Method of manufacturing mineral insulated electric cable and like elements

In a mineral insulated electric cable the powdered insulating material is impregnated with a quantity of a hydrophobic material, for example a silicone, which has been introduced into the powder in liquid form. The added hydrophobic material serves as a barrier against the ingress of moisture, and liquid dimethylpolysiloxane has been found particularly effective, the pressure of even a small quantity of this preventing any significant degree of moisture penetration into the cable.

This invention relates to mineral insulated electric cables and like 
elements, that is to say elements of the type comprising one or more 
electrical conductor wires enclosed within a tubular metal sheath, and 
insulated from the sheath by a filling of compacted powdered insulating 
material. 
Such elements will hereinafter be referred to simply as "mineral insulated 
electric cables" but this term is to be understood to include, in addition 
to wiring cables for the conduction of electric current for general 
purposes, elements of the construction described above which are designed 
to be employed for other purposes, for example sensing cables, heating 
cables, including sheathed wire electric heating elements, and 
thermocouple cables. The invention also includes within its scope the 
manufacture of such elements. 
The powdered insulating material which is most commonly used is magnesium 
oxide either fused or calcined or as sea-washed magnesia, and although 
magnesium oxide has a high electrical breakdown strength when dry the 
presence of even a small amount of moisture can reduce this significantly. 
Consequently the ingress of moisture is a major problem, particularly in 
the case of cables designed to operate athigh voltages. In the case of 
wiring cables it is therefore recommended that the ends of lengths of 
cable in store be provided with temporary seals to reduce moisture 
penetration. Nevertheless prior to forming a termination in a length of 
stored cable required for use it is invariably necessary to cut off an 
appreciable length, in some cases as much as 300 mm, from the ends of the 
cable in order to ensure that any damp powder, which would effect the 
insulating properties of the cable, is removed. This is clearly wasteful. 
Moreover terminations, formed in both this and other forms of cable, need 
to provide effective seals to prevent any subsequent moisture penetration 
in use. 
According, therefore, to the present invention in a mineral insulated 
electric cable the filling of powdered insulating material is impregnated 
with a quantity of a hydrophobic material which has been introduced into 
the powder in liquid form. The liquid hydrophobic material is readily 
absorbed by the powder and even a small amount of the liquid has been 
found sufficient to reduce moisture penetration to a marked extent. 
The hydrophobic material is preferably a liquid silicone and is preferably 
present in a proportion of 0.03% to 0.6% by weight with respect to the 
insulating material, for example approximately 0.11%. 
The liquid hydrophobic material should, of course, also have electrically 
insulating properties such that it does not have a deleterious effect on 
insulating properties of the powder filling. 
It is believed that the hydrophobic material forms a coating around the 
particles of powder and thereby provides a barrier to subsequent moisture 
penetration, and we have found liquid dimethylpolysiloxane, for example 
that sold by Dow Corning Limited as Silicone Fluid No. 17, to be a 
particularly efficient hydrophobic material suitable for this purpose. 
Thus a cable having a filling with between 1 and 20 mls of such a liquid 
added to each 5.0 kilograms of powdered magnesium oxide shows no 
significant moisture penetration even after many weeks storage, without 
the need for any separate end seals. 
Accordingly a cable in accordance with the invention can be stored without 
temporary end seals, and when required to be used the end does not require 
to be cut back further than is necessary to physically form the required 
termination, as there is no damp powder which needs to be removed. 
Moreover not only does the incorporation of liquid dimethylpolysiloxane 
within the filling have no detrimental effect on the electrical insulating 
properties of the filling, it has, in fact, been found to improve the 
electrical breakdown strength of the filling, particularly at high 
voltages, due, it is believed, to the exclusion of free moisture 
throughout the length of the cable. 
The sheath may be continuously formed from a ductile metal strip by bending 
the strip into tubular form and welding the edges together, whilst 
simultaneously introducing dry filling material and the conductor wire or 
wires into the sheath tube so formed, the liquid hydrophobic material 
being also introduced at this stage separately from the powder insulation. 
In such a case the or each said conductor wire may be guided into the 
sheath through a respective guide tube and the liquid hydrophobic material 
can also be introduced into the sheath through the guide tube or tubes, 
although a separate delivery tube could alternatively serve for such a 
purpose. 
Conveniently the powder insulation is introduced into the sheath tube 
through a delivery tube having its outlet downstream of the weld to avoid 
contamination of the weld, the liquid hydrophobic material also being 
delivered into the sheath tube, either through the conductor wire guide 
tubes or a separate delivery tube, downstream of the weld. 
Following the introduction of the conductor or conductors and the filling 
powder and liquid, the diameter of the sheath tube will usually be reduced 
by passing it through a series of reduction rollers or dies and annealing 
furnaces in known manner. 
The use of a liquid hydrophobic material has the further advantage that, 
during subsequent reducing and annealing processes, it acts as a 
lubricant, and this results in a pronounced reduction in the degree of 
abrasion of the conductor wire or wire sand of the inner surface of the 
sheath. 
Consequently the pronounced adherence of the filling powder to the 
conductor wire or wires, as is commonly experienced with mineral insulated 
electric cables as manufactured hitherto, is virtually avoided, and any 
loose powder on the surfaces of the wire or wires or on the inner surface 
of the sheath tube can be removed without difficulty when forming a 
termination. 
However other liquid aryl or alkylpolysiloxanes or mixtures thereof or any 
other suitable liquid hydrophobic, electrically insulating material might 
alternatively be used as an additive to the filling powder in a mineral 
insulated electric cable in accordance with the invention.

