Note: Descriptions are shown in the official language in which they were submitted.
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Method and Apparatus for Removing a Cable Core from a Cable
Sheath
The present invention relates to a method and apparatus for removing a cable
core
from a cable sheath.
Data, digital or coaxial transmission cables for transmitting data and digital
signals into
homes have been used for many years and are usually buried underground, often
beneath pavements and the front gardens of the homes to which data is being
supplied.
There are various different cables commonly in use, but a typical cable 1 is
shown in
io Figure 1 and generally comprises a core 2 formed from a bundle of
conductors 3 (only
four of which are shown in Figure 1). Each conductor 3 may comprise a number
of
stranded copper core filaments 4 twisted together and covered by an insulating
polymer
sheath 5. A plurality of conductors 3 may be twisted or otherwise bundled
together to
form the core 2. The conductor bundle forming the core 2 is received in an
outer sheath
6 to keep the conductor bundle together and to provide overall mechanical,
weather,
chemical and electrical protection. The sheath 6 can be formed from aluminium,
lead or
steel and may have a polymer coating. A filler material 7 is received between
the outer
sheath 6 and the core 2 and surrounds and fills the interstices of the core 2
formed by
the conductor bundle. This filler material 7 can be formed from polymer and is
usually
soft, although it is sometimes formed from a relatively hard polymeric
material.
Alternatively, it can sometimes be oil based. The filler material 7 protects
and supports
the core 2 and can act to seal any nicks or cuts in the outer sheath 6 should
they occur,
thereby preventing moisture ingress which may lead to consequential failure of
the
cable 1.
Some cables 1 are also provided with an envelope 8 which surrounds the core 2
and the
filler material 7 within the outer sheath 6. This envelope 8 may wind
helically around
the core 2 and filler material 7 and overlap so as to completely surround or
wrap the
filler material 7. The envelope 8 is commonly formed from paper, cellophane or
similar
material, but can also be formed from aluminum foil.
In another, unillustrated, cable type, the envelope is embedded within the
filler
material so that the filler is between the core and the envelope and between
the
envelope and the inner surface of the outer sheath. Some cables may also have
a
further, external envelope (not shown) which surrounds the filler material, in
addition
to the envelope that is embedded within the filler material. The external
envelope
separates the filler material that is on the outside of the envelope from the
inner surface
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of the outer sheath. The external envelope may be formed from cellophane, with
the
internal envelope formed from aluminium or paper.
Developments in technology, together with demand for better and faster data
transfer,
have recently necessitated the use of optical fibres for data transmission
purposes, as
optical fibre is capable of transmitting much larger quantities of data at
high speed
relative to traditional copper cored cables. Where optical fibre is required,
the standard
approach is to disconnect the existing cable and to lay an entirely new cable
containing
the optical fibre, with the old disconnected copper cored cable either being
removed
altogether or left in the ground. It will be appreciated that this approach
necessitates
io the digging of a trench in order to lay the new cable, which is disruptive,
time
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consuming and expensive.
US 7,814,654B2 discloses a method for removing a cable core from an outer
cable
sheath so that the optical fibre can be fed through the vacated outer cable
sheath
without having to dig a trench. In the method described in this document, a
flowable
medium is introduced under pressure into an annular space between the core and
the
sheath, so as to reduce friction between the core and the sheath to the extent
necessary
to allow the core to be drawn out of the sheath for subsequent insertion of
the new
optical fibre. However, this method is troublesome as it requires a supply of
pressurised
medium and a pump capable of pumping the pressurised medium along a length of
cable. Furthermore, it is necessary to minimise leakage and to provide means
for
collecting the pressurised medium after the core has been released to prevent
contamination of the surrounding ground.
The present invention seeks to provide a method and apparatus which
substantially
overcomes or alleviates problems associated with replacing cable core with
optical
fibre, such as those identified above.
According to the present invention, there is provided a method of removing a
metallic
cable core that extends through an outer cable sheath of a length of data
transmission
cable, the core being surrounded by a filler material contained within the
outer cable
sheath as an integral part of the cable, the method comprising the steps of:
(a) resistively heating the core by supplying electric current to it to reduce
the
viscosity of the surrounding filler material; and
(b) pulling the core out of one end of the outer cable sheath whilst the
viscosity
of the filler material is in a reduced state.
