Patent Publication Number: US-9851380-B2

Title: Power strip and electric power measurement system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 13/009,292 filed on Jan. 19, 2011, which is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-019166, filed on Jan. 29, 2010, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     It is related to a power strip and an electric power measurement system. 
     BACKGROUND 
     In recent years, there is a growing trend to save power consumption at home and office, in consideration of increasing electric power demand and the global environment. Along with the growth of the energy saving trend, efforts are made to frequently turn off electronic devices, to review the temperature set for air conditioning, and the like. 
     Various methods for measuring power consumption are proposed to figure out how much energy saving is actually achieved by such efforts. 
     However, any one of these methods has difficulty in accurately measuring power consumption of each of electronic devices. 
     For example, there is proposed a method in which a terminal for measuring power consumption is provided to an outlet at home, and measures the power consumption of an electronic device connected to the outlet. When a power strip is connected to one wall outlet and is connected to multiple electronic devices, however, this method has a problem that the power strip is incapable of measuring the power consumption of each of the electronic devices individually even though the power strip can measure the total power consumption of the multiple electronic devices. 
     In another method, an electric current sensor for measuring a power consumption is provided to a power supply line upstream of power division by a distribution board in a house. This method, however, has no way to figure out how much electric power is consumed in each of power supply lines downstream of the power division by the distribution board. 
     Note that, techniques related to the present application are disclosed in Japanese Laid-open Patent Publication Nos. 09-84146, 11-313441, and 2001-663330. 
     SUMMARY 
     According to one aspect discussed herein, there is provided a power strip including, a busbar electrically connected to a power source, a plurality of electrical outlets into which a plurality of power plugs are respectively insertable, a plurality of distribution bars which are branched out from the busbar and respectively supply the plurality of electrical outlets with electric currents of the power source, a plurality of electric current measurement units respectively measuring the electric current flowing through the plurality of distribution bars. 
     According to another aspect discussed herein, there is provided a power strip including, a first busbar electrically connected to one pole of a power source, a plurality of first contacts formed integrally with the first busbar, the first contacts allowing one of two plug blades of a power plug to be inserted therebetween, a second busbar electrically connected to the other pole of the power source, holding pieces provided integrally with the second busbar, a plurality of distribution bars whose main surfaces are held between the holding pieces, a plurality of second contacts provided to the distribution bars, the second contacts allowing the other one of the two plug blades of the power plug to be inserted therebetween, and a plurality of electric current measurement units to measure an electric current flowing through a corresponding one of the distribution bars, wherein the first busbar and the second busbar are fabricated by bending conductive plates having a same planar shape in a bending process. 
     According to still another aspect discussed herein, there is provided an electric power measurement system including a plurality of electric current measurement units each configured to measure an electric current flowing through a corresponding one of a plurality of distribution bars which are branched out from a busbar of a power strip, and respectively supply a plurality of electrical outlets with a electric power, and a program to multiply a voltage of the electric power and each of the measured electric currents, and thereby to calculate an amount of power consumed by each of a plurality of electronic devices connected respectively to the plurality of electrical outlets. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an external view of a power strip according to a first embodiment; 
         FIG. 2  is an external view of the power strip according to the first embodiment when the top and bottom covers of the power strip are removed; 
         FIG. 3  is an enlarged perspective view of an electric current measurement unit and the vicinity thereof in the power strip according to the first embodiment; 
         FIG. 4  is a circuit diagram of a hall element included in the power strip according to the first embodiment; 
         FIG. 5  is a plan view of a hall element included in the power strip according to the first embodiment; 
         FIG. 6  is a perspective view for describing a positional relation between a magnetic sensitive surface of the hall element, a second busbar and a distribution bar in the power strip according to the first embodiment; 
         FIG. 7  is an external view of the power strip according to the first embodiment in a state where the top cover of the power strip is removed; 
         FIG. 8  is a functional block diagram of a transmission circuit portion included in the power strip according to the first embodiment; 
         FIG. 9  is a schematic diagram for describing an electric power measurement system according to the first embodiment; 
         FIG. 10  is a perspective view for describing a simulation in a second embodiment; 
         FIGS. 11A and 11B  are diagrams each illustrating a simulation result of a magnetic field intensity in the second embodiment; 
         FIG. 12  is an enlarged perspective view of a distribution bar and a second busbar according to a third embodiment; 
         FIG. 13  is a side view of a distribution bar and a second busbar according to a fourth embodiment; 
