IE44196B1 - Fire resistant electric cables - Google Patents

Abstract

1500121 Fire resistant cable PIRELLI GENERAL CABLE WORKS Ltd 8 Dec 1976 [9 Jan 1976] 00825/76 Heading H1A A multi-core cable, e.g. a telephone cable, for resisting high temperatures comprises silicone rubber insulated conductors, a bare earth wire, a sheath consisting of a laminate of a thermoplastic polymeric composition/aluminium or copper foil and an extruded sheathing of a fire retardant polymeric composition around the laminate, the metal layer being on the inside of the laminate and contacting the earth wire throughout the length of the cable. The polymer for the laminate and the outer sheathing may be P.V.C., polyvinylidene chloride, polyethylene or polypropylene. In the case of the outer sheathing it may alternatively be a thermoplastic rubber and be mixed with a fire retardant, e.g. chlorinated paraffin, antimony trioxide, magnesium carbonate, zinc borate or aluminium hydrate. The earth wire(s) may be tinned copper laid helically or straight.

Description

This invention is concerned with electric cables having good fire resistant properties.

A known class of fire resistant cables is the so-called mineral insulated cable which contains no organic material at all. This type of cable has been developed to withstand heating to 800°C for 4 hours without its functioning being adversely affected. It is well known, however, that mineral insulated cables need special equipment for their installation and the formation of terminations with such cables is very timeconsuming.

We have now developed a multi-core electric cable that is easier to install than mineral insulated cable and which meets the major operating requirements of the latter as a wiring cable. In particular, during laboratory tests, the cable we have developed has continued to function after being heated for 4 hours at 800°C, though it is damaged by such treatment.

According to the present invention, there is provided a multi-core electric cable, which comprises two or more electric conductors, each of which is insulated with a coating of silicone rubber insulation, a hare earth Wire, and a sheath consisting of a laminate of a thermoplastic polymeric composition/aluminium or copper foil and an extruded sheathing of a fire retardant polymeric composition around the laminate, the sheath enclosing the insulated conductors and the earth wire with the metal layer of the laminate on the inside and in contact with the earth wire throughout the length of the cable.

The metal layer constitutes an efficient electro I static screen, which is particularly advantageous where these cables are installed near electronic equipment, such as computers.

The cable construction according to the invention is suitable for cables in which the crosssectional area of each conductor is up to 4 square millimetres and which comprise from 2 to 19 separate conductors. By way of example only, this construction may be used for and 5 pair 0.5 mm telephone and signal cables for use in a high fire risk area, such as a power station.

Silicone rubber compositions suitable for use as electrical insulation and adapted for coating on conductors by extrusion are well known and commercially available. Any of known compositions of this kind may be used in the cable according to the invention.

The earth wire is preferably formed of tinned copper. It may be laid up helically with the insulated conductors or laid straight, in each case in contact with the metal layer of the laminate.

Suitable thermoplastics polymers for the polymer/ aluminium or copper laminate are, for example, polyvinyl chloride (PVC), polyvinylidene chloride, polyethylene and polypropylene, of which polyvinyl chloride is preferred. Such laminates are either known in the art and generally available or can be produced in the same way as the known laminates; preferred laminates are those having a metal layer having a thickness of 100 to 250 microns. To form the sheath around the core consisting of the insulated conductors and the earth wire, the polymer/metal laminate, in the form of tape having a width somewhat greater than 44ΐθθ the circumference of the core, is fed longitudinally on to the core and is formed (by suitable guide surfaces) around the core so as to encircle the latter completely, with one edge of the laminate overlapping the other and forming a longitudinal seam along the length of the cable. The outer sheathing of fire retardant polymer composition is then extruded on to the outer surface of the laminate.

The outer coating may be formed of any fire retardant polymer composition. The polymer present in the composition may, for example be PVC, polyvinylidene chloride, polyethylene, polypropylene or a thermoplastic rubber, and the polymer may be cross-linked by any suitable means. The composition contains one or more fire retardants, the choice of fire retardant being mainly dependent on the particular polymer present. Suitable fire retardants are, for example, chlorinated paraffin, antimony trioxide, magnesium carbonate, zinc borate and aluminium hydrate. Suitable compositions of this kind are known in the art and generally available.

