BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method of assembling an electrical junction box suitable to be connected for example to a wire harness in a vehicle such as an automobile. 
     2. Description of Related Art 
     Recently, the increase of electrical and electronic component parts which are mounted in a vehicle has led to increase of circuits to be accommodated in electrical connection boxes and junction boxes in the vehicle. Thus, when forming branch circuits at a high density, it is necessary to mount a large number of component parts on a junction box or the like, which causes increase of manufacturing complexity. 
     In a junction box disclosed in JP-A-2000-92660 and shown in present FIG. 10, bus bars  5 A- 5 D are laminated one upon another between an upper case part  2  and a lower case part  3 , with insulation plates  4 A- 4 E interposed between the bus bars  5 A- 5 D. The upper case part  2  has a connector receiving portion  2   a,  a relay receiving portion  2   b  and a fuse receiving portion  2   c,  on which in use connectors  6 , relays  7  and fuses  8  are mounted respectively. Terminals of the connectors  6 , the relays  7  and the fuses  8  are connected to tabs  5   a  projecting from the bus bars directly or are connected to the bus bars through relaying terminals. The lower case part  3  has also a connector receiving portion  3   a  to connect connectors to tabs projecting from the bus bars. 
     In the junction box  1 , with the increase of the number of circuits, the area and the number of layers of the bus bars increase and thus the size of the junction box becomes large. If the connector, relay and fuse receiving portions are arranged on both the upper and lower case parts to connect connectors, relays and fuses to internal circuits of the junction box, it is possible to make the area of the junction box smaller than in the case where the receiving portions are mounted on only the upper case part or the lower case part. 
     However, if the connector, relay and fuse receiving portions are mounted on both the upper and lower case parts such that they are opposed vertically, the bent tabs of bus bars must overlap each other and thus cannot be easily arranged. In this case, it is necessary to form tabs on bus bars of other layers, which causes an increase of number of layers of bus bars, and thus leads to the increase of the height of the junction box. That is, the junction box is necessarily large. 
     Further, the above-described junction box is so constructed that the bus bars are connected to the connectors, the fuses and the relays. Thus, when the specification of the connection between the internal circuit and the fuses and/or the relays is altered, it is necessary to alter the entire internal circuit. Consequently the above-described junction box is incapable of allowing a circuit alteration easily. 
     Some proposals have been made for replaceable modules in electrical circuits of automobiles. 
     U.S. Pat. No. 5,179,503 shows a modular automobile power distribution box having replaceable modules carrying relays or fuses. The relays or fuses in each module are directly connected to terminals of leads of wire harnesses. A pair of bus bars connect power terminals to the fuses of three maxi-fuse modules. There is no discussion of interconnection of the modules. 
     U.S. Pat. No. 5,581,130 discloses removable multi-function modules in individual casings which are mounted on a circuit board. Each module is electrically connected to the power supply distribution layer of the board by a pin. Alternatively three modules are shown connected together by two electrical and mechanical coupling bars. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of assembling a junction box which can be made thin without increasing the number of layers of bus bars to be accommodated therein and which can cope with a circuit alteration easily. 
     According to the present invention, there is provided a method of assembling an electrical junction box adapted to provide electrical connection to a plurality of electrical connectors, a plurality of fuses and a plurality of relays. The method includes: 
     (i) assembling in a casing the following components: 
     (a) a connector circuit having at least one connector circuit insulation substrate having opposite main surfaces and a periphery around the main surfaces, a plurality of first bus bars fixed on the insulation substrate, the first bus bars being arranged to provide electrical connection to electrical connectors in use and having at least one first welding portion at an end of one of the first bus bars and standing up from one of the main surfaces and at least one second welding portion formed at an end of one of the first bus bars and extending laterally from the periphery of the insulation substrate; 
     (b) a fuse circuit having at least one fuse circuit insulation substrate, which is discrete from the connector circuit, and a plurality of second bus bars fixed on the fuse circuit insulation substrate, the second bus bars being arranged to provide electrical connection to fuses in use; and 
     (c) a relay circuit having at least one relay circuit insulation substrate, which is discrete from the connector circuit, and a plurality of third bus bars fixed on the relay circuit insulation substrate, the third bus bars being arranged to provide electrical connection to relays in use; 
     at least one of the fuse circuit and the relay circuit having a third welding portion at an end of a bus bar thereof and upstanding away from the insulation substrate thereof and a fourth welding portion at an end of a bus bar thereof and extending laterally from the insulation substrate thereof; 
     the assembling step including the steps of: 
     (ii) joining the first and third welding portions to form a welded connection by arranging them lying adjacent each other and welding them together; and 
     (iii) after step (ii), joining the second and fourth welding portions to form a welded connection by welding them together when superimposed one on the other. 
