US2246124A - Armored cable construction - Google Patents

Description

June 17, 1941. (:ARLSQNv 2,246,124

ARMORED CABLE CONSTRUCTION Filed Aug. 23, 1938 Fig.2.

Inventor; George .Carlson,

by Hi 5 Attorneg.

Patented June 17 1941 UNITED STATES PATENT OFFICE AaMoItEn CABLE CONSTRUCTION York Application August 23, 1938, Serial No. 226,265

2 Claims. (Cl. I'M-109T This invention relates to an armored cable construction, and more particularly to a cable employing apaper wrap iri combination with a spiral metallic armor. The invention further relates to an improved form of bushing for use with the cable in protecting the severed end of the armor. Y

It is an object of the invention to provide an improved construction and arrangement in cable and bushing constructions of the type described, which possess improved electrical and mechani-, cal characteristics, and which may be manufactured at low cost.

In the accompanying drawing Fig. l is a view of an armored cable with the bushing in operative position; Fig. 2 is a sectional view through a cable employing two conductors showing the 'manner of applying the insulating paper wrap;

Fig. 3 is a sectional view through a. modification of the cable employing three conductors; Fig. l is a sectional view through another modification of the cable employing four conductors; Fig. 5 is a perspective view of the bushing for use with the cable, and Fig. 6 is a diagrammatic view of the twisted conductors and paper wrap.

Referring to thedrawing the numeral it] refers to the improved form of armored cable shown most clearly by Fig. 1. In the form of the invention illustrated by Figs. 1 and 2 the cable employs two insulated conductors having conducting wires ll, each being insulated with a rubber compound i2 and an overall saturated braid I 3. In the manufacture of the cable the individual insulated conductors are run through a twisting machine, together with a longitudinal strip of waxed paper M, commonly known to the trade as krait paper. Upon entering the twisting machine the individual conductors are twisted together about the longitudinal paper as an axis so that the paper is wrapped about the individual conductors in an S-iorm, as shown by Fig. 2. In this form a central portion of the paper wrap remains placed between each of the insulated conductors, as shown at it, while the edges I 5 of the paper wrap encircle the entire periphery of the insulated conductors and completely cover them. As shown diagrammatically by Fig. 6, the edges of the d-shaped paper wrap appear twisted as they cover the twisted conductors. Inasmuch as the strip of paper it is fed into the twisting machine longitudinally and the conductors are twisted about the paper wrap, it will be apparent that the. paper wrap constitutes in effect a longitudinal wrap running lengthwise of the cable and having a length equal to the length of the cable. It will be seen that with the improved construction just described a considerable saving in paper is eflected over those constructions in which the paper is wrapped spirally about the twisted conductors due to the fact that a single strip of paper is disposed longitudinally of the cable rather than bly in overlapping convolutions such as shown at l8. Due to the fact that the longitudinal wrap l4 closely grips the periphery of each of the insulated conductors a twisted assembly of conductors is formed having a minimum uniform diameter because only a single layer of the paper l4 surrounds the insulation on each of the conductors. This makes it possible for the armoring machine to apply an armor of minimum diameter evenly over the entire length of cable without variations in the tightness of armor on the conductor assembly. At the same time the edges of the paper wrap encircling the conductors adequately protect them from damage during the armoring operation. Inasmuch as the diameter of the armor is reduced to a minimum by the reduced overall diameter of the twisted conductors and paper wrap a substantial saving is effected in the quantity of metallic armor used.

It will be apparent from an examination of Fig. 2 that the layer of paper M is disposed at all times between the individual conductors. Should the conductors inadvertently become crushed together during the armoring operations, thereby damaging the insulation on the individual conductors, the layer of paper will provide an additional insulating medium between the conductors capable of withstanding approximately 600 volts. It has been found that the provision of this additional insulating means between the conductors reduces by a substantial amount the number of rejects to be had in the commercial lengths of cable, which in the case of cables employed with voltages of lid to 600 are tested by placing a potential of 1501) volts across the conductors and between each conductor and the armor.

