Fire resistive cable system

Abstract

A fire-resistive cable system comprises an electrical cable housed in a fiberglass-reinforced thermosetting resin conduit. The electrical cable comprises a conductor and has only one couple of mica tapes surrounding the conductor. The couple of mica tapes are formed of a first mica tape and a second mica tape wound around the first mica tape. The mica layer of the first mica tape faces and contacts the mica layer of the second mica tape. The fiberglass-reinforced thermosetting resin conduit is made of a material comprising fibers of a glass selected from E-glass and E-CR-glass, and a resin.

Claims

1. A fire-resistive cable system comprising an electrical cable housed in a fiberglass-reinforced thermosetting resin conduit, wherein the electrical cable comprises a conductor and has one couple of mica tapes surrounding the conductor, the couple of mica tapes being formed of a first mica tape and a second mica tape wound around the first mica tape, each of the first and the second mica tape including a mica layer attached to a backing layer, and the mica layer of the first mica tape faces and contacts the mica layer of the second mica tape; and wherein the first fiberglass-reinforced thermosetting resin conduit is made of a material comprising fibers of a glass selected from E-glass and E-CR-glass, and a resin; and wherein the first mica tape is wound in a winding direction that is opposite to a winding direction of the second mica tape.

2. Fire-resistive system of claim 1, wherein the electrical cable further comprises at least one insulation layer surrounding the couple of mica tapes.

3. Fire-resistive system of claim 2, wherein the electrical cable further comprises a first insulation layer and a second insulation layer.

4. Fire-resistive system of claim 3, wherein the first insulation layer is formed of a silicone-based compound.

5. Fire-resistive system of claim 4, wherein the silicone-based compound includes a silicone-based rubber forming a matrix with a flame-retardant mineral filler incorporated into the matrix.

6. Fire-resistive system of claim 4, wherein the second insulation layer is made of a flame-retardant polymer.

7. Fire-resistive system of claim 1, wherein the resin of the conduit is a phenolic resin.

8. Fire-resistive cable system of claim 2, wherein the at least one insulation layer contacts and is applied directly onto the second mica tape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of an electrical cable, consistent with certain disclosed embodiments.

(2) FIG. 2 is a view of a fire-resistive cable system consistent with certain disclosed embodiments

DESCRIPTION OF THE EMBODIMENTS

(3) Reference will now be made in detail to the present exemplary embodiments, an example of which is illustrated in the accompanying drawing. The present disclosure, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

(4) Referring now to FIG. 1, an electrical cable 10 has a longitudinal axis 12. The electrical cable 10 includes, in order from the interior to the exterior, an electrical conductor 20, a couple of mica tapes 30, and one or more layers sequentially provided in radial external position with respect to the couple of mica tapes 30a. Such external layer(s) include a first insulation layer 40 and a second insulation layer 50. In some applications, an outer sheath (not illustrated) surrounding and, optionally, contacting the second insulating layer 50 can be present.

(5) The conductor 20 is made of an electrically conducting metal, preferably copper or copper alloy. Although shown in FIG. 1 as a single element, the conductor 20 may be either solid or made of stranded wires. For example, the conductor 20 may be 8 AWG (American wire gauge) (8.36 mm.sup.2) 7-strand compressed soft bare copper in accordance with the standards identified by ASTM International as ASTM B8 Class B concentric-lay-stranded copper conductors. The conductor 20 may also range in size from about 2 mm.sup.2 (14 AWG) to about 500 mm.sup.2 (1000 kcmil).

(6) The couple of mica tapes 30 is wound around the conductor 20. The couple of mica tapes 30 includes a first mica tape 32 and a second mica tape 34. The first mica tape 32 is disposed around the conductor 20 such that the first mica tape 32 contacts and is applied directly onto the conductor 20. The second mica tape 34 is disposed around the first mica tape 32 such that the second mica tape 34 contacts and is applied directly onto the first mica tape 32.

(7) Each of the first mica tape 32 and the second mica tape 34 are formed of a mica layer attached to a backing layer.

(8) The first mica tape 32 is wound onto the conductor 20 such that the backing layer of the first mica tape 32 faces and contacts the conductor 20, and the mica layer of the first mica tape 32 faces away from the conductor 20. Thus, the backing layer of the first mica tape 32 faces radially inward toward the axis 12 of the cable 10, and the mica layer of the first mica tape 32 faces radially outward away from the axis 12 of the cable 10.

