Bridging Tape Over Lap Seam Cable Shield

This invention is an improvement on the cable shown in U.S. Pat. No. 3,651,244, issued Mar. 21, 1972. The improvement relates particularly to the type of cable shown in FIG. 4 of that patent where a bridging element is used between the lap seam of the metal shield and the inside surface of an outer jacket.

In power cables having metal shields, it is advantageous to make the metal shield by longitudinally folding a metal tape around the core of the cable and leaving the seam of the shield as a lap seam with the edges free to move over one another circumferentially to accommodate expansion and contraction of the cable core with change in temperature. Power transmission cables are subject to substantial changes in temperature as the result of variations in the load on the cable from peak demand to slack loads when demand for electricity is low. In the usual construction, there is a plastic outer jacket extruded over the metal shield.

If the outer jacket adheres to the outside surface of the metal shield around the entire circumference of the cable, the stretching of the outer jacket, with increase in the diameter of the cable core by rising temperature, is concentrated at the region of the seam lap. In order to avoid excessive stretching of the outer jacket adjacent to the seam lap of the metal shield, bridging tapes which extend circumferentially in both directions beyond the outer lap of the seam, have been used. The outer extruded plastic jacket does not adhere to the bridging tape or the bridging tape is made of elastic material that stretches as the cable expands, and thus the stretching of the outer jacket is distributed circumferentially over the full width of the bridging tape.

This invention uses a bridging tape which is made of elastomeric material and which has a cross-section of different shape than those previously used; and these changes combine to provide a distribution of the stretch and a more streamline structure at the seam. The bridging element of this invention is made of relatively soft material which is thicker at the region of the outer lap of the metallic shield and with the thickness of the bridging element decreasing toward its longitudinal edges and preferably to a substantially feather edge.

Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.

In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views:

FIG. 1 is a diagrammatic, exploded view showing a bridging tape extending across a lap seam and disposed between the lap seam and an outer plastic jacket; and

FIG. 2 is a cross-section through a power cable with a corrugated shield and with a bridging element made in accordance with this invention.

The drawing shows a power cable 10 with a core 12. The core 12 is surrounded by a metal shield 14 which is a tape, usually made of aluminum or copper, folded longitudinally around the core 12. The shield 14 has a lap seam 16 where the square longitudinal edges of the shield are free to move circumferentially in accordance with increases and decreases in the diameter of the core resulting from change of temperature. The temperature changes in power cables are quite substantial because of the variations in the amount of current that the cables carry at various times.

The cable 10 has a outer jacket 18 extruded over the metal shield 14. The jacket is preferably made of polyethylene or polyvinyl chloride, or any of the other usual plastics used for outer jackets of electrial cables.

Because of the large amount of plastic material in the cable core, and the substantially higher coefficient of expansion of plastic, as compared with metal, with change of temperature, the edge portions of the metal at the lap seam 16 move over one another in a circumferential direction to accommodate the expansion of the core with rise in temperature. With the change in the circumference of the metal shield concentrated at the lap seam, substantial temperature rises would produce excessive stretching of the outer jacket 18, in the region of the lap seam, if the adherence of the outer jacket 18 to the metal shield was continuous around the entire circumference of the metal shield.

In order to distribute the stretching of the outer jacket 18 over a substantial angular extent of the metal jacket, a bridging element or "tape" 20 is placed over the metal shield 14 and extends circumferentially in both directions beyond the lap seam. This tape 20 is made of material that stretches to accommodate the circumferential movement at the lap seam of the metal shield over an angle equal to the circumferential extend of the tape 20.

The bridging element 20 may be adhered to the metal shield 14 or to the jacket 18 or to both. In the latter case there is no sliding movement, but the elastomeric material of the element 20 accommodates the changes of circumference by stretching within its elastic limit.

FIG. 1 shows the bridging tape 20 located between the metal shield 14 and the outer jacket 18. FIG. 1 is an exploded view of the cable construction with the curvature of the metal shield and outer jacket eliminated and with the parts spaced fron one another to illustrate the shape of the bridging tape 20 before it is subjected to pressure between the metal shield 14 and the outer jacket 18.

A high voltage power cable expands in diameter approximately 6.5% as the result of temperature increase when carrying normal current at the high end of its load cycle. The cable should be capable of expanding approximately 11% for emergency overloads. The overlap of the edges of the metal shield should be a little more than three times the increase in diameter to permit the change in circumference and still maintain the overlap of the edge portions of the metal shield 14.

FIG. 2 shows the bridging element of the present invention after it is compressed between the shield 14 and jacket 18. The cable core 12, for a typical power cable, is shown with a stranded conductor 30 surrounded by an extruded, semi-conducting plastic conductor shield 32. Insulation, preferably polyethylene or other conventional insulating material, is extruded over the conductor shield 32 and is indicated in FIG. 2 by the reference character 34.

An insulation shield 36 surrounds the outside of the insulation 34 and is an extruded layer of semi-conducting plastic material, such as polyethylene mixed with carbon black, in accordance with conventional practice. The metal shield 14 is corrugated in accordance with the conventional practice on large cables for greater flexibility.

The bridging tape 20 covers the lap seam 16 and extends for a substantial angular distance on both sides of the longitudinal lap seam 16. The bridging tape 20 is an elastomeric, resilient material, and because of its shape and resilience, it eliminates abrupt bulges of the extruded outer jacket 18. The bridging tape 20 has sufficient thickness at its center region to mask the sharp edge of the outer lap of the seam 16. Any indentation of the outer jacket 28 by the outer edge of the lap seam 16, acting through the bridging tape 20 is insignificant.

Beyond the center region of the bridging tape 20, the thickness of the bridging tape decreases toward both edges, and preferably decreases to substantially a "feather edge." This avoids any localized bulging of the outer jacket 18 at the longitudinal edges of the bridging tape 20. This freedom from abrupt bulges results from a combination of the softer material of which the bridging tape 20 is constructed, and also from the fact that the bridging tape 20 has a cross-section that tapers to the thin or feather edge at both ends.

The outer jacket 18, which is extruded over the bridging tape 20, may or may not adhere to the briding tape, depending upon the material of which the bridging tape is made. Silicone rubber is the preferred material for the bridging tape, but other rubber-like materials which are elastomeric and resilient can be used. Even if the outer jacket 18 adheres to the bridging tape 20, the outer jacket 18 can stretch circumferentially across the full width of the bridging tape because of the elastomeric characteristic of the bridging tape. As the cable core expands with heat, and the edge portions of the lap seam move circumferentially with heat, and the edge portions of the lap seam move circumferentially over one another to increase the circumference of the metal shield, the outer jacket 18 can stretch and stretch the bridging tape with it, if they are adhered to one another.

Other elastomeric resilient material of which the bridging tape can be made are ethylene-propylene copolymer and ethylene-propylene terpolymer.

The preferred embodiment of the invention has been illustrated and described, but changes and modifications can be made and some features can be used in other combinations as defined in the claims.