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As the filler material softens or liquifies at a lower temperature than the
material of the
outer sheath, it is possible to sufficiently reduce the viscosity of the
filler material so
that the friction between the core and the outer sheath is reduced, without
damaging or
altering the outer sheath, thereby enabling the core to be extracted from the
outer
sheath. Depending upon the length of the cable being treated, it should be
possible to
simply pull or draw the core out by hand. However, it is envisaged that the
core could
be pulled or drawn out of the outer sheath by a machine capable of applying
sufficient
load to the core. It has been found that, in its heated state, the filler
material effectively
becomes a lubricant that substantially reduces the force required to pull the
core out of
ro the outer sheath, that force being substantially lower than the overall
tensile strength of
the core. Ideally, the pulling step is carried out as soon as a sufficient
reduction in the
viscosity of the filler material has occurred and before its temperature drops
to a point
at which the filler material has regained its original viscosity. As it may be
difficult to
maintain a supply of current to the core simultaneously to the pulling step,
ideally the
/5 core is pulled immediately after disconnection of the current supply to
the core.
The method may include the step of terminating the supply of electric current
to the
core prior to carrying out step (b). In this case, the core is preferably
extracted
immediately after the supply of electrical current is terminated.
Step (a) may comprise heating the core by supplying it with electric current
having an
20 amperage range of between 3oA to 90A, a voltage of between 24V and iroV.
The
current may be supplied for a period of between 2 and 7 minutes.
The method may further include the step of attaching a draw-string to the end
of the
core prior to carrying out step (b), so that the draw-string is pulled through
the outer
sheath simultaneously with the drawing of the cable core out of the outer
sheath.
25 Alternatively, the draw-string may be fed through the outer sheath after
the cable core
has been pulled from the outer sheath in step (b).
If the cable from which a cable core is to be removed includes an envelope
between the
filler material and the outer sheath, the method preferably includes the
further step of
releasing said envelope from the outer sheath after extraction of the core in
step (b) by
30 jetting compressed air along the outer sheath.
The method preferably comprises the step of applying an initial pulling force
to the
envelope protruding from one end of the outer sheath prior to pulling the
envelope out
of the cable sheath from the opposite end, after jetting compressed air along
the outer
cable sheath. The method may include the step of applying an initial pulling
force to the
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envelope until a portion of the envelope breaks within the outer cable sheath
and prior
to pulling the remaining portion of the envelope from the opposite end of the
outer
cable sheath.
Preferably, lubricating agent is supplied or is contained in the compressed
air as it is
jetted into the outer cable sheath.
According to another aspect of the invention, there is provided apparatus for
removing
a metallic cable core extending through an outer cable sheath of a length of
data
transmission cable in which the cable core is surrounded by a filler material
contained
within the outer cable sheath as an integral part of the cable, the apparatus
including an
io electrical generator for generating current and wires extending from the
generator for
connecting the apparatus to a cable core for supplying electrical current to
the cable
core, wherein the apparatus includes a housing containing the generator and an
integral draw-string storage reel for supplying draw-string into the outer
sheath
following, or during, removal of the core.
In a preferred embodiment, a supply of draw string material is contained on
said draw
string storage reel in said housing. The draw string material is preferably
made from a
polymeric material.
The apparatus may also include a feeding mechanism for pushing draw-string off
the
reel and along the outer sheath.
According to another aspect of the invention, there is provided an apparatus
for
removing a metallic cable core extending through an outer cable sheath of a
length of
data transmission cable in which the cable core is surrounded by a filler
material
contained within the outer cable sheath as an integral part of the cable, the
apparatus
including an electrical generator for generating current and wires extending
from the
generator for connecting the apparatus to a cable core for supplying
electrical current to
the cable core, wherein the apparatus includes a user operable selector for
setting the
amperage to be generated, and an amp meter connectable directly to the cable
core to
provide a measure of the actual amperage in the cable core.
