METAL SCREEN FOR A POWER CABLE

Abstract: A metal screen for a power cable with a predetermined number of mutually twisted cores (1, 2, 3, 4). In accordance with the invention the screen is made up from a number of screening ribbons (8) corresponding to the number of cores, each ribbon including at least one elongate metal element (7), and extending along its associated, twisted core.

Description: METAL SCREEN FOR A POWER CABLE TECHNICAL FIELD The present invention relates to a metal screen for a power cable with a predetermined number of mutually twisted cable cores. BACKGROUND ART A power cable metal screen has the task of leading off the current passing through the insulation layers of the cable cores, and also to serve as a conductor for earth fault currents in operational disturbances such as dielec¬ tric breakdown in the cable insulation layers. Such a screen usually consists of a plurality of metal wires or ribbons along the cable surface, but may also be a solid metal tube, e g of lead. A metal screen, e g built up from copper wires, generally has considerably less conductive area than the main or phase conductors in the cable. There are standards prescribing that the screening conductor shall have a given density or largest average opening in respect of the cable circumference. The number of wires in the screen will thus be minimized and the wire diameter relatively small, usually about 1 mm. The problem with small diameter screen wires is that, due to insignificant stiffness and high friction against their surroundings, they can easily be deformed in conjunction with reeling the cable on or off a drum. The stresses will be even greater in the bending tests, which shall be performed according to requirements prescribed by standard, and in which the cable is bent in diametrically opposite directions. No wire breakage or other serious damage may arise in such a case. As uniform a substructure as possible, as well as a sufficiently small pitch of the normally spirally wound screening wires, are measures which are usually sufficient to satisfy prescribed requirements. The uniformity of a cable body, e g built up from three cable cores, may be improved with the aid of filler profiles between the cores and/or bandaging round the three cores. This measure requires extra material, however. Similarly, reduced pitch also requires more metal for standard requirements OMPI " prescribed for the conductivity of the screen to be satisfied. DISCLOSURE OF* INVENTION One object of the present invention is to provide a metal screen with which the risk of rupture is reduced simultaneously as good bending properties of the cable are .maintained. Another object is to provide a metal screen which is protected from corrosion. A third object is to provide a metal screen, which simplifies the manufacture of a screened power cable. This is achieved by the metal screen in accordance with the invention having been given the charac¬ terizing features disclosed in the claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described below while referring to the appended drawings, on which Figure 1 is a cross section of a power cable with a first embodiment of the metal screen in accordance with the invention, Figure 2 is a perspective view of a power cable with a second embodiment of the metal screen in accordance with the invention, Figure 3 is a cross section of a power cable with a third embodiment of the metal screen in accordance with the invention and Figure 4 is- a cross section of a power cable with a fourth embodiment of the metal screen in accordance with the invention. MODE FOR CARRYING OUT THE INVENTION A cross section of- a power cable with three cores is illustrated in Figure 1, each of the cores containing a conductor 1, e g of copper or aluminium, on which a semiconductive layer 2, e g of conductive plastics, has been extruded. An insulating layer 3 is extruded on said layer 2, and a second semiconductive layer 4, e g of conductive plastics, is extruded on the insulation layer 3. The cross section of each core is sector-shaped, and the three cores are laid in relation to each other such that their cylindrical backs face outwards to provide a substantially circular cross-sectional shape for the collected cores. Round the collected and twisted cores there is a first embodiment of a metal screen 5, in accordance with the invention, and on the screen 5 there is extruded a jacket 6, e g of plastics. V/IfO The screen 5 illustrated in Figure 1 includes a predetermined number of metal * wires 7, laid out in strip form on the back of the respective core and extending along the respective twisted core. So as to give the screen wires 7. a well defined location and to retain them in this position during movements and 5 bending of the cable, the wires 7 may be bedded into a layer of a material which is preferably electrically conductive and protects the wires against corrosion. The thickness of this layer exceeds by. some tenths of a millimeter the thickness of the wires. The layer in which the wires 7 are embedded can be applied by direct extrusion in the cable-making machine shortly after the 10 station where the cores and associated screening wires are simultaneously twisted and laid in their final position. The screening wires 7 will then be situated on the back of the respective core and at substantially uniform spacing. In a later production step, the plastics jacket 6 is extruded on the outside of the screen 5 with the screen wires 7. 