Grounded Power Cable

Abstract

An economical parallel conductor grounded cable that can, if desire, be insulated in a single operation, has the grounding wire between the phase conductors but close to one flat surface of an extrusion of insulation. This extrusion is slotted from the other flat surface to a point close to the ground and is covered by a second, non-adhering, extrusion of uniform wall thickness.

Description

BACKGROUND OF THE INVENTION

Six-hundred volt cables include types that are recognized in the art and particularly by Underwriters' Laboratories as types NM, NMC, and UF which can be installed under many building codes without conduit for conveying power within buildings. Such cables, since they are not proctected by conduits must themselves have sufficient toughness and fire resistance not to constitute a hazard. At the same time they should be easy to install and connect and inexpensive enough to sell in a very competitive market. Cables of the type on which the present invention is an improvement have two parallel phase conductors, each with its own sheath of extruded insulation and a grounding conductor laid between them, parallel to the phase conductors. A tough extruded jacket envelopes the grounding and insulated phase conductors. U.S. Pat. No. 3,600,500 describes a cable of this type. Other types of parallel-conductor cables are exemplified by U.S. Pat. No. 1,977,787 which has two power conductors with separate D-shaped insulations laid parallel within an overall braid covering and having the inside flat surfaces of the D's grooved to hold a stranded copper neutral; U.S. Pat. No. 2,081,634 wherein the insulation on the center conductor is grooved to support two additional insulated conductors with a rubber belt overall, and U.S. Pat. No. 2,729,696 for an unjacketed "rip cord" in which the insulation is slotted at both sides of the center conductor.

SUMMARY

We have invented a cable of improved simplicity and economy of manufacture comprising first and second power conductors and a first polymeric extrusion of electrical insulating composition directly encasing these power conductors. The extrusion comprises two opposite flat surfaces and two opposite curvilinear surfaces with the power conductors symmetrically spaced from both the flat and curvilinear surfaces. A grounding conductor which is solid in the sense of consisting of a single wire rather than being stranded from a plurality of wires, and may be surrounded by a longitudinal tape separator, is enclosed in the first extrusion parallel to the power conductors and is centered between the curvilinear surfaces close to one of the flat surfaces. A narrow slot, defined by walls in the first extrusion, extends from the other flat surface to a point close to the grounding conductor. A second extrusion of substantially uniform wall thickness directly surrounds, but does not adhere to, the first extrusion. In a preferred embodiment the thickness, between flat surfaces, of our first extrusion is at least 150 mils and its thickness between the grounding conductor and both the slot and the closest flat surface is between five and 20 mils.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a section of a cable made to our invention.

FIG. 2 shows a section of another embodiment of a cable of our invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1 a cable indicated generally by the numeral 10 has parallel phase conductors 11, 12 symmetrically encased in an extrusion 13 of an insulating composition such as polyvinyl chloride or, for example, flame retardant polyethylene. The extrusion 13 is applied directly to the surface of the metal of the conductors 11, 12 which may be aluminum or copper and may be tinned. The conductors 11, 12 may be solid, or stranded of a plurality of wires in a known manner. The extrusion 13 has opposite parallel flat surfaces 16, 17 and opposite semicircular surfaces 18, 19. Being symmetrically encased in the extrusion 13 the conductors 11, 12 are centered between the surfaces 16, 17 and a wall thickness 21 separating the conductor 12 from the surface 18 substantially equals a wall thickness 22 of the conductor 11 from the surface 19. Since the conductors 11, 12 will carry current at operating voltages the wall thickness of the extrusion 13 at any point around them must be sufficient to withstand the voltage stress under operating conditions. For 600 volt service the thickness 21, 22 should thus be 30 mils, according to industry standards. A solid, i.e. unstranded, grounded conductor 23 is also encased in the extrusion 13, centered between the conductors 11, 12 but offset to be close to the wall 17. A wall thickness 24 of the extrusion 13 between the conductor 23 and the surface 17 should be as low as can be consistantly extruded commercially without gaps which is about 5 mils and no greater than 20 mils to tear easily for exposure of the conductor 23 at termination. A slot 26, defined by walls 27, 28 in the extrusion 13 extends from the surface 16, to a point close to the conductor 23 leaving a thickness 29 of extrusion between the slot 26 and the conductor 23 comparable to the thickness 24. Considerations governing the thickness 24, hereinabove stated, also govern the thickness 29. A second extrusion 31 of tough polymeric composition directly surrounds the extrusion 13. The extrusion 31 has a uniform thickness sufficient to provide mechanical protection to the extrusion 13. The extrusion 31, which should be abrasion resistant, may, in fact, comprise the same basic polymer, such as polyvinyl chloride, as the extrusion 31 or it may be compounded for greater tensile strength, moisture resistance and toughness in a known manner at some sacrifice of electrical properties. Although the extrusion 31 is applied directly over the extrusion 13 it is essential that it does not adhere to the latter. Non-adherence can be accomplished by sufficient chilling of the surfaces 16, 17, 18, 19 before the second extrusion is applied, and by application of liquid or powdered anti-adhesive agents of known types.

