Electromagnetically shielded wire or cable

Abstract: A shield wire comprising: a conductor, an insulation layer provided around an outer periphery of the conductor; an electrically-conductive resin layer provided around an outer periphery of the insulation layer; a covering insulation layer formed around an outer periphery of the electrically-conductive resin layer; and shield device for shielding the shield cable from electromagnetic interference, the means formed to electrically contact the electrically-conductive resin layer.

Description: BACKGROUND OF THE INVENTION This invention relates to an electromagnetic interference prevention cable. More specifically, a high-frequency interference prevention and/or electromagnetic wave induction prevention wire is used for electrical connection of an electronic device such as an audio device and an office automatic device. In conventional electromagnetic and high-frequency circuits, various kinds of shield cables and shield plates have been used in order to prevent a wrong operation due to noises produced from such circuit. In the conventional high-frequency interference prevention, a static coupling and an electromagnetic coupling between the wires is interrupted by a shield cable or a shield plate, thereby removing unnecessary oscillation. However, such a method requires a highly-technical layout of shield cables and shield plates, and can not actually be achieved easily. In recent years, a computer control for electric devices and electric products has been remarkable. Electronic circuits of such devices have been highly integrated, and current flowing through elements have been microscopic, and there has arisen a problem that a wrong operation of the device may occur due to induction between wires of a wiring bundle. On the other hand, the products have become compact and lightweight, and also the space-saving and lightweight design of the wiring has been strongly desired. There is known a shield cable of this kind as shown in Fig. 11 in which an insulation layer 102, a shield layer 104 and a covering insulation layer 105 are provided around an outer periphery of a central conductor 101, and a drain wire 103 is provided along the shield layer 104 so as to facilitate an earth connection operation (Japanese Utility Model Application Examined Publication No. Sho. 53-48998). The shield layer 104 is made of electrically conductive metal such as a metal braid and a metal foil. In the conventional shield cable with the drain wire, a wire (conductor) of a circular cross-section is used as the drain wire 103, and therefore the diameter of the shield cable becomes large. This has prevented a small-size and space-saving design. In the case where an electrically-conductive resin is used as the shield layer 104, anisotropy is encountered when the drain wire 103 is provided parallel to the conductor 101 in the conventional manner. The result is that a uniform shielding effect can not be obtained. There is also disclosed a shield cable having no drain wire and utilizing an electrically-conductive resin. However, since high electrical conductivity can not be obtained, a practical use of it is difficult. Therefore, a metal braid or a metal foil is in practical use. However, the metal braid needs to have a high braid density, and therefore tends to be heavy and expensive. The metal foil lacks in flexibility, and becomes deteriorated due to corrosion, thus failing to provide sufficient durability. Thus, these problems have been encountered. Also, there are commercially available shield cables in which a metal foil, a metal braid or an electrically-conductive resin is provided, as an electrically-conductive layer, around a conductor insulator or a bundle of wires (Japanese Patent Application Unexamined Publication No. Sho. 64-38909). However, each of all the wires is formed into a shield wire, the wiring bundle has much space loss because of the circular cross-section of the wire. Thus, it is not suited for the space-saving purpose. Further, for connecting the electrically-conductive layer to the earth, a manual operation is required for separating the electrically-conductive layer from the internal conductor, and therefore the wiring can not be automated. Further, in the type of shield cable in which an electrically-conductive layer is provided around a bundle of several wires, induction between the wires within the bundle can not be prevented. When a metal foil or a metal braid is used as a shield layer, the construction is complicated, and therefore the efficiency of production of the cable is low, and a high cost is involved. On the other hand, recently, in order to achieve the space-saving of the wiring, tape-like cables have been increasingly used, and there has been marketed a shield cable in which such a tape cable is enclosed by a metal foil or a metal braid as described above. Even with this wire, induction within the tape cable can not be prevented (Japanese Patent Application Unexamined Publication No. Sho. 61-133510/86). Further, in the two, the type which uses metal as the shield electrically-conductive layer has a problem that it is heavy and inferior in durability. DE-A-2 654 846 describes a prior art shield means consisting of either a metal mesh or a plurality of