         FIGS. 14A and 14B  are each side views illustrating a case where a distribution bar is formed nonlinearly in the fourth embodiment; 
         FIG. 15  is a perspective view illustrating a magnetic core according to a fifth embodiment and an area around the magnetic core; 
         FIG. 16  is a top view of a first circuit board in the fifth embodiment as viewed from above the magnetic core; 
         FIG. 17  is a perspective view illustrating a magnetic core according to a sixth embodiment and an area around the magnetic core; 
         FIG. 18  is a side view of the magnetic core and L-shaped ribs according to the sixth embodiment; 
         FIG. 19  is a perspective view of a magnetic core according to a seventh embodiment; 
         FIG. 20  is a perspective view illustrating how the magnetic core is attached to a first circuit board in the seventh embodiment; 
         FIG. 21  is a perspective view illustrating a magnetic core according to an eighth embodiment and an area around the magnetic core; 
         FIG. 22  is an external view illustrating an inner surface of a top cover used in the eighth embodiment; 
         FIG. 23  is a perspective view of a first circuit board and a magnetic core in a ninth embodiment; 
         FIG. 24  is a partial perspective view illustrating a plate attachment method in the ninth embodiment (part 1); 
         FIG. 25  is a partial perspective view illustrating a plate attachment method in the ninth embodiment (part 2); 
         FIG. 26  is a perspective view of a magnetic core according to a tenth embodiment; 
         FIG. 27  is a top view of the magnetic core according to the tenth embodiment; 
         FIG. 28  is a perspective view illustrating portions of a distribution bar according to an eleventh embodiment; 
         FIG. 29  is a perspective view illustrating a state where the portions of the distribution bar are assembled into a unit in the eleventh embodiment; 
         FIG. 30  is a side view illustrating a state where the portions of the distribution bar are assembled into a unit in the eleventh embodiment; 
         FIG. 31  is a perspective view of a conductive plate serving as a source of each busbar in a twelfth embodiment; 
         FIG. 32  is a perspective view of a first busbar obtained by subjecting the conductive plate to a bending process in the twelfth embodiment; 
         FIG. 33  is a perspective view of a second busbar obtained by subjecting the conductive plate to a bending process in the twelfth embodiment; 
         FIG. 34  is a perspective view of a third busbar obtained by subjecting the conductive plate to a bending process in the twelfth embodiment; and 
         FIG. 35  is a perspective view of a power strip according to the twelfth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     First Embodiment 
       FIG. 1  is an external view of a power strip  1  according to a first embodiment. 
     The power strip  1  is used for distributing an alternating-current power source to multiple electrical outlets  1   a . The alternating-current power source is supplied through a power plug  2  and a power supply cord  3 . In addition, the power strip  1  has a bottom cover  5  and a top cover  6 , which are made of resin and are screwed to each other. 
     Multiple pairs of first openings  6   a  and a second opening  6   b , which correspond to the multiple electrical outlets  1   a , are formed in the top cover  6 . Each pair of the first openings  6   a  and the second opening  6   b  allows an external power plug  7  to be inserted thereinto. 
     Each of the first openings  6   a  has a substantially rectangular planar shape so as to allow a corresponding one of plug blades  8  and  9  of the power plug  7  to be inserted thereinto. In addition, each of the second openings  6   b  has a substantially semicircular planar shape so as to allow an earth terminal  10  to be inserted thereinto. 
       FIG. 2  is an external view of the power strip according to the first embodiment when the top and bottom covers  5  and  6  are removed. 
     As illustrated in  FIG. 2 , first to third busbars  11  to  13  are provided in the power strip  1 . The busbars  11  to  13  can be fabricated by subjecting a metal plate such as a brass plate to a punching process and then to a bending process, for example. 
     Among the busbars  11  to  13 , the first busbar  11  and the second busbar  12  are electrically connected to the poles A +  and A −  of an alternating-current power source AC, respectively, through the power supply cord  3  (see  FIG. 1 ), while the third busbar  13  is kept at the ground potential through the power cord  3 . 
     In addition, the first busbar  11  has multiple pairs of first contacts  11   a , each of the pairs allowing the plug blade  8  among the plug blades  8  and  9  of the external power plug  7  to be inserted therebetween. 
     Meanwhile, the second busbar  12  has multiple pairs of holding pieces  12   a  arranged at constant intervals in the extending direction of the second busbar  12 . 
     Each of the pairs of the holding pieces  12   a  holds the main surface of a distribution bar  17 , and a pair of second contacts  17   a  is provided at an end portion of the distribution bar  17 . Each pair of the second contacts  17   a  forms a pair with a corresponding one of the pairs of the first contacts  11   a  and also allows the plug blade  9  of the power plug  7  to be inserted therebetween. 
     The third busbar  13  has multiple pairs of third contacts  13   a , and each of the pairs of the third contacts  13   a  allows the earth terminal  10  of the power plug  7  to be inserted therebetween. 
     A first circuit board  20  is provided below the distribution bars  17 . 
     The first circuit board  20  is provided with multiple electric current measurement units  30  each configured to measure an electric current flowing through a corresponding one of the multiple distribution bars  17 . 
       FIG. 3  is an enlarged perspective view of each of the electric current measurement units  30  and the vicinity thereof. 