(9) The second mica tape 34 is wound onto the first mica tape 32 such that the mica layer of the second mica tape 34 faces and contacts the mica layer of the first mica tape 32, and the backing layer of the second mica tape 34 faces away from the conductor 20 and the first mica tape 32. Thus, the mica layer of the second mica tape 34 faces radially inward toward the axis 12 of the cable 10, and the backing layer of the second mica tape 34 faces radially outward away from the axis 12 of the cable 10.

(10) In embodiments in which the conductor 20 is made of stranded wires, the first mica tape 32 is preferably wound in an opposite winding direction than the stranding direction of the conductor 20 wires. Advantageously, the second mica tape 34 is wound in a winding direction opposite to the winding direction of the first mica tape 32. The opposite winding direction of the first and second mica tapes 32 and 34 assists in keeping the torque on the conductor 20 minimized so that twisting of the conductor 20 during exposure to fire can be minimized.

(11) For example, the first mica tape 32 may have a right hand winding direction or lay (RHL), and the conductor 20 (or at least an outer layer of wires contained therein) and the second mica tape 34 may have a left hand winding direction or lay (LHL), or vice versa. This lay of the mica tapes minimizes the torsion effect due to the mica tapes winding.

(12) Alternatively, both the first mica tape 32 and the second mica tape 34 may have, for example, a RHL, and the conductor 20 may have a LHL. With this winding configuration, the first and second mica tapes 32 and 34 exert a joined torque resistance, opposed to the torsion due to the conductor 20 winding.

(13) The first mica tape 32 and the second mica tape 34 are wound at an angle of from 30° to 60°, preferably of about 45°. Further, the first mica tape 32 and the second mica tape 34 both have an overlap percentage (e.g., the percentage of the width of the mica tape overlapping onto itself during winding) such that no gaps in the winding of the mica tapes are formed both during manufacturing and deployment of the cable 10. The overlap percentage can be, for example, of 25%.

(14) The mica layer of one or more of the mica tape 32, 34 preferably have dimensions (thickness and width) such that the tapes can be applied around the conductor 20 minimizing wrinkles and folds as much as possible. Wrinkles and folds may potentially cause the mica tapes to be vulnerable to damage. For example, the mica layer of one or both of the mica tapes 32, 34 has a nominal thickness of 0.005 inches (0.127 mm) and a nominal width of approximately 0.5 inches (12.7 mm). The term “thickness” used herein refers to the dimension of the mica tape extending radially with respect to the axis 12 of the cable 10 when the mica tape is applied to the cable 10. The term “width” used herein refers to the dimension of the mica tape orthogonal to the thickness and to the application direction of the mica tape.

(15) The layers sequentially provided in radial external position with respect to the couple of mica tapes 30, e.g., the first insulation layer 40 and/or the second insulation layer 50, are preferably extruded onto the couple of mica tapes 30. The first insulation layer 40 and/or the second insulation layer 50 may be formed of compounds that emit less smoke and little or no halogen when exposed to high sources of heat, e.g., low smoke zero halogen (LS0H) compounds, and that have low toxicity flame retardant properties.

(16) In the embodiment shown in FIG. 1, the first insulation layer 40 surrounds the second mica tape 34 such that the first insulation layer 40 contacts and is applied directly onto the second mica tape 34. The first insulation layer 40 has a nominal thickness selected according to the requirement of national or international standards, generally on the basis of the conductor size. The thickness of the first insulation layer 40 may be, for example, at least 0.045 inches (1.143 mm).

(17) The first insulation layer 40 may be formed of a silicone-based compound, such as a silicone-based rubber. The silicone-based rubber may form a matrix incorporating at least one mineral flame-retardant filler, e.g., to protect the material of the first insulation layer 40 during manufacturing and installation of the cables within the conduit. The mineral fillers cab be incorporated into the silicone-based compound by using a bonding agent, such as silane, and the silicone-based compound may be cured using a cure catalyst, such as peroxide.

(18) At higher temperatures experienced during fire conditions, e.g., at temperatures of greater than or equal to approximately 600° C., the silicone-based compound may form silicon dioxide ash. At these higher temperatures, the silicon dioxide ash formed by the first insulation layer 40 and the mica tapes of the couple 30 may link and form a continuous eutectic mixture that serves as a dielectric for the cable 10 to allow the cable 10 to continue operating.

(19) Alternatively, the silicone-based compound may be a ceramifiable polymer that ceramifies at higher temperatures experienced during fire conditions, e.g., at temperatures of approximately 600° C. to 900° C. At these higher temperatures, the ceramifiable polymer change from a flexible rubber-like material to a more solid, ceramic-like material.

(20) The second insulation layer 50 surrounds the first insulation layer 40 such that the second insulation layer 50 contacts and is applied directly onto the first insulation layer 40. The second insulation layer 50 may have a nominal thickness as prescribed by the relevant national or international standards.