tinsel wires. This shield means is used to electrically contact an electrically conductive resin layer in order to avoid electromagnetic interference. EP-A-2 0 279 985 discloses an electrically conductive thermoplastic plastic resin composition which is used for shielding cables from electromagnetic interference. This composition comprises a thermoplastic resin as a major component and carbon fiber as a minor component. The fiber comprising no more than 8% by volume of the composition. The thus generated electrically conductive resin has a restitivity between 1 and 500 ohm-cm. GB-A-2 047 947 discloses a shield flat cable comprising a plurality of parallel metal conductors, each of which is surrounded by an inner insulation layer. The plurality of insulated conductors is then coated by an electrically conductive polymer layer, on which is provided an outer insulation layer. Inside the conductive polymer layer, there is also provided a bare conductor, which serves as a drain wire. In view of the above described prior art it is an object of the invention is to provide a shield cable with a drain wire, which exhibits a uniform shield effect with respect to the direction of electromagnetic wave, and has a lightweight, compact and inexpensive construction. This is achieved by the features of claim 1. By providing a shield wire with an electrically-conductive resin layer and a drain wire such that said electrically-conductive resin layer includes vapor phase-growing carbon fiber and graphitized carbon fiber made of said phase-growing carbon fiber, said electrically-conductive resin layer having a volume resistivity of 10 -3 to 10 5 ohm-cm and said drain wire being a metal conductor of flat cross-sectional shape with a ratio of width to thickness of said drain wire being not less than 1, it is possible to produce a lightweight and inexpensive shield wire. According to one aspect of the invention, there is provided a shield wire with a drain wire wherein an insulation layer, an electrically-conductive resin layer and a covering insulation layer are sequentially provided around an outer periphery of a conductor; and a drain wire is provided in contiguous relation to the electrically-conductive resin layer; the drain wire is provided spirally in such a manner that the drain wire is either embedded in the electrically-conductive resin layer or disposed in contact with the electrically-conductive resin layer. Preferably, the electrically-conductive resin has a volume resistivity of 10 -3 to 10 4 Ω·cm so as to have a high electrical conductivity. At least one drain wire is spirally wound at a rate of not more than 200 turns per meter, or provided in parallel relation or intersecting relation to one another. In order to reduce the diameter of the shield cable, the ratio of the cross-sectional area (S1) of the electrically-conductive resin layer to the cross-sectional area (S2) of the drain wire is represented by S1/S2 < 1500. Preferably, the drain wire has a flattened ribbon-like shape. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are perspective views of high-frequency interference prevention cables ; Fig. 3 is a view showing a device for measuring an interference prevention effect of the above cables; Fig. 4 is a graph showing high-frequency interference prevention characteristics of Examples 1 and 2 and Comparative Examples 1 and 2; Fig. 5 is a view showing the principle of the operation of a conventional cable; Fig. 6 is as view showing the principle of the operation of the cable of the present invention; Fig. 7(a) is a perspective view of a shield cable with a drain wire provided in accordance with the present invention; Figs. 7(b) and 7(c) are cross-sectional views of a drain wire provided in accordance with the present invention; Fig. 8 is a view showing a device for measuring a shield effect of the above shield cable; Figs. 9(a) and 9(b) are views showing the manner of setting the shield cable in the above device; Fig. 10 is a graph showing shield characteristics of Example 3 and Comparative Examples 3 and 4, respectively; Fig. 11 is a perspective view of the prior art; DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION The invention will now be described in detail with reference to the drawings. Fig. 1 shows a high-frequency interference prevention cable A in which an electrically-conductive resin layer 2 is provided around an outer periphery of a conductor 1, and a covering insulation layer 3 is provided around the layer 2. In a high-frequency interference prevention cable A' shown in Fig. 2, an inner insulation layer 4 and a shield layer 5 composed of a metal braid are provided between a conductor 1 and an electrically-conductive resin layer 2. The shield layer 5 functions to prevent an electromagnetic wave induction. The electrically-conductive resin layer 2 is made of an electrically-conductive resin having a volume resistivity of 10 -3 to 10 5 Ω·cm, and preferably 10 -3 to 10 2 Ω·cm. The compositions of a matrix, an electrical conductivity-imparting material and the other additives of this electrically-conductive resin are not particularly limited. For example, as the matrix, there can be used a thermoplastic resin such as PE, PP, EVA and PVC, a thermosetting resin such as an epoxy or a phenolic resin, rubber