     The present invention also provides a junction box assembled by the above-described method. 
     The electrical junction box of the present invention has a modular construction. As described above, in the junction box of the present invention, the fuse circuit substrate and the relay circuit substrate are separately provided from the connector circuit substrate and are then joined by welding of the welding portions. 
     A connector module, which includes the connector circuit, has a welding portion formed by vertically projecting the end of the bus bar from the upper surface of the insulation substrate and a welding portion projected horizontally from the periphery of the insulation substrate. A fuse module and/or a relay module to be connected to the connector module respectively include the fuse circuit and the relay circuit, and have a welding portion formed by projecting an end of a bus bar from the periphery of an insulation substrate at one side thereof and bending it perpendicularly, i.e., vertically away from the insulation substrate. The fuse and/or relay module also have a welding portion formed by horizontally projecting the end of a bus bar from the periphery of the insulation substrate at the other side thereof. 
     The fuse module and/or the relay module are disposed at a predetermined position relative to the connector module by superimposing them on each other. The bus bars of the connector module and those of the fuse module and/or the relay module are connected to each other by superimposing and welding the corresponding vertical welding portions to each other and the corresponding horizontal welding portions to each other. 
     In the junction box of the present invention, the connector module, the fuse module and the relay module are separately formed by separately providing the connector connection circuit to be connected to external wires through a connector, the fuse circuit and the relay circuit. That is, the connector connection circuit bus bars, the fuse connection bus bars, and the relay connection bus bars are separately provided. 
     Alternatively, as described below, the fuse circuit and the relay circuit may be integrated. 
     In contrast, in the conventional method, in which electrically conductive sheets are punched to form unitarily the connector connection circuit, the fuse connection circuit and the relay connection circuit and form tabs to be connected to connectors, tabs to be connected to fuses, and tabs to be connected to relays and stacked in a single stack, the circuits thus being handled and arranged in a complicated manner. Consequently, in the conventional method, the area of the bus bars increases and a large number of bus bars are necessary. 
     On the other hand, in the present invention, because the circuits are separately provided and welded to join them, it is possible to avoid the complication of tabs overlapping each other and avoid a large number of bus bars. Thus, it is possible to form a thin or compact junction box. Further, because the circuits of the bus bars can be handled and arranged easily, the area of each bus bar can be reduced. Consequently, even though the bus bars are separately provided for the connector connections on the one hand and the fuse connections and the relay connections on the other hand, it is possible to reduce the total area of the bus bars and avoid increase of the area of the junction box. 
     Further, the connector module having the connector circuit, the fuse module having the fuse circuit and the relay module having the relay circuit are all separately provided, i.e., the respective insulation substrates are discrete from each other. Thus, if any one of the specification of the connector circuit, the fuse circuit, and the relay circuit is altered, only the design of any one of the modules need be changed. That is, the construction can cope with the alteration of the specification easily. 
     The fuse module and/or the relay module are for example located at a predetermined position relative to the connector module by superimposing the fuse module and/or the relay module on the connector module. The bus bars are connected to each other by welding the upstanding welding portions to each other and the lateral welding portions to each other. The connector connection bus bar is provided separately from the fuse connection bus bar or/and the relay connection bus bar but connected thereto by welding. Thus the construction does not reduce reliability of the electrical connections. The welding portions are superimposed on each other and connected to each other by ultrasonic welding, resistance welding, laser welding or gas welding. 
