Abstract:
This solar cell module comprises: a base member curved in the vertical direction and the horizontal direction; a plurality of solar cells disposed on the base member; first wiring members connecting adjacent solar cells to one another in the vertical direction and forming a plurality of strings; and second wiring members connected to the first wiring members that extend out in the vertical direction from the top of the solar cells located at an end of the columns of the strings. The interval between the at least some of the second wiring members and the solar cells of the strings connected to those wiring members is narrower toward the end portions in the horizontal direction of the string group than toward the central portion in the horizontal direction.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a solar cell module and a method of manufacturing a solar cell module. 
       BACKGROUND 
       [0002]    A solar cell module is known in which a group of strings of solar cells are placed over a substrate having a three-dimensional curvature (hereinafter referred to as “curved substrate”) (for example, refer to Patent Literature 1). As described in Patent Literature 1, a solar cell module which is three-dimensionally curved is desirably manufactured, from the viewpoint of productivity or the like, by placing the group of strings over the curved substrate after the group of strings have been manufactured on a flat surface. 
       CITATION LIST 
     Patent Literature 
       [0003]    Patent Literature 1: JP 2014-96511 A 
       SUMMARY 
       [0004]    When the group of strings are placed over the curved substrate, there is a possibility, for example, that a spacing between strings is narrowed at a part of the group of strings, the solar cells contact each other, and consequently, short-circuiting, cell cracking or the like occurs. In addition, in the group of strings placed over the curved substrate, because a large load tends to be applied on a wiring member, it is desired to reduce the load and to thereby improve the reliability. 
         [0005]    According to one aspect of the present disclosure, there is provided a solar cell module comprising: a substrate which is curved in a longitudinal direction and in a lateral direction; a plurality of solar cells placed over the substrate; first wiring members that connect adjacent solar cells to each other in a longitudinal direction to form a plurality of strings in each of which a plurality of the solar cells are arranged in one line; and second wiring members that are connected to the first wiring members which extend from a region above the solar cell positioned at an end of a row of the string in the longitudinal direction, and at least one of which connects adjacent strings in the lateral direction to form a group of strings, wherein a spacing between at least one of the second wiring members and the solar cell positioned at the end of the row of the string to which the wiring member is connected is narrower at a side of an end of the group of strings in the lateral direction than at a side of a central portion in the lateral direction. 
         [0006]    According to another aspect of the present disclosure, there is provided a method of manufacturing a solar cell module, comprising: connecting adjacent solar cells to each other by first wiring members in a longitudinal direction, to form a plurality of strings in each of which a plurality of the solar cells are arranged in one line; connecting second wiring members to the first wiring members which extend from a region above the solar cell positioned at an end of a row of the string in a longitudinal direction, to connect adjacent strings by at least one of the second wiring members to form a group of strings; and placing the group of strings over a substrate which is curved in a longitudinal direction and in a lateral direction, wherein when the group of strings is formed, the second wiring members are connected to the first wiring members in such a manner that a spacing between at least one of the second wiring members and the solar cell positioned at the end of the row of the string to which the wiring member is connected is narrower at a side of an end of the group of strings in the lateral direction than at a side of a central portion in the lateral direction. 
         [0007]    According to an aspect of the present disclosure, in a solar cell module which is three-dimensionally curved, a superior arrangement state of the solar cells can be obtained without occurrence of short-circuiting, cell cracking, or the like due to contact of the solar cells. In addition, load applied to the wiring member can be reduced and the reliability can be consequently improved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a perspective diagram of a solar cell module according to a first embodiment of the present disclosure, viewed from a light receiving surface side (also showing a cross section in the lateral direction of the solar cell module). 
           [0009]      FIG. 2  is a plan view of the solar cell module according to the first embodiment of the present disclosure, viewed from the light receiving surface side. 
           [0010]      FIG. 3  is a diagram showing a part of a cross section, in a longitudinal direction, of the solar cell module according to the first embodiment of the present disclosure. 
           [0011]      FIG. 4  is a diagram for explaining a method of manufacturing the solar cell module according to the first embodiment of the present disclosure.  FIG. 5  is a plan view of a solar cell module according to a second embodiment of the present disclosure, viewed from a light receiving surface side. 