The apparatus may comprise a controller, a timer, a programmable memory, and a
cable-type selection switch, and the memory may be configured to store data
such as
amperage, voltage and current supply time for a plurality of different cable
types, such
that user may select said parameters in dependence on a particular cable type
from
which a cable core is to be removed via said cable-type selection switch.
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It will be appreciated that the method and apparatus of the invention is
applicable to a
variety of cable lengths, but it is ideally suited to shorter lengths and, in
particular,
those lengths of cable which extend from beneath a road or pavement and under
a front
garden and into a property or building.
Embodiments of the invention will now be described, by way of example only,
with
reference to Figures 2a to 2e of the accompanying drawings, in which:
FIGURE 1 is a perspective view of an end portion of one type of traditional
copper
cored cable with the various layers shown stripped back to illustrate its
construction;
FIGURE 2a shows a schematic cross-sectional side view of a length of
traditional
/o copper cored cable prepared and readied for carrying out the method of
the invention;
FIGURE 2b shows the length of cable of Figure 2a with the cable core partially
removed
from the outer cable sheath;
FIGURE 2C shows the length of cable of Figure 2b after the cable core has been
completely removed and compressed air has been jetted down the cable from a
nozzle
inserted into one end;
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FIGURE 2d shows the length of cable of Figure 2C with the paper wrapper
partially
extracted from the outer sheath; and
FIGURE 2e shows the length of cable of Figure 2d with the paper wrapper
completely
removed and a draw-string extending through the outer sheath.
As described above, a perspective view of an end of one type of conventional
cable 1 is
shown in Figure 1 and comprises a core 2 formed from a bundle of conductors 3.
Each
conductor 3 may comprise a stranded copper core 4 formed from a number of
filaments
5 covered by an insulating sheath 6 formed from polyethylene. Groups of
conductors 3
may be twisted or bundled together to form the core 2. The insulated conductor
bundle
2 is received in an outer polymer sheath 7, also formed from polyethylene, to
keep the
conductor bundle 2 together and to provide overall mechanical, weather,
chemical and
electrical protection. A filler material 8 is received between the outer
sheath 7 and the
conductor bundle 2 and surrounds and fills the interstices of the conductor
bundle 2.
Cables 1 of this type are commonly used for local area networks and may be
buried
underground.
To remove the conductor bundle/core 2 from the outer sheath 7, a direct or
alternating
electric current is supplied to the core 2 so that its temperature is elevated
to a
sufficient level in order to melt the filler material 8 or to at least reduce
its viscosity to a
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level at which the core 2 may be removed from the outer cable sheath 7 by
pulling it
either by hand, or using an extraction tool. The force that is applied to the
core 2 to
extract it from the outer cable sheath 7 must be less than the tensile
strength of the core
2, as otherwise the core 2 will break within the outer cable sheath 7 and can
no longer
be removed. By heating the filler material 8, the friction between the core 2
and the
inner wall 9 of the outer cable sheath 7 is reduced so that the core 2 can
slide out of the
outer cable sheath 7 relatively easily and with little load applied to the
core 2. The filler
material 8 changes its state at a temperature which is lower than a
temperature which
would have any effect on the outer cable sheath 7. Therefore, the outer cable
sheath 7
/o remains undamaged during the current supply and heating step.
A section of cable 1 for which the core 2 is to be removed is shown in Figure
2a. Only
the end sections of the cable 1 are shown in Figure 2a, but the length of the
cable may
be in the order of io ¨ 20 metres or so. To provide the required current
necessary to
heat the core 2 and so melt or reduce the viscosity of the filler material 8,
an electrical
generator 10 (see Figure 2a) is provided. The generator io has connecting
wires ii
representing positive and negative poles. Clamps (not shown) are attached to
the end of
each of the wires 11 to enable them to be quickly and easily connected to, and
disconnected from, the cable core 2. The mating faces of each clamp that make
contact
with the core 2 are be flat, rather than serrated or toothed, so as to ensure
proper
connection between the clamps of the electrical generator io and the core 2.
