Patent Publication Number: US-2023154649-A1

Title: Covered electric wire and wire harness

Description:
TECHNICAL FIELD 
     The present disclosure relates to a covered electric wire and a wire harness. 
     BACKGROUND ART 
     A flat cable containing a flat-shaped conductor is commonly known. A flat cable occupies a smaller space for routing than a conventional electric wire configured with a conductor having a substantially circular cross section. 
     In a conventional flat cable, as disclosed in Patent Literatures 1 and 2, a flat rectangular conductor is often used as the conductor. The flat rectangular conductor is a single metal wire shaped to have a cross section formed into a square. In addition, Patent Literatures 3 and 4, which were applied by applicants, each discloses an electric wire conductor consisting of a wire strand containing a plurality of elemental wires twisted together and having a flat portion where a cross section of the electric wire strand intersecting an axial direction of the wire strand has a flat shape from a viewpoint of achieving both flexibility and space saving. 
     Citation List 
     Patent Literature 
     
         
         Patent Literature 1: JP 2014-130739 A 
         Patent Literature 2: JP 2019-149242 A 
         Patent Literature 3: WO 2019/093309 A 
         Patent Literature 4: WO 2019/093310 A 
         Patent Literature 5: JP 2018-129195 A 
         Patent Literature 6: JP 2018-006174 A 
       