Referring first to FIG. 1, the cable comprises an outer sheath 1 formed 
from a copper strip bent into tubular form and argon arc welded along the 
abutting edges. The sheath tube 1 contains a plurality of conductor wires 
2 (in this case two) separated from each other and from the sheath tube 1 
by a filling of powdered fused magnesium oxide 3, the powder being 
compacted around the conductors, following the introduction of the powder 
and conductors into the formed sheath tube, by a series of reduction 
stages, each followed by an annealing and quenching stage in known manner. 
In accordance with the invention the sheath 1 contains, in addition to the 
powdered magnesium oxide a quantity of a dimethylpolysiloxane which has 
been introduced into the sheath in liquid form so that it penetrates into 
the magnesium oxide powder 3 surrounding the conductor wires 2. 
The liquid dimethylpolysiloxane is introduced into the sheath tube 1 in the 
ratio of 3.5 mls of liquid to 5.0 kilogram of the powdered magnesium oxide 
and it has been found that even this small proportion of liquid imparts a 
marked hydrophobic quality to the filling which resists the penetration of 
moisture, and prevents any significant deterioration of the insulating 
properties of the filling adjacent severed ends of the cable for long 
periods without the need to provide additional seals, either during 
storage or when forming subsequent terminations. 
Consequently when forming a termination it is not necessary to cut back the 
end of the cable further than is necessary to physically form the 
termination. 
The cable may be manufactured by a continuous process, and one such process 
is illustrated in FIGS. 2 to 4 of the drawings. 
In such a process the cable sheath 1 is formed in a continuous manner from 
a thoroughly degreased copper strip 1a by means of a tube forming machine 
(not shown) which bends the downwardly fed strip into tubular form, and an 
argon arc welding head 4 which welds the abutting edges of the strip. The 
formed sheath tube 1, with the conductor wires 2 and the magnesium oxide 
powder 3 introduced into it, is fed vertically downwards to a reduction 
machine, which reduces the diameter of the tube and compacts the filling 
powder around the conductor wires. The reduced tube is then fed through an 
annealing furnace, and then through a water quenching tank in which the 
cable is turned in a catenary curve to continue travelling horizontally 
through further reduction machines, annealing furnaces and quenching 
tanks. The reduction machines and associated equipment have, however, been 
omitted from the drawing for simplicity. 
The conductor wires 2, which are also thoroughly degreased before their 
introduction into the sheath tube 1, are fed continuously downwards into 
the tube, as it is being formed, through a pair of vertical guide tubes 5 
rigidly located in desired positions within a powder delivery tube 7 
through which the magnesium oxide powder is fed. The powder is introduced 
into the delivery tube 7 from a hopper 8 which is kept replenished from a 
vibratory conveyor 9 supplied, in turn, from a powder reservoir. 
The lower ends of the powder delivery tube 7 and of the guide tubes 5 
terminate below the weld so that the magnesium oxide powder 3 is 
effectively fed into the already formed and welded tube, and is thereby 
prevented from contaminating the weld. 
The guide tubes 5 are preferably disposed, as shown in FIG. 4, on opposite 
sides of the plane containing the axis of the sheath tube 1 and the 
welding head 4, and a further tube 10, by which argon is introduced into 
the sheath tube to maintain an inert atmosphere in the weld area, extends 
downwards within the powder delivery tube 7 adjacent the seam edges of the 
sheath tube 1, the lower end communicating with the weld area through an 
opening 11 in the wall of the delivery tube. The opening 11 is sealed 
around its edges to the argon delivery tube 10 to prevent escape of the 
magnesium oxide powder in the vicinity of the weld. 
In accordance with the invention liquid dimethylpolysiloxane, for example 
Dow Corning Silicone Fluid No. 17, is introduced into the filling powder 
by being fed at a controlled rate through small bore pipes 6 into the 
conductor guide tubes 5 from a reservoir (not shown) by means of an 
adjustable low output pump 8. In a particular example in which the 
diameter of the sheath tube 1, before reduction, is of the order of 20 mm, 
and has an initial rate of travel past the welding head 4 of about 2 
meters a minute, with the magnesium oxide powder being fed into the 
delivery tube 7 at a rate of about 1.0 kilogram a minute, the 
dimethylpolysiloxane was introduced into the conductor wire guide tubes 5 
at approximately 0.7 ml a minute, although this ratio is not critical. 
Although a vertical cable forming process has been described, the invention 
is also applicable to mineral insulated cables formed by a so-called 
horizontal process, in which the strip, which is to form the sheath tube, 
and the conductor wires are fed horizontally past the welding position. In 
such ra case the liquid dimethylpolysiloxane is conveniently introduced 
into the powder filling through a separate delivery tube.