     Each of the electric current measurement units  30  includes a magnetic core  21  fixedly attached to the first circuit board  20  for a corresponding one of the distribution bars  17 . The magnetic core  21  is formed so as to cause a magnetic field generated around the electric current flowing through the busbar  17  to converge and is formed in a substantially ring shape along the path of the magnetic field. The material of the magnetic core  21  is not limited to a particular substance, but ferrite, which is a relatively easily available substance, is used in this embodiment as the material of the magnetic core  21 . 
     In addition, each of the electric current measurement units  30  includes a hall element  22  provided in a gap  21   a  of the magnetic core  21 . The hall element  22  estimates, on the basis of the intensity of the magnetic field in the gap  21   a , an electric current value flowing through the busbar  17 , and is mounted on the first circuit board  20  by soldering or the like. 
     The multiple electric current measurement units  30  are provided on the first circuit board  20 , which is a single piece of circuit board. Thus, reduction in the number of components and simplification of the assembly process can be achieved as compared with the case where circuit boards are provided for each of the electric current measurement units  30 . 
       FIG. 4  is a circuit diagram of the hall element  22 . 
     As illustrated in  FIG. 4 , the hall element  22  has a gallium arsenide based magnetic sensor  23  and an operational amplifier  24 . 
     When exposed to a magnetic field in a state where a voltage Vcc is applied between a power supply terminal  22   a  and a ground terminal  22   b , the magnetic sensor  23  generates a potential difference ΔV in accordance with the intensity of the magnetic field. The potential difference ΔV is amplified by the operational amplifier  24  and then is outputted to an outside from an output terminal  22   c.    
       FIG. 5  is a plan view of the hall element  22 . 
     As illustrated in  FIG. 5 , the magnetic sensor  23  is sealed by resin  26  so as to be positioned within the plane of a magnetic sensitive surface P M . Then, the hall element  22  detects a magnetic field component perpendicular to the magnetic sensitive surface P M  in the magnetic field penetrating the magnetic sensor  23  and then outputs an output signal corresponding to the magnitude of the magnetic filed component from the output terminal  22   c.    
     Note that, the terminals  22   a  to  22   c  are electrically connected to wiring in the first circuit board  20  (see  FIG. 3 ) by soldering or the like. 
     The hall element  22  as described above is small in size as compared with another magnetic field measurement element such as a current transformer. Thus, there is no concern about an increase in size of the power strip with the hall element  22 . 
     Further, the current transformer uses an inductive current, which is generated by a fluctuation of the magnetic field with time, to measure the magnitude of a magnetic field. Accordingly, the measuring object of the current transformer is limited to a magnetic field of alternating-current. Meanwhile, the hall element  22  has an advantage in that the intensity of a static magnetic field is measurable as well. 
     Moreover, the hall element  22  is inexpensive as compared with a current transformer. Accordingly, an increase in the cost of the power strip can be prevented with the hall element  22 . 
       FIG. 6  is a perspective view for describing a positional relationship between the magnetic sensitive surface P M  of the hall element  22  and each of the bars  12  and  17 . 
     The magnetic sensitive surface P M  is set so as to be in parallel with an extending direction D 1  of the distribution bar  17 . With this setting, a magnetic field H 1  generated from the electric current flowing through the distribution bar  17  penetrates the magnetic sensitive surface P M  in a substantially perpendicular direction. Thus, the current detection sensitivity of the hall element  22  improves with this configuration. 
     In addition, in this embodiment, the extending direction D 1  of the distribution bar  17  is set not to be in parallel with an extending direction D 2  of the second busbar  12 . Accordingly, a magnetic field H 2  generated at the second busbar  12  does not penetrate the magnetic sensitive surface P M  in a perpendicular direction. As a result, a risk that the hall element  22  provided for use in measuring the magnetic field H 1  generated at the distribution bar  17  accidentally detects the magnetic field H 2  generated at the second busbar  12  is reduced. Accordingly, occurrence of cross-talk, in which the influence of a magnetic field other than the magnetic field H 1  is included in the result of the magnetic field detection of the hall element  22 , can be prevented. Thus, the measurement accuracy for the magnetic field H 1  by the hall element  22  improves. 
     Specifically, when the extending direction D 1  of the distribution bar  17  is set perpendicular to the extending direction D 2  of the second busbar  12 , the magnetic sensitive surface P M  becomes also perpendicular to the extending direction D 2 . Thus, the magnetic field H 2  generated at the second busbar  12  has no component perpendicular to the magnetic sensitive surface P M , so that the measurement accuracy for the magnetic field H 1  by the hall element  22  further improves. 
       FIG. 7  is an external view of the power strip  1  in a state where the top cover  6  is removed. 
     As illustrated in  FIG. 7 , a transmission circuit portion  27  for housing a second circuit board  25  therein is allocated in the bottom cover  5 . 
