FIRE RESISTANT CABLE

Abstract

An electrical cable includes a conductor and a 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. Each of the first and the second mica tape includes a mica layer attached to a backing layer. The mica layer of the first mica tape faces and contacts the mica layer of the second mica tape.

Claims

1. An electrical cable comprising a conductor and a 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, wherein the mica layer of the first mica tape faces and contacts the mica layer of the second mica tape.

2. The electrical cable of claim 1, further comprising at least one insulation layer surrounding the couple of mica tapes.

3. The electrical cable of claim 1, wherein the couple of mica tapes is an inner couple of mica tapes surrounding the conductor, the electrical cable further comprising an outer couple of mica tapes surrounding the inner couple, the outer couple of mica tapes being formed of a third mica tape and a fourth mica tape wound around the third mica tape, the third and the fourth mica tape including a mica layer attached to a backing layer, wherein the mica layer of the third mica tape faces and contacts the mica layer of the fourth mica tape.

4. The electrical cable of claim 3, wherein the backing layer of the second mica tape faces and contacts the backing layer of the third mica tape.

5. The electrical cable of claim 3, further comprising at least one insulation layer surrounding the inner and the outer couple of mica tapes.

6. The electrical cable of claim 1, wherein the first mica tape is wound in a winding direction that is opposite to a winding direction of the second mica tape.

7. The electrical cable of claim 1, further comprising a first insulation layer and a second insulation layer.

8. The electrical cable of claim 7, wherein the first insulation layer is formed of a silicone-based compound.

9. The electrical cable of claim 8, wherein the silicone-based compound includes a silicone-based rubber forming a matrix with a flame-retardant mineral filler incorporated into the matrix.

10. The electrical cable of claim 7, wherein the second insulation layer is made of a flame-retardant polymer.

11. A method of manufacturing an electrical cable comprising a conductor and a 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, the method comprising: winding the first mica tape around the conductor so that the backing layer of the first mica tape faces the conductor; and winding the second mica tape around the first mica tape so that the mica layer of the second mica tape faces and contacts the mica layer of the first mica tape.

12. The method of claim 11, further comprising: winding a third mica tape around the second mica tape, the third mica tape being formed of a third mica tape including a mica layer attached to a backing layer, the backing layer of the second mica tape facing and contacting the backing layer of the third mica tape; and winding a fourth mica tape around the third mica tape, the fourth mica tape being formed of a fourth mica tape including a mica layer attached to a backing layer, the mica layer of the third mica tape facing and contacting the mica layer of the fourth mica tape.

13. The electrical cable of claim 3, further comprising a first insulation layer and a second insulation layer.

14. The electrical cable of claim 13, wherein the first insulation layer is formed of a silicone-based compound.

15. The electrical cable of claim 14, wherein the silicone-based compound includes a silicone-based rubber forming a matrix with a flame-retardant mineral filler incorporated into the matrix.

16. The electrical cable of claim 13, wherein the second insulation layer is made of a flame-retardant polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a cross-sectional view of an electrical cable, consistent with certain disclosed embodiments.

DESCRIPTION OF THE EMBODIMENTS

[0028] 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.

[0029] Referring now to FIG. 1, an electrical cable 10 has a longitudinal axis 12. The electrical cable 10 may include, in order from the interior to the exterior, an electrical conductor or conductor 20, an inner couple of mica tapes 30a, an outer couple of mica tapes 30b, and one or more layers sequentially provided in radial external position with respect to the inner and outer couples of mica tapes 30a, 30b. Such external layer(s) may include a first insulation layer 40, a second insulation layer 50, and, optionally, an outer sheath 60. In some applications, the outer sheath 60 can be omitted.

[0030] The conductor 20 may be 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. Alternatively, the conductor 20 may range in size from about 2 mm.sup.2 (14 AWG) to about 500 mm.sup.2 (1000 kcmil), but it is understood that the size of the conductor 20 may be greater than or less than this range. For example, for certain circuits and fire alarms, the size of the conductor 20 may be 16 AWG (1.31 mm.sup.2) and the conductor 20 may be tested at relatively lower voltages.

[0031] The inner and outer couples of mica tapes 30a, 30b are wound around the conductor 20. The inner couple 30a 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.