We have found that the combination of silicone rubber insulation for the conductors and a sheath of a fire retardant thermoplastics polymer, such as PVC, which has an internal lining of metal,, the metal lining being in contact with a bare earth wire which forms a part of the core, provides a cable having a favourable combination of properties. Thus the cable is mechanically robust so that it is suitable for surface mounting and - 4 44196 is easy to install. It also continues to function, even though damaged by very severe overheating.

Cables in accordance with the invention having from 2 to 19 cores and with the individual conductors o having cross-sectional areas of from 0.5 to 4.0 mm , were manufactured from the following materials: Conductors Non-insulated earth continuity conductors Core insulation Plain annealed copper complying with BS 6360 Tinned annealed copper complying with BS 6360 Silicone rubber complying with BS 6899 1969 PVC-coated aluminium foil Annealed aluminium strip with a film of PVC securely bonded to one surface. The metal strip was annealed, commercially pure aluminium having a thickness of 0.150 mm 0.0125 mm. The weight of the PVC layer was 2 gm per square metre with a tolerance of - 0.5 gm + gm per square metre.

PVC outer sheath Type 2 PVC complying with BS 6746 1969.

The make up, dimensions and weights of the cables that were manufactured are shown in Table 1 below. 419 6 Η Ε* Approximate weight 1 Β ΙΛ-3< ΙΛΙΛ sOcO ri Ή ο ιηο ο co ο m *η d d οι O OO ΙΛ o moo in d dd d mo in -4 O 01 -S* O ,-h fa Φ Η Ρ Η Φ Φ Η SO rl ri Π rlsO SO H SOSO so d d d sO r< ε (4 § • · · · a a a a a a a a • a a β φ ε r-r-co σ\ rococo OS co co σ\ o 0-0 d •tf in •Η > rl rl ri ri rl Ρ ο α ω Φ ,β S 8 COCO COCO CO co co CO co co eo co co co co co CO Φ ο Β a a- a a • · · · • · ft a a a a • a She thi Ο Ο Ο Ο o o o o o o o o ooo o O 3 0 fa •rt Φ μ ρ fa φ enco co in cncoco in cncoco in cncoco co CO φ φ 3 1 cnm os cninos^ cn ιησ\.3< cn m os OS in ε > 3 ε • · · · • · · · • « · · a a a a a 3 ο φ οι ¢4 ¢3 a oi w cn 03 ca ν cn 03 01 03 03 03 ♦rt β α Ή 3 ο η *rt « •ρ φ α! 3 ε SO M3 SOSO sO SO SO SO so so so so SO so SO SO SO Η J4 g a · a -a • t 4 a · a a a a a a φ ϋ oo oo o o o o OOO o ooo o o 22 Η +> cn m S» ri co -Ρ » -ΐί β cnco co in cncoco m cncoco in cncoco o 4 ί ε τ* cn c^ei ncnr-Gi π mn. oi π m n. rl X f β • · « ♦ • · · · a a a a a a a rl h ·Η « rl Ti rl 01 Η rl Ή 01 rl rl rl 01 rl r* rl K X 0 -Ρ 0 ’'Vb.Xk. W. S^KS^X Η 3 Β ri ri r< th d d d d rt H Ti ri ri ri rl 03 03 0 Ο +> ΰ 3 •β Ό φ ΰ «μ S cnco co in cncoco m cncoco m cncoco CO CO Ό β 0 α ncnts-oj n cn r* oi n cn is oi n m r- n- en Ο d b • » · « • » · » • »· a a a a a & 0 • ri rl ri N dddOl r< H rl 01 ri rl ri η φ 0 WW s s s 0 3 Β rl rl r< Ή ri rl rl ri ri rl ri rl ri rl rl rl Η Φ Ν •Η ιη Η« φ ε Η 3 ε Φ 1 3 3 « Η Ο lf\ ΙΛΟ o intno o in in o o m m m m •rj w «Α Φ « · a · 6 « · · a a a a a a a Β 0 3 Φ ri Ή 01-4« ri rl 03-4< rl ri 01 -Sjd rf rl 03 03 ri 0 fa ί h Β ο 93 Φ fa « - “ φ φ fi w b Ε C 0 (Μ cn <1* ts- CM OS d ο rl Ά I Lengths of certain of the above-described cables were subjected to the following tests.