15 A second embodiment of the metal screen in accordance with the invention is illustrated in Figure 2 for a cable of the same type as shown in Figure 1. The extrusion of the electrically conductive and corrosion-protecting layer for embedding- and fixing the screening wires 4, described -in conjunction with Figure 1, may have certain practical disadvantages, e g during changing drums, 20 wire rupture etc, if the extrusion is carried out in the same process as drawing the cable. In the embodiment illustrated in Figure 2 the cable includes three cores, as with the embodiment according to Figure 1, these cores being made up in the same way as those of Figure 1 with a conductor 1 surrounded, in . order, by a semiconductive layer 2, aa insulating layer 3 and a further 25 semiconductive layer 4. In the embodiment of the screen shown in Figure 2, the screen comprises a separate screening ribbon 8 for each core, i e three screening ribbons 8 in the present case. These screening ribbons 8 are * manufactured beforehand and, as with the embodiment according to Figure 1, include a predetermined number of metal wires 7 arranged in parallel and 30 baked into a ribbon of electrically conductive, corrosion-protecting material, e g an electrically conductive rubber or plastics material. When putting the three cores together into a cable the three screening ribbons 8 are applied on the back of the respective core, the cable body built up in this way then being wound with a band 9, after which the cable body, as in Figure 1 is provided 35 with a jacket 6, e g of plastics. In Figure 3 there is shown an embodiment of a cable with cores having circular cross section, as is the case with small cable dimensions. The cores shown in Figure 3 are otherwise built up in the same way as those in Figure 1 and Figure 2, i e with a conductor 1, in order surrounded by a semiconductive layer 2, an insulating layer 3 and a further semiconductive layer 4. To obtain a circular cross section of a cable having the cores illustrated in Figure 3, the intermediate spaces between the cores must be filled out. This can be attained, for example, by the type of screening ribbon 8 shown in Figure 2, which has been thickened out along its edges in the manner shown in Figure 3. The screening ribbons 8 shown in Figure 3 also include a predetermined number of screening wires 7, which have been embedded beforehand in a layer of material, e g conductive plastics or rubber. Instead of laying the screening wires 7 along the back of the respective " core as shown in Figure 1, and thereafter moulding the wires into a semiconductive material for fixing the wires in relation to the cores, the latter may be ' provided on their backs with longitudinal, parallel grooves 10 for accommodat¬ ing a predetermined number of mutually parallel wires 7 in a manner illustrated in Figure 4, the construction of the cores illustrated in Figure 4 otherwise corresponding to the construction of the cores in Figures 1 and 2, i e they include an inner conductor 1, in order surrounded by a semiconductive layer 2, an insulating layer 3 and an outer semiconductive layer 4, in which the grooves 10 are provided. No filling between the wires 7 is required in the embodiment illustrated in Figure 4, unless it is desired to protect the wires, e g against corrosion. In the embodiments of the screen in accordance with the invention which have been illustrated in Figures 1, 2 and 3, the screening wires 7 have namely been protected against corrosion thanks to being embedded in the semiconductive rubber or plastics material. • In order to fix the wires 7 against the respective core in the embodiment according to Figure 4, the cores with their respective screening wires 7 may be wound with a band or tape 9 before the jacket 6 is extruded onto the cable body. OMPI In bending a cable with several cores laid twisted, e g according to Figure 2, the cores glide relative each other in such a way that due to bending there occurs a deficiency in the core length on the outer surface of the cable (outer curve) which is compensated by a corresponding excess on its inner surface (inner curve). This means that a surface element on the individual core is neither stretched nor compressed as a result of the bending. This situation is utilized for the metal screen in accordance with the present invention, the metal elements of which, extending thus along their associated twisted core, are not subjected to any tension or compression forces when the cable is bent. Instead of using several mutually parallel metal wires, extending along their associated core, it is also possible to use a single metal ribbon per core, the width of this ribbon corresponding to the width of the back of the respective • core, and which in the same way as the screening wires extends along its associated twisted core. With this embodiment of the screen, the screen will also be able to function as a diffusion barrier against moisture in the surroundings.

Claims: C L A I M S 1 A metal screen for an electric power cable with a predetermined number of mutually twisted cores (1, 2, 3, 4), characterized in that the screen (5, 8) is made up from a number of screening ribbons corresponding to the number of cores, each ribbon comprising a pre-manufactured ribbon (8) of rubber or plastics material, which is extruded round a predetermined number of mutually substantially parallel metal wires (7), said screening ribbons each engaging against the outside of its respective twisted core such that the parallel metal wires in the respective ribbon extend parallel to the centre line of its associated core. 2 A screen as claimed in claim 1, characterized in that the rubber or plastics material is semiconductive. 3 A screen as claimed in claim 1, characterized in that the edges of each pre-manufactured ribbon are thickened.

Inventors: AXELSSON RUNE NILS, WRETEMARK SVEN GUNNAR, AXELSSON, RUNE, NILS, WRETEMARK, SVEN, GUNNAR
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1982
Main classification: H01B9
IPC codes: H01B9