    
     SUMMARY OF INVENTION 
     Problems to Be Solved by the Invention 
     When an insulation covering is formed at an outer surface of a conductor having a substantially circular cross section, a layer of the insulation covering with a highly uniformed layer can be formed on the entire surface of the conductor. In contrast, when the conductor has a cross section of a flat shape, it is difficult to form the insulation covering with a uniform thickness on the entire surface of the flat shape. For instance, in the covered electric wire  100  as illustrated in  FIG.  4   , the insulation covering  120  formed on the outer surface of the flat conductor  10  tends to be thicker in the positions corresponding corners  123  of the flat shape than in the positions corresponding to the plane regions  121 ,  122  (t′&gt;t) . When the insulation covering is thicker in some areas than in other areas, there is a possibility that sufficient flame retardancy may not be achieved in those areas. This is because, in those areas, the volume of the insulation covering which can be burned is larger and the distance from the surface of the insulation covering to the surface of the conductor is larger. As a result, it would be difficult to dissipate heat from the insulation covering to the conductor sufficiently in those areas. Especially, when the covered electric wires are installed in vehicles such as automobiles, low flame retardancy of the covered electric wire may facilitate spreading of fire through the covered electric wire in the event of a fire. 
     Thus, an object is to provide a covered electric wire which has excellent flame retardancy, with a structure where an insulation covering is applied on an outer surface of a conductor having a flat-shaped cross section and a wire harness that contains the covered electric wire. 
     Means of Solving the Problems 
     The present disclosure relates to a covered electric wire containing a conductor and an insulation covering which covers an outer surface of the conductor. The covered electric wire has a flat portion where a cross section of the conductor intersecting an axial direction of the covered electric wire has a flat shape which has a larger length in a width direction. The insulation covering contains, in the cross section of the flat portion, plane regions which contains, at the surface thereof: a width-directional region along the width direction; and a height-directional region intersecting the width direction. The insulation covering further contains a region having a rounded shape having a radius R at the junction between the width-directional region and the height-directional region, and the covered electric wire satisfies a relationship of R&gt;t between the radius R and a thickness t of the insulation covering at the plane region. 
     The present disclosure relates also to a wire harness containing the covered electric wire. 
     Advantageous Effects of the Invention 
     The covered electric wire according to the present disclosure shows excellent flame retardancy, with a structure where an insulation covering is applied on an outer surface of a conductor having a flat-shaped cross section. The wire harness according to the present disclosure contains such covered electric wire. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view of a covered electric wire according to one embodiment of the present disclosure. 
         FIG.  2    is an enlarged view of a vicinity of the corner of the covered electric wire according to one embodiment of the present disclosure, showing a case where a radius R of a rounded corner shape of an insulation covering is relatively small. A conductor is indicated by a circumscribing figure. 
         FIG.  3    is an enlarged view of a vicinity of the corner of the covered electric wire according to one embodiment of the present disclosure, showing the case where the radius R of the rounded corner shape of the insulation covering is relatively large. The conductor is indicated by a circumscribing figure. 
         FIG.  4    is a cross-sectional view showing a case where an insulation covering which covers the conductor having a flat shape is not provided with corner roundness. 
         FIGS.  5 A- 5 D  are photographs of cross sections of the covered electric wires having mutually different sizes of corner roundness. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     [Description of embodiments of the present disclosure] 
     Firstly, embodiments of the present disclosure are described. 
     A covered electric wire contains a conductor and an insulation covering which covers an outer surface of the conductor. The covered electric wire has a flat portion where a cross section of the conductor intersecting an axial direction of the covered electric wire has a flat shape which has a larger length in a width direction. The insulation covering contains, in the cross section of the flat portion, plane regions which contains, at the surface thereof: a width-directional region along the width direction; and a height-directional region intersecting the width direction. The insulation covering further contains a region having a rounded shape having a radius R at the junction between the width-directional region and the height-directional region, and the covered electric wire satisfies a relationship of R&gt;t between the radius R and a thickness t of the insulation covering at the plane region. 
     In the above-described covered electric wire, the insulation covering which covers the outer surface of the conductor having a flat-shaped cross section contains the region having a rounded shape at the corner formed as a junction between the width-directional region and the height-directional region as the plane region. Moreover, since the radius R of the rounded shape satisfies the relationship of R&gt;t with respect to the thickness t of the insulation covering at the plane region, and the corners have gently curved shapes. Thus, the thickness of the insulation covering is not excessively thicker at the corners than in the width-directional region and the height-directional region. As a result, in the corners, volume of the insulation covering is suppressed small, and the distance from the surface of the insulation covering to the conductor can be smaller. Thus, when contact of flame or combustion occurs at the insulation covering, in all regions including the corner, heat of the insulation covering can be efficiently dispersed to the conductor so that the temperature of the insulation covering can be decreased. In other words, the region having the rounded shape formed at the corner suppresses combustion of the insulation covering and increase flame retardancy of the covered electric wire. 
     Here, the insulation covering preferably contains a polyolefin and a flame retardant containing a metal compound. In this case, the resin component composing the insulation covering does not itself have excellent flame retardancy like a chlorine-containing polymer; so, there would be a possibility that the insulation covering cannot obtain sufficient flame retardancy at the corner without providing the rounded shape formed at the corner of the insulation covering. However, as described above, the formation of rounded shape satisfying the relationship of R&gt;t leads to excellent flame retardancy even at the corner. 
     The covered electric wire preferably satisfies a relationship of t&gt;0.10√s between the conductor cross-sectional area s and a thickness t of the insulation covering at the plane region. In this case, the thickness of the insulation covering will be relatively thick with respect to the cross-sectional area of the conductor. However, since the rounded shape with R&gt;t is formed at the corners so that the insulation covering will not be excessively thick at the corners, the corners of the insulation covering are helped to have sufficient flame retardancy. 
     The covered electric wire preferably satisfies a relationship of t′ &gt;0.8t between a thickness t′ of the insulation covering in the region having the rounded shape and a thickness t of the insulation covering at the plane region. Then, the thickness t′ of the insulation covering at the corner will not be excessively smaller compared to the thickness t of the insulation covering at the plane region, so that the influence caused by an excessively thin insulation covering such as reduced insulation and abrasion resistance can be suppressed. 
