GB1583954A - Electric cables - Google Patents

Description

(54) ELECTRIC CABLES (71) We, BICC LIMITED, a British Company, of 21 Bloomsbury Street, London WCIB 3QN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electric cables that comprise combustible material, by which is meant any material that can undergo combustion, even if only after pre-heating to an elevated temperature and/or only if heat is continually supplied to maintain combustion.

Most modern electric cables do not present any serious primary fire hazard, in that electrical faults are infrequent and when they do occur they seldom result in continued burning of the cable concerned. On the other hand, apart from mineral-insulated cables (which are only well suited to use in power circuits at relatively low voltages), practically all cables and non-metallic conduits include materials which will burn if exposed to fire conditions. Moreover when they do burn, cables and conduits are apt to produce rather large amounts of smoke, sometimes of a poisonous and/or corrosive and/or otherwise harmful kind. For example when a PVC compound burns a proportion of the chlorine it contains appears as hydrogen chloride gas and/or hydrochloric acid droplets in the smoke.

It is an object of the present invention to reduce the amount of smoke that will be evolved if a particular cable should burn without requiring alteration in its basic construction or its basic materials.

In accordance with the invention, an electric cable comprises at least one metallic conductor which has, or each of which has, insulation of combustible polymeric material applied directly to it, and an outer protective covering; and a close wrapping of a resinbonded mineral-fibre reinforced mica paper tape encloses at least a major part (as defined) of the total combustible material of the cable and preferably substantially all of it.

By a "major part" of the combustible material is meant, rather than any particular fraction of the total weight or volume of the combustible material, any part that, in the event of combustion, would contribute substantially to smoke hazard (for example very little PVC should be outside the tape whereas comparatively large amounts of polyethylene may be, since the latter burns relatively cleanly).

The precise structure of the mica paper constituent of the tape is not critical, but the particles should be sufficiently densely packed to make the mica paper self-supporting; on the other hand large flakes or splittings of mica do not form an adequate substitute for mica paper. Mica paper prepared from phlogopite is preferred, but muscovite mica paper can be used.

The reinforcing mineral fibres are preferably glass fibres, but other mineral fibres of high tensile strength (such as asbestos fibres) could be used. They may run only in the longitudinal direction of the tape, or they may run in more than one direction with or without being woven together; at present woven glass fabric with its warp running along the length of the tape is preferred.

Any type of bonding resin that adheres satisfactorily to the mica paper and the fibres and has adequate flexibility can be used (for example suitable silicone, polyester, epoxy, or phenolic resins). Since the resin is itself a combustible material, the minimum amount of resin compatible with satisfactory bonding should be used. The reinforced mica paper tape is preferably applied helically to the cable, but longitudinal taping could be used provided that the edges of the tape are overlapped and securely fixed down.

The mode of operation has not yet been completely established but it would appear that the mica paper acts as a stable barrier, on the one hand reducing the contact of external flame and hot gas with the enclosed combustible material, and so slowing down combustion, and on the other hand inhibiting the escape of smoke particles from within it and yet being sufficiently permeable to gas not to build up disruptive gas pressures within the barrier.

Conductors contained within the cable may be insulated entirely with conventional polymeric materials or the insulation may include incombustible layers of the same kind of glass/mica tape, glass fabric, or other suitable material; this not only further reduces the smoke hazard but also helps to maintain circuit integrity for a short period of fire exposure.

In cases where the mica paper tape layer needs mechanical protection, a thin extruded or braided layer of a low-smoke polymeric material (e.g. polypropylene or high-density polyethylene) can be used, or an incombustible braid (of wire or glass fibres for example) might be preferred for some applications.

Example I An armoured control cable in accordance with the invention comprises seven twisted pairs of polyethylene-insulated 1 5 mm diameter copper strands each made up of seven wires, 0 5 mm in diameter, laid up together in the usual way and lapped with polyethylene terephthalate tape to form a core 13 85 mm in diameter. Over this is extruded a bedding of a PVC compound containing finely divided dolomite filler with a radial thickness of 1 3 mm.