     Typically, when welding the welding portions formed at the ends of the bus bars, a welding portion formed at an end of one bus bar, and, for example, projecting at a periphery of the fuse module and/or the relay module, and bent upward, is arranged alongside and welded to a welding portion of an end of another bus bar projecting upward from an upper surface of the insulation member of the connector module. Thereafter a welding portion formed at an end of a bus bar, e.g., projecting laterally from a periphery of the fuse module and/or the relay module, is superimposed with and welded to a welding portion of an end of a bus bar projecting laterally from the periphery of the connector module. Thereby the connector module and the fuse module and/or the relay module are connected to each other. Thus, initially, the welding portions perpendicular to the insulation substrates of the connector module and the fuse module and/or the relay module are welded to each other. Thus, even though the position of the insulation substrate of the fuse module and/or the relay module is dislocated horizontally relative to the insulation substrate of the connector module owing to deformation of the bus bars caused by thermal expansion or shrinkage or compression caused by mechanical deformation thereof, such dislocation can be permitted. This is because the laterally projecting welding portions are superimposed on each other in such a way that their main surfaces are parallel with the dislocation direction of the insulation substrate of the fuse module and/or the relay module. Accordingly, it is possible to smoothly weld the laterally projecting welding portions to each other in a subsequent welding process. Further, there is no possibility that a large stress remains in the welding portions. 
     Although the fuse circuit and the relay circuit may be separately formed, it is possible to integrate them with each other to form a fuse/relay composite circuit which receives fuses and relays on the same substrate which has bus bars for fuses and relays on it. In this case, if it is necessary to connect the circuit of any of the bus bars for fuses to any of the bus bars for relays, the respective bus bars may be welded to each other, or the bus bars may be integrally formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of non-limitative example with reference to the accompanying drawings. In the drawings: 
     FIG. 1 is a schematic exploded perspective view showing a junction box of a first embodiment of the present invention. 
     FIG. 2 is a sectional view showing a state in which the junction box of FIG. 1 has been assembled. 
     FIG. 3 is a sectional view, showing the junction box of FIG. 2, taken along a line perpendicular to FIG.  2 . 
     FIG. 4 is a schematic perspective view of a connector module of the box of FIG. 1 viewed from the underside thereof. 
     FIG. 5 is a schematic view showing fuse connection bus bars. 
     FIG. 6 is a schematic view showing a relay connection bus bar. 
     FIGS. 7A and 7B show stages in the process of welding the connector module and the fuse module to each other. 
     FIG. 8 shows a comparative example of a process of welding a connector module and a fuse module to each other. 
     FIG. 9A is a plan view and FIG. 9B is a bottom view of a junction box of another embodiment of the present invention. 
     FIG. 10 is an exploded perspective view showing a conventional junction box. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below with reference to drawings. 
     FIG. 1 is a schematic exploded perspective view showing component parts constituting a junction box  10  which, in use, is mounted in a vehicle body and is connected to a wire harness of the vehicle body. The box has a casing formed of molded synthetic resin comprising upper and lower casing parts herein called lower case part  11  and upper case part  12 . In the casing are a connector module  13  constituting a connector circuit, a fuse module  14  constituting a fuse circuit and a relay module  15  constituting a relay circuit. Although not shown in FIG. 1, the casing formed by the lower case part  11  and the upper case part  12  accommodates an electronic control unit  40 , seen in FIGS. 2 and 3. 
     The lower case part  11  has a plurality of connector receiving portions  11   a  in the form of sockets projecting outwardly and a plurality of relay receiving portions  11   b  also in the form of sockets arranged in a row along one longitudinal side. The upper case part  12  has a fuse receiving portion  12   a  having sockets to receive a plurality of fuses, formed at one widthwise side. The remaining part of the upper case part  12  is formed as a closed portion  12   c  which covers and contacts the upper surface of the electronic control unit  40  housed within the case. The upper case part  12  also has a connector receiving portion or socket  12   e  on the periphery face of its other widthwise side. The lower case part  11  and the upper case part  12  are locked to each other by fitting their peripheral walls on each other. Suitable conventional locking fittings (not shown) may be provided. The upper case part  12  also has fixing brackets  12   d  projecting from its opposite ends, for fixing it to for example a vehicle body. 