           [0012]      FIG. 6  is a diagram for explaining a method of manufacturing the solar cell module according to the second embodiment of the present disclosure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0013]    Examples of embodiments of the present disclosure will now be described in detail with reference to the drawings. 
         [0014]    The drawings referred to in the embodiments are schematically drawn, and the size, ratio, or the like of the constituent elements drawn in the drawings may differ from the actual structure. The specific size, ratio, or the like should be determined in consideration of the following description. 
         [0015]    In the present specification, a term “longitudinal direction” and a term “lateral direction” are used as terms indicating directions. The longitudinal direction refers to a direction along which the solar cells of the string are arranged. The lateral direction refers to a direction orthogonal to the longitudinal direction, and is a direction along which the strings of the group of strings are arranged. Further, a description “providing a second member over a first member” is not intended to mean only the case where the first and second members are provided in direct contact with each other, unless otherwise specified. That is, such a description includes a case where another member exists between the first and second members. 
         [0016]    In the following, a surface in a solar cell module where the solar light is primarily incident (exceeding  50 % and up to  100 %) is referred to as a “light receiving surface”, and a surface opposite to the light receiving surface is referred to as a “back surface”. The terms of the light receiving surface and the back surface are also used for constituent elements of the solar cell or the like. 
       First Embodiment 
       [0017]    A solar cell module  10  according to a first embodiment of the present disclosure will now be described in detail with reference to  FIGS. 1 ˜ 4 .  FIG. 1  and  FIG. 2  are respectively a perspective view and a plan view of the solar cell module  10 , viewed from a light receiving surface side.  FIG. 3  is a diagram showing a part of a cross section in a longitudinal direction of the solar cell module  10 . 
         [0018]    As shown in  FIGS. 1 ˜ 3 , the solar cell module  10  comprises a plurality of solar cells  11 , a first protection component  12  provided on a side of a light receiving surface of the solar cell  11 , and a second protection component  13  provided on a side of a back surface of the solar cell  11 . The plurality of solar cells  11  are sandwiched and held between the first protection component  12  and the second protection component  13 , and are sealed by an encapsulant  14  (refer to  FIG. 3 ) filled between the protection components. 
         [0019]    The solar cell module  10  comprises a substrate which is curved in a longitudinal direction and in a lateral direction, and has a three-dimensionally curved shape. In the present embodiment, the first protection component  12  is the substrate which is curved in the longitudinal direction and in the lateral direction and which has a three-dimensional curvature. As will be described later in detail, the second protection component  13 , the encapsulant  14 , and a group of strings  30  (refer to  FIGS. 1 and 2 ) are placed over the first protection component  12 , to obtain the solar cell module  10  which is three-dimensionally curved. 
         [0020]    The solar cell module  10  comprises a first wiring member  21  which connects adjacent solar cells  11  in the longitudinal direction to form a plurality of strings  20  (refer to  FIGS. 1 and 2 ) in each of which a plurality of the solar cells  11  are arranged in a line. The first wiring member  21  is, for example, bent in a thickness direction of the module between adjacent solar cells  11 , and is attached using adhesive or the like to an electrode on the light receiving surface side of one solar cell  11  and to an electrode on the back surface side of the other solar cell  11  (refer to  FIG. 3 ). 
         [0021]    The solar cell module  10  comprises a second wiring member  31  which is connected to the first wiring member  21  extending, in the longitudinal direction, from a region above a solar cell  11  positioned at an end of a row of the string  20  (refer to  FIGS. 1 and 2 ). At least one of the second wiring members  31  connects adjacent strings  20  in the lateral direction, to form the group of strings  30 . That is, the group of strings  30  is formed from a plurality of the strings  20  and a plurality of the second wiring member  31 . In the present embodiment,  6  rows of strings  20  (referred to as strings  20   a,    20   b,    20   c,    20   d,    20   e , and  20   f  from left to right of  FIG. 2 ) are placed in the lateral direction to form the group of strings  30 . 
