NZ209204A - Screened fire and oil resistant cable construction - Google Patents

Description

i. ;>• ORIGINAL 209204 Pri,r, .. 2.3-% ^3 ' Complete ■; . . r. . HOiei C.3SS. ...... y— . 1 j , ^orojl/gp...,.

Publication Date: ... .*® JAN 79€|d.. P.O. Journal, ' ;o: ... 1^)03 ;NEW ZEALAND ;SUBSTITUTION OF APPUCAliT UNDEB SECTION 24 ■Sr/Vtv^^D "i j-CA&-eS &V A/Mf7ED ;PATENTS ACT 1953 ;COMPLETE SPECIFICATION ;o ;D ;"FIRE AND OIL RESISTANT CABLE ARRANGEMENT" ;WE, INTERNATIONAL STANDARD ELECTRIC CORPORATION,- a Corporation of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, New York, United States of America, 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: ;- 1 - ;i ;2 092 0 4 ;The present invention relates to cables and in particular to halogen free cables with improved flame/fire resistance and oil/abrasion resistance. ;Conventional flame retardant cables are usually made with halogen containing materials such as polyvinylcloride (PVC), polychloroprene (PCP), chlorsulphonated polyethylene (CSP. In a fire these materials give off thick, dense and corrosive smoke. In some cable installations this is not considered a drawback, but on sites like oil production platforms, hospitals and telephone exchanges such reactions are not acceptable. Cables containing halogen free materials have, however, also been known for some time. ;A cable having an insulation which, when it is exposed to very high temperatures or even flames, is transformed into an electrically insulating ash, may maintain its working capacity during and after a fire. An assumption is that the insulating ash is kept safe in position within the cable, and also that the cable is fastened securely to constructions which are stable during the fire to avoid bending and mechanical stress in the cable. Notwithstanding, known cables have evidenced a high percentage of failures during flame tests. ;One of the main problems concerned with installation of cables in potential fire areas is that the cable insulation may collapse during a fire so that the conductor(s) will shortcircuit to the metal screen of the cable. This problem ;and several solutions have been described in US patent Nos. 3.180.925 and 4.280.016. ;The cables concerned are usually built up from one or more individually insulated conductors and/or optical fibres, which are embedded in a filler sheath of halogen free material. The cables also include a braided metal screen as well as outer protective covers. ;One theory of how the failures occur in such conventional cables when subjected to high temperatures is as follows: ;When the insulation and the inner sheaths are burnt to ashes, the cable diameter tends to increase slightly. The braid which is applied around the inner sheath, is then radially expanded, and this again results in a longitudinal contraction of the braid. As this longitudinal contraction occurs simultaneously with the radial expansion, the frictional forces between the burnt insulation and the multiwire conductor elements become very large. Therefore, the longitudinal contraction of the braid will be frictionally transferred to the helical, outer elements of the multiwire conductor and further to the centre wire. Because of their helical form, the outer elements of the multiwire conductor are free to contract in the longitudinal direction. The centre wire, on the other hand, in a conventional multiwire conductor, is a straight element, and a contraction in longitudinal direction beyond the elasticity limit, is impossible. Instead, there will be built up a ;h ;» ;I ;I'M ;209204 ;longitudinal, compression force until the centre wire kinks ;Jy;- ;|j in an uncontrolled manner. The other elements of the multi- ;W ;| wire conductor will then also be forced out in a kink and ;| may, if a large kink occurs, make contact with the metal j 5 braid, which finally results in a shortcircuit of the cable. ;Uncontrolled kinks as described occur mostly in cables and ;| conductors with cross-sectional conductor areas in the order of 0.5 to 2.5 mm2 and when the number of conductors within the cable is low (<10). ;i\ 10 An object of the present invention is to provide improved ;^ I ;^ electric cables (power and tleecom), which will maintain circuit integrity under high temperature conditions. A further object is to provide fire resistant cables which have j ;high oil and abrasion resistance. ;15 When installing cables in the mentioned high fire risk areas it is essential when passing from one area to another or from one room or compartment to another that the wall bushings and glands are gas and fluid tight. In order to obtain and maintain gas tight glands it is important that 20 the cable insulation and in particular the filler sheath will not flow under the pressure of the gland. A vulcanized or crosslinked insulation or sheath has this property. ;One undesirable property of vulcanized (and thermoplastic) filler sheaths is, however, that they will increase in volume ;25 in a fire and cause undesirable radial pressure on the screen ;- 4 - ;2 Ci Q ? ;wires. A still further object of the present invention is to solve this problem. ;Another problem with cables to be used, for example in the oil industry, is that the commonly used outer protective 5 covers are not oil and abrasion resistant. Some sheaths of PVC would be oil resistant, but such sheaths are not acceptable due to their high halogen content. When subjected to oil the sheath may absorb oil which would be detrimental to the cable core. The oil absorption may also cause expans-| 10 ion of the filling sheath. It is a still further object of the present invention to also solve these problems. ;It has been found that there must be a close relationship between the properties of the filler sheath and the metal screen. ;15 It has in particular been found that the braid angle of the metal screen is very critical when constructing fire resistant cables. The braid angle a {Fig. 1) is defined as the angle which the individual wires form with a plane at right angles to the cable axis. From a production point of ;20 view it will be desirable to use a large braid angle which ;'■w would allow a high production speed. Experiments have, ;however, shown that braid angles above 50° are undesirable on cables with vulcanized sheaths, as radial expansion of such ^ sheaths results in axial contraction. ;O ;25 According to the present invention there is provided a ;- 5 - ;209204 ;fire and oil resistant cable arrangement comprising at least one insulated conductor and/or optical fibre embedded in a halogen free vulcanized filler sheath, a metallic braided screen surrounding said filler sheath as well as outer halogen free protective covers, wherein the conductor insulation includes one or more layers of mica, glass fibres or similar inorganic particles or compounds and is encompassed with an extruded layer of self-extinguishing material, and wherein the filler sheath comprises a material which is capable of letting off the water of crystallization when subjected to high temperatures in an amount large enough to obtain an oxygen index between 40 and 50, and wherein the metallic screen comprises wires which are braided with an angle of 35° to 45° relative to a plane substantially perpendicular to the cable axis, and wherein the outer protective covers include an extruded sheath of self extinguishing material. ;The conductors and possibly the optical fibres involved should be provided with a layer of mica, glass fibre or similar unorganic material which should be covered with an extruded layer e.g. of EPR or silan vulcanized EVA. ;The extruded conductor insulation as well as the extruded inner and outer sheaths are halogen free and contain amounts of aluminium trihydrate or similar material containing sufficient crystallization water to give the crosslinked polyolefin copolymeric material self-extinguishing properties. E.g. hydro- ;magnecite may be an alternative material. While the oxygen index of the conductor insulation should be above 24, that of the filler sheath should be between 40 and 50, whereas that of the outer sheath should be above 30. ;Whereas the optional outer oil and abrasion resistant layer of Nylon is halogen free, the material in itself is combustible, but the layer is used in accordance with the present invention is so thin, - in the order of 0.2 to 0.6mm -that when placed on top of the self-extinguishing outer protective sheath it will not sustain a fire. ;Tests have shown that the cables claimed at least satisfy the following requirements: ;- In IEC 331 the cable must stay in operation at the normal working voltage exposed to a flame at 750°C for three hours. The cables defined pass the test at 1000°C. ;- IEC 332 - 3, 1982 is a flame propagation test carried out on cables mounted on a ladder. The test is divided into three categories A, B and C. The cables in question pass category A which is the most difficult to pass. The number of cable lengths, 3.5m long, to give 7 liters of combustible (organic) material per meter is mounted on a ladder and placed in the test oven and exposed to a flame for 40 min. The cables pass the test if the height of the burned area does not go higher than 2.5m above the burner. ;DIN 57472/VDE 0742 part 813 Draft. ;* 209204 Indirect determination of corrosiveness of combustion gases. The combustion gases from a sample of each of the cable materials are led into water. The pH value and the electrolytical conductivity are measured. The test is passed if none of the 5 measured pH values is lower than 4 and if none of the measured electrolytical conductivity values exceeds 100 p. S cm"^.

