US 3230300 A
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18, 1966 s. RUBINSTEIN ETAL 3,230,300
ALUMINUM SHEATHED BUILDING WIRE Filed Oct. 29, 1965 F45W/C CORE MP5 OPEA/ \S /PIQL ST 55L 72/ 5 INVENTOR 2:4 am QM BY @Iu Mu ML 3 AT TORNEYS United States Patent 3,230,300 ALUMINUM SHEATHED BUILDING WIRE Solomon Rubinstein, Fanwood, and George S. Eager, Jr.,
Upper Montclair, N.J., assignors to General Cable Corporation, New York, N.Y., a corporation of New Jersey Filed Oct. 29, 1963, Ser. No. 319,822 6 Claims. (Cl. 174-106) This application is a continuation-impart of our application Serial No. 61,513, filed October 10, 1960, now abandoned.
This invention relates to electrical conductors of types known as building Wire. The term building wire as use-d in the industry applies to the various types of single and multiple conductor constructions which are within the jurisdiction of the National Electrical Code. More particularly, this invention relates to an improved construction of aluminum sheathed building wire.
Building wires having conductors arranged in a central core with an overlying tightly fitting aluminum sheath are known to the art. The aluminum sheath may be extruded as a tube and drawn down over the cable core, or it may be extruded directly thereon, for example in accordance with the disclosure set forth in the C. E. Howard Patent No. 3,031,523, April 24, 1962. Aluminum sheaths extruded directly onto the core are particularly advantageous in building wire use because of the flexibility of the extruded sheath as compared to that of a sheath which has been Work-hardened by being drawn down.
Unfortunately, in some instances, any ground which may occur between a conductor and the aluminum sheath, particularly under conditions of a high resistance load, tends to generate an are which travels lengthwise along the cable. This are and the sparks emitted from the cable during such arcing create a fire hazard which is extremely dangerous, particularly in building wire.
An arc may be formed between a cable conductor and the metal sheath if the cable is damaged, for example if it is crushed. The distance between conductor and sheath may vary along the cable, particularly in the damaged section, and the insulation and valley filler material interposed between conductor and sheath result in an irregular arc, different from that produced in a welding operation. If the cable has a lead sheath the are probably will not persist for more than a few seconds, because the melting point of the lead is low and the lead melts rapidly and drips away, making it practicially impossible for the arc to be maintained. In cable sheathed with interlocking helical steel, armor commonly known as BX cable, the melting point of the steel is much higher than that of the current carrying conductor, and the electrical resistance i relatively high, and an are between conductor and armor usually burns out in a matter of a few seconds. An arc established between a conductor and the aluminum sheath may travel along the cable throwing oif sparks reminiscent of a 4th of July sparkler. Just why this is so is not fully understood. Obviously, the fire hazard is substantial.
It is, therefore, the primary object of this invention to provide an improved aluminum sheathed building wire construction in which an are caused by a short circuit between conductor and sheath will be quickly extinguished and will not travel for any substantial distance along the length of the cable.
Applicants have discovered that if the cable core has applied thereover in open helical lay a tape of metal of substantially higher metling point than the aluminum sheath, and relatively high electrical resistance, an arc formed between the conductor and the sheath will be extinguished in a matter of a few seconds, and before the arc has been able to travel along the cable more than a few inches, at most. This metal tape must be made 'of flexible material and it must be applied with an open helical lay, preferably with a coverage of about onehalf, or up to about two-thirds, so as not to make the cable too stiff for convenient installation procedures. The tape must be fairly inexpensive, or the cost of the cable will be so high as to limit or prevent its use. The tape must be sufiiciently thick so that it will not be melted or de'stroped so rapidly as to permit an arc between the conductor and the aluminum sheath to travel along the cable. The smallest size of conductor approved for building wire is size 14, and applicants have found that the tape should have a minimum thickness of about 10 mils. Preferably the electrical resistance of the tape should be higher than that of the aluminum sheath and its resistance longitudinally of the cable is further increased by reason of the fact that the tape is applied in an open helix.
This invention may be more easily understood by reference to the following description taken in combination with the accompanying drawing which is a partially sectioned view of an illustrative building wire constructed in accordance with the present invention.
In the figure there is shown a building wire comprising a core of conductors 10, each covered by a conventional insulation 12, and protective covering 13. The conductors are cabled together with fibrous fillers 14 which fill out the core to round shape, all in accordance with conventional practice.
