MINERAL INSULATED CABLE

Abstract: A method of manufacturing mineral insulated cable comprises transporting a strip (1) of metal and one or more elongate conductors (4) along their length, continuously forming the strip into a tube that encloses the or each conductor, and welding opposed longitudinally extending edges of the strip 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. The mineral insulant comprises amorphous silica that contains from 0.01 to 30 % by weight of a binder that causes the silica to resist deformation of the tube when the tube is subjected to the reduction operations. Preferred binders include mullite in the range of about 13 to 20 % by weight, and lime in the range of from 1 to 20 % by weight.

Description: MINERAL INSULATED CABLE This invention relates to the manufacture of mineral insulated electric cable, 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 upto 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 (the disclosure of which is incorporated herein by reference), 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 80 to 99% 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. The annealing temperature usually lies in the range of 350 to 650°C, depending on the speed of the cable preform through the annealing stage. A number of materials have been proposed for use as mineral insulant in mineral insulated cables, although throughout their history the vast majority of such cables have employed magnesium oxide in view of its thermal stability, high insulation resistance and handling properties. However, recently mineral insulated cables have been employed increasingly for signal transmission rather than power supply, and the bandwidth of the signals has been continually increasing to the point that the distance along which the signals can be transmitted by the mineral insulated cable has been severely restricted by the dielectric properties of the magnesium oxide mineral insulant. Thus, magnesium oxide has a relative permitivity (ε r ) of about 4.58 which leads to a capacitance in the order of about 230 pFm "1 for a typical MI cable as compared with capacitance values of about 100 to 120 pFm "1 for a soft insulated cable. Clearly it would be an advantage to employ a mineral insulant such as silica with a relative permitivity of about 2.28 as the mineral insulant, since this would reduce the overall capacitance of the mineral insulated cables to the same values as those of soft insulated cables. However, we have observed that if silica is employed as the mineral insulant in a continuous manufacturing process as described above, it is not possible to produce a cable that is free of defects. In particular, we have observed regular longitudinal splitting of the metal sheath adjacent to the sheath weld line. The splitting usually occurs in the drawing stage of the process for example after the second bank of rollers. It is possible to reduce the tendency of the preform to split, but not to r%move it, by increasing the annealing temperature. For example, if the annealing temperature is raised to a value in the range of from 450 to 550°C, the preform will split at the third bank of rollers rather than at the second. It is, in general, not possible to increase the annealing temperature yet further because the cable becomes too soft and oxidises. According to the present invention, the mineral insulant contains from 0.01 to 20% by weight of a binder that causes the silica to resist deformation of the tube when the tube is subjected to the reduction operations. The present invention is based at least in part on our observation that, in a continuous process, during the diameter reduction operations when the cable preform is subject to forces applied sequentially at different points on the preform, a silica mineral insulant will allow the cable preform to deform under the applied forces whereas a magnesia mineral insulant will not, or will only allow deformation to a much lesser extent. Thus, during the diameter reduction operation with a silica mineral insulant, the cross-section of the preform is deformed into an ellipse by rollers bearing on opposite sides of the preform. However, in each reduction operation, a number of pairs of rollers are employed, alternate pairs bearing on the the preform at an angle of 90° to the adjacent roller pairs. This has the result that the cross-section of the cable preform rapidly oscillates between two ellipses whose major and minor axes alternate in orientation as the preform passes through the pairs of rollers. This causes rapid work hardening of the metal sheath (faster than that caused by the simple reduction of diameter of the preform), and consequent splitting of the sheath, usually at a point adjacent to the weld line. It is believed that the splitting of the metal sheath may be exacerbated by so-called 'reassertion' of the mineral insulant. Reassertion of the mineral insulant is a phenomenon in which the mineral insulant applies a radial force on the metal sheath immediately after the reduction operation to such an extent that the diameter of the sheath is caused to increase. For silica mineral insulant, increases in cross-sectional area in the range of from 3.5 to 7% are typical, the degree of