such as silicone rubber, EPDM, CR and fluororubber, or a styrene-type or an olefin-type thermoplastic elastomer or ultraviolet curing resin. Vapor phase-growing carbon fiber and graphitized carbon fiber are combined, as the electrical conductivity-imparting material, with the matrix to produce the electrically-conductive resin having a desired volume resistivity. Additives such as a process aid, a filler and a reinforcing agent can be added. Fig. 5 shows an electric loop P produced when using a conventional cable a . In order to eliminate this loop, various layouts have been tried as described above. In this Figure, reference character L denotes a reactance of a wire, and reference numeral C denotes a capacitance between the wires and a capacitance between the wire and the earth. Fig. 6 shows an electric loop P' obtained when using the cable of the present invention having the electrically-conductive resin layer with a volume resistivity of 10 -3 to 10 5 Ω·cm. R (resistor) is inserted in the closed loop, so that the circuit current is attenuated, thereby reducing the resonance. Thus, in the high-frequency interference prevention cable of the present invention, R is naturally inserted in the electric loop (resonance circuit) produced when using the conventional cable. Therefore, the resonance due to the wiring in the high-frequency circuit as well as the leakage of the high frequency is prevented. For preventing the electromagnetic induction, the shield layer is provided on the cable, as described above. Comparative Example 1 An ordinary wire, having a copper conductor (whose cross-sectional area was 0.5 mm 2 ) and an insulation coating (polyvinyl chloride) (whose outer diameter was 1.6 mm) coated on the conductor, was used as a standard sample. Example 1 An electrically-conductive resin having a volume resistivity of 10° Ω·cm was coated on a copper conductor (whose cross-sectional area was 0.5 mm 2 ) to form a 0.4 mm-thick resin coating thereon. Then, PVC was coated on the resin coating to form thereon a PVC layer 2.4 mm in outer diameter, thereby preparing a high-frequency interference prevention wire (measuring sample) as shown in Fig. 1. The above standard sample and the above measuring sample were separately set in a central portion of a copper pipe 6 (inner diameter: 10 mm; length: 100 cm) of a measuring device B shown in Fig. 3, and a high-frequency interference prevention effect (interference with the copper pipe) was measured. In this Figure, reference numeral 7 denotes a FET probe, and reference numeral 8 denotes a spectrum analyzer. Referring to the measuring method, in the above device B, the components of the frequency, produced in the sample by induction when an electric field was applied to the copper pipe, were analyzed by the spectrum analyzer. The standard sample with no shield was first measured, and then the measuring sample was set in the device, and one end of the shield layer was grounded, and the measuring sample was measured. The measurement results of the two cables are indicated respectively by a curve a (Comparative Example 1) and a curve b (Example 1) in Fig. 4. Comparative Example 2 An insulation coating (PVC) having an outer diameter of 1.6 mm was formed on a copper conductor having a cross-sectional area of 0.5 mm 2 , and a metal braid was provided on the insulation coating to form a shield structure (outer diameter: 2.1 mm) thereon. Then, a covering insulation layer (PVC) was formed on the shield structure to prepare a shield cable having an outer diameter of 2.9 mm. Example 2 An electrically-conductive resin was coated on the shield braid of Comparative Example 2 to form thereon an electrically-conductive resin layer having a thickness of 0.4 mm and a volume resistivity of 10 0 Ω·cm, thereby preparing a high-frequency interference prevention cable as shown in Fig. 2. A high-frequency interference prevention effect was measured with respect to the above two cables in the same manner as described above. The results thereof are indicated by a curve c (Comparative Example 2) and a curve d (Example 2) in Fig. 4. As is clear from Fig. 4, with respect to Comparative Example 1 (curve a ), the cable resonated with the copper pipe, and a large interference due to induction is recognized. However, with respect to Example 1 (curve b ), it will be appreciated that this interference is greatly reduced. Similarly, in Comparative Example 2 (curve c ) a better electromagnetic wave induction prevention effect than that of Comparative Example 1 (curve a) is obtained, but the cable resonated with the copper pipe, and a large interference is recognized. In Example 2 (curve d), the interference- is greatly reduced. As described above, by using the high-frequency interference prevention cable of the present invention, the interference due to the resonance in the high-frequency circuit can be prevented, and the use of the conventional shield plate and the difficulty of the layout are omitted, thereby achieving the space-saving. Further, by the addition of the shield layer, the electromagnetic wave induction can be prevented at the same time, thereby eliminating a wrong operation of the circuit. A second embodiment of the invention will now be described in detail. Fig. 7(a) shows a shield cable C