     The covered electric wire preferably has a flatness ratio of four or smaller, where the flatness ratio is estimated as w/h, defining the length of the conductor along the width direction as w and the length of the conductor along the height direction as h. Then, since the conductor will not have an excessively flat shape, it is possible to suppress decrease in flame retardancy of the entire covered electric wire, which may occur when the conductor has a higher flatness and concomitantly the insulation covering has a larger volume. 
     The conductor preferably contains a wire strand containing a plurality of elemental wires twisted together. Then, the conductor is more likely to be both flexible and space saving. In addition, use of a wire strand helps formation of a conductor having a flat portion even if the conductor cross-sectional area is large. When a conductor cross-sectional area is larger, a thicker insulation covering tends to be formed, and thus the effect of improving flame retardancy by forming the rounded shapes at the corners is enlarged relatively. 
     The wire harness according to the present disclosure contains the covered electric wire according to the present disclosure. As described above, the covered electric wire according to the present disclosure has a rounded shape at the corners of the insulation covering in the flat portion, and the radius R at the rounded shape is sufficiently large, satisfying the relationship of R&gt;t with respect to the thickness t of the insulation covering at the plane region. Thus, the covered electric wire shows excellent flame retardancy. Its excellent flame retardancy can also be exploited in the wire harness. 
     [Detailed description of embodiments of the present disclosure] Hereinafter, detailed description of a covered electric wire and a wire harness according to one embodiment of the present disclosure will be provided with figures. In the present description, concerning the shapes of parts of the insulation covering, concepts for describing the shape or arrangement of parts such as straight line, circular arc, flat, parallel, and vertical may include deviations with reference to the concepts in geometry within the allowable range for this type of covered electric wires such as deviations in a length of approximately  ± 15%, and in an angle of approximately  ± 15°. In the present specification, the cross section of a covered electric wire or a conductor shall refer to the cross-sectional cut intersecting to an axial direction (i.e., a longitudinal direction). 
     Outline of the Covered Electric Wire 
       FIG.  1    shows a cross-sectional view of a covered electric wire  1  according to one embodiment of the present disclosure. In addition,  FIG.  2    shows an enlarged view of a vicinity of the corner of the covered electric wire  1 . The covered electric wire  1  according to the present embodiment has a conductor  10  and an insulation covering  20 . The insulation covering  20  covers the outer surface of the conductor  10  for entire periphery. In  FIG.  2   , the conductor  10  is illustrated by a circumscribed figure. 
     The conductor  10  has a flat shape at least in some area along the axial direction of the covered electric wire  1 . In other words, the conductor  10  has a flat portion where its cross section has a flat shape. In the present embodiment, the entire region of the conductor along the axial direction of the conductor  10  constitutes the flat portion. 
     Here, the concept that “the cross section of the conductor  10  has a flat shape” indicates a state where a width w, which is a length of the largest line among lines that extend in the cross section in parallel to edges constituting the cross section and encompass the entire cross section, is larger than a height h, which is a length of a line perpendicular to the above-mentioned longest line and encompass the entire cross section. While the cross-sectional shape of the conductor  10  may have any specific shape as long as it is flat, it is preferable in the present embodiment that the cross section of the conductor  10  has a cross-sectional shape that can be approximated as a rectangle except that the cross section has the rounded shape at the four corners  13 . This is because such a cross-sectional shape helps the covered electric wire  1  to improve the space saving property and to form the insulation covering  20  having a uniform thickness in the plane regions  21 ,  22 , which will be explained later. Here, the concept that “the cross-sectional shape of the conductor  10  is rectangular” indicates a state that the circumscribed figure of the conductor  10  can be approximated as a rectangle within an error range of approximately +15° in terms of a mutual relationship between the edges. 
     As described later, in the present covered electric wire  1 , the insulation covering  20  is provided regions having a round shape at the corners  23  from a viewpoint of obtaining the effect of decreasing the thickness of the insulation covering  20  at the corners  23 . From a viewpoint of increasing the effect, it is preferable that the conductor  10  also has a rounded shape (i.e., a round-chamfered shape; or a rounded corner shape) at the corners  13 , or in other words, at the junctions between the width-directional edges  11  and the height-directional edges  12 . It is preferable that a radius R of the rounded corner shape at the conductor  10  is 33% to 66% of the length of the height-directional side (short side) of the conductor  10 . Further, the radius R of the rounded corner shape at the corner  13  of the conductor  10  may be evaluated based on the circumscribed figure of the conductor  10  as illustrated in  FIG.  2   ; however, it is not necessarily clear whether the circumscribed figure has the rounded corner shape since  FIG.  1    contains reduced number of elemental wires  15  constituting the conductor  10  for clarity of understanding easily. When the number of the elemental wires  15  constituting the conductor  10  is even larger (for example, when the number of the elemental wires  15  is 150 or larger), the rounded corner shape in the circumscribed figure can be clearly recognized. 
     The configuration of the conductor  10  is not specifically limited as long as the conductor  10  has the flat portion. For instance, the conductor  10  may have a single-wire structure made of an integrally formed metallic material such as a metal foil or a metal plate. However, in the present embodiment, the conductor  10  consists of a wire strand comprising a plurality of the elemental wires  15  twisted together. When the conductor  10  is constructed as the wire strand, the covered electric wire  1  can have both the space saving property due to the flat shape of the conductor  10  and the flexibility due to the twisted structure. For instance, the wire strand having the flat shape can be prepared by pressing of the raw wire strands containing a plurality of the elemental wires  15  twisted together and having a substantially circular cross section. 
     In the conductor  10  constructed as the wire strand, at least some of the elemental wires  15  constituting the conductor  10  may be deformed in cross section from the circular shape concomitantly with formation of the conductor  10  into a flat shape. However, from a viewpoint of securing excellent flexibility of the conductor  10 , it is preferable that a deformation ratio from the circular shape of the elemental wires  15  is smaller than in the outer circumferential region in the cross section than in the inner region. In addition, in the cross section of the conductor  10 , it is preferable to leave a vacant space that can accommodate one or more elemental wires  15 , or even two or more elemental wires  15  among the elemental wires  15 . 
     In the covered electric wire  1 , the cross-sectional area of the conductor is not specifically limited. However, in general, in a covered electric wire, when the cross-sectional area is larger, a thickness of the insulation covering provided at the outer surface of the conductor tends to be larger. In the covered electric wire according to the present embodiment, the thicker the insulation covering  20  is, the larger the effect of suppressing the thickness of the insulation covering  20  at the corner  23  small by providing the rounded corner shape at the corner  23  of the insulation covering  20  as explained later. Thus, from a viewpoint of acquiring the larger effect due to formation of the rounded corner shape, it is preferable to use the conductor  10  having relatively large cross-sectional area. For instance, the cross-sectional area is preferably 15 mm 2  or larger, and more preferably 50 mm 2  or larger at a nominal value. 
