Interlocked metal-clad cable

Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition

Reexamination Certificate

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Details

C174S1020SP, C174S1130AS

Reexamination Certificate

active

06486395

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a Metal-Clad Cable which has an interlocked metal tape armor as part of a metallic sheath assembly which provides a low impedance and a reliable ground path and functions as an equipment grounding conductor. The present invention has a conductor assembly containing at least two electrically insulated conductors, where the at least two electrically insulated conductors are cabled together longitudinally into a bundle and enclosed within an overall cover, preferably nonmetallic. A bare bonding/grounding conductor is cabled externally over the overall cover of the conductor assembly with the same lay and in concert with the conductors cabled within the conductor assembly. Metal tape is helically applied to form an interlocked armor around the cabled conductor assembly and bare bonding/grounding conductor. The bare bonding/grounding conductor is generally a similar size to the insulated conductors present in the conductor assembly; however, as the gauge of the conductors increases, it is acceptable for the size difference between the bare bonding/grounding conductor and the insulated conductors to increase (collectively hereinafter, “preferably full size”). Fillers may then be cabled internally within the conductor assembly to fill in undesirable spaces in the conductor assembly. This sizing, along with the use of fillers where desired, enables the bare bonding/grounding conductor to be in intimate contact with the inner curves of the convolutions in the interlocked metal tape armor along a substantial length of the cable. The armor and bare bonding/grounding conductor act in concert to provide a metallic sheath assembly which has an ohmic resistance value equal to or lower than the ohmic resistance requirements necessary to qualify as an equipment grounding conductor. In addition, an electrically insulated grounding conductor may be cabled within the conductor assembly to provide a redundant grounding path for cable which is to be used in patient care areas of Health Care Facilities, in addition to other applications.
(b) Description of the Prior Art
Underwriter Laboratory Standard for Safety for Armored Cables UL 4 (hereinafter “AC Cable,” and hereinafter “UL 4”) provides construction guidelines and performance standards for interlocked steel or aluminum armored cables which contain two, three, or four electrically insulated conductors of size No. 14 AWG to 1 AWG. Cables covered by UL 4 are for use as type AC Cable in accordance with Section 333, and other applicable sections, of the National Electrical Code (hereinafter “NEC”).
NEC Section 333 provides minimum requirements for type AC Cable, including construction requiring a flexible metal tape armor and an internal bonding strip of copper or aluminum in intimate contact with the armor for its entire length, in accordance with NEC Section 333-19; electrically insulated conductors with an overall moisture-resistant and fire-retardant fibrous covering, in accordance with NEC Section 333-20; and an adequate equipment grounding path, in accordance with NEC Sections 333-21 and 250-2(d).
NEC Section 250-2(d) requires that the fault current path shall be permanent and electrically continuous, capable of safely carrying the maximum fault likely to be imposed upon it, and shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions.
Underwriter Laboratory Standard for Safety for Metal-Clad Cables UL 1569 (hereinafter “MC Cable” and hereinafter “UL 1569”) provides construction guidelines and performance standards for metal-clad cables which may or may not contain optical fiber members and which contain one or more electrically insulated conductors of size No. 18 AWG to 2000 kcmil. Cables may, but are not required to, contain fillers. If fillers are used, they must be cabled or laid straight with the electrically insulated conductors. Cables covered by UL 1569 are for use as type MC Cable in accordance with NEC Section 334, and other applicable sections of the NEC.
NEC Section 334 provides minimum requirements for type MC Cable, including construction requiring (1) an armor of continuous and close fitting interlocking metal tape, or (2) a smooth metallic sheath, or (3) a corrugated metallic sheath, in accordance with NEC Sections 334-1 and 334-22; solid or stranded copper conductors with a minimum conductor size of 18 AWG, or solid or stranded aluminum or copper-clad aluminum conductors with a minimum conductor size of 12 AWG, in accordance with NEC Section 334-20; and, an adequate equipment grounding path, in accordance with NEC Sections 334-23 and Article 250.
UL 1569 Tables 6.1 and 6.2 set forth the smallest acceptable sizes for copper, aluminum or copper-clad aluminum grounding conductors in cable containing 90 degree Celsius and 75 degree Celsius circuit conductors, respectively.
NEC Section 250-96 requires that the cable armor which serves as a grounding conductor must be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed thereon.
NEC Section 250-118 requires, in applicable part, that the equipment grounding conductor running with or enclosing the circuit conductors be one or more or a combination of the following: (1) a copper or other corrosion-resistant conductor which may be solid or stranded, and electrically insulated, covered, or bare; (9) armor of type AC cable as provided in NEC Section 333-21; or (11) the metallic sheath or the combined metallic sheath and grounding conductors of type MC cable.
NEC Article 517 sets forth the requirements for wiring of Health Care Facilities. NEC Section 517-13(a) and (b) set forth the requirements for wiring of patient care areas. Pursuant to 517-13, the grounding terminals of all receptacles and all noncurrent carrying conductive surfaces of fixed electric equipment likely to become energized that are subject to personal contact, operating at over 100 volts, must be grounded by an electrically insulated copper conductor. If type MC or AC cable is used, the outer metal armor or sheath of the cable must be an acceptable grounding return path and also must qualify as an equipment grounding return path in accordance with NEC Section 250-118.
Metal sheathed cables, such as AC Cable and MC Cable, are available in armor composed of steel or aluminum interlocked metal tape. The edges of the helically wrapped metal tape armor interlock to form a series of “S” shaped convolutions along the length of the cable. MC Cable is also available in sheaths made of smooth or corrugated metal. MC Cable having interlocked metal tape armor and AC Cable are similar in appearance but differ in internal construction, performance, permitted uses, and safety features.
AC Cable contains a conductor assembly composed of two, three, or four electrically insulated conductors which are individually enclosed in fibrous covers. The individually enclosed conductors are cabled into a bundle wherein the conductors are twisted longitudinally together with a left-handed lay in accordance with the lay requirements defined in Section 5.5 of UL 4. The conductor assembly may contain one or more grounding conductors. If a bare grounding conductor is present, it may be individually enclosed in a fibrous cover before being cabled with the other conductors or the cabling assembly must be collectively enclosed in a fibrous cover to prevent the bare conductor from contacting the armor. During the armoring process, a bonding strip is laid lengthwise along the conductor assembly. The bonding strip and the conductor assembly are fed together into an armoring machine, where interlocking metal tape is helically applied around the conductor assembly to form an armor around the cabled conductor assembly and bonding strip. The bonding strip is composed of a strip of a thin bare copper, aluminum, or copper-clad aluminum conductor which runs longitudinally along the cable in direct contact with and electrically parallel

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