  may be, for example, an electrical bus on the transfer switch panel  12 . It is understood that the electrical conductors  18 ,  19  are electrically connected to the input conductors in a conventional manner. A transfer switch  28  is operable to electrically connect the electrical conductors  18 ,  19  to either the utility power supply (through the main electrical panel  14 ) or the electric generator  16 . The transfer switch  28  may be of conventional design and, as known in the art, is designed to electrically isolate the electrical conductors  18 ,  19  from the electric generator  16  when utility power is being provided to the electrical conductors  18 ,  19 , and electrically isolate the electrical conductors  18 ,  19  to the load from the utility power supply when the electric generator  16  is feeding electrical power to the electrical conductors  18 ,  19 . 
     The electrical conductors  20 ,  22  are interconnected between the electrical conductors  18 ,  19  to the load on the transfer switch panel  12  and an electrical bus  30  of the electrical panel  14 . The electrical bus  30  is electrically connected to the utility power supply and thus utility power may be fed to the electrical conductors  18 ,  19  of the transfer switch panel  12  to power loads connected to the transfer switch panel  12  via conductors  20 ,  22 . Alternately, conductors  20 ,  22  may be connected directly to the utility power supply. 
     The electric generator  16  feeds electrical power to the electrical conductors  18 ,  19  when utility power is interrupted. It will thus be appreciated that the transfer switch panel  12  may include components and circuitry to detect the loss of utility power and automatically electrically connect the electrical conductors  18 ,  19  to the electric generator. Such technology is generally well known and thus will not be described in greater detail. Moreover, it is understood that the transfer switch panel  12  may include components to cause the electric generator  16  to start automatically upon the loss of utility power. 
     The electrical panel  14  has a neutral bus  32 , which connects to the neutrals  34  of the one or more loads (not shown) that are fed electrical power through the transfer switch panel  12 . In contrast to conventional transfer switch arrangements, the present invention provides a neutrals transfer apparatus  36  that includes a switch  38  connected between the load neutrals  34  and the neutral bus  32  of the electrical panel  14  via a conductor  40 , to selectively connect the load neutrals  34  to the neutral bus  32  when utility power is feeding electrical power to the electrical conductors  18 ,  19  of the transfer switch panel  12 . In addition, neutrals transfer switch  38  is also connected to the neutral of the generator  16  via a conductor  42 , to selectively connect the load neutrals  34  to the neutral of the generator  16  when generator  16  is feeding electrical power to the electrical conductors  18 ,  19  of the transfer switch panel  12 . 
     The neutrals transfer switch  38  is operable to automatically make the transfer of the load neutrals  34  between the neutral bus  32  and the neutral of the generator  16 . In one embodiment, the neutrals transfer switch  38  includes a contactor  44  having a pair of coils  46 ,  48  each of which is wrapped about an armature  50 , such that energization of the coils causes the armature  50  to selectively move between first and second positions. When the armature  50  is in the first position, the contactor  44  electrically connects the load neutrals  34  to the neutral bus  32  of the electrical panel  14 . Conversely, when the armature  50  is in the second position, the contactor  44  electrically connects the load neutrals  34  to the neutral of the generator  16 . 
     The neutrals transfer switch  38  connects to the neutral bus  32  of the main electrical panel via conductor  40 , and the load neutrals  34  connect to the neutrals transfer switch  38  via a conductor  52 . 
     The neutral of the electric generator  16  connects to the neutrals transfer switch  38  via neutral conductor  42 . Representatively, the generator neutral may be connected to the neutral conductor  42  through a neutral bus  43  in the transfer switch panel  12  and a conductor  45  that extends between transfer panel neutral bus  43  and the neutral connection of generator  16 . 
     The neutrals transfer switch  38  automatically connects the load neutral bus  34  to the main panel bus  32  or the generator neutral conductor  42  based on which power supply is supplying electrical power to the transfer switch panel  12 . Doing so avoids the possibility of the neutrals transfer switch  38  erroneously connecting the load neutral bus  34  to the generator neutral conductor  42  when the utility power supply is feeding power to the transfer switch panel  12 . That is, switching of the neutrals transfer switch  38  is not controlled based on electrical current flow between a generator conductor  24  or  26  and the generator neutral conductor  42 , which can result in the load neutrals  34  being electrically connected to the electric generator  16  after the transfer switch  28  has been switched to the utility position upon the restoration of utility power if the electric generator is still running. In such a scenario, utility power can be fed to the loads without the neutrals of the loads being connected to the utility neutral, i.e., neutral bus  32 . This can result in either some loads receiving electrical power at a much higher, and potentially, damaging voltage or an unsafe current to flow on the ground wire. 