according to the present invention with a drain wire in which an insulation layer 12 is coated on a conductor 11 of copper, and a drain wire 13 is spirally wound around this insulation layer at a rate of ten turns per meter, and further an electrically-conductive resin layer 14 is coated, and a covering insulation layer 15 is provided for insulating purposes. Preferably, the drain wire 13 is turned at least twice per meter. In the illustrated embodiment, although the drain wire 13 is wound on the outer periphery of the insulation layer 12, that is, disposed inwardly of the electrically-conductive resin layer 14, the drain wire may be wound around the outer periphery of the electrically-conductive resin layer 14 in so far as the former is in contact with the latter. Also, the drain wire may be embedded in the inner surface of the electrically-conductive resin layer 14. As shown in Figs. 7(b) and 7(c), it is preferred that a ribbon-like metal conductor of a flattened cross-section (hereinafter referred to as "flattened square conductor") be used as the drain wire 13. This flattened square conductor can be subjected to plating. The ratio of the width W to the thickness t of the flattened square conductor is preferably not less than 1, and more preferably not less than 10. Alternatively, a flattened braid formed by braiding narrow conductors into a ribbon-like configuration can be used. With respect to the relation between the cross-sectional area (S2) of the drain wire 13 and the cross-sectional area (S1) of the electrically-conductive resin layer 14, it is preferred that S1/S2 < 1500 be established. In so far as this requirement is satisfied, either a single wire or a plurality of wires can be used. In the case of the plurality of wires, the wires can be wound in parallel to each other, or in intersecting relation. The electrically-conductive resin layer 14 is made of an electrically-conductive resin having a volume resistivity of not more than 10 4 Ω·cm. The compositions of a matrix, an electrical conductivity-imparting material and the other additives of this electrically-conductive resin are not particularly limited. For example, as the matrix, there can be used a thermoplastic resin such as PE, PP, EVA and PVC, a thermosetting resin such as an epoxy or a phenolic resin, rubber such as silicone rubber, EPDM, CR and fluororubber, or a styrene-type or an olefin-type thermoplastic elastomer or ultraviolet curing resin. Vapor phase-growing carbon fiber and graphitized carbon fiber are combined, as the electrical conductivity-imparting material, with the matrix to produce the electrically-conductive resin having a desired volume resistivity. Additives such as a process aid, a filler and a reinforcing agent can be added. In the shield cable with the drain wire according to the present invention, the drain wire is wound on the inner or the outer surface of the electrically-conductive resin layer, and is disposed in contact therewith. Anisotropy due to the shield effect is overcome. Despite the fact that there is used the electrically-conductive resin layer having a volume resistivity of 10 4 to 10 -2 Ω·cm, excellent shield characteristics can be obtained, and as compared with the conventional metal braid and the metal foil, the cable can be lightweight and be produced at lower costs, and deterioration due to corrosion is eliminated, thereby enhancing the durability and reliability. Further, by the use of the flattened drain wire, the diameter of the shield cable can be reduced, and by spirally winding the drain wire, excellent shield effects can be obtained up to a high-frequency region. Example 3 A flattened square conductor, composed of a copper conductor (1.5 mm x 0.1 mm) subjected to plating (tinning: 1 µm thickness), was spirally wound at a rate of ten turns per meter on a wire (outer diameter: 1.1 mm) composed of a copper conductor (whose cross-sectional area was 0.3 mm 2 ) coated with PVC. Then, an electrically-conductive resin (volume resistivity: 10° Ω·cm), containing a vapor phase-growing carbon fiber as an electrical conductivity-imparting material, was coated thereon to form thereon an electrically-conductive resin layer having a thickness of 0.5 mm. Then, a covering insulation layer was provided on the electrically-conductive resin layer to prepare a shield cable with the drain wire. This shield cable was placed in an eccentric manner in a copper pipe 116 (inner diameter: 10 mm; length: 100 cm) of a measuring device D of Fig. 8, and the anisotropy of the shield effect was confirmed. In Fig. 8, reference numeral 17 denote a FET probe, and reference numeral 18 denotes a spectrum analyzer. Referring to the measuring method, the induced voltage (Vo) induced in the cable when applying an electric field to the copper pipe was measured, and then the induced voltage (Vm) induced in the cable when connecting the drain wire to the ground was measured. The initial attenuation amount at each frequency was determined by the following formula: S = 20 log Vo Vm where S represents the shield effect, Vo represents the initial induced voltage, and Vm represents the induced voltage after the shielding. The measurement results are indicated by a curve e in Fig. 10. Comparative Examples 3 to 5 A copper conductor (drain wire) having