To this bedding is applied an armouring formed by 32 galvanised steel wires each 16 mm in diameter, with a left-hand lay of 164 mm. An extruded serving of the same PVC compound with a radial thickness of 18 mm covers the armour wires. An openweave glass-fibre tape, 50 mm wide and 0 434 mm thick, is lapped over the serving with a right-hand lay and an overlap of 25%.

Finally, the whole is enclosed in a glassfibre reinforced mica paper tape, 50 mm wide and 0 15 mm thick applied with a right-hand lay and with an overlap of 25%, to give an overall diameter of about 25'5 mm.

Example 2 A radiating cable has a central conductor of plain annealed copper wire 2'3 1 mm in diameter, a semi-airspace dielectric of the "thread and tube" type, 9.5 mm in diameter, and a plain annealed copper wire braid to give a nominal characteristic impedance of 75 ohm. The braid is enclosed in a bedding layer of the compound described in our Provisional Specification no. 10267/77 (Serial no. 1583956) as compound EPR2, to an overall diameter of 13 1 mm.Over this bedding layer is applied a lapping of openweave glass-fibre tape 35 mm wide and 0 434 mm thick with a right-hand lay and an overlap of 25%, followed by a glass reinforced mica paper tape, 35 mm wide by 0 15 mm thick applied with right hand lay and with 25% overlap, and then a braid of flat strip of polypropylene, and the overall diameter is 16 mm.

Example 3 This is the same as Example 2 except that the bedding layer is of the compound described in our Provisional Specification 10268/ 77 (Serial no. 1583957) as compound EVA 1.

Examples 4 and 5 These are similar to Examples 2 and 3 respectively, except that in each case the copper braid is replaced by an apertured corrugated copper tape of conventional design, in order to achieve reduced attenuation.

The reinforced mica type material is the same in all examples, and includes a mica paper layer, 0 05 mm thick (applied on the inside) and an open weave glass cloth backing 0 07 mm thick and having on average about 13 glass yarns per cm longitudinally and 7 yarns per cm transversely, bonded together with a silicone resin constituting about 20% by weight of the material. This tape was supplied by Jones Stroud Insulations Ltd.

WHAT WE CLAIM IS: 1. An electric cable that comprises at least one metallic conductor which has, or each of which has, insulation of combustible polymeric material applied directly to it, and an outer protective covering wherein a close wrapping of a resin-bonded mineral-fibre reinforced mica paper tape encloses at least a major part (as defined) of the total combustible material of the cable.

2. An electric cable that comprises at least one metallic conductor which has, or each of which has, insulation of combustible polymeric material applied directly to it, and an outer protective covering, wherein a close wrapping of a resin-bonded mineralfibre reinforced mica paper tape encloses substantially all the combustible material of the cable.

3. A cable as claimed in either of the preceding claims in which the mineral fibres run only in the longitudinal direction of the tape.

4. A cable as claimed in claim 1 or claim 2 in which the mineral fibres constitute a woven fabric.

5. A cable as claimed in claim 1 or claim 2 or claim 4 in which the mineral fibres are bonded to the mica paper by a silicone, polyester, epoxy or phenolic resin.

