MINERAL INSULATED CABLE

Abstract: A method of forming a mineral insulated cable comprises: i) coating a particulate mineral insulant, for example silica, with not more than 5 % by weight of an uncured silicone oil; ii) subjecting the coated mineral insulant to a heat-treatment step in order at least partly to polymerise the silicone oil and to allow any hydrogen evolved during the polymerisation to be removed; iii) introducing the resulting mineral insulant into a metal tube that contains one or more elongate conductors that extend along the length thereof and are isolated from one another and from the tube, to form a cable preform; and iv) subjecting the cable preform to a number of drawing and annealing steps, whereby the preform is reduced in diameter, the annealing steps being such that the cable preform does not reach a temperature exceeding 450 DEG C. The method enables the intrinsic hydrophilic nature of the mineral insulant to be removed so that moisture ingress at the ends of the cable or in the event of damage to the sheath is prevented. This can be achieved without affecting the flowability of the mineral insulant powder to any signifiant extent, so that the cable can be manufactured by a dry-filling process such as the "vertical-fill" or continuous process.

Description: MINERAL INSULATED CABLE This invention relates to the manufacture of mineral insulated electrical cables, that is to say, cables which comprise at least one elongate electrical conductor and a surrounding metal sheath, the or each elongate conductor being insulated from the sheath, and from any other conductor, by means of compacted mineral insulating powder. Such cables have been manufactured for many years, and are widely employed for example where performance may be needed at high temperatures for indefinite periods, such as in systems intended to operate during fires. The cables were originally manufactured by a so-called 'vertical-fill' process in which the conductors are inserted into a vertically oriented metal tube, and mineral insulant is poured into the tube while compacting it, to form a cable preform. The cable preform is then subjected to a number of die drawing and annealing operations to reduce its cross-sectional area by about 99%, thereby to form the finished cable. In recent years, manufacturing economics have required a move to continuous processes, at least for the more commonly sold sizes of cable. One such process is described in EP-A-0 384 778, in which a strip of metal and one or more elongate conductors are transported along their length and the strip is continuously formed into a tube that encloses the or each conductor, opposed longitudinally extending edges of the strip are welded together, mineral insulant is inserted into the tube to form a cable preform, and the cable preform is subjected to one or more reduction operations in which its diameter is reduced to form the cable. Reduction operations have traditionally been performed in the 'vertical -fill' method by pulling the cable preform through a number of dies, the steps being separated by annealing stages. In the continuous process, the die reduction stage is replaced by banks of shaped rollers arranged in pairs about the cable preform, alternate roller pairs in each bank being arranged at 90° to one another, so that the cross-sectional area of the preform is reduced by about 40 to 70 percent as it passes through each bank of rollers. An annealing stage is located between the banks of rollers and after the last bank of rollers. Typically, the drawing stage will comprise three banks of rollers, each with 14 pairs of rollers, and three annealing stages. Whichever process is used, annealing temperatures lying in the range of 450 to 650°C are normally employed, depending on the speed of the cable preform through the annealing stage. While such processes are generally satisfactory, it would be preferred if the cable could be rendered more resistant to moisture ingress, for example at the ends of the cable, or in the region of any damage to the cable sheath. According to one aspect, the present invention provides a method of forming a mineral insulated cable which comprises the steps of: (i) coating a parti culate mineral insulant with not more than 5% by weight of an uncured silicone oil; (ii) subjecting the coated mineral insulant to a heat-treatment step in order at least partly to polymerise the silicone oil and to allow any hydrogen evolved during the polymerisation to be removed; (iii) introducing the resulting mineral insulant into a metal tube that contains one or more elongate conductors that extend along the length thereof to form a cable preform, the or each conductor being isolated from the tube and from any other conductor that may be present by means of the mineral insulant; and (iv) subjecting the cable preform to a plurality of drawing and annealing steps, whereby the preform is reduced in diameter, the annealing steps being such that the cable preform does not reach a temperature exceeding 450°C. The method according to the invention enables the manufacture of a mineral insulated cable in which the intrinsic hydrophilic nature of the mineral insulant is removed, so that moisture ingress at the ends of the cable or in the event of damage to the sheath is prevented. Furthermore, rendering the mineral insulant hydrophobic according to the process of the present invention does not affect the flowability of the mineral insulant powder (before compaction by the drawing stage) to any significant extent, so that the cable can be manufactured by a dry-filling process (i.e. by a process in which the mineral insulant is introduced into the tube as a free-flowing