         [0022]    The solar cell module  10  desirably comprises a terminal portion  15  (refer to  FIGS. 1 and 2 ) to which at least a part of the second wiring members  31  is connected. In the present embodiment, the terminal portion  15  is provided at a back side of the second protection component  13 , and four second wiring members  31  placed on the side of one end in the longitudinal direction of the solar cell module  10  are connected to the terminal portion  15 . Of the four second wiring members  31 , two connect adjacent strings  20 , and the remaining two connect one row of strings  20  and the terminal portion  15 . It is desirable that an electric power cable connected to an external device is connected to the terminal portion  15 , and a bypass diode for stabilizing output is provided. 
         [0023]    The solar cell  11  comprises a photoelectric conversion unit which produces carriers upon receiving solar light. The photoelectric conversion unit has, as electrodes for collecting the produced carriers, a light receiving surface electrode formed over the light receiving surface of the photoelectric conversion unit and a back surface electrode formed over the back surface, for example (both of which are not shown in the figures). The wiring members  21  are connected to the electrodes. However, the structure of the solar cell  11  is not limited to this structure, and the structure may be, for example, a structure in which the electrode is formed only over the back surface of the photoelectric conversion unit. Desirably, the back surface electrode is formed in a larger area than the light receiving surface electrode, and a surface having a larger electrode area (or a surface on which the electrodes are formed) may be considered as the back surface of the solar cell  11 . 
         [0024]    The photoelectric conversion unit comprises a semiconductor substrate made of, for example, crystalline silicon (c-Si), gallium arsenide (GaAs), indium phosphide (InP), or the like, an amorphous semiconductor layer formed over the semiconductor substrate; and a transparent conductive layer formed over the amorphous semiconductor layer. As a specific example, a structure may be employed in which an i-type amorphous silicon layer, a p-type amorphous silicon layer, and a transparent conductive layer are sequentially formed over one surface of an n-type monocrystalline silicon substrate, and an i-type amorphous silicon layer, an n-type amorphous silicon layer, and a transparent conductive layer are sequentially formed over the other surface. The transparent conductive layer is desirably formed from a transparent conductive oxide in which a metal oxide such as indium oxide (In 2 O 3 ) and zinc oxide (ZnO) is doped with Sn, Sb, or the like. 
         [0025]    For the first protection component  12  and the second protection component  13 , for example, a glass substrate, a resin substrate, a resin film, or the like may be employed. For the first protection component  12 , a member having a light transmissive characteristic is used, and from the viewpoint of heat resistivity and endurance, a glass substrate is desirably used. A thickness of the glass substrate is, for example, about 2˜6 mm. For the second protection component  13 , a transparent member may be used or a non-transparent member may be used. For the second protection component  13 , for example, a resin film is used. A thickness of the resin film is, for example, about 50˜300 μm. 
         [0026]    In the present embodiment, as described above, as the substrate which is curved in the longitudinal direction and in the lateral direction, the first protection component  12  is used. The first protection component  12  is not particularly limited so long as the component has a curved surface which is curved in the longitudinal direction and in the lateral direction, and has, for example, a curved surface with a three-dimensional curvature such as a shape corresponding to a part of a spherical surface that is cut out. The curvature of the first protection component  12  is not particularly limited, and may be constant over the entire region of the first protection component  12  or different at a part of the regions. In the following, a structure is described in which the curvature of the first protection component  12  is approximately constant. The first protection component  12  is, for example, a transparent glass substrate which is three-dimensionally curved with an approximately constant curvature, and has an approximately rectangular shape in the plan view. In the present specification “approximately” is intended to include, for example, in the case of “approximately constant”, cases of completely constant and also cases of substantially constant. 
         [0027]    The encapsulant  14  has a function to fill a gap between the solar cell  11  and the protection components, to seal the solar cell  11 . The encapsulant  14  desirably has, as a primary constituent, a resin that can be applied for a lamination process to be described below. As the resin, ethylene vinyl acetate copolymer (EVA), polyvinylbutyral (PVB), or the like can be exemplified. The encapsulant  14  may include various additives such as an oxidation prevention agent, a flame resisting agent, an ultraviolet absorption agent, or the like, and the encapsulant  14  placed at the back surface side of the solar cell  11  may contain pigments such as titanium oxide. 
         [0028]    As shown in  FIGS. 1 and 2 , in the solar cell module  10 , a spacing between at least a part of the second wiring members  31  and the solar cell  11  positioned at an end of a row of the string  20  to which the wiring member is connected is narrower at a side of an end of the group of strings  30  in the lateral direction than at a side of a central portion in the lateral direction. 