6. A cable as claimed in any one of the preceding claims in which the tape is longitudinally applied.

7. An armoured control cable substantially as described in Example 1.

8. A radiating cable substantially as

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. may be insulated entirely with conventional polymeric materials or the insulation may include incombustible layers of the same kind of glass/mica tape, glass fabric, or other suitable material; this not only further reduces the smoke hazard but also helps to maintain circuit integrity for a short period of fire exposure. In cases where the mica paper tape layer needs mechanical protection, a thin extruded or braided layer of a low-smoke polymeric material (e.g. polypropylene or high-density polyethylene) can be used, or an incombustible braid (of wire or glass fibres for example) might be preferred for some applications. Example I An armoured control cable in accordance with the invention comprises seven twisted pairs of polyethylene-insulated 1 5 mm diameter copper strands each made up of seven wires, 0 5 mm in diameter, laid up together in the usual way and lapped with polyethylene terephthalate tape to form a core 13 85 mm in diameter. Over this is extruded a bedding of a PVC compound containing finely divided dolomite filler with a radial thickness of 1 3 mm. To this bedding is applied an armouring formed by 32 galvanised steel wires each 16 mm in diameter, with a left-hand lay of 164 mm. An extruded serving of the same PVC compound with a radial thickness of 18 mm covers the armour wires. An openweave glass-fibre tape, 50 mm wide and 0 434 mm thick, is lapped over the serving with a right-hand lay and an overlap of 25%. Finally, the whole is enclosed in a glassfibre reinforced mica paper tape, 50 mm wide and 0 15 mm thick applied with a right-hand lay and with an overlap of 25%, to give an overall diameter of about 25'5 mm. Example 2 A radiating cable has a central conductor of plain annealed copper wire 2'3 1 mm in diameter, a semi-airspace dielectric of the "thread and tube" type, 9.5 mm in diameter, and a plain annealed copper wire braid to give a nominal characteristic impedance of 75 ohm. The braid is enclosed in a bedding layer of the compound described in our Provisional Specification no. 10267/77 (Serial no. 1583956) as compound EPR2, to an overall diameter of 13 1 mm.Over this bedding layer is applied a lapping of openweave glass-fibre tape 35 mm wide and 0 434 mm thick with a right-hand lay and an overlap of 25%, followed by a glass reinforced mica paper tape, 35 mm wide by 0 15 mm thick applied with right hand lay and with 25% overlap, and then a braid of flat strip of polypropylene, and the overall diameter is 16 mm. Example 3 This is the same as Example 2 except that the bedding layer is of the compound described in our Provisional Specification 10268/ 77 (Serial no. 1583957) as compound EVA 1. Examples 4 and 5 These are similar to Examples 2 and 3 respectively, except that in each case the copper braid is replaced by an apertured corrugated copper tape of conventional design, in order to achieve reduced attenuation. The reinforced mica type material is the same in all examples, and includes a mica paper layer, 0 05 mm thick (applied on the inside) and an open weave glass cloth backing 0 07 mm thick and having on average about 13 glass yarns per cm longitudinally and 7 yarns per cm transversely, bonded together with a silicone resin constituting about 20% by weight of the material. This tape was supplied by Jones Stroud Insulations Ltd. WHAT WE CLAIM IS:

1. An electric cable that comprises at least one metallic conductor which has, or each of which has, insulation of combustible polymeric material applied directly to it, and an outer protective covering wherein a close wrapping of a resin-bonded mineral-fibre reinforced mica paper tape encloses at least a major part (as defined) of the total combustible material of the cable.

2. An electric cable that comprises at least one metallic conductor which has, or each of which has, insulation of combustible polymeric material applied directly to it, and an outer protective covering, wherein a close wrapping of a resin-bonded mineralfibre reinforced mica paper tape encloses substantially all the combustible material of the cable.

3. A cable as claimed in either of the preceding claims in which the mineral fibres run only in the longitudinal direction of the tape.

4. A cable as claimed in claim 1 or claim 2 in which the mineral fibres constitute a woven fabric.

5. A cable as claimed in claim 1 or claim 2 or claim 4 in which the mineral fibres are bonded to the mica paper by a silicone, polyester, epoxy or phenolic resin.

6. A cable as claimed in any one of the preceding claims in which the tape is longitudinally applied.

7. An armoured control cable substantially as described in Example 1.

8. A radiating cable substantially as

described in Example 2 or Example 3 or Example 4.

9. An electric cable as claimed in claim 1 or claim 2 and substantially as described with reference to the Examples considered collectively.