     The first and second circuit boards  20  and  25  are provided with connectors  35  and  36 , respectively, and a communication cable  37  is connected between these connectors  35  and  36 . 
     The communication cable  37  has a function to supply the first circuit board  20  with an electric power which is supplied through the power supply cord  3  and which is required for driving the hall elements  22  (see  FIG. 3 ). The communication cable  37  also has a function to transmit an output signal of each of the hall elements  22  to the second circuit board  25 . 
       FIG. 8  is a functional block diagram of the transmission circuit portion  27 . 
     As illustrated in  FIG. 8 , the transmission circuit portion  27  has a frequency sensor  31 , an AD converter  32 , an arithmetic unit  33  and an output port  34 . The frequency sensor  31  detects a frequency of the alternating-current power flowing through the power supply cord  3 . The AD converter  32  digitalizes an analog signal outputted from the hall elements  22 . 
     The transmission circuit portion  27  functions as follows. 
     Firstly, each of the hall elements  22  outputs an analog current signal S IA  indicating an electric current flowing through a corresponding one of the distribution bars  17 . 
     The analog current signal S IA , which is an analog value, is digitalized by the AD converter  32  and becomes a digital current signal S ID . 
     The frequency sensor  31  is a photocoupler, for example, and detects the frequency of the alternating-current power flowing through the power supply cord  3  connected to the alternating-current power source AC, and then outputs a frequency signal S P  rising from “0” to “1” in synchronization with the frequency. For example, when the frequency of the alternating-current power is 50 Hz, the frequency signal Sp also rises from “0” to “1” at the frequency of 50 Hz. 
     The arithmetic unit  33  measures the frequency at which the signal rises in the frequency signal S P , and then identifies the frequency as a frequency T of the alternating-current power. Further, the arithmetic unit  33  uses 64/T as the sampling frequency and then receives the digital current signal S ID  at the sampling frequency. 
     Note that, the arithmetic unit  33  is not limited to a particular processor, but an 8-bit MPU (Micro Processing Unit) is used as the arithmetic unit  33  in this embodiment. 
     Thereafter, the arithmetic unit  33  formats the digital current signal S ID  to comply with the USB (Universal Serial Bus) standard and then outputs the formatted signal to the output port  34  as an output signal S OUT . 
     Note that, the standard of the output signal S OUT  is not limited to the USB standard, and the digital current signal S ID  may be formatted to comply with an optional standard such as wired LAN (Local Area Network), wireless LAN or the like. 
     In addition, a multiplier may be provided to the arithmetic unit  33 . In this case, the voltage of the alternating-current power source AC is multiplied by the digital current signal S ID  to find the electric energy consumed by each of the electronic devices connected respectively to the distribution bars  17 . In this case, the electric energy with respect to the distribution bars  17  is outputted as the output signal S OUT . 
     Next, an electric power measurement system using the power strip  1  is described. 
       FIG. 9  is a schematic diagram for describing an electric power measurement system  60  according to this embodiment. 
     When the power strip  1  is used, the power plug  2  is inserted into a wall outlet  48  as illustrated in  FIG. 9 . 
     Then, power plugs  41   a  to  44   a  of first to fourth electronic devices  41  to  44  are inserted into the electrical outlets  1   a  of the power strip  1 , respectively. Note that, it is not necessary to use all of the electrical outlets  1   a  for connecting electronic devices, and there may be an unused electrical outlet  1   a  among the multiple electrical outlets  1   a.    
     Further, a signal cable  45  such as a USB cable is used to connect an electronic computer  46  such as a personal computer to the output port  34  of the power strip  1 . 
     In this configuration, a value of the electric current supplied to each of the electronic devices  41  to  44  from the respective electrical outlets  1   a  is inputted to the electronic computer  46  as the output signal S OUT . 
     The electronic computer  46  is provided with a storage unit  46   a  such as a hard disk drive. The storage unit  46   a  stores therein a program  47  for individually computing the electric power that is consumed by the respective electronic devices  41  to  44 . Here, the program  47  computes the electric power by multiplying the voltage of the power source by the electric current included in the output signals S OUT . 
     The method for storing the program  47  in the storage unit  46   a  is not limited to any particular method. For example, the electronic computer  46  may use an unillustrated CD (Compact Disk) drive or the like of the electronic computer  46  to read the program  47  stored in a recording medium  49  such as a CD and thereby to store the program  47  in the storage unit  46   a.    
     When used, the program  47  is loaded into a RAM (Random Access Memory)  46   b , and an arithmetic unit  46   c  such as a CPU calculates the power consumption of the electronic devices  41  to  44  individually for each of the electronic devices  41  to  44  by use of the program  46 . Then, the result of the calculation is displayed on a monitor  51  for each of the electrical outlets  1   a.    