a cross-sectional area of 0.3 mm 2 was extended along and parallel to a wire (outer diameter: 1.1 mm) composed of a copper conductor (whose cross-sectional area was 0.3 mm 2 ) coated with PVC (see Fig. 11). Then, an electrically-conductive resin (volume resistivity: 10 0 Ω·cm), containing a vapor phase-growing carbon fiber as an electrical conductivity-imparting material, was coated thereon to form thereon an electrically-conductive resin layer having a thickness of 0.5 mm. Then, a covering insulation layer was provided on the electrically-conductive resin layer to prepare a shield cable C' with the drain wire. The shield wire C' was placed at the bottom of the copper pipe 116 with the drain wire 103 being eccentric to the lower side (Comparative Example 3) as shown in Fig. 9(a). Also, the shield wire C' was placed at the bottom of the copper pipe 116 with the drain wire 103 being eccentric to the upper side (Comparative Example 4) as shown in Fig. 9(b). In the same manner as described above for Example 3, the anisotropy of the shield effect was measured. The results thereof are indicated by curves f and g in Fig. 10. Also, there was prepared a cable with a drain wire of Comparative Example 5 which differed from the cable of Example 3 in that instead of the electrically-conductive resin having a volume resistivity of 10 0 Ω·cm, an electrically-conductive resin having a volume resistivity of 10 5 Ω·cm was used. The measurement results of this cable was indicated by a curve h in Fig. 10. As is clear from Fig. 10, the anisotropy was recognized in the curves f and g representing the cables each having the parallel drain wire; however, the anisotropy was not recognized in the curve e (Example 3) representing the cable having the spirally-wound drain wire, and the cable represented by the curve e exhibited far better shield effect at high frequency than the cable represented by the curve h . As described above, the shield cable with the drain wire according to the present invention does not exhibit anisotropy, and has excellent shield effect up to high-frequency regions, and with the use of the flattened drain wire, the diameter of the cable can be reduced. Further, since the electrically-conductive resin having a volume resistivity of 10 -3 to 10 4 Ω·cm is used as the shield layer, excellent processability can be achieved, and the lightweight and compact design can be achieved, and the shield effect generally equal to that achieved by a metal braid can be achieved. By the addition of the drain wire and the shield layer, an easy earth connection can be made in addition to the electromagnetic wave shield effect.

Claims: A shield wire comprising: a conductor (11); an insulation layer (12) provided around an outer periphery of said conductor (11); an electrically-conductive resin layer (14) provided around an outer periphery of said insulation layer; a covering insulation layer (15) formed around an outer periphery of said electrically-conductive resin layer (14); and shield means (13) for shielding said shield wire from electromagnetic interference, said shield means (13) formed to electrically contact said electrically-conductive resin layer, and said shield means (13) including a drain wire (13) spirally wound along the length of the shield wire, characterized in that said electrically-conductive resin layer (14) includes vapor phase-growing carbon fiber and graphitized carbon fiber made of said vapor phase-growing carbon fiber, said electrically-conductive resin layer (14) having a volume resistivity of 10 -3 to 10 5 ohm-cm and said drain wire being a metal conductor of flat cross-sectional shape with a ratio of width to thickness of said drain wire being not less than 1. A shield wire as claimed in claim 1, wherein said drain wire (13) is turned at least twice per meter. A shield wire as claimed in claim 1 or 2, wherein said drain wire (13) is spirally wound around said outer periphery of said electrically-conductive resin layer (14). A shield wire as claimed in claim 1 or 2, wherein said drain wire (13) is spirally disposed inwardly of said electrically-conductive resin layer (14). A shield wire as claimed in claim 1 or 2, wherein said drain wire (13) is spirally embedded in the inner surface of said electrically-conductive resin layer (14). A shield wire as claimed in any of claims 1 to 5, wherein the ratio of the cross-sectional area of said electrically-conductive resin layer (14) to that of said drain wire (13) is met to express the following condition: S 1 /S 2 < 1500 where S 1 : cross-sectional area of said electrically-conductive resin layer (14); S 2 : cross-sectional areas of said shield means (13). A shield wire as claimed in any of claims 1 to 6, wherein said drain wire (13) is formed as a flattened braid consisting of braided narrow metal conductors.

Inventors: IKEGAYA AKIRA, KATSUMATA MAKOTO, USHIJIMA HITOSHI, YAMANASHI HIDENORI, IKEGAYA, AKIRA, KATSUMATA, MAKOTO, USHIJIMA, HITOSHI, YAMANASHI, HIDENORI, IKEGAYA, AKIRA, C/O YAZAKI PARTS CO., LTD., KATSUMATA, MAKOTO, C/O YAZAKI PARTS CO., LTD., USHIJIMA, HITOSHI, C/O YAZAKI PARTS CO., LTD., YAMANASHI, HIDENORI, C/O YAZAKI PARTS CO., LTD.
Applicants: YAZAKI CORP, YAZAKI CORPORATION
Application year: 1991
Main classification: H01B11
IPC codes: H01B7, H01B11