At the end of the cable core 2 which is to be connected to the generator io,
the
conductors 3 are divided into two groups and the insulation material stripped
back to
reveal the filaments of each conductor 3. All the filaments of the conductors
belonging
to the same group are electrically connected together, such as by twisting the
filaments
together. The positive wire from the generator 10 is then connected to one
group and
the negative wire from the generator io is connected to the other group, using
the
clamps. At the opposite end of the cable length, the insulation material of
all the
conductors is stripped back, but rather than separate the conductors into
groups, all the
conductors are electrically connected to each other. When the generator 10 is
activated,
current will flow in both directions, i.e. from the generator io along the
core via one
conductor group and then back down the core in the opposite direction via the
other
conductor group.
Heat is generated in the core due to resistive heating. The amount of heat
released is
proportional to the square of the current according to Joule's first law. By
using a
relatively high current at a relatively low voltage, a sufficient amount of
heat is
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generated in the core to melt the filler material. The exact current and
voltage required
is largely dependent on the cable iwhose core is to be extracted, although the
Applicant's have found that a supply capable of delivering 30 to 90 Amps with
a voltage
of between 24 and noVolts is sufficient for most cable types. Generally, the
current
should be applied to the core 2 for a period of between 2 and 7 minutes.
The generator 10 includes a gauge for indicating the generated current. A
separate
meter, connectable directly to the cable core, may provide a further amperage
reading
indicative of the actual amperage in the cable core 2, so that account may be
taken of
any losses in the wires 11 or connections to the core 2 via the clamps.
io The exact parameters to be used for a particular cable type may be
determined from
data obtained from previous tests conducted on similar cable types, so that an
operator
will know exactly what voltage, amperage and time period is required once the
cable-
type has been determined by visual inspection. The operator may then input the
required settings into the generator in prior to heating. In a further
development, the
/5 electrical generator 10 may be provided with a controller, a memory, and
a cable-type
selector. In this case, the required amperage and voltage that have been
previously
determined for a particular cable type may be pre-programmed into the memory
so
that an on-site operator is only required to input the cable-type via the
selector and the
generator 10 will use the previously stored settings held in the memory. The
generator
20 10 may also include a timer to control the period for which the current
is applied. An
appropriate time period may also be determined based on previous tests .and
can be
stored in the memory together with amperage and voltage values. By having data
for
different cable types stored in the memory of the apparatus, the speed of the
process
can be greatly increased. The possibility for an operator to use the wrong
settings,
25 which could result in damage to the outer sheath 6 or an inability to
properly extract
the cable core 2, is also reduced.
Once the filler material 7 has been heated to a temperature at which its
viscosity is
reduced sufficiently, the electrical generator 10 is disconnected from one end
of the
core 2 and then a force is applied to pull the core 2 from the outer sheath 6
from the
30 opposite end. Figure 2b shows the cable of Figure 2a after the generator
10 has been
disconnected and the core 2 has been partially removed from the outer sheath 6
of the
cable 1 by pulling it in the direction of arrow 'IV.
If the cable 1 to which the method is being applied does not have an envelope
or paper
wrapper 8, then it is possible to attach, tie or hook a draw-string to the end
of the core 2
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so that the draw-string is pulled into and through the outer sheath 6 from one
end as
the core 2 is pulled out of the opposite end. This speeds up the process, as
it is not
necessary to insert a draw-string into the outer sheath 6 in a separate Method
step
following core removal. This method also applies to cables in which there is
filler
material surrounding the envelope and there is no external envelope between
the filler
material and the inner surface of the outer sheath, as the filler in this
region will also
melt/reduce in viscosity to allow the core and envelope to be removed as one.
In the cable length shown in Figures 2a to 2d, a paper wrapper envelope 8 is
shown.