     When a multiplier is provided to the arithmetic unit  33  (see  FIG. 8 ), the aforementioned calculation does not have to be performed by the electronic computer  46 , and the electrical energies included in the output signals S OUT  for the respective electrical outlets  1   a  are displayed on the monitor  51 . 
     Then, the user can monitor the monitor  51  and thereby realize in real time how much electric power is consumed by each of the electronic devices  41  to  44 . The user can thus obtain information for determining whether or not to reduce, for the purpose of energy saving, the electric power consumed by each of the electronic devices  41  to  44 . 
     In addition, a database  46   d  may be provided in the electronic computer  46 , and the total electric power of the electronic devices  41  to  44  consumed in a predetermined period may be stored in the database  46   d . Accordingly, additional information for determining whether or not to reduce the power can be obtained. 
     According to the embodiment described above, as described with reference to  FIG. 9 , the amounts of power consumption of the electronic devices  41  to  44  connected to the power strip  1  can be individually monitored. 
     Moreover, as described with reference to  FIG. 6 , the distribution bars  17  branch from the second busbar  12 . Thus, the extending direction D 1  of the distribution bars  17  and the extending direction D 2  of the second busbar  12  are not in parallel with each other, so that a risk that the hall elements  22  accidentally measure the magnetic field H 2  generated at the second busbar  12  is reduced. As a result, the electric current flowing through each of the distribution bars  17  can be detected by a corresponding one of the hall elements  22  with high accuracy, and the reliability of the calculated value of the amount of the power consumption of each of the electronic devices  41  to  44  can be enhanced. 
     Second Embodiment 
     In this embodiment, a preferable positional relationship between the magnetic core  21  and the hall element  22  is described. 
       FIG. 10  is a perspective view for describing a simulation performed by the inventors of the present application. 
     As illustrated in  FIG. 10 , the magnetic field intensity on a gap surface  21   b  of the magnetic core  21  which faces the gap  21   a  is simulated. 
       FIGS. 11A and 11B  are diagrams each illustrating a simulation result thereof. 
     A distance W 1  between the distribution bar  17  and the hall element  22  in  FIG. 11A  is different from a distance W 2  between the distribution bar  17  and the hall element  22  in  FIG. 11B . In  FIG. 11A , the hall element  22  is arranged at a position closer to the distribution bar  17  than in  FIG. 11B . 
     As illustrated in  FIGS. 11A and 11B , it is verified that the magnetic field intensity is drastically reduced in an edge portion  21   e  of the gap surface  21   b  at a position closer to the distribution bar  17 . 
     Based on the above finding, in order to ensure the measurement accuracy of the magnetic field detected by the hall element  22 , it is preferable that the magnetic sensor  23  is positioned near the center of the gap surface  21   b  where a spatial fluctuation of the magnetic field is small. 
     However, when the hall element  22  is mounted on the first circuit board  20  (see  FIG. 3 ), a certain amount of a positional shift is expected between the hall element  22  and the first circuit board  20 . Therefore, it is difficult to accurately position the magnetic sensor  23 , and the magnetic sensor  23  may be positioned out of the area near the center of the gap surface  21   b . Specifically, when the distance W 1  is set smaller as illustrated in  FIG. 11A , the magnetic sensor  23  is more likely to be positioned in a region where the magnetic field is weak in the edge portion  21   e . In this case, it is difficult to ensure the measurement accuracy of the magnetic field detected by the hall element  22 . 
     To deal with this problem, the distance W 2  is preferably arranged as large as possible in considering the positional shift of the hall element  22  at the time of mounting the hall element  22 , so that the magnetic sensor  23  is prevented from being positioned in the edge portion  21   e  even when the positional shift occurs. In this configuration, the magnetic sensor  23  is not located in the edge portion  21   e  but is surely located near a center portion C of the gap surface  21   b . Thus, a spatially almost uniform magnetic field near the center portion C is measurable by the magnetic sensor  23 , so that the measurement reliability of the magnetic field improves. 
     Furthermore, the area of the gap surface  21   b  may be made sufficiently larger than the area of the magnetic sensor  23 . This configuration increases a region where the magnetic field is substantially uniform in the gap surface  21   b , and accordingly can reduce a risk that the magnetic sensor  23  is positioned in a region where the magnetic field spatially drastically changes such as in the edge portion  21   e . Thus, the measurement accuracy of the magnetic field detected by the hall element  22  can be improved. 
     However, when the distance W 2  is too large due to the increase in the area of the gap surface  21   b , the magnetic sensor  23  is positioned excessively apart from the distribution bar  17 . In this case, there arises a concern that the detection sensitivity of the hall element  22  of the magnetic field decreases because the magnetic field intensity at the magnetic sensor  23  is reduced. 
     To deal with this problem, a height B 2  of the gap surface  21   a  is preferably kept around 1.5 to 2.5 times of a height A 2  of the hall element  22  in order to prevent the magnetic field at the magnetic sensor  23  from being weakened due to the increase in the distance W 2 . 
