Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B7/00—Insulated conductors or cables characterised by their form
  • H01B7/17—Protection against damage caused by external factors, e.g. sheaths or armouring
  • H01B7/29—Protection against damage caused by extremes of temperature or by flame
  • H01B7/295—Protection against damage caused by extremes of temperature or by flame using material resistant to flame
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B7/00—Insulated conductors or cables characterised by their form
  • H01B7/17—Protection against damage caused by external factors, e.g. sheaths or armouring
  • H01B7/29—Protection against damage caused by extremes of temperature or by flame

Definitions

• This invention relates to fire-resistant wires capable of maintaining electrical circuit integrity when the wires, or cables containing them, are exposed to fire.
• Mica paper reinforced by a backing material of woven glass or of polyethylene film is produced as tape that can be spirally wrapped onto electrical conductors to give a degree of fire resistance.
• the mica wrapped conductor is then normally coated with a conventional polymer to impart the required electrical/mechanical properties. In a fire, the polymer is destroyed but the electrical integrity of the cable is maintained by the mica layer and the insulating char from the polymer. It is generally understood that the more mica the better from a fire performance point of view and tapes are available with different weights of mica (e.g. 80, 120, 160 g./sq m).
• mica tapes are applied by spiral wrapping with an overlap of typically 30% to 50% (of tape width) to maintain protection when the wrapped conductor is flexed, since the overlaps tend to open on flexing.
• High overlaps are preferred to achieve the required degree of fire resistance. It is a problem that the thicker, heavier tapes are harder to wrap successfully, especially on small diameter conductors, leading to tape damage, wrinkling and a poor wrapped surface making subsequent processing more difficult and final appearance and/or performance of the wire less satisfactory. Therefore, if a higher level of fire protection is needed, the options are normally to increase the weight of the mica tape and suffer a deterioration in the wrap quality, or to apply 2 layers of tape giving a greater diameter.
• Sketch A indicates an "ideal" 50% overlap wrap on a wire
• Sketch B indicates a more realistic degree of overlap achieved in practice
• Sketch C indicates a wire having a single wrap with a typical 33% overlap
• Sketch D indicates a double layer of wraps similar to that shown in Sketch B;
• Sketch E indicates a stranded wire having a double layer of wraps similar to that shown in Sketch C;
• Sketch G indicates increases in wrap thickness caused by the aforementioned mica splicing tapes.
• the successive turns of mica wrap M of perceived width W would have an overlap of exactly 50%, giving a double thickness of mica tape over the entire surface of the wire conductor C from a single wrap.
• this ideal overlap is difficult to achieve, and Sketch B indicates a more realistic result where gaps G occur, taking into account the finite thickness of the mica tape (M) which necessitates a lower degree of overlap to achieve a smooth wrap.
• Sketch C indicates a single wrap of mica tape M of perceived width W with an overlap (shaded) of about 33 % .
• the perceived width W is slightly greater than the true width of the tape M owing to the spiral wrapping angle of the tape about the conductor axis.
• two mica tapes Ml and M2 are preferably used as indicated in Sketches D and E to ensure that at least two layers of the mica wrap are present at all points.
• Sketch F shows, with some exaggeration for clarity, the unevenness which occurs in these known wrappings of mica tape M on conductor C, and indicates the undesirable breakage of the brittle reinforcing fibres F (usually glass) which tends to occur due to stresses in the overlapped tapes.
• the invention provides an electrical wire having a conductor carrying at least one under-layer of fire-resistant material over which is spirally wrapped a covering tape of inorganic fire-resistant material with adjacent turns of the tape overlapping one another by less than 10%, preferably less than 5%, more preferably less than 3 % , of the tape width.
• the under-layer also comprises a tape of inorganic fire-resistant material spirally wrapped around (preferably in contact with) the conductor.
• the overlapping regions of the covering tape will preferably be positioned between the overlapping regions of the underlayer tape, so as to maintain protection on flexing of the wire which will tend to open the small overlaps of the covering tape.
• spiral is used herein in the sense of winding continually and advancing as if along a cylinder, the cylinder being in fact represented by the conductor in this invention, which is preferably of substantially circular cross-section although other cross-sections are not necessarily excluded.
• tape is used herein to refer to any elongate body of material capable of being spirally wrapped around a wire conductor, without limitation to any particular format, construction or materials except as specifically stated herein.
• This invention is especially useful for wires having a coating of organic polymer material of thickness less than 0.8 millimetre, preferably less than 0.6 mm, more preferably less than 5 mm, and especially less than 0.4 mm, overlying and preferably in contact with the outermost surface of the said wrapped tape(s).
• the adjacent turns of the underlayer tape overlap one another by not less than 2%, preferably not less than 4%, more preferably not less than 8%, of the respective tape width.
• the outer covering tape will have turns overlapping less than the turns of the underlayer tape(s).
• the preferred fibrous support of the covering tapes may be able to enhance the smoothness by providing a degree of "play” or looseness which enables the reduced overlaps to partially amalgamate or settle closely against (or partially “into") each other, thus relieving some of the overlap tension and unevenness.
• the conventional larger overlaps extending nearer to the central portions of the tape are apparently unable to do this.
• the preferred fibrous backing appears to produce a greater smoothing effect than is obtained by similarly reduced overlapping of polymer-backed mica tapes. The aforementioned breakage of brittle fibres is also reduced, presumably due to reduction in tension in the overlaps according to the present invention.
• the fibrous support of each tape is an open-weave cloth, preferably a square weave having the weft fibres or filaments substantially normal to the length of the tape.
• adjacent fibres in the weave are preferably spaced apart by a distance at least equal to the average fibre thickness, more preferably at least twice the average fibre thickness, and preferably not more than 20 times (more preferably not more than 10 times) the average fibre thickness.
• the preferred fibrous materials are woven glass cloths.
• the covering tape backing preferably comprises a substantially continuous film of organic polymeric material, for example polyethylene or polyester film. Tapes having polymer coatings or films applied over the aforementioned fibrous (cloth) backings may be advantageous in combining the preferable qualities of the fibre or cloth and the polymeric (film) backings.
• the overlapping portions of the covering (outer) tape are positioned substantially mid-way between the overlapping portions of the underlayer tape.
• Figure 1 shows schematically in partial section a two-layered reduced- overlap construction according to this invention.
• Figure 2 indicates the smoother appearance of the wrapping in comparison with the known result indicated in Sketch F.
• the wire conductor 10 is shown carrying a first spiral wrap of glass-cloth-backed mica tape 20, the preferred open square weave of the glass cloth support of the tape being suggested by one illustrated strip 22.
• the tape is wrapped so that successive turns overlap by about 10% of the tape width (not to scale) and, as shown schematically, these overlaps 24 tend may to flatten slightly as indicated to provide a somewhat smoother wrapping. Larger overlaps may also be acceptable in this under-layer.
• a second covering wrap of a similar mica tape 30 is shown having its overlaps of about 2% of tape width positioned in the preferred arrangement mid-way between those of the underlying tape 20. These minimal overlaps tend to produce noticeably smoother wrapping, as aforesaid.
• two glass-cloth-backed mica tapes of 120 g/m 2 weight wrapped as shown in Figure 1 can advantageously achieve substantially the same level of fire protection with the coverage tending towards the theoretical minimum of only 240 g/m 2 (slightly more on average over the whole wire, due to the small overlaps).
• a splicing tape 40 occurs on the mica tapes, it appears to benefit to some extent from the "amalgamation" effect of the reduced overlaps according to the present invention, as indicated schematically at splice overlaps 42 and 44, thus reducing substantially the problematic splice-tape- thickness-doubling effect illustrated in Sketch G.
• a thin (0.35 mm) coating of polymeric insulation 50 can be extruded by methods known per se over the wrapped tapes 20 and 30 with only a relatively small and acceptable thickening 55 occurring at the splice.
• a thinner, smoother wire is thus produced according to this invention, with improved production efficiency, to meet the same fire-resistance standards which previously required much higher percentage overlaps of lighter tapes and/or thicker polymer coatings.
• Fire resistance may be measured on fire test samples consisting of 2 of mica-wrapped wires with no additional polymer insulation twisted together with an overall copper braid applied.
• the fire tests are performed according to IEC331 (published by International Electrotechnical Commission, 1 rue de Varembe, Geneva) with the flame enhanced to 950°C ⁇ 50°C. Voltage is applied to the conductors with the braid connected to earth. Tests are terminated if the cable maintains circuit integrity for more than 60 minutes.
• the present invention may be practised using any of the known materials and production methods suitable for the production of insulated electrical wires, especially fire-resistant electrical wires.
• a preferred example will now be described by way of further illustration of the invention.
• Conductor 7 strands of 0.52 mm tin plated copper conductors assembled into a round circular conductor to a nominal diameter of 1.50 mm.
• a tube extrusion or similar technique (known per se) is used to enable the slight diameter increase caused by splices in the wrapped product to be accepted.
• Diameter increases due to tape splices will produce an increase in insulation diameter of the same order as of the wrap itself. These splices need not be removed as fire resistant properties and insulation electrical and mechanical properties are not impaired.
• Insulated wires so produced may be constructed by known methods into multicore electrical cables consisting of further extrusions and possibly steel armour layer as required.