         [0029]    The group of strings  30  has, for example, a long-length wiring member  31 L having a length longer than a length in a lateral direction of two rows of the strings  20 , among the second wiring members  31  connected to the terminal portion  15 . In the present embodiment, as shown in  FIG. 2 , strings  20   a  and  20   f  positioned at the ends of the group of strings  30  are connected to the terminal portion  15  by long-length wiring members  31 La and  31 Lb, respectively. In addition, adjacent strings  20   b  and  20   c,  and adjacent strings  20   d  and  20   e  are respectively connected by second wiring members  31   a  and  31   b.  and one end of each of these wiring members is connected to the terminal portion  15 . 
         [0030]    A spacing between the long-length wiring member  31 La and the solar cell  11  positioned at an end of the row of the string  20   a  to which the wiring member is connected is narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction. Similarly, a spacing between the long-length wiring member  31 Lb and the solar cell  11  positioned at an end of the row of the string  20   f  to which the wiring member is connected is narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction. In other words, the spacings between the long-length wiring members  31 La and  31 Lb and the solar cells  11  of the strings to which the wiring members are connected are narrower at the side of the end, of the first protection component  12  which is the curved substrate, in the lateral direction than at the side of the central portion in the lateral direction. In addition, the spacing becomes narrower as a distance from the terminal portion  15  is increased in the lateral direction of the group of strings  30 . 
         [0031]    Because the second wiring members  31  are symmetrical in the left and right direction with respect to the central portion of the group of strings  30  in the lateral direction, in the following description, the side including the string  20   a  and the long-length wiring member  31 La will be exemplified for the description. 
         [0032]    In the string  20   a,  desirably, a plurality of first wiring members  21  extend in the longitudinal direction from a region above the solar cell  11  positioned at the end of the row and are connected to the long-length wiring member  31 La. In the string  20   a  (in the present embodiment, other strings are similar), for example, two adjacent solar cells  11  are connected by three first wiring members  21  arranged in the lateral direction. Desirably, a length of the first wiring member  21 , more specifically, at least a length of extension (extension length) of the first wiring member  21  from a region above the solar cell  11  positioned at the end of the row of the string  20   a  in the longitudinal direction, becomes shorter toward the end of the group of strings  30  in the lateral direction. 
         [0033]    Specifically, the extension lengths of the three first wiring members  21  (referred to, from left to right in  FIG. 2 , as first wiring members  21   a,    21   b,  and  21   c ) of the string  20   a  are in a relationship: the length of the first wiring member  21   a &lt; 21   b &lt; 21   c.  The long-length wiring member  31 La is connected to a tip of each of the first wiring members. With such a configuration, the spacing between the long-length wiring member  31 La and the solar cell  11  of the string  20   a  is set narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction. 
         [0034]    The long-length wiring member  31 La is desirably curved along the curved surface of the first protection component  12 . The long-length wiring member  31 La is curved, for example, along a virtual curve α having a constant curvature along the curved surface of the first protection component  12 . With such a configuration, for example, a distortion of the long-length wiring member  31 La can be reduced, and the load applied to the wiring member can be reduced. When the long-length wiring member  31 La is curved along the virtual curve α, it becomes easier for a part of the strings  20  (for example, string  20   a ) to move toward the inner side of the group of strings  30 . In the present embodiment, this movement is suppressed by adjusting the spacing between the long-length wiring member  31 La and the solar cell  11  of the string  20   a.  That is, even when the long-length wiring member  31 La is curved along the curved surface of the first protection component  12 , narrowing of the spacing between the strings  20  at a part thereof can be prevented, and consequent contact of the solar cells  11  can be prevented. 
         [0035]    In the present embodiment, the spacings between the second wiring members  31   a  and the solar cells  11  of the strings  20   b  and  20   c  are approximately equal to each other. In addition, the spacings between the second wiring members  31   b  and the solar cells  11  of the strings  20   d  and  20   e  are approximately equal to each other. The extension lengths of the first wiring members  21  in these strings are also approximately equal to each other. Alternatively, the spacing between the second wiring members  31   a  and  31   b  and the solar cells  11  of the strings to which the wiring members are connected may be set narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction. Alternatively, the spacing between the second wiring member  31  and the solar cell  11  of the string  20  may be adjusted by setting the extension lengths of the first wiring members  21  approximately equal to each other and changing a connection position with the second wiring member  31 . 