     Third Embodiment 
     The present embodiment is different from the first embodiment only in the form of the distribution bar  17 , and the other configuration of the present embodiment is the same as that of the first embodiment. 
       FIG. 12  is an enlarged perspective view of the distribution bar  17  and the second busbar  12  according to the present embodiment. 
     As illustrated in  FIG. 12 , the distribution bar  17  is inserted between the pair of holding pieces  12   a  of the second busbar  12  during the assembly process. Here, the portion of the distribution bar  17 , which is to be held between the holding pieces  12   a , is preferably chamfered in advance to form a chamfered portion  17   g  at a corner portion of the distribution bar  17 . In this configuration, the chamfered portion  17   g  is allowed to be in sliding contact with the holding pieces  12   a , thereby allowing the distribution bar  17  to be smoothly and easily inserted between the pair of the holding pieces  12   a  during the assembly process. 
     The method for forming the chamfered portion  17   g  is not particularly limited. However, an embossing process using a mold is preferably used to form the chamfered portion  17   g  in considering that this method takes less machining time than a cutting process. 
     Fourth Embodiment 
     This embodiment is different from the first embodiment only in the form of the distribution bar  17 , and the other configuration of the embodiment is the same as that of the first embodiment. 
       FIG. 13  is a side view of the distribution bar  17  and the second busbar  12 . 
     In the example of  FIG. 13 , the distribution bar  17  is linearly formed. The linearly formed distribution bar  17  is supported by ribs  5   a  of the bottom cover  5 , each of the ribs  5   a  having a height L 0 . 
       FIGS. 14A and 14B  are each side views illustrating the case where the distribution bar  17  is formed nonlinearly. 
     In the example of  FIG. 14A , an extension portion  17   c  extending toward the first circuit board  20  is provided to the distribution bar  17  on a side near the second contacts  17   a.    
     In this configuration, a distance D 1  between the first circuit board  20  and the distribution bar  17  near the second contacts  17   a  is made smaller than a distance D 2  between the first circuit board  20  and the distribution bar  17  near the second busbar  12 . 
     Thus, a height L 1  of the ribs  5   a  supporting the distribution bar  17  is made smaller than the height L 0  in  FIG. 13 . Thus, the bottom cover  5  can be formed thinner in this configuration. 
     Meanwhile, in the example of  FIG. 14B , the distribution bar  17  is formed in a bridge shape, so that the distribution bar  17  near the second contact  17   a  can be positioned closer to the first circuit board  20 . In this configuration as well, the distance D 1  is made smaller than the distance D 2 . Thus, a height L 2  of the ribs  5   a  can be made smaller than the height L 0  in  FIG. 13 . Accordingly, it is possible to achieve a reduction in the thickness of the bottom cover  5  in this configuration. 
     Fifth Embodiment 
       FIG. 15  is a perspective view illustrating the magnetic core  21  according to the present embodiment and an area around the magnetic core  21 . 
     The present embodiment is different from the first embodiment only in the attachment method for the magnetic core  21 , and the other configuration of the present embodiment is the same as that of the first embodiment. 
     As illustrated in  FIG. 15 , in this embodiment, openings  20   a  are formed in the first circuit board  20 , and L-shaped ribs  5   b  are respectively inserted into the openings  20   a.    
     The method for forming the L-shaped ribs  5   b  is not particularly limited. However, it is preferable to vertically form the L-shaped ribs  5   b  integrally with the bottom cover  5  on the inner surface of the bottom cover  5  (see  FIG. 7 ) in consideration of reduction in the number of components. 
       FIG. 16  is a top view of the first circuit board  20  as viewed from above the magnetic core  21 . 
     As illustrated in  FIG. 16 , the two L-shaped ribs  5   b  are provided diagonally with respect to the magnetic core  21  having a rectangular shape in a cross-sectional view. Here, the two L-shaped ribs  5   b  cooperatively hold the magnetic core  21 . 
     Accordingly, the magnetic core  21  no longer has to be adhered to the first circuit board  20 , and the number of steps required for the adhesion can be reduced. 
     Moreover, the magnetic core  21  is not fixed onto the first circuit board  20  in this structure. Thus, even when the first circuit board  20  thermally expands, a fluctuation in a width G of the gap  21   a  following the thermal expansion does not occur. Thus, a change in the magnetic field in the gap  21   a  caused by a fluctuation in the width G can be suppressed. Accordingly, the measurement accuracy of the magnetic field detected by the hall element  22  can be maintained. 
     Sixth Embodiment 
       FIG. 17  is a perspective view illustrating the magnetic core  21  and an area around the magnetic core  21 . 
     The present embodiment is different from the fifth embodiment only in the form of the ribs  5   b , and the other configuration of the present embodiment is the same as that of the fifth embodiment. 