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• Insulated Conductors (AREA)
• Insulating Bodies (AREA)

Applications Claiming Priority (5)

Publications (2)

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• 1995
  • 1995-07-12 EP EP95924456A patent/EP0770259B1/de not_active Expired - Lifetime
  • 1995-07-12 KR KR1019970700213A patent/KR100363600B1/ko not_active IP Right Cessation
  • 1995-07-12 CN CN95194100A patent/CN1080443C/zh not_active Expired - Lifetime
  • 1995-07-12 AU AU28951/95A patent/AU696852B2/en not_active Expired
  • 1995-07-12 JP JP50479296A patent/JP4015694B2/ja not_active Expired - Lifetime
  • 1995-07-12 DE DE69512242T patent/DE69512242T2/de not_active Expired - Lifetime
  • 1995-07-12 WO PCT/GB1995/001638 patent/WO1996002920A1/en active IP Right Grant
  • 1995-07-12 CA CA002194993A patent/CA2194993C/en not_active Expired - Lifetime
  • 1995-07-13 MY MYPI95001987A patent/MY113998A/en unknown
  • 1995-07-13 IL IL11457995A patent/IL114579A/en not_active IP Right Cessation
  • 1995-07-17 TW TW084107380A patent/TW374182B/zh not_active IP Right Cessation

Non-Patent Citations (1)

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