GB2056158A

GB2056158A - Mineral insulated cable assembly and method of making the same

Info

Publication number: GB2056158A
Application number: GB8023863A
Authority: GB
Original assignee: Okazaki Manufacturing Co Ltd
Current assignee: Okazaki Manufacturing Co Ltd
Priority date: 1979-07-27
Filing date: 1980-07-22
Publication date: 1981-03-11
Also published as: FR2462771A1; DE3022736A1; NL8003259A

Abstract

A mineral insulated cable assembly comprises a metal sheath (1), metal conductors (3), and a mineral insulator (6) fllling the space therebetween. The mineral insulator (6) is impregnated with heat-resisting silicon oil which does not vaporize under high temperature, even up to about 600 DEG C. The manufacture of the mineral insulated cable assembly includes the step of soaking a pellet of mineral insulator in heat-resisting silicon oil to impregnate the pellet with the silicon oil. <IMAGE>

Description

SPECIFICATION Mineral insulated cable assembly and method of making the same This invention relates to a mineral insulated cable assembly and a method of making the same.

As is well known, a mineral insulated cable (abbreviated to Ml cable) includes magnesium oxide (MgO) as an insulator, which densely packs the space between a metal sheath, such as a copper pipe, and a metal conductor inserted in the sheath. With this distinctive construction, Ml cable has outstanding characteristics such as high resistance to flame, heat, corrosion, water or radioactivity, and further has a large current carrying capacity and a high mechanical strength.

Because of these characteristics, Ml cables are used as wiring material under special conditions in such places as high temperature locations, explosion-proof areas, on board ship, chemical factories, atomic reactors, signal transmitters, and the like.

Recently, from the view point of saving resources, the use of mineral insulated cable has been greatly extended since it does not require replacement within a short period as do prior conventional cables.

However, one property of the MgO insulator of the Ml cable is that it is highly hygroscopic. Thus, if the insulator is laid open to air, it instantly absorbs moisture from the air and the insulating resistance thereof then descreases substantially.

When the MgO has absorbed moisture, it becomes necessary to cut off that moistened portion, or to heat the portion to remove the moisture from the cable. Various prior art treatments for removing moisture or for resisting ingress of moisture from cable-ends or conductor connections are known.

For example, one cable-end treatment utilizes a number of tools and instruments and requires that the steps of cutting a cable assembly, taking off the sheath thereof, assembling a gland, pot and disc, and applying a filling compound should be carried out successively within a short time.

In another method of treating a conductor connection disclosed in Japanese Patent Publication No. 16114/1978, an Ml cable assembly is protected from moisture by brazing a sleeve to the assembly and blowing an inert gas on the connecting portions.

However, in all such prior methods for removing or resisting ingress of moisture, a workman has to carry out a complicated operation within a short working time on the wiring spot.

When the wiring is complicated and includes many connecting points, or the length of the wiring is relatively long, it is almost impossible to complete the required work in the short time available. Also in the prior art methods, there is a danger of moisture being absorbed during the working time, so after the work has been completed it is necessary to monitor the degree of insulating resistance of the cable assembly over a long period. Furthermore, if there are many connecting points, it is difficult to find a wrong connection. Thus, the prior methods not only require a complicated operation, but also provide unreliable connections.

An object of this invention is to provide an Ml cable assembly and a method of manufacturing the same which do not require a complicated operation on the spot, and resist ingress of moisture substantially absolutely so the cable assembly never suffers from a drop in insulating resistance.

An Ml cable assembly in accordance with the present invention comprises a metal sheath forming an outermost layer of the assembly, one or more metal conductors located within the sheath and a mineral insulator filling the space between said metal sheath and said metal conductor or conductors, the particles of said mineral insulator being impregnated with heatresisting silicon oil.

The heat-resisting silicon oil must have the property of not vaporizing under high temperatures, even up to about 6000C (11 1 OOF).

Among the silicon oils publicly known, "AEROSEAL" sold by ARI INDUCTIRES INC. in the U.S. satisfies the above condition. Of course, it is possible to use other silicon oils having the same characteristic.

The method for manufacturing an Ml cable assembly in accordance with this invention comprises the steps of firing powdered mineral insulator to form a pellet of insulator, soaking said pellet in heat-resisting silicon oil under ambient temperature to impregnate the pellet with said silicon oil, inserting a metal conductor or conductors inside said pellet, locating a metal sheath over said pellet to form a cable assembly, reducing the outer diameter of said cable assembly under ambient temperature to crush the pellet into powder, annealing said cable assembly at a high temperature not exceeding about 6000C (1 1 1 OOF), and repeating said reducing and annealing steps alternately several times to obtain a predetermined outer diameter of the cable assembly.