     As illustrated in  FIG. 17 , an extension portion  5   c  is provided to each of the L-shaped ribs  5   b , and a pawl  5   d  is further formed at a tip end of the extension portion  5   c.    
       FIG. 18  is a side view of the magnetic core  21  and the L-shaped ribs  5   b.    
     As illustrated in  FIG. 18 , the pawl  5   d  is provided to hold a top surface  21   c  of the magnetic core  21 . 
     Here, the magnetic core  21  is pressed against the first circuit board  20  by the pawls  5   d  provided to the L-shaped ribs  5   d . Thus, the magnetic core  21  can be prevented from being displaced from the L-shaped ribs  5   b.    
     Seventh Embodiment 
       FIG. 19  is a perspective view of the magnetic core  21  according to the present embodiment. 
     In the present embodiment, grooves  21   e  are provided respectively to side surfaces  21   d  of magnetic core  21  as illustrated in  FIG. 19 . The other configuration of the present embodiment is the same as that of the sixth embodiment. 
       FIG. 20  is a perspective view illustrating how the magnetic core  21  is attached to the first circuit board  20 . 
     Each of the grooves  21   e  has a width and a depth large enough to allow the pawl  5   d  to fit into the groove  21   e . Thus, when the magnetic core  21  is moved down toward the first circuit board  20  in the course of the attachment, the pawls  5   d  are placed inside the grooves  21   e . Accordingly, the magnetic core  21  can be prevented from being damaged by sliding contact with the pawls  5 . 
     Eighth Embodiment 
       FIG. 21  is a perspective view illustrating the magnetic core  21  according to the present embodiment and an area around the magnetic core  21 . 
     In the present embodiment, an elastic body  38  is provided to the top surface of the magnetic core  21  as illustrated in  FIG. 21 . The other configuration of the present embodiment is the same as that of the fifth embodiment. 
       FIG. 22  is an external view illustrating an inner surface of the top cover  6  used with the elastic body  38 . 
     As illustrated in  FIG. 22 , an inner surface  6   c  of the top cover  6  has a partial region R with which the elastic body  38  is in contact. 
     The elastic body  38  has a function to press the magnetic core  21  against the first circuit board  20  while being in contact with both of the partial region R of the inner surface  6   c  and the top surface of the magnetic core  21 . 
     The elastic body  38  is used to regulate the movement of the magnetic core  21  in its height direction M (see  FIG. 21 ) of the magnetic core  21 . Thus, the elastic body  38  can prevent magnetic core  21  from being displaced from the first circuit board  20 . 
     The material of the elastic body  38  is not particularly limited, but soft sponge or rubber, which is unlikely to damage the magnetic core  21 , is preferably used as the material of the elastic body  38 . Further, a spring expandable in the height direction M may be used as the elastic body  38 . 
     Note that, a single elastic body  38  may be used commonly for all of the magnetic cores  21 , instead of providing the multiple elastic bodies  38  for the magnetic cores  21  as illustrated in  FIG. 21 . 
     Ninth Embodiment 
       FIG. 23  is a perspective view of the first circuit board  20  and the magnetic core  21  in the present embodiment. 
     In this embodiment, as illustrated in  FIG. 23 , a plate  39  is provided. The plate  39  presses the multiple magnetic cores  21  against the first circuit board  20  while being in contact with the top surfaces of the multiple magnetic cores  21 . The other configuration of the present embodiment is the same as that of the fifth embodiment. 
       FIGS. 24 and 25  are each perspective views illustrating an attachment method of the plate  39 . 
     As illustrated in  FIG. 24 , projections  5   e  each including a screw hole formed therein are provided on the inner surface of the bottom cover  5 . 
     As illustrated in  FIG. 25 , the projections  5   e  are inserted through openings  20   b  of the first circuit board  20  and then fixed to the plate  39  by screws  40 . 
     The plate  39  is used to regulate the movement of the magnetic cores  21  in it height direction. Thus, the plate  39  can prevent the magnetic cores  21  from being displaced from the first circuit board  20 . 
     The material of the plate  39  is not particularly limited, but a resin plate is preferably used as the material of the plate  39  in consideration of preventing the magnetic cores  21  from being damaged when the plate  39  is brought in contact with the magnetic cores  21 . 
     Tenth Embodiment 
       FIG. 26  is a perspective view of the magnetic core  21  according to the present embodiment, and  FIG. 27  is a top view of thereof. 
     As illustrated in  FIGS. 26 and 27 , magnetic shields  50  are vertically stood on the first circuit board  20  in the present embodiment. The other configuration of the present embodiment is the same as that of the fifth embodiment. 
     Each of the magnetic shields  50  is provided beside the gap  21   a  of the magnetic cores  21 , and functions to prevent an unnecessary magnetic field from entering into the gap  21   a  from outside of the magnetic core  21 . 