     A material constituting the conductor  10  is not specifically limited, and various metal materials can apply to the conductor  10 . Representative metal materials constituting the conductor  10  can include copper, copper alloys, aluminum, and aluminum alloys. In particular, since electric conductivity of aluminum and aluminum alloys is lower than that of copper and copper alloys, the cross-sectional area of the conductor made of aluminum and aluminum alloys tends to be large in order to secure necessary electric conduction. Considering as described above, that the effect of providing the rounded corner shape at the corner  23  of the insulation covering  20  is larger when the cross-sectional area of the conductor  10  is larger, the conductor  10  should preferably be made of aluminum or aluminum alloys. 
     In the covered electric wire  1  according to the present embodiment, the insulation covering  20  covers the outer surface of the conductor  10 . In the cross section, corresponding to the fact that the conductor  10  has the flat shape, the entire covered electric wire  1  including the insulation covering  20  also has the flat shape. In addition, cross section of the entire covered electric wire  1  including the insulation covering  20  can be approximated as the rectangular except for the rounded corner shape of the corner  23 . Here, the concept that “the cross-sectional shape of covered electric wire  1  is rectangular” indicates a state that the circumscribed figure of the covered electric wire  1  can be approximated as a rectangle within an error range of approximately +15° in terms of a mutual relationship between the edges. Further, in the cross section, the outer surface of the insulation covering  20  has width-directional regions  21  extending along the edges in the width direction and the height-directional regions  22  extending the edges in the height direction, and the both regions  21 ,  22  constitute plane regions that can be approximated as a straight line. Further, the thickness t of the insulation covering  20  is uniform in the entire of the width-directional regions  21  and the height-directional regions  22  within an error range of approximately +15° except the corners  23  and in the vicinity thereof. 
     In the cross section of the present covered electric wire  1 , four corners  23  of the insulation covering  20 , or in other words, junctions between the width-directional regions  21  the height-directional regions both configured as plane regions are provided with rounded shapes (a round-chamfered shape; a rounded corner shape; corner roundness). The rounded corner shape can improve the flame retardancy of the insulation covering  20  at the corners  23 . Detailed description of the shape of the insulation covering  20  at the corners  23  and the effect due to that shape will be provided later. 
     The present description mainly deals with the embodiment in which the insulation covering  20  is in close contact with the outer surface of the conductor  10 . It is preferable for the insulation covering  20  to be in close contact with the conductor  10  in order to improve the flame retardancy of the insulation covering  20 ; however, a vacant space may exist between the conductor  10  and the insulation covering  20 . Even when such a vacant space exists, the configuration described below can be suitably applied to the insulation covering  20  with regard to the structure thereof such as the shape of the insulation covering  20  at the corner  23  except for the existence of the vacant space. In addition, as described above, the present description mainly deals with the case where the cross-sectional shape of the conductor  10  can be approximated as a rectangle; however, the cross section of the conductor  10  may take a shape other than the shape that can approximate a rectangle. Also in that case, it is preferable to provide the rounded corner shape at the corner  13  of a figure approximating the cross section of the conductor  10  (corresponding to an intersection of two straight lines, an intersection of a straight line and a curved line, or an intersection of two curved lines); and the corner  13  of the conductor  10  and the corner  23  of the corresponding insulation covering  20  should preferably have the structure described below. Examples of the approximate figure of the flat shaped conductor having corners  13  other than a rectangle include ovals (i.e., a shape containing a rectangular shape and arcs joined on both sides; an elliptical shape of the oval; or a running-track shape) and polygons such as hexagons and octagons. When the conductor  10  has an oval shape, the corner  13  is formed as the intersection of a straight line and a curved line, and when the conductor  10  has a polygonal, the corner  13  is formed as the intersection of two straight lines. 
     The material composing the insulation covering  20  is not specifically limited as long as it is insulating material; however, it is preferable that it is mainly composed of organic polymers. From a viewpoint of increasing the flame retardancy of the insulation covering  20 , constituent material of the insulation covering  20  preferably has flame retardancy. However, if the constituent material of the insulation covering  20  has very high flame retardancy, the insulation covering  20  is more likely to achieve sufficient flame retardancy regardless of the shape of the insulation covering  20 , such as with or without the rounded corner shape at the corner  23 . When the flame retardancy of constituent material of the insulation covering  20  itself is not so high, the effect of flame retardancy by providing the rounded corner shape at the corner  23  of the insulation covering  20  is relatively higher. Specifically, the organic polymer contained in the insulation covering  20  should not consist of a polymer with high flame retardancy such as polyvinyl chloride (PVC) , but an organic polymer is preferable which does not have chlorine or high flame retardancy; for example, a polyolefin such as polyethylene is preferable. Flame retardancy should preferably be imparted to the organic polymer by addition of a flame retardant. Moreover, as a flame retardant to be added, it is more preferable to use a flame retardant containing a metal compound such as magnesium hydroxide or other metal hydroxide, rather than a brominated flame retardant or other flame retardant which exhibits high flame retardancy in small amounts. 
     A material in which a flame retardant containing a metal compound is added to a polyolefin such as polyethylene shows excellence when it is used in non-halogen wire covering material due to its high heat resistance and can be suitably used in the present embodiment. From a viewpoint of increasing the heat resistance, polyolefin should particularly preferably be crosslinked. From a viewpoint of exhibiting sufficient flame retardancy, it is preferable that the amount of the flame retardant is 50 parts by mass or larger with respect to 100 parts by mass of the polyolefin. On the other hand, from a viewpoint of keeping the mechanical property of the insulation covering  20  such as wear resistance, it is preferable that the amount of the flame retardant is 200 parts by mass or smaller with respect to 100 parts by mass of the polyolefin. In addition to the polyolefin and the flame retardant, the insulation covering  20  may contain a polymer other than the polyolefin and may also contain an additive other than the flame retardant. 
     The method for forming the insulation covering  20  is not particularly limited; however, it is preferable that the layer of the insulation covering  20  at the outer surface of the conductor  10  is formed by extrusion of the composition containing the ingredients mixed together. Depending on the shape of the die for the extrusion, the insulation covering  20  having a predetermined rounded corner shape can be formed at the corners  23  of the cross section. 