powder, as distinct from processes in which the mineral insulant is formed into a paste), for example by the "vertical-fill" or continuous process as described above. The method according to the invention is preferably conducted so that the cable preform does not reach a temperature exceeding 400°C and especially not exceeding 380°C during the annealing steps, since to high annealing temperatures will degrade the polymerised silicone coating on the mineral insulant. Annealing temperatures quoted herein are the temperatures reached by the mineral insulant in the cable preforms rather than the annealing furnace temperatures, since the temperature reached by the cable preforms will depend wter alia on the dwell time in the furnace. However, the annealing temperature is preferably at least 350°C. The annealing temperatures employed in the process according to the present invention are significantly lower than those employed conventionally, for example in the region of 575°C, in order to prevent any degradation of the polymerised silicone in the mineral insulant. The silicone oil is preferably purely aliphatic, and is preferably a medium molecular weight aliphatic silicone. The silicone oil is added to the mineral insulant during the coating step to an amount of not more than 5%, by weight (based on the total weight of the mineral insulant and the silicone oil). A relatively low quantity such as this is used in order to reduce or prevent any gas evolution from the end of the cable or from any damaged portion of the cable sheath during prolonged exposure to fire. Preferably the silicone oil is employed in an amount of not more than 2% by weight and especially not more than 1% by weight. However, the silicone oil is normally employed in an amount of at least 0.2% by weight since quantities significantly below this may not provide sufficient hydrophobic nature to the mineral insulant, and more preferably at least 0.5% by weight (all percentages being based on the total weight of the mineral insulant and the silicone oil). Normally the quantity of silicone oil used will be 0.75% by weight. The process according to the present invention, at least in its broadest aspect, is applicable to the production of mineral insulated cables employing any mineral insulant, for example magnesium oxide, alumina or boron nitride. However, materials such a magnesium oxide have the disadvantage, at least in some applications where the cable is intended to carry signals instead of, or in addition to electrical power, that the relative permitivity (ε r ) of the mineral insulant is relatively high (in the region of 4.6 for magnesium oxide) with the result that the capacitance of the cable is relatively high and its characteristic impedance relatively low. In view of this, it is preferred, at least in some cases, for the mineral insulant to have a relatively low relative permitivity, for example of not more than 3. Thus, preferably, at least a major part of the mineral insulant comprises amorphous silica (which has a relative permitivity in the region of about 2.3). In such a case, the mineral insulant more preferably comprises at least 80% by weight amorphous silica, most preferably at least 90% by weight silica, and especially substantially entirely silica (based on the total inorganic content). If other mineral insulants are employed in addition to the amorphous silica, conventional mineral insulants such as magnesium oxide, alumina and boron nitride may be employed, preferably magnesium oxide. As one example of a process for forming a mineral insulated cable according to the invention, particulate amorphous (fused) silica mineral insulant, is mixed for about 15 to 20 minutes with 0.75% by weight medium weight silicone oil (DC 1107 sold by Dow Corning), and the coated silica is heat treated for one hour at 150°C in order to polymerise the silicone oil and drive off any hydrogen gas generated during the polymerisation. A length of a mineral insulated cable preform is then formed by a "vertical-fill" method in which a copper tube of 50 to 60 mm diameter is held vertically and a solid copper conductor is held inside the tube by means of a die at the bottom of the conductors so that it is spaced from the tube. The coated mineral insulant, which is freely-flowable after polymerisation of the silicone oil, is introduced into the tube an packed down at the bottom by vertical oscillation of the die. As more mineral insulant is introduced into the tube, the die rises, and introduction of the insulant is terminated when the die reaches the top of the tube. The cable preform so formed is then subjected to a number of drawing steps in which the preform is pulled through dies of decreasing diameter so that the diameter of the preform is reduced from the original 50 to 60 mm down to about 5 mm (about 30 die drawing steps). After each two to three draws, the preform is annealed at 375°C for about one hour. According to another aspect, the invention provides a mineral insulated cable, which comprises a metal tube, one or more elongate conductors located within the tube and which extend along the length of the tube, and mineral insulant that fills the tube and which isolates the or each conductor from the tube and (where more than one conductor is present) from one another, wherein the mineral insulant comprises particles that are coated with not more than 5% by weight of a polymerised silicone oil. Preferred materials, designs and compositions for the cable are as described above. Although the cables have been described only with reference to one sheath, it is quite possible for the cable to include more than one sheath, for example to be in the form of a triax cable.