reassertion generally increasing with the annealing temperature. According to the invention, the addition of a small quantity of a binder material to the silica, will reduce the tendency of the silica to flow, and will thereby enable the silica to resist the oscillating elliptical deformation of the cable preform without preventing reduction of the diameter thereof as it passes through the roller pairs. The binder material will preferably exhibit the similar temperature resistance to the silica, and so is preferably a refractory, for example a metal oxide, especially an oxide of calcium, aluminium, or magnesium, or a material such as boron nitride. The relative permitivity of the binder is not of critical importance since it will usually be employed at relatively low concentrations in the mineral insulant. The binder may comprise a mixed metal oxide, and especially a mixed oxide of aluminium, silicon and magnesium. Such mixed oxides include periclase, spinel, corundum, forsterite, cordierite, mullite, clinoenstatite, and cristobalite. As stated above, the mineral insulant contains at least 0.01% binder, preferably at least 0.1% binder, and especially at least 1% binder (all percentages expressed herein being by weight). However, the insulant should not contain more than about 30% binder since such levels will cause the overall relative permitivity of the mineral insulant to be unacceptably increased. Preferably the mineral insulant contains not more than 25% binder, and especially not more than 20% binder. It is possible to employ alternative materials as the binder to those mentioned above. For example, a polymerised silicone resin may be employed (although this may require a reduction in the annealing temperature). The use of a polymerised silicone resin coating in a mineral insulant is described in our copending UK patent application No. 9702827.8, the disclosure of which is incorporated herein by reference. Indeed it could be possible to employ incompletely polymerised silicone, and to complete polymerisation during the annealing stage. It is possible, very generally, to estimate the ability of any proposed binder to reduce the tendency of the cable to split by observing the effect of the binder on the bulk properties of the mineral insulant, as described in more detail below. Essentially, binders may be classified into a number of categories depending on the degree to which they prevent the silica mineral insulant from flowing after compaction. Preferably the binder is one that causes the at least some of the silica to agglomerate into lumps after compaction at a defined pressure. Some binders may cause higher degrees of agglomeration, for example causing the silica to be formed into a pellet after compaction, and such binders may also be employed. However, other binders do not cause any significant change in the ability of the silica to flow, and such binders are not considered suitable. The mineral insulated cable in accordance with the present invention may be of any form that is currently employed, for example it may have any number of internal conductors, and may, if desired, include an intermediate screen (triaxial construction). Often the cable will employ copper conductors and a copper sheath, which would, for example, be suitable for a power and signal cable that is employed in conditions where it could be subjected to a fire. However, other materials could be employed, for example, if the cable is intended to be employed for applications in which it will experience prolonged use at high temperatures, materials such as stainless steel, or nickel based alloys may be used. Examples of nickel based alloys include alloys of nickel with copper, e.g. having between 25% and 75% nickel, such as cupronickel, and Monel, and other alloys comprising nickel with chromium and/or with cobalt, e.g. Ni,Cr,Fe,Co alloys. Examples of such alloys include those sold under the trademarks 'Inconel' and 'Incoloy'. Alternatively pure nickel may be employed. Such cables may be employed for example as thermocouples (where two conductors of different composition are employed) or cables for capacitive transducers (for example for monitoring rotor blade tip clearances). A process according to the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a typical continuous process for the manufacture of mineral insulated cable; and Figure 2 is a schematic cross-sectional view of a mineral insulated cable undergoing reduction by means of rollers; and Figure 3 is a graphical representation showing the agglomeration characteristics of a range of binders. Referring to the accompanying drawings, in a continuous process for the manufacture of mineral insulated cable, metal (e.g. copper) strip 1 is unwound from supply spools 2, cleaned, degreased and edge-trimmed at station 3. The strip 1 is then continuously formed into a tube around a number of copper conductors 4 by means of forming rolls (not shown) at forming station 6 until the strip 1 encloses the conductors, and opposite longitudinally extending edges of the strip face each other. The opposite edges are then welded together at a TIG arc welding station 8. Downstream of the arc welding station the welded tube so formed is filled with mineral