If necessary these new cable sheaths and covers will also effectively protect the cable core against undue influence of drilling mud, oils and other hydrocarbons. The resistance 10 against abrasion and termite attacks is also excellent.

The above mentioned objects and features of the present invention will clearly appear from the following detailed description of embodiments of the invention, taken in conjunction with the drawing, which schematically shows a halogen 15 free, oil and fire resistant cable.

In experiments for finding the desired braid angle, speciments of cable, 900mm long, where subjected to flames at temperatures in the range of 750°C - 1000°C for up to 3 hours. The diameter and lengths of the cable specimens were measured 20 before and after the fire exposure.

These experiments showed that for braid angles over 50° the diameter over the braid increased by more than 10%, and the length of the cable samples shrunk with more than 10 mm. In all these samples with braid angle larger than 50° the 25 cable conductors showed waves and kinks, and in some cases 209204 the conductors also shortcircuited to the metal screen.

The experiment also included a number of cable specimens having a braid over the filler sheath in which the braid wires formed less than 45° with a plane perpendicular to the cable axis. As a conclusion the samples having braid angles between 35° and 45° showed less than 10% increase of the diameter over the braid after the fire, and a cable length shrinkage of less than 10mm. Preferably the braid angles should be chosen to be between 38° - 42° in which range the diameter increase/cable length shrinkage were shown to be in the order of 5% and 5mm respectively. A too small braid angle can lead to a stiff cable which is difficult to strip, and the production speed should also be reduced.