The s-shaped paper wrap constitutes a proin cylindrical form over both the conductors.

By using the S-shaped wrap it is possible to manuiahture a cable which will withstand severe twisting or bending without injury to the inner insulating and protective layer of paper or to the insulated conductors themselves.

In using the improved cable, a severed end of the armor is easily stripped by pulling the armor longitudinally of the cable because the con-= volutions of armor slide easily over the plain longitudinally disposed paper strip. This construction makes it possible to strip the cable quickly thereby saving considerable time in installing lengths of the cable. It is also a simple matter to remove the excess length of paper over the stripped end of the cable simply by tearingit ed at the point at which the armor was severed. It will be seen that by using a longitudinal strip of paper twisted together with the conductors to S-shaped formation in the combination with a spiral'metallic armor that it is possible to secure an improved form of armored cable having the advantages above described. These advantages make for better electrical characteristics of the cable, a reduction in the cost oi manufacture, and a. greater simplicity and saving of time in the use of the cable by skilled electricians. While the invention has been described thus far in connection with a two conductor cable, it will be apparent that the invention may be applied to cables employing a plurality of conducdoctors I! and 20 together and about the single remaining conductor 2!. Cables having three conductors frequently have one of the conductors grounded or at a neutral potential. In such situations the longitudinal paper strip is disposed between the conductors of high potential to provide additional insulation; for example, .in the aaaaraa 1 As shown in Fig. 5 the bushing 26 is of substantially cylindrical iorm'and is slightly tapered and provided with a slot 21. The bushing may be made of insulating material such as sheet fiber. One edge of the bushing is bent inwardly to form a separating or dividing plate 28 which extends substantially across the diameter of the cylindrical bushing forming the compartments 29 and 3d. The bushing is provided with an overhanging head or edge 36.

In the use of the bushing it is grasped by the hand andinserted over the insulated conductors, which may pass through the compartments Z9 and 30;, and then pushed down firmly over the severed end of the armor. The bead 3i overlies the rough severed edge of the armor and forms a smooth surface upon which the insulated conductors may rest, while the wall of the bushing forms a liner for the end portion of the armor. The dividing plate 28 takes a position between the insulated conductprs and effectively insulates them from each other.

It frequently happens that a careless workman, in severing the spiral armor preparatory to stripping a length of the cable, will inadvertently cut through the layers of insulation upon the inner conductors. When this happens it is likely that steel slivers or filings incident to severing of the armor will be embedded within the insulation surrounding the individual conductors, and thus cause a short circuit to developbetween the individual conductors of the BX cable. In

those cases where the insulation on the individual conductors has been damaged by the severing of construction 01' Fig. '3 either of the conductors I! or 20 may constitutethe neutral with the high potential existing between the remaining conductor and the conductor 2|.

In the modification shown by Fig. 4 a cable is shown having four conductors 22, 23, 24 and 25. The conductors-are twisted in pairs about the longitudinal paper strip l4 and if there is any -diflereuce in potential between the pairs of conductors, the paper wrap is placed between those pairs subject to the greatest diflerence in potbntial.

v To protect the severed end of the armor from damaging the insulation on the individual conductors, an improved form of bushin s placed over the end of the armor.

the metallic armor, the dividing plate prevents any possibility of a short circuit occurring between the ccnductcrs at this point. The dividing plate provides an effective continuation of that portion oi the. paper strip which passes between the individual conductors and thus forms in effect a complementary extension of'the shaped paper wrap at that point where the paper 'wrap has been torn off in preparing the cable for wiring.

What I claim as new and desire to secure by Letters Patent oi the United States is:

1. An armored cable comprising a plurality of insulated electric conductors, a longitudinal strip of insulating material, said insulated conductors being twisted together around said material to form an S-shaped wrap closely engaging the outer peripheries oi said conductors and having a central portion positioned between said conductors and a helical metallic armor closely engaging and surrounding said S-shaped wrap and in-

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