Note: Descriptions are shown in the official language in which they were submitted.
METHOD OF AND APPARATUS FOR MAKING MINERAL
INSULATED CABLE
The present invention relates to a method of and
apparatus for making a mineral insulated cable.
Mineral insulated cable comprises an outer metal tubular
sheath, usually made of copper, containing one or more
conductors embedded in an insulating mineral, usually
magnesium oxide. Mineral insulated cable is used in
applications where the cable has to withstand high
temperatures or fires, for instance in emergency lighting
systems and fire alarm systems. Such cables have
conventionally been made by either a batch process or a
continuous process.
In a known batch process, preformed blocks of mineral
insulant having through-holes are inserted into a metal
tube which will form the outer sheath in the finished
cable. The holes in the blocks are aligned and conductor
rods are inserted through the aligned holes. This
arrangement forms a blank which is then further
processed, for instance by repeated drawing or rolling
and annealing to reduce the cross section and provide a
finished cable. In alternative batch processes, the
conductors are embedded in mineral insulant in powder
form, the metal tube being arranged vertically and the
powder being inserted from above. A ram may be used to
compact the powder within the tube.
By their very nature, such batch processes are capable of
producing cables of limited maximum length. Also, these
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processes have a relatively low rate of production, and
the finished cable made by such processes is relatively
expensive.
A known continuous process is illustrated in Figure 1 of
the accompanying drawings, which illustrates manufacture
of a mineral insulated cable having two conductor cores.
The conductors are made from a pair of copper rods 1
which are supplied continuously through bores in a spacer
block 2. Copper strip 3 for forming the cable outer
sheath is likewise continuously supplied to a tube
forming mill illustrated diagrammatically by a pair of
rollers 4 and 5. Powdered magnesium oxide 6 is fed under
gravity from a hopper 7 through a tube 8 so as to fill
the outer sheath. A welding station 9 welds the tube
seam in the immediate vicinity of the rollers 4 and 5 so
as to form a completed blank 10. The completed blank 10
is continuously fed to a plurality of rolling stages 11
and annealing stages 12, only one of each being shown in
Figure 1.
In practice, the continuous process illustrated in Figure
1 has to be performed vertically, at least up to the
first rolling stage 11. This requires a considerable
vertical space.
According to a first aspect of the invention, there is
provided a method of making mineral insulated cable,
comprising supplying preformed blocks of mineral insulant
having at least one groove and laying at least one
conductor in the or each groove.
Preferably, the preformed blocks are supplied
continuously and the or each conductor is continuously
laid in the or each groove. Preferably, a metal tube is
continuously formed around the blocks. Although the
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method according to the first aspect of the invention can
be used with advantage in various processes, such as the
batch process described hereinbefore, it is particularly
advantageous when used in a continuous process.
Preferably, the method comprises at least one further
step of cross section reduction, such as drawing or
rolling, with the or each further step being followed by
an annealing step.
The preformed blocks may be supplied as sets of blocks
having opposing faces provided with corresponding
grooves, the blocks of each set being brought together
such that the corresponding grooves form at least one
duct containing a respective conductor. For instance,
the sets may comprise pairs of blocks, each of which is
hemi-cylindrical and has at least one longitudinally
extending groove in a flat surface.
In an alternative arrangement, the blocks may be formed
as substantially cylindrical blocks with at least one
longitudinally extending peripheral groove for receiving
a respective conductor. After the or each conductor has
been laid in the respective groove, mineral insulant in
the form of blocks or powder may be introduced into the
or each groove so that the or each conductor becomes
embedded. Alternatively, a subsequent cross section
reduction step may be sufficient to close the mineral
insulant around the or each conductor.
The blocks may be held in place around the or each
conductor, prior to forming the metal tube, by a
plurality of rollers. Alternatively, the blocks may be
held in place by winding an elongate material
therearound. For instance, the elongate material may be
a thread, such as a glass fibre thread, wound helically
around the blocks. The elongate material may
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alternatively be an electrically insulating tape,
preferably self-adhesive, wound so as to cover or
partially cover the blocks. Electrically insulating tape
may alternatively be applied longitudinally around the
blocks and formed into a tube. The tape may, for
instance, be a silicone rubber which can have the
advantage of being self-amalgamating. However, other
types of tape may be used, such as mica tape or
polytetrafluoroethylene tape.