     In the present invention switching of the neutrals transfer switch  38  is controlled based on which power supply is feeding electrical power to the transfer switch panel  12 . Since the transfer switch  28  can be either an automatic or manual transfer switch, it can connect the electrical conductors  18 ,  19  to the electric generator  16  when utility power is interrupted. Similarly, the transfer switch  28  will restore connection of the electrical conductors  18 ,  19  to the utility power supply when utility power is restored. When utility power is available, the voltage potential between electrical conductor  22  and the electrical conductor  18  on the load side of the transfer switch  28  will be 240 volts. Conversely, if transfer switch panel  12  is switched to the generator, the voltage potential between the electrical conductor  22  and the electrical conductor  18  on the load side of the transfer switch  28  will drop to zero volts. When the transfer switch  28  is switched to electrically connect the electrical conductor  18  on the load side to the electrical generator  16 , the voltage potential between conductor  26  of the electric generator  16  and the electrical conductor  18  on the load side of the transfer switch  28  will be 240 volts. The present invention exploits these differences in voltage potentials to control switching of the neutrals transfer switch  38 . Although the example described above discusses a 240 volt potential, it is further contemplated that other voltage potentials may be used to control switching of the neutrals transfer switch  38 . For example, the voltage potential may vary by location between about 230 volts to 250 volts. Similarly, some foreign utilities generate 200-210 volts. As still another option, the switch  38  may be configured to operate on a 400 volt or a 575 volt utility grid. The 400 volt utility grid may supply a potential between 400-480 volts. Still other voltage potentials may be utilized without deviating from the scope of the present invention. 
     According to one embodiment of the invention, the power transfer arrangement  10  includes a set of sensing leads  54 ,  56 , and  58  that are connected to provide operating information to the neutrals transfer apparatus  36 . The sensing leads  54 ,  56 , and  58  may be configured to detect the amplitude of voltage or current present on its corresponding conductor. As illustrated, the sensing leads  54 ,  56 , and  58  detect the amplitude of voltage on the conductors connected to the neutrals transfer apparatus  36 . When the voltage between leads  54  and  56  is present, for example at 240 volts, the neutrals transfer switch  38  electrically connects neutral conductor  52  to the neutral bus  32  of the electrical panel  14  via neutral conductor  40 . On the other hand, if the voltage between leads  58  and  56  is 240 volts, the neutrals transfer switch  38  electrically connects the neutral conductor  52  to the neutral of the generator  16  via neutral conductor  42 . It can thus be appreciated that the switching of the neutrals transfer switch  38  depends on which power supply is feeding electrical power to the electrical bus  18  of the transfer switch panel  12 . 
     The load neutrals  34  and the neutrals transfer switch  38  may be incorporated into either the housing of transfer switch panel  12  or the housing of the electrical panel, although it is understood that any other satisfactory configuration and arrangement may be employed as desired. 
     As noted above, in one embodiment, the neutrals transfer switch  38  is in the form of a contactor  44  having a pair of coils  46 ,  48  that when energized cause the armature  50  to switch between the first and second positions as noted above. It will thus be appreciated that when utility power is feeding the electrical bus  18 , the voltage between sensing leads  54  and  56  will be present, for example at 240 volts, which causes the first coil  46  to move the armature  50  to the first position which switches the load neutrals to the utility neutral bus  32 . When the electric generator  16  is feeding electrical power to the electrical bus  18 , the voltage between sensing leads  58  and  56  will be 240 volts, which causes the second coil  48  to move the armature  50  to the second position which switches the load neutrals  34  to the generator neutral conductor  42 . 
     In an alternate embodiment, which is schematically shown in  FIG. 2 , sensors  60 ,  62  are be used to provide logic HIGH and LOW signals  66 ,  68  to a microprocessor  64  that in turn provides appropriate command signals  70  to the neutrals transfer switch  38 . As illustrated, the sensors  60 ,  62  are configured to detect the amplitude of the voltage potential between two conductors. Optionally, the sensors  60 ,  62  may be configured to detect the amplitude of current carried by the conductor. 
     Voltage sensor  60  measures the voltage potential between the transfer switch panel  12  and the electrical panel  14  across conductors  54  and  56  and voltage sensor  62  measures the voltage potential between the electrical generator  16  and the transfer switch panel  12  across conductors  56  and  58 . According to the amplitude of the voltage potential detected between the respective conductors, the sensor  60 ,  62  transmits a digital logic signal  66 ,  68  to the microprocessor  64 . The microprocessor  64  then provides the appropriate command signals  70  to the neutrals transfer switch  38  to connect the neutral conductor  52  either to the neutral bus  32  of the electrical panel  14  via neutral conductor  40  or to the neutral of the generator  16  via neutral conductor  42 . 
     The present invention may be embodied in a transfer switch panel or, alternately, in an add-on device for a transfer switch panel. 
     While not specifically shown and described herein, it will be appreciated that the present invention takes advantage of a solidly connected ground wire between the primary and secondary power supplies. This is typically required by many electrical codes, such as the U.S. National Electrical Code. Since each power supply is connected to the neutral (“bonded”) in each system, then the neutral current may return to the source on the ground wire when there is no neutral connection. In this regard, it will be appreciated that the neutral switch apparatus described herein is designed to prevent electrical current from returning on the ground wire. This is important because the U.S. National Electrical Code, for example, prohibits the ground wire from carrying normal electrical current. 
     Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.