     FIG. 4 is a schematic perspective view showing the bottom side of the connector module  13  of FIG.  1 . The connector module  13  has a plurality of layers of connector connection bus bars  16  laminated one upon another with interposed insulation plates in a stack  17 . In this embodiment, there are four layers of the connector connection bus bars. In FIG. 1, the connector module  13  is schematically shown, with one insulation plate of the stack  17  as the uppermost layer. By molding in resin, it is possible to form the insulation plate stack  17  and a multi-layer integral assembly of the bus bars  16 . 
     Each layer of the bus bars  16  of the connector module  13  has circuit portions  16   a  having a required configuration obtained by punching a conductive plate. At least one tab  16   b  stands perpendicularly from each circuit portion  16   a  towards the lower case part  11  to project through a terminal hole  11   c  of one of the connector receiving portions  11   a  of the lower case part  11 . A connector (not shown) secured on one end of a wire harness is in use fitted in the connector receiving portion  11   a  to connect a terminal in the connector to the tab  16   b.    
     As shown in FIG. 1, the connector connection bus bars  16  have welding tabs  16   c,    16   d,  projecting from holes of the insulation plate stack  17  at the upper side of the connector circuit  13  and welding tabs  16   e  projecting from the periphery of the insulation plate stack  17 . The welding tabs  16   c,    16   e  are welded to fuse connection bus bars  20  formed on the fuse module  14 . The welding tabs  16   d  are welded to relay connection bus bars  30  formed on the relay module  15  which project upwardly through a slot in the connector module  13 . The welding tabs  16   c  and the welding tabs  20   a  of the bus bars  20  are placed adjacent each other vertically. Similarly the welding tabs  16   d  and the welding tabs  30   a  of the bus bars  30  are placed adjacent each other extending vertically. The welding tabs  16   e  are welded to welding tabs  20   b  of the fuse connection bus bars  20  by laminating them on each other vertically and horizontally. There are thus a plurality of sets of each of the welding tabs  16   c,    16   d  and  16   e  in each case parallel with one another. 
     The fuse module  14 , partly shown schematically in FIG. 5, has the fuse connection bus bars  20  arranged between two superimposed insulation plates  22 A and  22 B (see FIG.  1 ), formed by molding resin. The fuse connection bus bars  20  in use connect to terminals  25   a  or  25   b  of replaceable fuses  25 . As shown in FIG. 5 (where the upper insulation plate is not shown), each fuse connection bus bar  20  has horizontal portions  20   e - 1  and  20   e - 2  fixed to the substrate  22 B, terminal portions  20   c - 1  and  20   c - 2  formed by bending the fuse connection bus bar  20  upward at one end of the horizontal portion  20   e - 1  and  20   e - 2 , and pressure connection grooves  20   d - 1  and  20   d - 2  formed at the upper end of the terminal portions  20   c - 1  and  20   c - 2  respectively to allow the terminals  25   a  and  25   b  of the fuse  25  to be fitted in the grooves  20   d - 1  and  20   d - 2  respectively. The end of one bus bar  20  opposite to its end to be connected to the terminal  25   a  projects to form the welding portion  20   a.  The end of the other bus bar  20  opposite to its end to be connected to the terminal  25   b  projects laterally to form the welding portion  20   b.  The welding portion  20   a  is projected horizontally from one edge of the substrate  21  in its longitudinal direction and bent vertically upward so that in the assembled position the welding portion  20   a  and the welding portion  16   c  of a connector bus bar  16  lie adjacent each other, as shown in FIG.  7 . The welding portion  20   b  is projected horizontally from the other longitudinal side of the substrate  21  and stepped vertically downward so that in the assembled condition the welding portion  20   b  and the welding portion  16   e  of a connector bus bar  16  are superimposed, as also shown in FIG.  7 . It is possible to fix the bus bar  20  to the insulation plate  22 B by means of caulking. In this case, a caulking projection (not shown) is formed on the insulation plate  22 B and inserted through a hole formed on the bus bar  20 . Then, the caulking projection is deformed to fix the bus bar  20  at a predetermined position. 
