MINERAL INSULATED CABLE

Description

[0001] 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.

[0002] 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. GB-A-2056158 discloses a mineral insulated cable, the mineral insulator being impregnated with heat-resisting silicone oil.

[0003] 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 as claimed in claim 1.

[0004] 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.

[0005] 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 inter 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] According to another aspect, the invention provides a mineral insulated cable as claimed in claim 10.

[0010] Preferred materials, designs and compositions for the cable are as described above.

[0011] 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

1. A method of forming a mineral insulated cable which comprises the steps of:

(i) coating a particulate mineral insulant with not more than 5% by weight of an uncured silicone oil based on the total weight of the mineral insulant and the 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 4, 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 the 1% by weight of silicone oil.

7. A method as claimed in any one of claim 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 based on the total weight of the mineral insulant and the 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.

Ansprüche

1. Verfahren zum Bilden eines mineralisolierten Kabels, umfassend:

(i) Beschichten eines besonderen mineralischen Isolierstoffs mit nicht mehr als 5 Gewichtsprozent eines unvernetzten Silikonöls, basierend auf dem Gesamtgewicht des mineralischen Isolierstoffs und des Silikonöls,

(ii) Unterziehen des beschichteten mineralischen Isolierstoffs einem Wärmebehandlungsschritt, damit das Silikonöl mindestens teilweise polymerisiert wird und ein beliebiger Wasserstoff, der während der Polymerisation entsteht, entfernt werden kann;

(iii) Einführen des sich ergebenden mineralischen Isolierstoffs in ein Metallrohr, das einen oder mehrere verlängerte Leiter umfasst, die sich entlang seiner Länge erstrecken, um eine Kabelvorform zu bilden, wobei der oder jeder Leiter mittels des mineralischen Isolierstoffs gegen das Rohr und gegen jeden Leiter isoliert ist / sind, der vorhanden sein kann; und

(iv) Unterziehen der Kabelvorform mehreren Zieh- und Temperschritten, wobei die Vorform im Durchmesser verringert wird, wobei die Temperschritte so sind, dass die Kabelvorform keine Temperatur erreicht, die die 450 °C übersteigt;

2. Verfahren nach Anspruch 1, wobei die Kabelvorform während der Temperschritte keine Temperatur erreicht, die über 400 °C liegt.

3. Verfahren nach Anspruch 1 oder 2, wobei mindestens ein größerer Teil des mineralischen Isolierstoffs ein Mineral umfasst, das eine relative Permittivität von nicht mehr als 3 besitzt.

4. Verfahren nach Anspruch 3, wobei mindestens ein größerer Teil des mineralischen Isolierstoffs amorphes Siliziumoxid umfasst.

5. Verfahren nach einem der Ansprüche 1 bis 4, wobei der mineralische Isolierstoff nicht mehr als 2 Gewichtsprozent Silikonöl umfasst.

6. Verfahren nach Anspruch 5, wobei der mineralische Isolierstoff nicht mehr als 1 Gewichtsprozent Silikonöl umfasst.

7. Verfahren nach einem der Ansprüche 1 bis 6, wobei der mineralische Isolierstoff mindestens 0,2 Gewichtsprozent Silikonöl umfasst.

8. Verfahren nach Anspruch 7, wobei der mineralische Isolierstoff mindestens 0, 5 Gewichtsprozent Silikonöl umfasst.

9. Verfahren nach einem der Ansprüche 1 bis 8, wobei das Silikonöl ein aliphatisches Silikonöl ist.

10. Mineralisoliertes Kabel, das ein Metallrohr, einen oder mehrere verlängerte Leiter, die sich innerhalb des Rohrs befinden und die sich entlang der Länge des Rohrs erstrecken und einen mineralischen Isolierstoff umfasst, der das Rohr füllt und der den oder die Leiter gegen das Rohr und (wenn mehr als ein Leiter vorhanden ist) gegen einander isoliert, wobei der mineralische Isolierstoff Teilchen umfasst, die mit nicht mehr als 5 Gewichtsprozent eines polymerisierten Silikonöls, basierend auf dem Gesamtgewicht des mineralischen Isolierstoffs und des Silikonöls, beschichtet ist.