         [0036]    The solar cell module  10  having the above-described structure can be manufactured by laminating the group of strings  30  using resin sheets forming the first protection component  12 , the second protection component  13 , and the encapsulant  14 . In a laminating device, the first protection component  12 , a first resin sheet forming the encapsulant  14 , the group of strings  30 , a second resin sheet forming the encapsulant  14 , and the second protection component  13  are layered in sequence over a heater. From the viewpoint of productivity or the like, the group of strings  30  is placed over the first protection component  12  after being manufactured on a flat surface, as will be described later. The layered structure is heated, for example, to a temperature at which the resin sheet forming the encapsulant  14  is softened under a vacuum state. Then, the heating is continued while the constituent members are pressed toward the heater side under an atmospheric pressure, to laminate the members, and to consequently obtain the solar cell module  10 . 
         [0037]      FIG. 4  shows the group of strings  30  manufactured on a flat surface (and before being placed over the substrate). 
         [0038]    As shown in  FIG. 4 , the group of strings  30  is manufactured by forming, on a flat surface, the string  20  by connecting the plurality of solar cells  11  in the longitudinal direction by the first wiring members  21 , and connecting the second wiring member  31  to the first wiring member  21  of each string. In the present embodiment, the second wiring members  31   a  and  31   b  have an L-shape, large portions thereof extend in the lateral direction, and the second wiring members  31   a  and  31   b  are connected respectively to the first wiring members  21  of the strings  20   b  and  20   c  and the wiring members  21  of the strings  20   d  and  20   e.    
         [0039]    Meanwhile, the long-length wiring members  31 La and  31 Lb also have an L-shape, but are not formed to extend straight in the lateral direction, unlike the second wiring members  31   a  and  31   b,  and are tilted in the longitudinal direction to be distanced away from the string  20  toward the central portion of the group of strings  30  in the lateral direction. By placing the group of strings  30  having such a shape over the first protection component  12  and laminating with the above-described constituting members, the long-length wiring member  31 La is curved along the curved surface of the first protection component  12 , and the solar cell module  10  having the above-described structure is obtained. 
         [0040]    Specifically, an example manufacturing method of the solar cell module  10  includes the following steps: 
         [0041]    (1) connecting adjacent solar cells  11  in the longitudinal direction by the first wiring member  21 , to form a plurality of strings  20  in each of which a plurality of solar cells  11  are arranged in one line; 
         [0042]    (2) connecting the second wiring member  31  to a first wiring member  21  which extends from a region above a solar cell  11  positioned at an end of the row of the string  20  in a longitudinal direction, to connect adjacent strings  20  by at least a part of the second wiring member  31 , to consequently form the group of strings  30 ; and 
         [0043]    (3) placing the group of strings  30  over the first protection component  12  which is the substrate which is curved in the longitudinal direction and in the lateral direction. 
         [0044]    When the group of strings  30  is formed, a spacing between at least a part of the second wiring members  31  and the solar cell  11  positioned at the end of the row of the string  20  to which the wiring member is connected is set to be narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction. That is, the second wiring member  31  is connected to the first wiring member  21  so that the spacing is narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction. 
         [0045]    When the group of strings  30  is formed, the long-length wiring member  31 La is connected to the first wiring member  21  in such a manner that the spacing between the long-length wiring member  31 La and the solar cell  11  of the string  20   a  is narrower at the side of the end of the group of strings  30  in the lateral direction than at the side of the central portion in the lateral direction (similarly for the case of the long-length wiring member  31 Lb). In the present embodiment, a plurality of first wiring members  21  extend from a region above the solar cell  11  positioned at the end of the row of the string  20   a,  the lengths of the first wiring members are set to be shorter toward the end of the group of strings  30  in the lateral direction, and the long-length wiring member  31 La is connected to the tip of each of the first wiring members. 