     The covered electric wire  1  according to the present embodiment may be used alone or as a constituent member of the wire harness according to the present embodiment. The wire harness according to the present embodiment include the covered electric wire  1  described in the above embodiment. The wire harness may include a plurality of the above-described covered electric wires  1  and may also include other types of covered electric wires in addition to the above-described covered electric wires  1 . Preferably, a plurality of the above-described covered electric wires  1  should be arranged in array(s) in the width and/or height direction. In this case, the specific arrangement structure of the plurality of the covered electric wires  1  is not particularly limited. For example, in a suitable form, the plurality of the covered electric wires  1  are lined up in the width direction and fixed to a common sheet material by fusion or other means. In this case, it is particularly preferable that the heights of the plurality of covered electric wires  1  lined up are uniform. 
     Description of the Insulation Covering 
     In the covered electric wire  1  according to the present embodiment described above, the insulation covering  20  which covers the outer surface of the conductor  10  has the rounded corner shapes at the corners  23  as junctions between the width-directional regions  21  and the height-directional regions  22  both configured as the plane regions. Here, as illustrated in  FIGS.  2  and  3   , the radius of the rounded corner shape at the corner  23  of the insulation covering  20 , or in other words, the radius of a circular arc approximating the corner roundness is defined as R.  FIG.  2    shows a case where the radius R is relatively small and a center  02  of the rounded corner shape is located outside of the conductor  10  while  FIG.  3    shows the case where the radius R is relatively large and the center  02  of the rounded corner shape is located in the conductor  10 . 
     Defining thicknesses of the insulation covering  20  in the plane regions, which is to say, in the width-directional region  21  and the height-direction region  22  as t, the covered electric wire  1  according to the present embodiment satisfies a relationship of R&gt;t between the radius R of the rounded corner shape of the insulation covering  20  and the thicknesses t of the plane regions  21 ,  22 . In other words, the radius R of the rounded corner shape at the corners  23  of the insulation covering  20  is larger than the thicknesses t of the insulation covering  20  in the plane regions  21 ,  22 . As the radius R of the rounded corner shape is larger, the degree of roundness of the rounded corner shape is greater and the corner  23  exhibits a more gentle curve. 
     In the covered electric wire  1 , the thickness direction of the corner  23  corresponds to a diagonal direction of the flat shape of the conductor  10 . Thus, if the corners  23  of the insulation covering  20  are formed to have no rounded corner shapes or formed to have the rounded corner shapes with a small radius R, then thicknesses t′ of the insulation covering  20  at the corners  23  would be larger, and the volume occupied by the insulation covering  20  would be larger at the corners  23 . Further, the distance from the surface of the insulation covering  20  to the surface of the conductor  10  would be greater. As a result, if the insulation covering  20  contacts flame or is burned, a large amount of the insulation covering material would be burned, and the heat generated in the insulation covering  20  would be less likely to be dissipated into the conductor  10 . Then, the insulation covering  20  would be more prone to be burned. In addition, if the insulation covering  20  is subjected to burning, the burning would be more likely to continue for an extended period of time. 
     In contrast, when the corners  23  of the insulation covering  20  are formed to have rounded corner shapes with a sufficiently large radius R, the thicknesses t′ of the insulation covering  20  at the corners  23  are small and volumes of the insulation covering  20  at the corners are small. In addition, the distance from the surface of the insulator covering  20  to the surface of the conductor  10  is small at the corner  23 . Then, even if flame contacts the insulation covering  20  or the insulation covering  20  burns, the temperature rising of the insulation covering  20  and the progress of combustion are suppressed. This is because volume of the insulation covering which could burn is small, and because the distance along which heat to be dissipated from the insulation covering  20  to the conductor  10  have to be conducted is small, whereby the heat generated in the insulation covering  20  is easily dissipated to the conductor  10 . As a result, the insulation covering  20  is less prone to cause combustion. Even if the insulation covering  20  causes combustion, the combustion is stopped in a short time due to self-extinguishing. Thus, when the corners  23  of the insulation covering  20  are formed to have rounded corner shapes with a sufficiently large radius R, the flame retardancy at the corners  23  is increased. 
     In particular, when the radius R of the rounded corner shape at the corner  23  of the insulation covering  20  satisfies the relationship of R&gt;t with respect to the thicknesses t of the insulation covering  20  in the plane regions  21 ,  22 , the effect of improving flame retardancy by reduction of the thicknesses of the insulation covering  20  at the corners  23  is sufficiently high. For example, when the thicknesses t of the insulation covering in the plane regions  21 ,  22  are the same as the thicknesses of a conventional covered electric wire having a circular cross section, sufficiently high flame retardancy can be obtained in the entire insulation covering  20 , including the corners  23 , in comparison with the conventional covered electric wire having the circular cross section. In particular, even if the covered electric wire  1  is installed in a vehicle such as an automobile, and the insulation covering  20  catches fire in the event of fire, the high flame retardancy of the insulation covering  20  helps self-extinguishing at an early stage and suppress spread of the fire through the covered electric wire  1 . A relationship of R&gt;1.5t or R&gt;2t is preferred for obtaining higher flame retardancy. A relationship R&gt;r+0.5t; is further preferred. In addition, when a relationship of R=r+t is satisfied, the thicknesses t of the insulation covering  20  at the plane regions  21  and  22  and the thickness t′ of the insulation covering  20  at the corner  23  are the same. 
     When the corners  23  of the insulation covering  20  are formed to have rounded corner shapes, it is effective for achieving tight fitting between an external material and the covered electric wire  1 , in addition to improving the flame retardancy. For example, when a rubber seal is placed on the outer surface of the insulation covering  20  for the purpose of waterproofing, the rubber seal will fit tightly to the outer surface of the insulation covering  20 . Then, high waterproofness can be achieved at the ends of the covered electric wire  1 . 
     From a viewpoint of improving the flame retardancy at the corner of the insulation covering  20 , there is no upper limit to the radius R of the rounded corner shape at the corners  23 . However, it is preferable to keep the radius R of the rounded corner shape to be reasonably small and to secure sufficient thicknesses t′ of the insulation covering  20  at the corners  23 , whereby the properties that the insulation covering  20  has, such as insulation and wear resistance, are sufficiently exhibited even at the corners  23 . Specifically, the thicknesses t′ of the insulation covering  20  at the corners  23  should preferably maintain a relationship of t′&gt;0.8t between the thicknesses t at the plane regions  21  and  22 . A relationship of t′≥t is more preferred. 
     Here, explanation about a relationship between the thicknesses t of the insulation covering  20  at the corners  23  and the radius R of rounded corner shape is provided. As illustrated in  FIG.  3   , a situation is assumed where the center O2 of the rounded corner shape (having a radius R) at the corner  23  of the insulation covering  20  is located inside the center O1 of the rounded corner shape at the corner  13  of the conductor  10  by a distance a in the width and height directions, respectively. In this case, assuming a straight line (straight line L1) connecting the center O2 and the center O1 and extending to the surface of the insulation covering  20 , the radius R of rounded corner shape of the insulation covering  20  can be expressed by following formula 1. 
     