There are two prior methods of manufacturing an Ml cable assembly. One is to make an aforementioned pellet, and the other is to inject powdered insulator into the space between the metal sheath and conductor directly. In this invention, the former method is adopted for the purpose of distributing silicon oil in said insulator uniformly.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a partial sectional view of the components of an Ml cable in accordance with the present invention; Fig. 2 is a partial sectional view of the assembled Ml cable; and Fig. 3 is a partial enlarged sectional view of the completed Ml cable.

Referring first to Fig. 1, a metal sheath 1 of copper or aluminum having predetermined diameter and length, a plurality of pellets 2 of mineral insulator, and metal conductors 3 of copper are prepared for further steps. The pellets 2 are made, in a well known procedure, by extruding powdered MgO with the addition of water and a binder (bonding material) and firing at a temperature of 1 4000C (25500 F) for about four hours. Holes 4 in which the metal conductors 3 are to be inserted may be formed in a desirable number in the latter extrusion step. The pellets after firing are, in accordance with this invention, impregnated with a heat-resisting silicon oil such as the aforementioned AEROSEAL by soaking them in a bath of the silicon oil under ambient temperature.

This soaking treatment is continued for about 65 hours under ambient or vacuum pressure conditions. If desired, only a part of the pellet can be impregnated by soaking a particular part in silicon oil. Since the length of one pellet is limited by its mechanical strength, one metal sheath is provided with a plurality of pellets.

Fig. 2 shows an assembled Ml cable after completing the steps of inserting the pellets 2, made by the aforementioned process, into a metal sheath 1, and the step of inserting metal conductors 3 into holes 4 of the pellets. This assembly is then passed through a die for a working treatment which reduces the diameter of the metal sheath 1, whereby the pellets 2 are crushed into powder which fills the spaces between adjacent pellets. After this first reducing step, the assembly is treated by an annealing process to eliminate residual stress. This annealing step is performed at a temperature near and below 6000C (111 0 F). In this step. since the aforementioned heat-resisting silicon oil is used, no silicon oil vaporizes.After completion of this annealing process, the reducing process and annealing process are repeated alternately, as required.

Fig. 3 illustrates a section of the Ml cable assembly produced by the aforegoing process.

This cable assembly does not absorb moisture from a cut end thereof since, as already explained, the interstices between particles of powdered material 6 of magnesium oxide are filled with silicon oil 5.

When the locations of the end portions or connecting portions of the cable are known from the beginning, it is sufficient to impregnate only the required portions with silicon oil.

Based upon the above mentioned construction, in an Ml cable assembly in accordance with this invention, moistureproof silicon oil fills the micro interstices between particles of powdered insulation to form a moistureproof seal (said interstices being hitherto a cause of absorbing moisture), whereby there is no danger of moisture being absorbed from cut ends or connecting portions of the cable. Thus, even when the cable is cut halfway, the Ml cable assembly of this invention does not suffer from the ingress of moisture from the cut end, and there is no need for difficult cable-end treatment on the spot.

Furthermore, due to the reliable moistureproofness whereby reliable Ml cable is supplied, there is no need for subsequent monitoring of the insulating resistance.

It is further understood that the foregoing description is a preferred embodiment of the invention and that various changes and modifications may be made in the invention without departing from the scope of the appended

Claims

claims. CLAIMS

1. A method of making a mineral insulated cable assembly having a mineral insulator enclosed within a metal sheath and one or more metal conductors located within said insulator, comprising the steps of; firing powdered mineral insulator to form a pellet of insulator, soaking said pellet in heat-resisting silicon oil under ambient temperature to impregnate the pellet with said silicon oil, inserting metal conductor or conductors inside said pellet and locating a metal sheath over said pellet to form a cable assembly, reducing the outer diameter of said cable assembly under ambient temperature to crush the pellet into powder, annealing said cable assembly at a high temperature not exceeding about 6000 C, and repeating said reducing and annealing steps alternately several times to obtain a predetermined outer diameter of the cable assembly.

2. A mineral insulated cable assembly comprising a metal sheath forming an outermost layer of the assembly, one or more metal conductors located within the sheath and, a mineral insulator filling the space between said metal sheath and said metal conductor or conductors, the particles of said mineral insulator being impregnated with heat-resisting silicon oil which does not vaporize under high temperatures, even up to about 6000 C.