[0032] Each of the first mica tape 32 and the second mica tape 34 are formed of a mica layer attached to a backing layer. The mica layer may be formed of one or more types of mica (e.g., muscovite and/or phlogopite). The mica layer may include a mica paper or sheet, which may be impregnated or coated with a binding agent (e.g., silicone resin or elastomer, acrylic resin, and/or epoxy resin). The backing layer may be formed of a supporting fabric (e.g., woven glass and/or glass cloth). The mica layer may be bonded to the backing layer using the binding agent. Alternatively, the mica layer may be impregnated or coated with the binding agent, and reinforced with the backing layer.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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 on the conductor 20.

[0038] The first mica tape 32 and the second mica tape 34 may be 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 may 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%.

[0039] The cable 10 of FIG. 1 also includes an outer couple of mica tapes 30b formed of a third mica tape 36 and a fourth mica tape 38. The third mica tape 36 surrounds or is wound around the second mica tape 34 and may contact the second mica tape 34 directly. The direct contact between the backing layer of the second mica tape 34 and the backing layer of the third mica tape 36 is advantageous for the fire-resistance performance of the couples 30a and 30b of mica tapes. The fourth mica tape 38 is wound around the third mica tape 36 such that the fourth mica tape 38 contacts and is applied directly onto the third mica tape 36.

[0040] The third mica tape 36 and/or the fourth mica tape 38 may be formed of materials similar to those used to form the first mica tape 32 and/or the second mica tape 34. It is understood that the first, second, third, and fourth mica tape 32, 34, 36, and 38, or sub-combinations thereof, may be formed using different or the same mica layer and backing layer.

[0041] The third mica tape 36 and the fourth mica tape 38 may be applied in a similar manner as the first mica tape 36 and the second mica tape 34, as described below.

[0042] The third mica tape 36 is wound onto the second mica tape 34 such that the backing layer of third mica tape 36 faces and may contact the backing layer of the second mica tape 34, and the mica layer of the third mica tape 36 faces away from the conductor 20 and the first and second mica tapes 32 and 34. Thus, the backing layer of the third mica tape 36 faces radially inward toward the axis 12 of the cable 10, and the mica layer of the third mica tape 36 faces radially outward away from the axis 12 of the cable 10.

[0043] The fourth mica tape 38 is wound onto the third mica tape 36 such that the mica layer of the fourth mica tape 38 faces and contacts the mica layer of the third mica tape 36, and the backing layer of the fourth mica tape 38 faces away from the conductor 20 and the first, second, and third mica tape 32, 34, and 36. Thus, the mica layer of the fourth mica tape 38 faces radially inward toward the axis 12 of the cable 10, and the backing layer of the fourth mica tape 38 faces radially outward away from the axis 12 of the cable 10.

[0044] The third mica tape 36 may be wound in an opposite winding direction than the fourth mica tape 38. Also, the third mica tape 36 may be applied in the same winding direction as the first mica tape 32, and the fourth mica tape 38 may be applied in the same winding direction as the second mica tape 34. For example, the third mica tape 36 may have a RHL and the fourth mica tape 38 may have a LHL, or vice versa. The third mica tape 36 and the fourth mica tape 38 may be wound at an angle of, for example, 45. Further, the third mica tape 36 and the fourth mica tape 38 may both have an overlap percentage of, for example, 25%.

[0045] The mica layer of one or more of the mica tape 32, 34, 36, 38 may 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 more of the mica tape 32, 34, 36, 38 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.

[0046] The layers sequentially provided in radial external position with respect to the inner and outer couples of mica tapes 30a, 30b, e.g., the first insulation layer 40, the second insulation layer 50, and/or the outer sheath 60, may be extruded onto the inner and outer couples of mica tapes 30a, 30b. The first insulation layer 40, the second insulation layer 50, and/or the outer sheath 60 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 (LSOH) compounds, and that have low toxicity flame retardant properties.

[0047] In the embodiment shown in FIG. 1, the first insulation layer 40 surrounds the fourth mica tape 38 such that the first Insulation layer 40 contacts and is applied directly onto the fourth mica tape 38. Alternatively, if the third and fourth mica tape 36 and 38 are omitted, then 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 may have 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).

[0048] 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 may be incorporated into the silicone-based compound using a bonding agent, such as silane, and the silicone-based compound may be cured using a cure catalyst, such as peroxide.

[0049] 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 couples of mica tapes 30a, 30b may link and form a continuous eutectic mixture that may serve as a dielectric for the cable 10 to allow the cable 10 to continue operating.

[0050] The silicone-based compound may also 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 may change from a flexible rubber-like material to a more solid, ceramic-like material.

[0051] 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.

[0052] 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. No. 6,495,760, U.S. Pat. No. 6,552,112, U.S. Pat. No. 6,924,031, U.S. Pat. No. 8,097,809, EP0893801, and EP0893802.