In Fig. 1 a cable according to the present invention comprises a cable core including one or more insulated conductors and/or optical fibres. The cable core 1 may be designed to transfer power, telecom and data. In a conventional manner the conductors 2 may be provided with initial layers or windings 4 containing mica, glass fibres or other materials which after a fire provided a cintered insulated layer on the conductor. Optical fibres 3 may be protected in the same or similar manner and/or these may be of the metal clad type to provide fire protection.

The conductor/fibre insulation also includes an extruded self-extinguishing layer 5 over the mica tape so that during installation, - when the filler sheath and the outer protective cable layers are removed, - there is no need to apply special self-extinguishing covers or tapes over the conductor insulation.

To improve the fire resistance, the conductors may be of the multiwire type described in the abovementioned US patents Nos. 3.180.925 and 4.280.016.

The insulated conductors and/or optical fibres and/or groups of conductors/fibres which may be enclosed within a screen 6 of copper/polyester or aluminium/polyester laminate, are embedded in a filler sheath 7 which to a substantial degree contains aluminium trihydrate or similar material as hydro magnecite with bound water of crystallization which at high temperatures gives off water.

The filler sheath 7 is vulcanized to satisfy the hot set test in IEC 92-3 Amendment 3, Appendix GA at 200°C, to provide a cable core suitable for giving fluid and gas tight passage through bushings and glands, and it has an oxygen index between 40 and 50.

Immediately over the filler sheath 7 there is provided a braided metal screen 8 comprising preferably tinned copper wires. The wires of this screen form 35° to 45° with a plane at right angles to the cable axis. As explained above the combination of such a screen 8 and the filler sheath 7 gives a cable which efficiently withstands fires because the cable 2 09204 is prevented from undue shrinkage and the cable conductors are prevented from forming waves and kinks.

The outer protective sheaths include a polyester tape winding 9 and a self-extinguishing sheath 10 having an oxygen index above 30 as well as an optional thin extruded sheath 11 of Nylon which effectively protects the cable core against abrasion and damaging hydrocarbons like oil and drilling mud. Several series of experiments have been made to investigate the properties of outer sheaths. Samples of cables being provided with outer sheaths of Nylon (trade names Vestamid 1540E, Vestamid L 1801E), Polyuretan, EVA (trade name 407 AE1), PE, PVC, PCP, CSP and CP, were tested according to the following procedures: 1. Oil-ageing in ASTM No. 2 after IEC 92-3 and Bureau Veritas - 1977 - Chapter 18. 2. Ageing of dumb-bell test specimens in drilling mud.

Only two of the sheaths showed satisfactory results in tensile strength, elongation at break and longitudinal swell, namely the Nylon sheath and the PVC sheath. However, the PVC sheath is not acceptable due to its content of halogen materials. While Nylon in general gives satisfactory results, Nylon 11 and 12 are preferred as outer protective oil and abrasion resistant layer.

Claims (8)

209204 € What we claim is:

1. A fire and oil resistant cable arrangement comprising at least one insulated conductor and/or optical fibre embedded in a halogen free vulcanized filler sheath, a metallic braided screen surrounding said filler sheath and outer halogen free protective covers surrounding the braided screen, wherein the conductor insulation includes one or more layers of mica, glass fibres or similar inorganic particles or compounds which provide a cintered insulation layer after exposure to fire, and encompassed by an extruded layer of self-extinguishing material, wherein the filler sheath comprises a material which Is capable of giving off the water of crystallization when subjected to high temperatures in an amount large enough to obtain an oxygen index between 40 and 50, wherein the metallic screen comprises wires which are braided with an angle of 35° to 45° relative to a plane substantially perpendicular to the cable axis, wherein the outer protective covers include an extruded sheath of self-extinguishing material.

2. A cable arrangement as claimed in claim 1, wherein the (^ filler sheath comprises aluminium trihydrate.

3. A cable arrangement as claimed in claim 1 or 2, wherein the outer protective covers Include an external sheath of extruded nylon.

A cable arrangement as claimed In any one of the preceding claims, wherein the braid angle Is 38° to 42°. ' ' ' O 209204

5. A cable arrangement as claimed In claim 3 or claim 4, wherein the nylon sheath is constituted by nylon 11 or 12.

6. A cable arrangement as claimed in claims 3 to 5, wherein the nylon sheath has a thickness In the order of 0.2-0.6mm.

7. A cable arrangement as claimed in any one of the preceding claims, wherein the self-extinguishing properties of the said extruded layer and of the said extruded outer sheath are obtained by adding sufficient amounts of aluminium trihydrate, or a like material capable of giving off water of crystallization, to crosslinkable polyolefin copolymers to obtain oxygen Indexes above 24 and 30 respectively of these layers .

8. A fire and oil resistant cable arrangement, substantially as herein described with reference to the figure of the drawing. STANDARD TELEPHONES AND CABLES PTY. LIMITED C.H. Matthews Secretary 13