       
         
           
             R 
             = 
             
               t 
               ′ 
             
             + 
             r 
             + 
             2 
             a 
           
         
       
     
     Meanwhile, assuming a straight line (i.e., straight line L2) connecting the center  02  to the junction point P between the plane regions  21  of the insulation covering  20  and the rounded corner shapes at the corners  23 , the radius R of the rounded corner shape of the insulation covering  20  can be expressed by following formula 2. 
     
       
         
           
             R 
             = 
             t 
               
             + 
               
             r 
               
             + 
               
             a 
           
         
       
     
     The right-hand sides of formulas 1 and 2 are equal, so, the distance a can be expressed as follows. 
     
       
         
           
             a 
             = 
               
               
             
               
                 
                   
                     t 
                     − 
                     
                       t 
                       ′ 
                     
                   
                 
               
               / 
               
                 
                   
                     2 
                       
                     - 
                       
                     1 
                   
                 
               
             
           
         
       
     
     Substituting formula 3 into formula 1 or formula 2 and rearranging the obtained formula, the radius R of the rounded corner shape of the insulation covering  20  can be expressed as follows. 
     
       
         
           
             
               
                 R 
                 = 
                 r 
                 + 
                 
                   
                     
                       1 
                       / 
                       
                         
                           
                             2 
                               
                             − 
                               
                             1 
                           
                         
                       
                     
                   
                 
                   
                 ⋅ 
                   
                 
                   
                     2 
                     t 
                     − 
                     
                       t 
                       ′ 
                     
                   
                 
               
             
             
               
                 
                   
                     = 
                     r 
                     + 
                     2 
                       
                     . 
                       