This envelope 8 surrounds the filler material and so there is no filler
between the
io envelope 8 and the inner surface 9 of the outer sheath 6. This paper
wrapper 8 is not
extracted together with the core 2 when the core 2 is pulled from the outer
sheath 6, as
the friction between the envelope 8 and the outer sheath 6 is not reduced as a
result of
heating the core 2. However, as the wrapper 8 may at least partially block the
outer
sheath 6 and prevent subsequent insertion of the draw-string and/or the new
optical
fibre, it must be removed prior to any attempt to insert the draw-
string/optical sheath
into the outer sheath 6. The same method also applies to cables in which there
is an
internal envelope and an external envelope surrounding the filler material so
that the
region between the external envelope and the inner surface of the outer sheath
is dry.
As the wrapper 8 may be adhered to the inner surface 9 of the outer sheath 6,
it cannot
simply be removed by pulling it from one end, as the wrapper 8 will simply
break inside
the outer sheath 6 making it even harder to extract. However, the Applicant's
have
found that the paper wrapper 8 can be released from the inner surface 9 of the
outer
sheath 6 by jetting compressed air or an inert gas ('A' in Figure 2c),
preferably
containing a small amount of lubricating agent such as oil, down the outer
sheath 6
from one end. The compressed air or gas and oil mixture passes around and
between
the overlapping windings that form the wrapper 8 and detaches it from the
inner
surface, as well as separating the overlapping regions of the windings from
each other.
Figure 2C shows a nozzle 12 pushed into the end of the outer sheath 6 for the
supply of
compressed air and lubricant from a compressed air/lubricant source (not
shown)
down the outer sheath 6. The paper wrapper 8 is also shown in a state in which
it has
now been partially detached from the inner surface 9 of the outer sheath 6 and
is held
loosely within the outer sheath 6.
Once compressed air has been forced down the outer sheath 6, it may then be
possible
to pull the envelope 8 out of the outer sheath 6 from one end. However,
certain types of
envelope 8 comprise a helically wound ribbon of cellophane material extending
around
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the filler 7 that also includes a number of yarns or strings that extend
around the
cellophane. It has been found that, with envelopes of this type, the friction
between the
wrapper 8 and the inner surface of the outer sheath 9 is still relatively high
even after
jetting compressed air along the outer sheat, so it is still not possible to
slide it out.
However, it has been found that if the wrapper is initally pulled from one
end,
preferably until it snaps inside the outer sheath 6, and the broken portion is
drawn out
from that end of the outer sheath before pulling the remaining portion from
the
opposite end of the outer sheath 6 (by pulling it in the direction of arrow
'C' in Figure
2d), the remaining portion will slide out relatively easily. The initial
pulling/snapping
lo step from one end tends to pull the strings or yarns taught or
straighter within the outer
sheath thereby pulling the cellophane away from the inner surface, making it
easier to
subsequently draw the remaining portion of the envelope out from the opposite
end of
the outer sheath 6.
Removal of the paper wrapper 8 by the application of compressed air and
lubricant is
/5 preferably carried out immediately after core 2 extraction and when any
remaining
filler material 7 remaining in the outer sheath 6 has not fully cooled and so
still has a
relatively low viscosity. Wrapper removal is made more difficult if any
remaining filler
7 is allowed to regain its normal state, especially if the ambient temperature
is relatively
low, as the residual filler 7 can increase the friction between the paper
wrapper 8 and
20 the inner surface 9 of the outer sheath 6.
Once the paper wrapper 8 has been extracted, a draw-string 16 can be pushed
into the
now empty sheath 6 using a known feeding mechanism, which may be integrated
into
the generator housing 13. An optical fibre can be attached to one end of the
draw-string
and the draw-string pulled to draw the optical fibre through the outer sheath
6.
25 The electrical generator 10 has a housing 13 containing the circuitry
required for
generating the current at the required level. However, the housing 13 may also
be
provided with a draw-string storage reel 14 within the housing 13 on which is
wound a
length of draw string 16, which may be made from steel or, more preferably, a
polymer
material. The housing 13 may also contain a feeding mechanism 15 for feeding
draw
30 string from the reel 14 for insertion through a vacated cable sheath 6.
Many modifications and variations of the invention falling within the terms of
the
following claims will be apparent to those skilled in the art and the
foregoing
description should be regarded as a description of the preferred embodiments
of the
invention only.