insulant supplied from hopper 9, the mineral insulant being introduced into the tube by means of a lance 10 that extends along the interior of the tube from the forming station 6 to a position beyond the welding station 8. After the welded tube has been filled with the mineral insulant, the cable preform 12 so formed is passed through three banks of shaped rollers (only the first bank 14 being shown) whose profiles cause the diameter of the tube to be reduced until the the final desired diameter of the cable is achieved. An annealing furnace (not shown) is located between adjacent banks of rollers, and after the last bank of rollers. Figure 2 shows schematically, and exaggerated for the sake of clarity, the mineral insulated cable preform as is passes between a pair of rollers in one of the banks. As can be seen, the rollers do not exert a radially uniform force on the preform, but instead apply the force on opposite sides of the preform in the direction shown by the arrows, thereby causing the preform sheath to be deformed into a generally elliptical shape. These rollers alternate with similar rollers oriented at 90° to them, with the result that, the metal sheath of the preform is caused to oscillate rapidly between ellipses having vertical and horizontal major axes, thereby causing rapid work hardening of the sheath. The degree to which the preform is distorted by the rollers is believed to depend on the ability of the mineral insulant within the sheath to flow and thereby to accommodate the pressure of the rollers, with the result that when amorphous silica is employed as the mineral insulant, the sheath exhibits in the region of 200 splits km "1 during manufacture. A number of binders were examined for their ability to prevent flowing of the amorphous silica insulant. The binders were mixed with the silica and a 20 ml quantity of the resulting mixture was pressed in a pelletiser for 15 minutes at a pressure of 4.5 kN. After 15 minutes the pressure was released and the powder was removed from the press and examined. The binders were graded into six catagories depending on the properties of the powder as shown in the table. TABLE CATAGORY DESCRIPTION Powder falls straight out of press Some pressure required to remove powder from press, but no agglomeration Some pressure required to remove powder from press, and 0 - 30% lumps Some pressure required to remove powder from press, and 30 - 70% lumps Some pressure required to remove powder from press, and powder had formed an almost complete pellet Some pressure required to remove powder from press. Powder had formed completely into a pellet. The results of the compression test on a range of binders is shown in figure 3. In addition, figure 3 shows the number of splits per kilometre of cable when the mineral insulant is employed in a cable manufactured by the process described in EP- A-0 384 778 (where tested). From the figure, it can be seen that AZ 44 and microspheres show no cohesion on compaction. Amorphous silica showed a tendency to compact, but did not coalesce to any great extent. Additions of other powders to silica increased its dendency to coalesce up to cagatories 2-3. From these tests, preferred binders are mullite in the range of about 13 to 20%, and lime in the range of from 1 to 20% by weight.

Claims: Claims: 1. A method of manufacturing mineral insulated cable 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, characterised in that the mineral insulant comprises amorphous silica that contains from 0.01 to 30% by weight of a binder that causes the silica to resist deformation of the tube when the tube is subjected to the reduction operations. 2. A method as claimed in claim 1, wherein the or at least one reduction operation comprises passing the cable preform between pairs of shaped rollers. 3. A method as claimed in claim 2, wherein the or at least one reduction operation comprises passing the cable preform between a number of different pairs of shaped rollers, some of the roller pairs bearing on the preform at a different circumferential position to that at which other roller pairs bear on the preform. 4. A method as claimed in any one of claims 1 to 3, wherein the cable preform is subjected to a plurality of reduction operations that are separated by an annealing operation. 5. A method as claimed in claim 4, which includes at least three reduction operations and at least two annealing operations. 6. A method as claimed in any one of claims 1 to 5, wherein the binder comprises an oxide of calcium, aluminium or magnesium, or boron nitride. 7. A method as claimed in any one of claims 1 to 5, wherein the binder comprises a mixed oxide of silicon and aluminium (and optionally one or more other elements). 8. A method as claimed in claim 7, wherein the binder comprises a mixed oxide of silicon, aluminium and magnesium. 9. A method as claimed in claim 8, wherein the binder comprises mullite. 10. A method as claimed in any one of claims 1 to 8, wherein the mineral insulant additionally contains a quantity of an agent to remove or prevent absorption of moisture. 11. A method as claimed in claim 10, wherein the agent comprises a silicone coating on the silica.

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