A fabric core tape 16 is applied over the core in an overlying helix with overlapping adjacent turns; A steel tape 18 is wrapped over the core tape in an axially extending helix having adjacent turns spaced from each other. Applicants have found that for building wire having size 14 conductors a tape of 10 mils thickness made from SAE 1020 steel and applied in a helix with about one-half coverage is eifective to extinguish an arc in short order. SAE 1020 steel is a typical low carbon steel having a melting point of approximately 2760 F., and an electrical resistivity at 32 F. of 60 ohms/cit. mil ft. The composition of SAE 1020 steel is approximately 0.45% manganese, 0.25% silicon, 0.20% carbon, and the balance iron. An aluminum sheath 20 is applied over the steel tape. The aluminum sheath may be directly extruded on the cable, or it may be preformed in a tube and worked down over the cable. For building wire applications, direct extrusion is preferred since the sheath is then soft and not work-hardened, thus permitting maximum flexibility in the cable.
The open spiral form of the steel tape helix allows required cable flexibility during handling and installation. It has been found advisable to provide about one-half to two-thirds coverage of the cable by the overlying steel helix as the best compromise between arc extinguishing and cable flexibility. However, the exact amount of coverage is not critical.
The steel tape, because it has a higher melting point than the aluminum sheath, tends to extinguish any are which may develop between the conductor and the sheath to preclude the development of self-propagating arc traveling along the conductor length. As an alternate to a steel tape, other metallic tapes, such as nickel based alloys having a comparable melting point to that of steel, perform as well in extinguishing an arc. The melting point of the tape should be considerably higher than that of the aluminum sheath, and preferably approximately 2760 F.
The exact reason why such a tape applied in this manner disclosed in the present application extinguishes an are between a conductor and the aluminum sheath in a matter of a few seconds and a few inches of travel along the cable, at most, is not fully understood, but its effectiveness for this purpose has been demonstrated by tests.
In order to obtain test information on aluminum sheathed cable without tape and with various tape wraps over the core, the following cable samples were prepared:
2/C No. 14 AWG aluminum sheathed cable, SBR
insulation, .035" extruded aluminum sheath:
(a) no metal tape over core. (b) steel tape wrap over core. (c) copper tape wrap over core. (d) neoprene coated fabric wrap over core.
Each of these samples was tested in accordance with 'the following procedure:
The sheath and insulation from one conductor was removed for 1' inch on both ends of the sample. On one end the bared conductor was bent back so that it touched the sheath. The other-end of this conductor was connected to one terminal of a 110 volt D.C. source of power while the sheath was connected through a current limiting load and ammeter to the second terminal. The
current was adjusted for 25 amperes. By tapping the cable the momentary make and break of the conductor with-the sheath caused an arc to be struck. The duration of the arc and the length of the cable which was burned by the arc was noted.
The following results were obtained as an average of twenty tests on each sample:
Duration of Selfsustaining arc Travel of between current are along carrying conduocable in tors and aluminum inches sheath, seconds No core tape 120 12 Copper core tape 5inch x 5 mils open spiral 120 12 Neoprene Coated Fabric Tape (full coverage, 20 mils thick) 85 8 Mild steel core tape (M inch x 5 mils open spiral) 5 This invention may be variously embodied and modified within the scope of the subjoined claims.
What is claimed is:
1. An aluminum sheathed building wire construction which is highly resistant to propagation of an arc established by short circuit between conductor and sheath, said construction comprising insulated conductors arranged in a core, a low carbon steel tape having a minimum thickness of about 10 mils applied in an open helical wrap directly over the core, and an enclosing aluminum sheath, the steel tape being in contact with the aluminum sheath so as to extinguish any arc which may be formed between a conductor and the sheath.
2. An aluminum sheathed building wire construction which is highly resistant to propagation of an arc established by short circuit between conductor and sheath, said construction comprising a core which includes an insulated conductor, a metal tape having a thickness of at least about 10 mils, a melting point substantially above that of aluminum, and flexibilityv and, electrical resistance of the order of a low carbon steel tape of similar size, the saaid tape being applied in an open helical wrap directly over the core, and an enclosing aluminum sheath, the metal tape being in contact with the aluminum sheath so as to extinguish any are which may be formed between conductor and sheath.
3. An aluminum sheathed building wire construction according to claim 2 in which the metal tape is a low carbon steel tape.
4. An aluminum sheathed building wire construction according to claim 2 in which there is a fabric tape applied in an overlapping wrap over the core and under the open metal tape wrap.
5. An aluminum sheathed building wire construction according to claim 2 in which the metal tape covers about one-half of the core surface.-
6. An aluminum sheathed building wire construction according to claim 2 in which the metal tape covers about one-half to two-thirds of the core surface.
References Cited by the Examiner UNITED STATES PATENTS 1,935,323 11/1933 Kemp et al. 174107 2,591,794 4/1952 Ebel 174-106 X 2,603,684 7/1952 Holmes 174106 2,662,931 12/1953 Mougey 174106 ROBERT K. SCHAEFER, Primary Examiner. DARRELL L. CLAY, JOHN F. BURNS, Examiners. D. A. KETTLESTRINGS, Assistant Examiner.