     The relay module  15  has a construction similar to that of the fuse module  14 . More specifically, the relay module  15  has a large number of relay connection bus bars  30  fixed between upper and lower insulation plates  31 A and  31 B (see FIG.  1 ). The bus bars  30  are separated from each other and individually connected to terminals of relays in use. As shown schematically in FIG. 6 (where the insulation plate  31 B is not shown), each bus bar  30  of the relay module  15  has a horizontal portion  30   b,  a terminal portion  30   c  formed by bending the bus bar  30  upwards at one end of the horizontal portion  30   b,  and a pressure connection groove  30   d  formed at the outer end of the terminal portion  30   c  to allow a terminal  35   a  of a relay  35  to be fitted therein. The welding tab  30   a  is formed by bending the other end of the horizontal portion  30   b  in the shape of an “L” to permit the welding tab  30   a  and the welding tab  16   d  of one of the connector bus bars  16  to be aligned adjacent each other vertically in the assembled state. 
     In assembling the junction box  10  of FIG. 1, initially the fuse module  14  is disposed over one portion of the connector module  13 . Then, as shown in FIG. 7A, the upstanding welding portion  20   a  is welded to the adjacent welding portion  16   c  projecting vertically from the upper surface of the insulation plate  17  of the connector module  13 . Thereafter, as shown in FIG. 7B, the welding portion  20   b  projecting horizontally from the periphery of the fuse module  14  at the other side thereof is welded to the welding portion  16   e  projecting horizontally from the periphery of the connector module  13  with the former superimposed on the latter. In this manner, the connector module  13  and the fuse module  14  are connected to each other. 
     In welding the welding portions  16   c  and  20   a  to each other and the welding portions  16   e  and  20   b  to each other, there is a possibility that the connector module  13  and the fuse module  14  are dislocated (incorrectly located) from each other due to deformation of the bus bars  16 ,  20  caused by thermal expansion or contraction, or by mechanical deformation thereof caused by compression. To prevent adverse effects of such dislocation, as described above, the welding portions  16   c,    20   a  both project perpendicularly to the insulation plate  17  of the connector module  13  and to the insulation plates  22 A,  22 B of the fuse module  14  and are welded to each other when vertically aligned. In this case, even though the position of the fuse module  14  is dislocated horizontally relative to the connector module  13  owing to the deformation of the bus bars  16 ,  20  during welding of the welding portions  16   c  and  20   a  to each other and the welding portions  16   e  and  20   b  to each other, the dislocation of the fuse module  14  can be allowed. This is because the horizontally projecting welding portions  16   e  and  20   b  are superimposed on each other in such a way that the horizontal surfaces thereof to be welded to each other are parallel with the dislocation direction of the fuse module  14 . Accordingly, it is possible to smoothly weld the horizontally projecting welding portions  16   e  and  20   b  to each other in a subsequent welding process. Further, because there is no possibility that significant stress remains in the welding portions  16   e  and  20   b,  it is possible to prevent separation of the welded portions  16   e  and  20   b.    
     By way of comparative example, let it be supposed that the welding surface of welding portions  16   c′,    20   a′  and that of welding portions  16   e′,    20   b′  are perpendicular to the insulation plates  17 ′,  22 A′,  22 B′, as shown in FIGS. 8A and 8B. In this case, when the welding portions  16   c′  and  20   a′  ( 16   e′  and  20   b′ ) are welded to each other, the welding portions  16   e′  and  20   b′  ( 16   c′  and  20   a′ ) have a gap therebetween or strike against each other owing to a dislocation among the insulation plates  17 ′,  22 A′,  22 B′. Thus, an operator has difficulty in performing a welding operation. Because the direction of the dislocation among the insulation plates  17 ′,  22 A′,  22 B′ and the direction of the connection between the welding portions  16   e′  and  20   b′  ( 16   c′  and  20   a′ ) intersect with each other, it is impossible to absorb the dislocation. Therefore, a high residual stress may be generated at the welding portions, which may cause the separation of the welding portions. 