11. Kabel nach Anspruch 10, wobei der mineralische Isolierstoff eine relative Permittivität von nicht mehr als 3 besitzt.

12. Kabel nach Anspruch 10 oder 11, wobei mindestens ein größerer Teil des mineralischen Isolierstoffs amorphes Siliziumoxid umfasst.

13. Kabel nach einem der Ansprüche 10 bis 12, wobei der mineralische Isolierstoff nicht mehr als 2 Gewichtsprozent des polymerisierten Silikonöls umfasst.

14. Kabel nach einem der Ansprüche 10 bis 13, wobei der mineralische Isolierstoff mindestens 0,5 Gewichtsprozent des polymerisierten Öls umfasst.

Revendications

1. Procédé de formation d'un câble isolé à isolant minéral, qui comprend les étapes suivantes:

(i) enrober un isolant minéral particulaire avec au maximum 5 % en poids d'une huile de silicone non durcie, par rapport au poids total de l'isolant minéral et de l'huile de silicone ;

(ii) soumettre l'isolant minéral enrobé à une étape de traitement thermique afin de polymériser au moins partiellement l'huile de silicone et de permettre l'élimination de tout hydrogène dégagé pendant la polymérisation;

(iii) introduire l'isolant minéral résultant dans un tube métallique qui contient un ou plusieurs conducteurs allongés qui s'étendent suivant sa longueur pour former une préforme de câble, le ou chaque conducteur étant isolé du tube et de tout autre conducteur qui peut être présent au moyen de l'isolant minéral ; et

(iv) soumettre la préforme de câble à une pluralité d'étapes d'étirage et de recuit, par lesquelles le diamètre de la préforme est réduit, les étapes de recuit étant telles que la préforme de câble n'atteigne pas une température supérieure à 450°C.

2. Procédé selon la revendication 1, dans lequel la préforme de câble n'atteint pas une température supérieure à 400°C pendant les étapes de recuit.

3. Procédé selon la revendication 1 ou la revendication 2, dans lequel au moins une majeure partie de l'isolant minéral comprend une substance minérale ayant une permittivité relative non supérieure à 3.

4. Procédé selon la revendication 3, dans lequel au moins une majeure partie de l'isolant minéral comprend de la silice amorphe.

5. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel l'isolant minéral ne comprend pas plus de 2 % en poids d'huile de silicone.

6. Procédé selon la revendication 5, dans lequel l'isolant minéral ne comprend pas plus de 1 % en poids d'huile de silicone.

7. Procédé selon l'une quelconque des revendications 1 à 6, dans lequel l'isolant minéral comprend au moins 0,2 % en poids d'huile de silicone.

8. Procédé selon la revendication 7, dans lequel l'isolant minéral comprend au moins 0,5 % en poids d'huile de silicone.

9. Procédé selon l'une quelconque des revendications 1 à 8, dans lequel l'huile de silicone est une huile de silicone aliphatique.

10. Câble isolé à isolant minéral, qui comprend un tube métallique, un ou plusieurs conducteurs allongés disposés à l'intérieur du tube et qui s'étendent suivant la longueur du tube, et un isolant minéral qui remplit le tube et qui isole le ou chaque conducteur du tube et (lorsqu'il y a plus d'un conducteur) de tout autre conducteur, dans lequel l'isolant minéral comprend des particules qui sont enrobées avec au maximum 5 % en poids d'une huile de silicone polymérisée, par rapport au poids total de l'isolant minéral et de l'huile de silicone.

11. Câble selon la revendication 10, dans lequel l'isolant minéral a une permittivité relative non supérieure à 3.

12. Câble selon la revendication 10 ou la revendication 11, dans lequel au moins une majeure partie de l'isolant minéral comprend de la silice amorphe.

13. Câble selon l'une quelconque des revendications 10 à 12, dans lequel l'isolant minéral ne comprend pas plus de 2 % en poids de l'huile de silicone polymérisée.

14. Câble selon l'une quelconque des revendications 10 à 13, dans lequel l'isolant minéral comprend au moins 0,5 % en poids de l'huile polymérisée.