         [0046]    As described above, according to the solar cell module  10  having the above-described structure, distortion of the wiring member can be reduced and the load on the wiring member can be reduced. According to the solar cell module  10 , even when the second wiring member  31  is curved along the curved surface of the first protection component  12 , the spacing between the strings  20  can be easily maintained at a constant spacing. With such a configuration, a superior arrangement state of the solar cells  11  (strings  20 ) can be obtained without occurrence of the short-circuiting, cell cracking, or the like due to contact between the solar cells  11 . In addition, the load on the first wiring member  21  can be reduced as well as the load on the second wiring member  31 , and, for example, a superior outer appearance and a high reliability can be obtained. 
       Second Embodiment 
       [0047]    A solar cell module  50  according to a second embodiment of the present disclosure will now be described in detail with reference to  FIGS. 5 and 6 . In the following, constituent elements similar to those in the above-described embodiment are assigned the same reference numerals and will not be described again. 
         [0048]    As shown in  FIG. 5 , the solar cell module  50  differs from the solar cell module  10  in that seven strings  20  form a group of strings  51 , and that second wiring members  52  are not symmetrical in the left and right direction with respect to the central portion of the group of strings  51  in the lateral direction. In the solar cell module  50  also, spacings between long-length wiring members  52 La and  52 Lb and solar cells  11  positioned at ends of the rows of the strings to which the wiring members are connected are narrower at a side distanced from the terminal portion  15  in the lateral direction of the group of strings  51  than at a side closer to the terminal portion  15 . 
         [0049]    The solar cell module  50  differs from the solar cell module  10  in that both long-length wiring members  52 La and  52 Lb connect adjacent strings  20 , unlike the long-length wiring members  31 La and  31 Lb. Extension lengths of six first wiring members  21  connected to the long-length wiring member  52 La (similarly for the long-length wiring member  52 Lb) are shorter at a side of the group of strings  51  distanced from the terminal portion  15  than at a side closer to the terminal portion  15 . In other words, the extension lengths of the six first wiring members  21  are in a relationship: the length of the first wiring member  21   a &lt; 21   b &lt; 21   c &lt; 21   d &lt; 21   e &lt; 21   f.    
         [0050]    As shown in  FIG. 6 , in the manufacturing process of the solar cell module  50 , similar to the case of the solar cell module  10 , first, the group of strings  51  is manufactured on a flat surface. In this case also, a plurality of first wiring members  21  are set to extend from a region above the solar cell  11  positioned at an end of the row of the string  20 , and the lengths of the first wiring members are set in such a manner that the length becomes shorter toward the end of the group of strings  51  in the lateral direction. The long-length wiring members  52 La and  52 Lb are respectively connected to the tips of the first wiring members. The group of strings  51  thus manufactured is placed over the first protection component  12  and laminated with the constituting members, so that the long-length wiring members  52 La and  52 Lb are curved along the curved surface of the first protection component  12  and the solar cell module  50  having the above-described structure is obtained. 
         [0051]    In the above-described embodiments, a configuration is exemplified in which the spacing between the long-length wiring member and the solar cell of the string is varied. Alternatively, a configuration may be employed in which a spacing between a wiring member having a shorter length than a length of two rows of strings in the lateral direction and the solar cell of the string is narrower at the side of the end of the substrate in the lateral direction than at the side of the central portion in the lateral direction. Alternatively, a configuration may be employed for the solar cell module in which a spacing between all second wiring members, including a second wiring member that connects only adjacent strings, and the solar cell of the string is narrower at the side of the end of the substrate in the lateral direction than at the side of the central portion in the lateral direction. 
       Reference Signs List 
       [0052]      10 ,  50  SOLAR CELL MODULE;  11  SOLAR CELL;  12  FIRST PROTECTION COMPONENT;  13  SECOND PROTECTION COMPONENT;  14  ENCAPSULANT;  15  TERMINAL PORTION;  20 ,  20   a,    20   b,    20   c,    20   d,    20   e,    20   f  STRING;  21 ,  21   a    21   b,    21   c,    21   d,    21   e,    21   f  FIRST WIRING MEMBER;  30 ,  51  GROUP OF STRINGS;  31 ,  31   a,    31   b,    52  SECOND WIRING MEMBER;  31 L,  31 La,  31 Lb,  52 La,  52 Lb LONG-LENGTH WIRING MEMBER.