     Then, in the present invention the welding portion  30   a  of the bus bar  30  is superimposed on and welded to the welding portion  16   d  of the bus bar  16 , with the relay module  15  disposed over one side of the connector module  13 . The welding portions are welded to each other by ultrasonic welding, resistance welding, gas welding or laser welding. 
     As described above, the fuse module  14  is mounted over the connector module  13  at one widthwise side thereof, and the relay module  15  is disposed under the connector module  13  at one longitudinal side thereof to integrate the three modules. Then, this subassembly of the three modules  13 ,  14  and  15  is accommodated in the lower case part  11 . In this case, the terminal portions of the connector module  13  are disposed in the connector receiving sockets  11   a,  and the terminal portions  30   c  of the relay module  15  are disposed in the relay receiving sockets  11   b.    
     Then, the electronic control unit  40  is mounted on the connector module  13  at a portion thereof on which the fuse module  14  is not mounted (see FIGS.  2  and  3 ). The electronic control unit  40  has, at one side thereof, a connector portion  44  having bent and projecting conductive pins  43  connected with electrical conductors  42  fixed to a substrate  41  thereof. The electrical conductors  42  are connected to tabs  16   f  projecting from the bus bars  16  of the connector module  13 . The conductors  42  are connected to a large number of electronic component parts  45  mounted on the substrate  41 . The electronic component parts  45  fixed to the substrate  41 , with the electronic component parts  45  projecting downward. The welding tabs of the bus bars  16 ,  20  and  30  are disposed in a dead space below the electronic control unit  40 . 
     After the electronic control unit  40  is mounted on the connector module  13 , the upper case part  12  is mounted on the lower case part  11 . At this time, the terminal portions  20   c  of the bus bars  20  fixed to the fuse module  14  become located in the fuse receiving portions  12   a.    
     At this time, the connector portion  44  of the electronic control unit  40  is fitted in a notch  12   e  of the upper case  12 . The assembling of the junction box  10  is completed by locking the upper case  12  and the lower case  11  to each other. 
     As shown in FIGS. 5 and 6, when the fuses  25  and the relays  35  are inserted into the fuse accommodation portion  11   a  and the relay accommodation portion  11   b,  respectively, they are fitted in and connected to the pressure connection grooves  20   d  of the fuse connection bus bars  20  and the pressure connection grooves  30   d  of the relay connection bus bars  30 , respectively. 
     In the first embodiment, the vertically projecting welding portions  16   c  and  20   a  and the horizontally projecting welding portions  16   e  and  20   b  are welded to each other respectively to connect the connector module  13  and the fuse module  14  to each other. But the connector module  13  and the relay module  15  can be connected to each other by welding the welding portions in a similar manner. 
     In the first embodiment described above, the fuse module and the relay module are separate from each other. Instead, the fuse module and the relay module may be integral with each other to form a composite module. In this case, as shown in FIGS. 9A and 9B, a fuse receiving portion  12   a′  and a relay receiving portion  12   b′,  in which terminal portions of bus bars of the composite module are disposed, are formed in the upper case  12 ′. Formed in the lower case  11 ′ is a connector receiving portion  11   a′  in which terminal portions of bus bars of the connector module are disposed. 
     The junction box of the present invention is not limited the above-described embodiments. For example, each of the fuse module and the relay module may be divided into two parts, respectively. In this case, when the specification of any one of the fuses or the relays is altered, it is possible to replace only the module associated with the fuse or the relay which should be altered. However, if the fuse module and the relay module are divided into three or more parts, many assembling stages are required. Thus, it is preferable to divide the fuse module and the relay module into at most two parts, respectively, in the case of a large junction box. 