                     41 
                       
                     
                       
                         2 
                         t 
                         − 
                         
                           t 
                           ′ 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     Here, when the relationship of t′&gt;0.8t is established as described above, based on formula 4, the range of radius R of the insulation covering  20  is expressed as R&lt;r+1.48t. Combining it with the above-explained requirement R&gt;t as described above, which defines the lower limit of radius R, a range of t&lt;R&lt;r+1.48t is obtained as a preferable range of the radius R of the rounded corner shapes at the corners  23 . 
     As described above, since the covered electric wire  1  according to the present embodiment, has the rounded corner shapes at the corners  23  of the insulation covering  20  and the radius R of the rounded corner shape satisfies R&gt;t, reduction of the flame retardancy due to large thicknesses of the insulation covering  20  at the corners  23  can be suppressed even when the insulation covering  20  is formed thickly in the plane regions  21 ,  22 . Rather, when the thicknesses of the insulation covering  20  in the plane regions  21 , 22  are larger, the relative effect of improving flame retardancy by forming the rounded corner shape at the corners  23  can be increased. From this viewpoint, it is preferable that the thicknesses t of the insulation covering  20  at the plane regions  21 , 22  satisfies a relationship of t&gt;0.10√s with respect to the conductor cross section s. The thickness satisfying this relationship is regarded as relatively large for general insulation coverings provided in standard covered electric wires having circular cross sections, or is still larger. A relationship of t&gt;0.20√s or t&gt;0.30√s is more preferred. In addition, if the thickness of the insulation covering  20  is too large, not only the corners  23  but also the insulation covering  20  as a whole may not have sufficient flame retardancy. Therefore, an approximate relationship of t&gt;0.50√s is preferred. 
     In the covered electric wire  1 , the flatness ratio w/h of the conductor  10  is not particularly limited, since effect of improvement of the flame retardancy can be obtained by providing the rounded corner shape at the corners  23  of the insulation covering  20 , regardless of the degree of the flatness ratio of the flat shape of the conductor  10 . However, it is preferable that the flatness ratio w/h of the conductor  10  is two or larger. Then, the flat shape of the conductor  10  is more effective in saving space, and is also effective in improving flame retardancy since the length of the outer surface of the conductor is longer and heat is more easily dissipated from the insulation covering  20  to the conductor  10  through the interface. On the other hand, the flatness ratio w/h is preferably six or smaller, and more preferably four or smaller. With this arrangement, a situation is prevented where the flame retardancy of the insulation covering  20  is reduced because of the increase of the volume of the insulation covering  20  that covers the conductor  10  due to a larger flatness ratio and a larger length of the outer surface of the conductor  10 . 
     Example 
     Hereinafter, examples are explained. It should be noted that the present invention is not limited by these examples. In this section, here, the relationship between the radius R of the rounded corner shape of the insulation covering and flame retardancy was investigated. 
     Preparation of Samples 
     First, conductors composing covered electric wires were prepared. Wire strands comprising a plurality of aluminum alloys elemental wires twisted together and having a circular cross section were prepared and then the obtained wire strands were subjected to compressing with rollers to prepare the conductors having the flat shape. The prepared wire strands had a conductor cross-sectional area (nominal value) of 130 mm 2  (diameter of elemental wires: 0.42 mm, and number of elemental wires: 931), or 60 mm 2  (diameter of elemental wires: 0.32 mm, and number of elemental wires: 741). The flatness ratios w/h of the conductors were varied between 2.5 to 4 by changing the compression ratio, and plural types of conductors were prepared for each conductor cross-sectional area of the conductor. The radius r of the rounded corner shape of the conductor corner was 4.0 mm when the cross-sectional area of the conductor was 130 mm 2 , and was 2.6 mm when the cross-sectional area of the conductor is 60 mm 2  . 
     Insulation covering were formed on the outer surfaces of the prepared conductors by extruding. In this process, the thickness of the insulation covering and the radius R of the rounded corner shapes of the corners were varied by changing the shape and size of the die used. As constituent material of the insulation covering, material in which magnesium hydroxide was added in an amount of 70 parts by mass with respect to  100  parts by mass of cross-linked polyethylene was used. 
     The covered electric wires of samples 1-12 were prepared in the above-described manner. Photographs were taken of the cross sections of the prepared samples to confirm that the dimensions of the respective portions in the conductors and insulation coverings were as predetermined. The cross-sectional samples were prepared by cutting the covered electric wires vertically to the axial direction of the covered electric wires embedded in acrylic resin. 
     Evaluation of Flame Retardancy 
     Flame retardancy was evaluated by performing a combustion test on the above-obtained covered electric wires. Specifically, each covered electric wire was cut to a length of 30 cm, held horizontally, and a reducing flame having a length of 35 mm was brought into contact with the center of the covered electric wire. The insulation covering was observed to catch fire within 30 seconds of flame contact, and after ignition, flame was taken away from the covered electric wire. The time taken for flame to be extinguished spontaneously after flame was taken away (i.e. , flame extinguishing time) was then measured. When a covered electric wire exhibited the flame extinguishing time within 30 seconds, the covered electric wire can be evaluated as having sufficient flame retardancy. If the flame extinguishing time is within 30 seconds as evaluated by a similar combustion test for a general covered electric wire having a circular cross section, the covered electric wire is regarded as having sufficiently high flame retardancy to prevent spreading of fire to other parts. 
     Results 
     Table 1 shows the flame extinguishing time obtained in the combustion test along with the dimensions of respective portions of the covered electric wires. In addition,  FIGS.  5 A to  5 D  show photographs of the cross sections of samples 1, and 3 to 5, respectively, before the combustion test was performed.  
     