The use of electrically insulating tape to surround the
blocks has advantages in addition to holding the blocks
in place. The insulating properties of the finished
cable between the or each conductor and earth are
improved. When a continuous production process has to be
interrupted, the ingress of moisture into the blocks is
reduced or eliminated and this avoids possible problems
caused by degrading of the insulation, expansion of the
blocks, and production of steam within the cable during
subsequent heat treatment, such as annealing.
According to a second aspect of the invention, there is
provided an apparatus for making mineral insulated cable,
comprising means for supplying preformed blocks of
mineral insulant having at least one groove and means for
laying at least one conductor in the or each groove.
It is thus possible to provide a method and apparatus
which can be performed horizontally or in any convenient
arrangement, thus reducing the cost of manu~acturing
plant. The conductors are held accurately in place
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without the need for any guidance, which reduces or
eliminates the possibility of metal particles or slivers
being produced during guidance of the conductor or
conductors and entering the insulant. It is not
necessary to use fused magnesium oxide, and hence damage
to the conductor surface is reduced or eliminated. Thus,
it is not necessary to use over-sized conductors in order
to achieve a desired current rating. The density of the
mineral insulant can ~sily be varied in order to obtain
mineral insulated cable with desired properties. A much
higher rate of production can be achieved compared with
any known process for making mineral insulated cables.
Thus, the cost of the cable can be reduced and a cable
with better defined geometry and properties can be made.
The invention will be further described, by way of
example, with reference to the accompanying drawings, in
which:
Figure 1 is a diagram illustrating a known continuous
process for manufacturing mineral insulated cable, as
hereinbefore described;
Figure 2 is a diagram illustrating a method of and
apparatus for making mineral insulated cable constituting
a first preferred embodiment of the present invention;
Figure 3 is a cross sectional view of a preformed block
of mineral insulant for use in the method illustrated in
Figure 2;
Figure 4 is a cross sectional view of parts of a mineral
insulated cable before tube forming to form an outer
sheath;
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Figure 5 is a cross sectional view of a finished mineral
insulated cable constituting a preferred embodiment of
the invention;
Figures 6 to 13 are cross sectional views of different
shapes of preformed blocks which may be used in preferred
methods;
Figure 14 is a diagra~ illustrating a method of and
apparatus for making mineral insulated cable constituting
a second preferred embodiment of the present invention;
and
Figures 15 to 19 are cross-sectional views on lines I-I
to V-V, respectively, of Figure 14.
The method and apparatus illustrated diagrammatically in
Figure 2 show all the steps required to make preformed
blocks and finished mineral insulated cable. In the
first step 21, a mineral insulating powder, such as
magnesium oxide, is mixed and supplied to a powder
granulating step 22. The granules of insulant are
supplied to a tablet making step 23 which forms the
mineral into the desired shape of the preformed blocks.
These blocks are then supplied to a heat treatment step
24 which ensures that the blocks have a sufficiently
stable form for the subsequent steps.
The preformed blocks 25 have the shape shown in Figure 3
i.e. substantially hemi-cylindrical with a diameter of
approximately 1" (approximately 2.5. centimetres) and a
length of approximately 8" (approximately 20
centimetres). The flat surface of the block has two
longitudinally extending grooves 26 which are also
hemi-cylindrical in shape with a diameter of
approximately 1/5" (approximately 5 millimetres).
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The preformed blocks 25 are automatically supplied in
facing pairs at 27 and 28 so as to entrain therebetween
two copper conductors 29 supplied in the form of
continuous rods. The opposing grooves 26 of the pairs of
blocks 25 form continuous ducts containing the conductors
29.
The blocks 25 and the conductors 29, together with a
continuous strip 30 of- copper, are supplied to a
tube-forming mill 31 in which the strip 30 is formed into
a tube around the blocks. The resulting seam is welded
at 32 to form a continuous blank which is then supplied
to a plurality of further processing steps. Figure 3
shows, purely by way of example, three rolling steps 33
to 35, each of which is followed by a respective
annealing step 36 to 38, the final annealing step 38
being followed by a coiling step 39 for the finished
mineral insulated cable.