     As a material of the magnetic shield  50  having such a function, a material having a high magnetic permeability can be used, for example. The material having a high magnetic permeability has a characteristic of capturing an external magnetic field and allowing the external magnetic field to penetrate the material itself. Accordingly, when the material having a high magnetic permeability is used as the material of the magnetic shield  50 , the magnetic shield  50  captures an external magnetic field attempting to enter the gap  21   a . Thus, a reduction in the measurement accuracy of the magnetic field by the hall element  22  due to the external magnetic field can be prevented. 
     Among the materials having a high magnetic permeability, a ferromagnetic material having a high permeability and a high saturation magnetic flux density and having a low retention force is preferably used as the material of the magnetic shield  50 . As an example of such a material, electromagnetic soft iron, electromagnetic steel sheet, permalloy alloy, an amorphous material of a compound of iron, silicon and boron, and a microcrystalline ribbon obtained by causing the amorphous material to crystallize and the like can be used. 
     In addition, the magnetic shields  50  may be fixed onto the first circuit board  20  by any method such as adhering, soldering or the like. 
     Note that, each of the magnetic shields  50  is preferably formed in a size within a range large enough to effectively prevent the external magnetic field from entering into the gaps  21   a . For example, a width A 1  of the magnetic shield  50  is preferably larger than the width G of the gap  21   a  but smaller than a width B 1  of the magnetic core  21 . In addition, a height A 2  of the magnetic shield  50  is preferably larger than the height of the hall element  22  but smaller than a height B 2  of the gap  21   a.    
     Eleventh Embodiment 
       FIG. 28  is a perspective view illustrating portions of the distribution bar  17  according to the present embodiment. The present embodiment is different from the first embodiment only in the form of the distribution bar  17 , and the other configuration of the embodiment is the same as that of the first embodiment. 
     As illustrated in  FIG. 28 , the distribution bar  17  is separated into a sensing portion  17   b  and a contact portion  17   c  in the present embodiment. 
     Among these portions, the sensing portion  17   b  is provided with posts  17   d . Meanwhile, the contact portion  17   c  is provided with a pair of holding pieces  17   e  in addition to the pair of second contacts  17   a.    
       FIG. 29  is a perspective view illustrating a state where the portions  17   b  and  17   c  are assembled into a unit. 
     As illustrated in  FIG. 29 , one end of the sensing portion  17   b  is held between the pair of holding pieces  17   e  of the contact unit  17   c , while the other end thereof is held between the pair of holding pieces  12   a  of the second busbar  12 . 
     Moreover, the posts  17   d  of the sensing portion  17   b  are fixed onto the first circuit board  20  by adhering, soldering or the like. 
       FIG. 30  is a side view illustrating a state where the portions  17   b  and  17   c  are assembled into a unit. 
     As described above, the posts  17   d  are fixed onto the first circuit board  20  in the present embodiment. Thus, even if a force is applied to the sensing portion  17   b  when the plug blade  8  or  9  is inserted into or removed from the pair of first contacts  17   a , a distance X between the sensing portion  17   b  and the hall element  22  does not change. 
     Thus, it is possible to prevent a fluctuation in the magnetic field intensity at a portion of the sensing portion  17   b  where the hall element  22  exists due to a change in the distance X. 
     Twelfth Embodiment 
     In the present embodiment, a method of manufacturing the first to third busbars  11  to  13  described in the first embodiment is described. 
       FIG. 31  is a perspective view of a conductive plate  55  serving as an original plate for the busbars  11  to  13 . 
     The conductive plate  55   a  is formed by processing a brass plate with a mold and provided with multiple projections  55   a.    
       FIGS. 32 to 34  are perspective views of the first to third busbars  11  to  13  obtained by subjecting the conductive plate  55  to a bending process. 
     As illustrated in  FIGS. 32 to 34 , by the aforementioned bending process, the multiple projections  55   a  are formed into the first contacts  11   a  provided integrally with the first busbar  11 , the second contacts  12   a  provided integrally with the second busbar  12 , or the third contacts  13   a  provided integrally with the third busbar  13 . 
     In this manner, it is possible to easily manufacture the first to third busbars  11  to  13  by changing the portions to be bent or the bending direction of the single flat conductive plate  55  in this embodiment. Thus, the manufacturing cost of the first to third busbars  11  to  13  can be inexpensive. 
       FIG. 35  is a perspective view of the power strip  1  including the first to third busbars  11  to  13  manufactured by the aforementioned manner. Here, illustration of the covers  5  and  6  (see  FIG. 1 ) is omitted in  FIG. 35 . 
     In this embodiment, the manufacturing cost of the busbars  11  to  13  can be inexpensive as described above. Thus, a reduction in the cost of the power strip  1  incorporating the busbars  11  to  13  therein can be achieved. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.