     The connector connection bus bars of the connector module, the fuse connection bus bars of the fuse module, and the relay connection bus bars of the relay module may be welded to each other in any of the following three patterns, selected according to the circuit design: 
     (1) A connector connection bus bar and a fuse connection bus bar are welded to each other. 
     (2) A connector connection bus bar and a relay connection bus bar are welded to each other. 
     (3) A connector connection bus bar is welded to a fuse connection bus bar and to a relay connection bus bar. 
     In case (3), the fuse connection bus bar may be welded to a welding portion of the connector connection bus bar at one end thereof; the relay connection bus bar may be welded to the welding portion of the connector connection bus bar at the other end thereof; and a tab provided at a third portion of the connector connection bus bar may be connected to a connector. 
     In the first embodiment, the connector module and the electronic control unit are accommodated in the lower and upper case parts. Additionally, it is possible to add a circuit including electrical wires connected to pressure contact terminals on the base circuit. The wires may also connect to connectors which fit in the connector receiving portion. It is also possible to add a circuit formed as an electrically conductive portion of an FPC (flexible printed circuit), a PCB (printed circuit board) or a highly electrically conductive resin molded with insulating resin. 
     As is apparent from the foregoing description, in the method of the present invention, in connecting the connector module to the fuse module and/or the relay module, after the vertically projecting welding portions are welded to each other by superimposing them on each other, the horizontally projecting welding portions are welded to each other by superimposing them on each other. Thus, even though the position of the fuse module and/or the relay module is dislocated horizontally relative to the connector module after the vertically projecting welding portions are welded to each other, such dislocation of the fuse module and/or the relay module causes no problem and can be allowed. This is because the horizontally projecting welding portions are superimposed on each other in such a way that the horizontal surfaces thereof to be welded to each other are parallel with the dislocation direction of the fuse module and/or the relay module. Accordingly, it is possible to smoothly weld the horizontally projecting welding portions to each other in a subsequent welding process. Further, there is no possibility that a great stress remains in the welding portions. 
     As is apparent from the foregoing description, in the junction box of the present invention, the connector connection bus bars are separately provided from the fuse connection bus bars and the relay connection bus bars, using discrete substrates. Thus, tabs for connecting the connector connection bus bars, the fuse connection bus bars, and the relay connection bus bars to connectors, fuses and relays, respectively are disposed at different positions and do not overlap each other. Accordingly, it is unnecessary to increase the number of layers of the bus bars to provide them with tabs. Consequently, it is possible to form a thin junction box or otherwise to achieve a compact and logical lay out. For example, in the case of the construction of the first embodiment, the number of bus bars can be reduced from six layers required in the conventional junction box to four layers. Thus, it is possible to reduce the thickness of the junction box. 
     Further, as described above, because the fuse connection tabs and the relay connection tabs are separate from the bus bars of the base circuit, it is easy to handle and arrange the bus bars of the base circuit. Thus, it is possible to reduce the area of the bus bars and hence the area of the junction box. Consequently, in the case where the bus bars are divided and the ends of the bus bars are welded to each other, the area of the entire bus bars is not large and hence the area of the junction box is not increased. 
     Further, if the specification of the fuses and the relays is altered, the fuse module, the relay module or the composite module of the fuse module and the relay module is replaced. Thus, it is unnecessary to alter the entire upper and lower cases including the base circuit. That is, the construction can permit the alteration of the specification quickly and at low cost. 
     While the invention has been illustrated by the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.
An electrical junction box for a vehicle has a connector circuit having bus bars fixed on an insulation substrate to connect to electrical connectors, a fuse circuit having bus bars fixed on an insulation substrate to connect to fuses, and a relay circuit having third bus bars fixed on an insulation substrate to connect to relays in use. In assembling the box, in order to avoid generation of stress and allow for dislocation of the circuits, the method includes the steps of: (i) joining upstanding welding portions of bus bars of the connector circuit and the fuse or relay circuit by arranging them adjacent each other and welding them together; and (ii) after step (i), joining laterally projecting welding portions of bus bars of the connector circuit and the fuse or relay circuits by welding them together when superimposed one on the other.