Figure 4 illustrates the partly formed blank as supplied
to the forming mill 31, whereas Figure 5 illustrates the
finished blank after the welding step 32. In fact, the
rolling and annealing steps 33 to 38 do not alter the
form, so that Figure 5 also illustrates the finished
mineral insulated cable, having a weld seam at 40.
Figure 6 illustrates the pairs of blocks 25, showing the
cylindrical ducts 41 provided by the opposed grooves 26.
Figure 7 illustrates two blocks 42 which have grooves
arranged to provide a single duct 43 for a single core
cable. The step of laying the conductors in the grooves
of the blocks may be performed in any suitable way. For
instance, as described above, the blocks 25 are brought
together around the continuously fed conductors 29.
However, in an alternative configuration, the lower
blocks of the pairs are supplied so as to define two
continuous grooves with the conductors being laid in the
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grooves from above. The upper blocks may then be placed
on top so as to complete the laying in of the conductors.
Figures 8 and 9 illustrate two alternative forms of
blocks 44 and 45. The blocks 44 shown in Figure 8 are
continuously supplied so as to define two continuous
diametrically opposite grooves 46. The blocks 45 in
Figure 9 differ in that the grooves 47 are side-by-side
and extend downwardly~- The conductors are laid into the
grooves 46 from the side whereas the conductors are laid
into the grooves 47 from above. In order that the
conductors be embedded within the mineral insulant, it
may be sufficient merely to perform the rolling
operations so that the mineral insulant closes around the
conductors. However, it is also possible to fill the
grooves 46 or 47, after the conductors have been laid
therein, with mineral insulant. Suitably shaped
preformed lengths of mineral insulant may be provided for
this purpose. Alternatively, mineral powder or granules
may be used, particularly with the blocks 46 shown in
Figure 9.
Figure 10 shows a set of four identical blocks 48, each
of which is generally quarter-cylindrical in shape with
grooves extending longitudinally along the two flat
surfaces of each block. When placed together as shown in
Figure 10, the blocks 48 define four ducts 49 for
receiving conductors in order to provide a four core
cable. The blocks 50 shown in Figure 11 differ in that
each is generally third-cylindrical in shape, these
blocks being used to provide a three core cable.
Figures 12 and 13 illustrate two possible forms of
dissimilar pairs of blocks. The blocks 51 and 52 ln
Figure 12 differ from the blocks 25 in Figure 6 in that
the block 51 has a longitudinal tongue 53 which extends
between ducts 54 into a correspondingly shaped groove in
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the block 52. Figure 12 shows the block 51 disposed
above the block 52, but the reverse arrangement is
possible and may have advantages in that the tongue 53
assists in correctly locating the conductors during
laying in.
The lower block 55 in Figure 13 is similar to the block
45 shown in Figure 9 but has a centre limb of reduced
height for co-operating with a preformed upper block 46
to close the conductors within ducts 57.
Figure 14 illustrates another process for continuously
forming mineral insulated cable. Preformed blocks 60 of
mineral insulant, such as magnesium oxide, are
continuously supplied in the direction of production,
indicated by arrow 61, so as to form a column. As shown
in Figure 15, the blocks 60 are substantially identical
to the blocks 25 shown in Figure 6 and are arranged in
the column with their grooves 62 aligned and facing
upwardly.
As the blocks move along the production line, copper
conductors 63 are supplied from reels 64 or the like and
are laid into the grooves 62 as illustrated in Figure 16.
Further insulating blocks 65 are continuously supplied
from above and are positioned on top of the blocks 60 so
as to enclose fully the conductors 63, as shown in Figure
17.
The blocks 60 and 65 and the conductors 63 are next
covered with a layer of insulation in the form of an
insulating tape 66 supplied from a reel 67 or the like.
As the blocks 60 and 65 and the conductors 63 move in the
direction of production, the reel 67 is rotated around
the axis of the cable and supplies the tape 66 so as to
form a continuous layer around the blocks 60 and 65. The
tape 66 is electrically insulating and preferably self-
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adhesive so as to adhere to the outer curved surfaces ofthe blocks 60 and 65. For instance, the tape 66 may be a
silicone rubber provided on one surface with an adhesive.