       
         
          TABLE  1 

           
               
               
               
               
               
               
               
               
             
               
                 Sample No. 
                 Conductor 
                 Insulation covering 
                 flame extinguishing time [sec.] 
               
               
                 Cross-sectional area [mm2] 
                 Flatness ratio w/h 
                 Radius r at the rounded corner shape [mm] 
                 Thickness t of plane region [mm] 
                 Radius r at the rounded corner shape [mm] 
                 Ratio R/t 
               
             
            
               
                 1 
                 130 
                 4 
                 4 
                 1.6 
                 1.6 
                 1.0 
                 36 
               
               
                 2 
                 2.5 
                 1.6 
                 1.6 
                 1.0 
                 50 
               
               
                 3 
                 3 
                 1.6 
                 2.5 
                 1.6 
                 23 
               
               
                 4 
                 3 
                 1.6 
                 3.5 
                 2.2 
                 12 
               
               
                 5 
                 3 
                 1.6 
                 5.5 
                 3.4 
                 4 
               
               
                 6 
                 60 
                 3 
                 2.6 
                 1.5 
                 1.5 
                 1.0 
                 43 
               
               
                 7 
                 3 
                 1.5 
                 2.5 
                 1.7 
                 22 
               
               
                 8 
                 3 
                 1.2 
                 1.5 
                 1.3 
                 17 
               
               
                 9 
                 3 
                 1.2 
                 2.5 
                 2.1 
                 13 
               
               
                 10 
                 2.5 
                 1.5 
                 1.5 
                 1.0 
                 73 
               
               
                 11 
                 3.5 
                 1.5 
                 1.5 
                 1.0 
                 48 
               
               
                 12 
                 4 
                 1.5 
                 1.5 
                 1.0 
                 49 
               
            
           
         
       
     
     First, from at the cross-sectional photographs in  FIGS.  5 A to  5 D , it is confirmed that, in all the photographs, the insulation covering formed on the outer surface of the conductors have plane regions having highly uniform thickness and have corners having the rounded corner shape. The larger the radius R of the rounded corner shape, the more gently curved the corners are. 
     Table 1 shows that in samples 1, 2, 6, and 10-12 having a relationship of R≤t, or in other words, R/t≤1.0, the flame extinguishing time is over 30 seconds, and thus sufficient flame retardancy is not obtained. In contrast, in samples 3-5, and 7-9 having a relationship of R&gt;t, or in other words, R/t&gt;1.0, flame extinguishing time is within 30 seconds in all cases despite the different conductor configurations and the thickness t of the insulation covering in the plane regions. This indicates that the ratio of radius R of the corner of the insulation covering and thickness t in the plane region is a good indicator about flame retardancy. In addition, when a relationship of R&gt;t is satisfied, the insulation covering obtains excellent flame retardancy. 
     In each of the group consisting of samples 3-5 and the one consisting of samples 8 and 9, only the radius R of the rounded corner shape is changed among the parameters while the relationship of R&gt;t is satisfied. Comparing the test results of those samples, the larger the radius R, the shorter the flame extinguishing time. From this result, it can be said that the larger the radius R of the rounded corner shape is, in the corner of the insulation covering, the higher flame retardancy becomes. 
     Although embodiments of the present invention have been described above in detail, the present invention is not limited to the particular embodiment(s) disclosed herein, and various changes and modifications may be made without deviating from the scope of the present invention.  
     
       
         
           
               
               
             
               
                 List of Reference Numerals 
               
             
            
               
                   1 ,  100   
                 Covered electric wire 
               
               
                 
                   10 
                 
                 Conductor 
               
               
                 
                   11 
                 
                 Length of the conductor in the width direction 
               
               
                 
                   12 
                 
                 Length of the conductor in the height direction 
               
               
                 
                   13 
                 
                 Corner of the conductor 
               
               
                 
                   15 
                 
                 Elemental wire 
               
               
                   20 ,  120   
                 Insulation covering 
               
               
                   21 ,  121   
                 Width-directional regions (Plane region) 
               
               
                   22 ,  122   
                 Height-directional regions (Plane region) 
               
               
                   23 ,  123   
                 Corner (Joint portion) 
               
               
                 a directions 
                 Distance between centers  01  and  02  along the width and height 
               
               
                 L 1 , L 2 
 
                 Straight line for analysis 
               
               
                 h 
                 Height of the conductor 
               
               
                 
                   01 
                 
                 Center of the rounded corner shape of the conductor 
               
               
                 
                   02 
                 
                 Center of the rounded corner shape of the insulation covering 
               
               
                 P 
                 Junction between the rounded corner shape and plane region 
               
               
                 r 
                 Radius of the rounded corner shape of the conductor 
               
               
                 R 
                 Radius of the rounded corner shape of the insulation covering 
               
               
                 t 
                 Thickness of the insulation covering at plane region 
               
               
                 t′ 
                 Thickness of the insulation covering at corner 
               
               
                 w 
                 Width of the conductor