US2308274A - Armored cable - Google Patents

Description

Jan. 12, 1943. o. A. FREDEFelcKsoN 2,308,274

ARMORED CABLE Filed Aug. 8, 1959 QE-ff,

INVENTOR Z 0 /rfede/zb/nsolz 2M ATTO RN EYS 'Pnented Jan. 12, 1943 ARMORED CABLE Otto A. Frederickson, Glen Ridge, N. J., assignor to National Electric Products Corporation Application August 8, 1939, Serial No. 288,913

4 Claims.

This invention has to do with the provision of a -ground return path for current in an armored cable.

It is common practice to ground the armor of an armored cable but it is recognized that the armor itself does not aiord a conductive path that can be relied upon to carry the current to ground without excessive heating upon breakdown of the insulation surrounding the conductors. The spiral turns of the armor must have some freedom of movement in order to give the cable its requisite flexibility, and this in itself involves some impairment of the conductive path. When installed, the resistance of the conductive path through the armor often increases owing to the accumulation of dust or other resistive material in the turns. It has been proposed to introduce into the cable a separate wire serving as a ground return, either in contact with the inner face of the armor or within an insulating jacket surrounding the main conductors. While satisfactory for providing a ground return, such expedients admit of abuse in that the wire intended as a ground return is sometimes connected as a current carrying conductor in the main circuit, for instance, to make a two-conductor cable serve the purpose oi a three-conductor cable. 'I'his creates a hazard and defeats the purpose of the ground return wire.

The object of the present invention is to pro vide an improved ground return, easily incorporated in the cable structure during the usual manufacturing operations and of adequate and assured conductivity and free from the hazards and abuses that are inherent in former constructions.

A preferred form of the present invention is shown in the annexed drawing, in which Figure 1 shows an armored cable with parts broken away to disclose the interior conductors and the ground return, and

Figure 2 is a similar view showing how a bushing is inserted at a point where the cable is connected.

The cable shown has two main conductors I (the number being immaterial) each of which has rubber insulation il surrounded by a protecting jacket I2 which may be saturated and coated with suitable compounds to give moisture resistance or flame resistance or both and, if desired, may be coated with a illm of wax in accordance with my Patent 1,635,829. The spiral metallic armor i3 surrounds the pair of conductors in the usual way. Between this armor and the conductors is the ground return of the present invention and with it may be associated a paper or other suitablejacket or ller.

I prefer to surround the pair of conductors with a flexible non-conductive jacket, commonly in the form of a spirally wound paper wrapping il. For this I may employ any ksuitable arrangement, such as paper folded to a double thickness, as shown, giving a iiat strip which is wound about the conductors with each turn over-lapping the preceding turn to give complete coverage. Or I may employ crumpled paper as shown in my Patent 1,828,772. Paper wrappings, whether spiral or longitudinal, are at present in general use in flexible armored cable, having quite generally displaced the former braided fabric Jacket; but it is within the contemplation of this invention dealing with the ground return to use any of these different forms of non-conductive jacket.

The ground return I5 consists oi a strip of metal foil, such as aluminum or copper foil, which is similarly Wound with its turns overlapped` Such a Jacket, formed of a spirally wrapped strip of metal foil, has a resistance of only about a half ohm per hundred feet of cable. This furnishes a much greater conductivity than the armor alone, which can be made with difiiculty to the present standard of an ohm and a half for the same length.

Preferably the strip of metal foil is wound on immediately after the paper wrapping in a single operation employing wrapping machines of the type commonly used for applying paper wrapping. The flexible metallic armor is applied immediately after application of the strip of metal foil.

When the cable is to be connected to the terminals of an outlet, or elsewhere, the armor is removed in the usual way for a short distance back from the point of connection. The paper and metal foil thus exposed are then unwound back to the armor and are broken oi at a point I8 within the armor a short distance in from its end. This is done by drawing out the last few turns within the armor, as is now commonly done with paper alone (see my Patent 1,687,013) whereupon the paper and foil are torn or broken by giving the free end a sharp pull with the hand. The foil is fragile and lends itself well to this step. In fact, the paper and foil can be unwound and torn 01T together, the foil adding no appreciable resistance as compared with the paper alone. An insulating protective bushing i1, such for instance as those shown in my Patents 1,687,013, 1,801,549 and 1,829,612 is then inserted at the end or the armor inthe space made available by removal of the paper and foil. The fragility of the foil is such as to make it equally useful where the paper and foil are torn of! at the point where the armor is cut rather than at a point within the armor back of the point where it is cut.

It is possible to omit the paper wrapping and to apply the metal foil directly to the insulated conductors. The foil wrapping as a whole has considerable strength and serves to hold the conductors together against undesirable movement within the armor. The strip of paper and the strip of metal foil may be applied together, the foil overlying the paper, but this is less desirable than to apply them separately, one wrapping over the other, because when together the paper interrupts the continuous contact of the turns of metal foil with one another and thus de- ;reases the conductivity of the path afforded by the foil. A In its broadest aspect, the invention is in the provision of a ground return in the form of a fragile metal strip, whether wrapped spirally or otherwise applied, of high conductivity but easily torn by hand at a free end to afford entry of the bushing in the manner described. The fact that the metal strip must be torn away to permit the bushing to be inserted, and the fact that a metal strip of this sort can not be attached effectively to a binding post or other point of electrical connection, make it practically impossible to use the ground return as a main conductor, as may be done when a wire is provided for the ground return. Nevertheless an adequate ground return path is provided.

Modifications of the preferred form illustrated are possible, embodyinl the inventive idea in its several aspects as denned in the following claims.

I claim:

1. A ilexible armored cable having one or more insulated conductors, a flexible non-conductive Jacket for said conductors, an outer spirally wound metal armor having its turns interlocked to permit bending, 'and a Jacket of metal foil in contact with the inside of said armor to form a ground return path oi' aured conductivity.

2. A exible armored cable having one or more insulated conductors, a nexible non-conductive Jacket for said conductors, an outer spirally wound metal armor having its turns interlocked to permit bending, and a spirally wound strip ot metal foil in contact with the inside of said armor to form a ground return of assured conductivity.

3. A iiexible armored cable having one or more insulated conductors, a exible non-conductive jacket for said conductors, an outer spirally wound metal armor having its turns interlocked to permit bending, and a Jacket of metal foil in contact with the inside o1' said armor to form a ground return of assured conductivity, said jacket being of such fragility as to permit its being torn by hand in making a connection.

4. A ilexible armored cable having one -or more insulated conductors. a exible non-conductive jacket for said conductors, an outer spirally wound metal armor having its turns interlocked to permit bending, and a jacket of spirally wrapped thin metallic strip in contact with the inside of said armor to form a ground return of assured conductivity, said strip being of such fragility as to permit its being torn by hand in making a connection.

O'I'IO A. FREDERICKSON.

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