Although Figure 18 indicates that the edges of the
adjacent turns of the tape abut against each other, the
pitch of the tape may be such that the edges overlap in
order to ensure complete enclosure of the blocks 60 and
65. It is also possible to adopt a coarser winding pitch
such that the layer of tape does not completely enclose
the blocks 60 and 65. Such an arrangement ensures that
the blocks are held in place for subsequent production
steps, but does not provide the advantages associated
with complete enclosure, such as improved insulating
properties and exclusion of moisture from the blocks 60
and 65.
Various other types of tape may be used, such as
polytetrafluoroethylene and mica tape. In general, the
tape 66 is required to have electrical insulating
properties and must withstand subsequent heat treatment
of the mineral insulated cable. Also, the insulating
material of the tape should not break down in an
undesirable way at the high temperatures at which the
cable is required to be able to operate, for instance as
a fire-proof cable. It is preferable for the material of
the tape not to contain carbon, as this could impair the
insulating properties of the cable when subjected to
elevated temperatures. It is also generally preferable
that the material of the tape should not break down and
produce substantial quantities of gas, which could cause
the cable to rupture when subjected to elevated
temperatures. Where the tape is provided with an
adhesive, the adhesive should preferably have similar
properties so as not to compromise the performance of the
cable.
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11
Although a winding arrangement has been shown for
helically winding the tape 66 around the blocks 60 and
65, other techniques may be used. For instance, tape of
width equal to or greater than the circumference of the
blocks 60 and 65 may be supplied longitudinally and may
be wrapped around the blocks in a manner similar to a
tube forming mill.
In cases where improved insulation provided by the layer
of tape 66 is not necessary, a thread may be wound
helically around the blocks 60 and 65 so as to hold them
in place on the conductors 63 for subsequent production
steps. For instance, a fibre glass thread may be used
for this purpose and will not impair the insulating
properties of the cable. Alternatively, the blocks 60
and 65, or only the blocks 65, may be held in position by
means of rollers.
The next step in the production process comprises the
forming of a metal tube around the layer of tape 66.
Figure 14 shows a copper strip 68 of sufficient width
being supplied continuously from a reel 69. The strip 68
is formed into a tube by a rolling mill (not shown), for
instance of the type illustrated in Figure 1, and the
edges of the strip are welded together at a welding
station 70 so as to form a weld seam 71 as shown in
Figure 19. The cable is then supplied to a plurality of
rolling or drawing steps alternating with annealing steps
so as to reduce the cross section to the final desired
size of the mineral insulated cable, after which the
cable is stored in a coiling step or the like.
The continuous process for producing mineral insulated
cable can operate at great speed and the length of cable
produced is limited only by mechanical handling and
inspection considerations. The preformed blocks 60 and
65 provide excellent geometrical stability which allows
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12
the insulating properties of the cable to the maximised.
The absence of any abrasive steps in the process prevents
the ingress of copper particles or slivers or other
material into the mineral insulant so that the insulating
properties are not compromised. Further, the copper
conductors suffer little or no surface damage and their
cross sections do not therefore have to be over-specified
in order to ensure adequate electrical conductivity in
the finished cable. ~lso, "hot-spots" causing high
potential gradients are not created by the process so
that the insulating properties are not compromised.
The provision of an insulating layer around the blocks
further improves the insulating properties of the cable,
but has additional advantages. For instance, if the
production process has to be stopped and then restarted,
the layer prevents the ingress of moisture into the
blocks which might impair the insulation performance and
might cause problems during subsequent production steps.
For instance, during heat treatment such as annealing,
any moisture trapped within the blocks could generate
steam and, in severe cases, could rupture the outer metal
tube or cause substantial distortion. The provision of
the layer of insulating tape avoids this.
Because the mineral insulant is supplied in the form of
preformed blocks, there is little or no loose mineral
powder at any stage in the cable production. Thus, there
is substantially no contamination at the welding stage of
the outer tube. Also, there is little or no loss of
insulant material or production of powder dust so that
the process is very clean and does not waste raw
materials.
Because of the geometrical stability in cables made by
this method, it is possible to alter the conductor-to-
conductor spacing compared with each conductor-to-sheath
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13
spacing in order to maximise the dielectric performance
of the cable. For instance, the conductor-to-conductor
spacing may be made greater than the conductor-to-sheath
spacing and this provides a cable with better insulating
properties than one in which the spacings are the same
or, alternatively, allows the diameter of the cable to be
reduced.