Claims: Claims: 1. A method of forming a mineral insulated cable which comprises the steps of: (i) coating a paniculate mineral insulant with not more than 5% by weight of an uncured silicone oil; (ii) subjecting the coated mineral insulant to a heat-treatment step in order at least partly to polymerise the silicone oil and to allow any hydrogen evolved during the polymerisation to be removed; (iii) introducing the resulting mineral insulant into a metal tube that contains one or more elongate conductors that extend along the length thereof to form a cable preform, the or each conductor being isolated from the tube and from any other conductor that may be present by means of the mineral insulant; and (iv) subjecting the cable preform to a plurality of drawing and annealing steps, whereby the preform is reduced in diameter, the annealing steps being such that the cable preform does not reach a temperature exceeding 450°C. 2. A method as claimed in claim 1, wherein the cable preform does not reach a temperature exceeding 400°C during the annealing steps. 3. A method as claimed in claim 1 or claim 2, wherein at least a major part of the mineral insulant comprises a mineral having a relative permitivity of not more than 3. 4. A method as claimed in claim 3, wherein at least a major part of the mineral insulant comprises amorphous silica. 5. A method as claimed in any one of claims 1 to 5, wherein the mineral insulant includes not more than 2% by weight of silicone oil. 6. A method as claimed in claim 5, wherein the mineral insulant includes not more than 1% by weight of silicone oil. 7. A method as claimed in any one of claims 1 to 6, wherein the mineral insulant includes at least 0.2% by weight of silicone oil. 8. A method as claimed in claim 7, wherein the mineral insulant includes at least 0.5% by weight of silicone oil. 9. A method as claimed in any one of claims 1 to 8, wherein the silicone oil is an aliphatic silicone oil. 10. A mineral insulated cable, which comprises a metal tube, one or more elongate conductors located within the tube and which extend along the length of the tube, and mineral insulant that fills the tube and which isolates the or each conductor from the tube and (where more than one conductor is present) from one another, wherein the mineral insulant comprises particles that are coated with not more than 5% by weight of a polymerised silicone oil. 11. A cable as claimed in claim 10, wherein, the mineral insulant has a relative permitivity of not more than 3. 12. A cable as claimed in claim 10 or claim 11, wherein at least a major part of the mineral insulant comprises amorphous silica. 13. A cable as claimed in any one of claims 10 to 12, wherein the mineral insulant includes not more than 2% by weight of the polymerised silicone oil. 14. A cable as claimed in any one of claims 10 to 13, wherein the mineral insulant includes at least 0.5% by weight of the polymerised oil.

Inventors: DEEGAN EDWARD, FREESTONE JAMES, GRIFFITHS CARL, DEEGAN, EDWARD, FREESTONE, JAMES, GRIFFITHS, CARL
Applicants: BICC PLC, DEEGAN EDWARD, FREESTONE JAMES, GRIFFITHS CARL, BICC PUBLIC LIMITED COMPANY, DEEGAN, EDWARD, FREESTONE, JAMES, GRIFFITHS, CARL
Application year: 1998
Main classification: H01B13
IPC codes: H01B13