The thicknesses of insulating wall separating the grounding conductor 23 from the phase conductors 11,12 exceeds the thicknesses 21, 22 but if additional insulation is desired between the grounding and phase conductors a longitudinal separator tape 32 may be applied over the conductor 23 during the operation of extruding the extrusion 13. This tape 32 is shown in FIG. 2 with a slight overlap but it may be butt lapped or applied with a slight gap if the gap does not face one of the phase conductors. A polyester film tape, one to three mils thick, is preferred for the separator 32, but tapes of other film forming polymers or paper tapes can also be applied.

Our cable can be manufactured in two different operations, first extruding the extrusion 13, and, separately extruding the second extrusion 31. However, it is preferable to apply the extrusions in tandem in a single operation both for considerations of economical manufacture and for the protection of the first extrusion from separation at the thin points 24, 29.

A cable made with No. 12 Awg phase conductors as per FIG. 1 will exemplify suitable dimensions of cables made to our invention.

EXAMPLE

No. 12 Awg conductors 11, 12, solid aluminum, diameter 81 grounding conductor 23, solid aluminum, diameter 81 distance between centers of conductors 11, 12 220 greatest distance between surfaces 18, 19 361 distance between surfaces 16, 17 170 wall thickness of extrusion 31 30 thickness 24 15 thickness 29 8 width of slot 26, w 15

The distance between the flat surfaces 16, 17 of the cable of the Example with No. 12 Awg conductors was 170 mils. This distance should be varied, of course, with conductor size, but, in no case be less than 150 mils.

In making a splice or termination in our cable 10 a suitable length of the second extrusion 31 is cut away. Because of its freedom from adhesion this length can readily be removed from the underlying portion of the extrusion 13. The half of the extrusion 13 encasing the conductor 11 is manually pulled apart from the half encasing the conductor 12, tearing the thicknesses 24 and 29 to expose the conductor 23.

The foregoing description has been exemplary rather than definitive of our invention for which we desire an award of Letters Patent as defined in the appended claims.

Claims

We claim:

1. A cable comprising:

A. first and second parallel power conductors,

B. a first polymeric extrusion of electrical insulating composition directly encasing said power conductors, said first extrusion comprising two opposite flat surfaces and two opposite curvilinear surfaces, said power conductors being symmetrically spaced from said flat and said curvilinear surfaces,

C. a solid grounding conductor, parallel to said power conductors, said grounding conductor being enclosed in said first extrusion, and said grounding conductor being centered between said curvilinear surfaces and close to one of said flat surfaces,

D. walls defining a narrow slot in said first extrusion extending from the other of said flat surfaces to a point close to said grounding conductor,

E. a second extrusion of abrasion resistant polymeric composition directly surrounding said first extrusion, said second extrusion being of substantially uniform wall thickness and said second extrusion being free from adhesion to said first extrusion.

2. The cable of claim 1 wherein the thickness of said first extrusion between said flat surfaces is at least 150 mils and the thickness of said first extrusion at a point of one of said flat surfaces close to said grounding conductor is between five and 20 mils.

3. The cable of claim 1 wherein the thickness of said first extrusion between said flat surfaces is at least 150 mils and the thickness of said first extrusion between said slot and said grounding conductor is between five and 20 mils.

4. The cable of claim 2 wherein the thickness of said first extrusion between said flat surfaces is at least 150 mils and the thickness of said first extrusion between said slot and said grounding conductor is between five and 20 mils.

5. The cable of claim 1 comprising a longitudinal tape separator directly surrounding said grounding conductor.

6. The cable of claim 2 comprising a longitudinal tape separator directly surrounding said grounding conductor.

7. The cable of claim 3 comprising a longitudinal tape separator directly surrounding said grounding conductor.

8. The cable of claim 4 comprising a longitudinal tape separator directly surrounding said grounding conductor.