(21) The second insulation layer 50 may be formed of a thermoplastic polymer or of a thermosetting polymer. For example, the second insulation layer 50 may be formed of a polyolefin, an ethylene copolymer (e.g., ethylene-vinyl acetate (EVA) or linear low density ethylene (LLDPE)), and/or a mixture thereof. Examples of polymers or polymeric mixtures suitable for the second insulation layer 50 are described in U.S. Pat. Nos. 6,495,760, 6,552,112, 6,924,031, 8,097,809, EP0893801, and EP0893802.

(22) The polymer of the second insulation layer 50 is added with a non-halogen, inorganic flame retardant filler, such as magnesium hydroxide and/or aluminum hydroxide in an amount suitable to confer flame-retardant properties to the second insulation layer 50 (for example from 30 wt % to 70 wt % of inorganic flame retardant filler with respect to the total weight of the polymeric mixture).

(23) The cable 10 constructed as described above may be used in various conditions, such as the conditions specified for a Type RHW-2 cable in the National Electrical Code® (NEC®). The cable 10 may have a voltage rating of from 400 to 600 volts and may be fire rated at from 400 to 600 volts.

(24) One or more of the cables 10 may be deployed in a conduit 100 according to FIG. 2, where three cables 10 are illustrated. The cross-section of conduit 100 is circular, though other shapes can be envisaged.

(25) In the fire-resistive cable system, the fittings typically associated to the conduit are preferably made of a fiberglass-reinforced thermosetting resin, too.

(26) The conduit fill, i.e. the percentage of the hollow section of the conduit that is filled by the cable 10, may be up to 25% for 2-hour vertical rated cables and up to 35% for 2-hour horizontal rated cables, but it is understood that the conduit fill may also be less than these values. For a conduit including four of the cables 10 with 17% fill, the nominal diameter of the conduit may be approximately 1.5 inches (38.10 mm), the outer diameter of the conduit may be approximately 1.74 inches (44.20 mm), and the inner diameter of the conduit may be approximately 1.61 inches (40.89 mm). For a conduit including four size 8AWG cables 10 with 27% fill, the nominal diameter of the conduit may be approximately 1.0 inches (25.4 mm), the outer diameter of the conduit may be approximately 1.683 inches (42.75 mm), and the inner diameter of the conduit may be approximately 1.183 inches (30.05 mm). It is understood that the diameters may be greater than or less than these values.

(27) The cable is suitable for passing stringent fire resistive testing that challenges the capacity of the cable to carry current in the presence of fire and of water.

(28) While mica tape manufacturers may typically recommend that the mica surface of the mica tape face and/or be in contact with the conductor, the Applicant has found to the contrary that it is more effective for improving fire resistance to sandwich together the mica layers of two adjacent mica tapes. Sandwiching the mica layers could assure the integrity of the mica layers which, together with the deployment in a fiberglass-reinforced thermosetting resin conduit, allows the cable to resist higher temperatures, thereby improving the fire resistance of the cable, and therefore protecting the electrical performance of the electrical conductor.

(29) The system comprises a cable including one couple of mica tapes, and such a construction may be sufficient for various sizes of the cable to pass fire wall tests when tested both in vertical and in horizontal configuration, when the cable is deployed in a fiberglass-reinforced thermosetting resin conduit.

Example

(30) A number of cable/conductor systems according to the disclosure and comparative cable/conductor systems have the construction features according to Table 1.

(31) TABLE-US-00001 TABLE 1 System Cable 1 2 3 4 1A 1B 2A 2B 2C Conductor 750 MCM 8AWG 500 KCM 250 KCM 750 MCM 750 MCM 8AWG 8AWG 8AWG Size (380 mm.sup.2) (8.36 mm.sup.2) (253.35 mm.sup.2) (126.67 mm.sup.2) (380 mm.sup.2) (380 mm.sup.2) (8.36 mm.sup.2) (8.36 mm.sup.2) (8.36 mm.sup.2) Number of 2 2 2 2 4 2 4 2 2 Mica Tapes (1 couple) (1 couple) (1 couple) (1 couple) (2 couples) (1 couple) (2 couples) (1 couple) (1 couple) Mica Tape 25% 25% 25% 25% 25% 25% 25% 25% 25% Overlap Mica Facing up/down up/down up/down up/down up/down up/down up/down up/down down/down (x2) (x2) (x2) Mica Tape up = RHL up = RHL up = RHL up = RHL up = RHL up = RHL up = RHL up = RHL down = RHL Winding down = LHL down = RHL down = RHL down = RHL down = LHL down = LHL down = LHL down = RHL down = RHL Direction Conduit FRE FRE FRE FRE EMT EMT EMT EMT EMT No. of 2 4 2 3 2 2 4 4 4 cables in conduit FRE = fiberglass-reinforced thermosetting resin conduit (extra heavy wall Breathsaver ® by FRE Composites ®) EMT = zinc-free steel conduit (by Allied Tube and Conduit ®)