Fire Resistance A gas flame at a temperature of 800°C was applied to the cable with working voltage applied to the cable. After six hours, there was no electrical breakdown.

The PVC sheath also meets the requirements of BS 4o66 for flame retardant cables.

Another fire resistance test is defined in C.E.G.B. Specification Ο999θ^ which requires the cable to be subjected to a gas flame at a temperature of 1000°C and to continue to operate for a minimum of twenty minutes. Application of this test to the cable of the invention showed that after one hour at this temperature, the cable was still functioning satisfactorily.

Mechanical The cable was bent through 180° on a mandrel ten times the diameter of the cable, straightened and then bent through l80° in the opposite direction. This procedure was repeated five times; no kinking or creasing of the outer sheath developed.

Voltage The breakdown voltage of samples of cables according to the invention (having 2 cores with a nominal cross-sectional area of 2.5 mm and having 3 cores with a ο nominal cross-sectional area of 1.5 mm ) was determined 4419 6 following the bending test described above. The results are given in Table 2 below.

In the case of the 3 x 0.15 mm cable, failure took the form of flashover at the ends. The sheath of a further sample was then stripped back 150 mm to eliminate end flashover and the breakdown voltage again determined. The results are given in Table 2 below.

Table 2 2 Size mm C ire colour Voltage at Breakdown kV Form of failure After bending 2 x 2.5 Red 11 Breakdown Black 11 Breakdown 3 x 1.5 Red 11 Flashover Yellow 12 Flashover Blue 13 Breakdown With 15Ο mm tails 3 x 1.5 Red 16 Breakdown Yellow 18 Breakdown Blue 19 Breakdown Moisture Resistance The breakdown voltage of lengths of cores of These voltage breakdown strengths give a verygood margin of safety over voltage surges from contactors and fluorescent tubes which may well be of the order of 5kV. certain of the above-described cables were determined, both before and after the bending test described above. Further samples of the same cores were immersed in water for 24 hours and the breakdown voltage then determined.

The results of these tests are given in Table 3 below.

Table 3 Description 3 x 1.5 mm2 ο 2 x 2.5 mm o 2 x 4 mm Breakdown voltage of cable/earth lOkV 10kV lOkV Breakdown voltage after bending test lOkV lOkV lOkV Breakdown voltage on core immersed in water at 20°C for 24 hours lOkV lOkV lOkV Lengths of the cable were stored in containers so as to maintain a 100% relative humidity condition in the cable. After eight weeks, the cables were opened up and no corrosion of the aluminium foil or the bare earth conductor was found.

Claims (6)

1. CLAIMSϊ1. A multi-core electric cable which comprises two or more electric conductors, each of which is insulated with a coating of silicone rubber insulation, a bare earth wire, and a sheath consisting of a laminate of a thermoplastics polymeric composition/aluminium or copper foil and an extruded sheathing of a fire retardant polymeric composition around the laminate, the sheath enclosing the insulated conductors and the earth wire with the metal layer of the laminate on the inside and in contact with the earth wire throughout the length of the cable.

2. A multi-core cable according to claim 1, in which the thermoplastics polymer---^ of the polymer/metal laminate is polyvinyl chloride, polyvinylidene chloride, polyethylene or polypropylene.

3. · A multi^core cable according to claim 1 or 2, in which the metal layer of the laminate has a thickness of 100 to 250 microns.

4. A multi-core cable according to claim 1 or 2, in which the outer sheathing is formed of a composition comprising polyvinyl chloride, polyvinylideno chloride, polyethylene, polypropylene or a thermoplastic rubber, and a fire retardant for said polymer.

5. - A multi-core cable according to claim 1 or 2, in which the outer sheathing is formed of a composition comprising cross-linked polyethylene and a fire retardant therefor.

6. A multi-core electric cable substantially as described herein and having any one set of the dimensions 5 set forth in Table I.