[0053] 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).

[0054] The outer sheath 60 surrounds the second insulation layer 50 such that the outer sheath 60 contacts and is applied directly onto the second insulation layer 50. The outer sheath 60 may be formed of a polymeric material, such as high-density polyethylene (HDPE).

[0055] The cable 10 constructed as described above may be a thermoset wire that 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 72 to 600 volts and may be fire rated at from 72 to 600 volts.

[0056] One or more of the cables 10 may be deployed in a conduit (not shown). In some embodiments, the conduit may include four of the cables 10, but it is understood that more or fewer than four of the cables 10 may be included in the conduit.

[0057] The conduit fill, e.g., the percentage of a section of the conduit that is filled by the cable 10, may range from approximately 14% to 40%, but it is understood that the conduit fill may also be less than this range. 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 of the cables 10 with 40% 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 1.163 inches (29.54 mm), and the inner diameter of the conduit may be approximately 1.049 inches (26.64 mm). It is understood that the diameters may be greater than or less than these values.

[0058] The cable of the invention 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.

[0059] 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 allows the cable of the invention to resist higher temperatures, thereby improving the fire resistance of the cable, and therefore protects the electrical performance of the electrical conductor.

[0060] The cable of the invention may advantageously include four mica tapes. When four of the mica tapes are applied, the third mica tape and the fourth mica tape are preferably applied in a similar manner as the first mica tape and the second mica tape. That is, the third mica tape may be applied with the mica layer facing up (away from the second mica tape), and the fourth mica tape may be applied with the mica layer facing down towards the third mica tape. As a result, the mica layers of the third mica tape and the fourth mica tape may be sandwiched together.

[0061] The cable of the invention may include one couple of mica tapes, and such a construction may be sufficient for various sizes of the cable to pass horizontal fire wall tests. The Applicant has found that including two couples of mica tapes may improve the fire resistance of the cable of the invention, e.g., to be able to pass both horizontal and vertical fire wall tests.

Example

[0062] A number of cables according to the invention and comparative cables have the construction features according to Table 1.

TABLE-US-00001 TABLE 1 Cable 1 2 3 4* 5* 6* Conductor 750 MCM 8AWG 8AWG 8AWG 8AWG 10AWG Size (380 mm.sup.2) (8.36 mm.sup.2) (8.36 mm.sup.2) (8.36 mm.sup.2) (8.36 mm.sup.2) (5.26 mm.sup.2) Number of 4 4 2 2 1 1 Mica Tapes (2 couples) (2 couples) (1 couple) (1 couple) Mica Tape 25% 25% 25% 25% 50% 50% Overlap Mica up/down up/down up/down down/down down down Facing (x2) (x2) Mica Tape up = RHL up = RHL up = RHL down = RHL RHL RHL Winding down = LHL down = LHL down = RHL down = RHL Direction Cables marked with an asterisk (*) are comparative cables.

[0063] 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 and one mica tape with the mica layer facing down 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. Down means that there is one mica tape and the mica layer of the mica tape faces down.

[0064] 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.

[0065] 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 mm.sup.2) 7-strand compressed soft bare copper in accordance with ASTM B8 Class B concentric-lay-stranded copper conductors. Four 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.

[0066] All of the cables of Table 1 had an insulating layer of LSOH low toxicity flame retardant silicon insulation applied over the mica tape(s), and a polymeric flame retardant layer of LSOH low toxicity flame retardant polyolefin (UNIGARD RE HFDA-6525 from The Dow Chemical Company) applied over the insulating layer.

[0067] The cables of Table 1 were tested according to UL-2196 with the configuration test given in Table 2. Table 2 also reports the outcome of such tests.

TABLE-US-00002 TABLE 2 Cable 1 2 3 4* 5* 6* Conduit V H H V V V V V V V V Position Conduit 19 32 40 17 40 14 16 21 36 32 26 Fill (%) Failing 0 of 5 0 of 1 0 of 1 0 of 5 0 of 1 0 of 1 0 of 5 1 of 2 2 of 2 1 of 1 2 of 2 Samples

[0068] Conduit position refers to the mounting orientation of the conduit on the fire wall, i.e., vertical (V) or horizontal (H).

[0069] As shown in Table 2, all of the cables according to the invention passed the fire-test at different test conditions, while the comparative cables had at least one sample failing, which is not acceptable by the standards used for the present testing.

[0070] 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 or spirit 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 and spirit of the invention being indicated by the following claims.