(32) Systems alphanumerically named are comparative. “Mica facing” refers to the directions that the mica layers of the mica tapes are facing. For example, “up/down” means that there is one couple of mica tapes including one mica tape with the mica layer facing up (away from the conductor) and one mica tape with the mica layer facing down (towards the conductor) such that the mica layers are sandwiched together. “Up/down (×2)” means that there are two couples of mica tapes with each couple having the “up/down” orientation. “Down/down” means that there is one couple of mica tapes, and the mica layer of each mica tape faces down (towards the conductor).

(33) “Mica tape winding direction” refers to the winding direction of the mica tapes. “Up=RHL” means that the mica tape with the upward-facing mica layer has RHL, “down=LHL” means that the mica tape with the downward-facing mica layer has LHL, and “down=RHL” means that the mica tape with the downward-facing mica layer has RHL.

(34) All of the cables of Table 1 were Type RHW-2 cable having a voltage rating of 600 volts and a fire rating of 480 volts includes 8 AWG (8.36 mm2) 7-strand compressed soft bare copper in accordance with ASTM B8 Class B concentric-lay-stranded copper conductors. Layers of mica tape (Cablosam® 366.21-30 from Von Roll Switzerland Ltd) having a nominal thickness of approximately 0.005 inches (0.127 mm) and a nominal width of approximately 0.5 inches (12.7 mm) are applied on top of the conductor.

(35) All of the cables of Table 1 had an insulating layer of LS0H low toxicity flame retardant silicon insulation applied over the mica tape(s), and a polymeric flame retardant layer of LS0H low toxicity flame retardant polyolefin (UNIGARDTM RE HFDA-6525 from The Dow Chemical Company) applied over the insulating layer.

(36) The systems of Table 1 were tested according to 2-hour Horizontal and 2-hour Vertical UL-2196 test as from Table 2. Table 2 also reports the outcome of such tests.

(37) TABLE-US-00002 TABLE 2 System 1 2 3 4 1A 1B 2A 2B 2C Conduit V H V H V H V H V H V H V H V H V Position Conduit 21 28 16 27 20 27 14 33 19 32 18 30 17 40 14 34 21 Fill (%) +/− + + + + + + + + + + − + + + − + − “Conduit position” refers to the mounting orientation of the conduit on the fire wall, i.e., vertical (“V”) or horizontal (“H”). The positive (+) and negative (−) signs indicate, respectively, that the system passed or not passed the test.

(38) As shown in Table 2, all of the cable systems according to the disclosed features passed the 2 hours fire-test both in vertical and horizontal conditions, thus demonstrating the fire resistance of a cable having one single couple of mica tape in “sandwich” configuration housed in a fiberglass-reinforced thermosetting resin conduit.

(39) When a metal (steel) conduit is used for housing the electric cable, only cables with two couples of mica tape in “sandwich” configuration pass the 2 hours fire-test both in vertical and horizontal conditions.

(40) In particular, System 1A, having the same conductor size of System 1, but two couples of mica tapes and a conduit made of steel, passed both the 2-hour Horizontal and 2-hour Vertical tests by virtue of said additional mica tapes. It should be noted that the conduit fill of the vertical test is lower than that of System 1, accordingly such system with a cable with four mica tapes in a steel conduit can be certified for less conduit fills.

(41) System 1B, having the same conductor size and mica tapes number of System 1, but a conduit made of steel, passed the 2-hour Horizontal test only, but in vertical configuration it lasted 1 hour only, accordingly such system with a steel conduit cannot be 2-hour vertical rated.

(42) System 2A having the same conductor size of System 2, but two additional mica tapes and a conduit made of steel, passed both the 2-hour Horizontal and 2-hour Vertical tests by virtue of said additional mica tapes. It should be noted that the conduit fill of the vertical test is lower than that of System 2, accordingly such system with a cable with four mica tapes in a steel conduit can be certified for less conduit fills.

(43) System 2B having the same conductor size and mica tapes number of System 2, but a conduit made of steel, passed the 2-hour Horizontal test only, but in vertical configuration it lasted 1 hour only, accordingly such system with a steel conduit cannot be 2 hour vertical rated.

(44) It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the cable disclosed herein without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims.