Note: Descriptions are shown in the official language in which they were submitted.
~4~36~5
This invention relates to telephone cable elements~
Telephone cables are always formed from elements
consisting of insu]ated wires ~rouped in twos or fours, known
respectively as pairs and quads. The pairs may consist of either
two coaxial conductors or helically twisted wires. In this latter
case they are known as symmetrical pairs. The quads are either
formed from four twisted wires (star quad), or from twisted wire
pairs which are themselves twisted, and known as D~ (Dieselhorst-
Martin) quads or symmetrical pair quads. These pairs or quads
form the elementary circuits of a cable. The present invention
relates in particular to star quads, DM quads and symmetrical
pairs.
The proximity of the conductors grouped in a cable means
that the telephone circuits are not totally independent of each
' other. Interactions are produced, and parasite signals may be
detected in a particular circuit, generated by the passage of
signals over other circuits. This phenomenon is known as cross-
talk and is manifested in practice by the awareness of a telephone
eonversation transmitted over a neighbouring circuit.
Corss-talk i5 influenced by the electrical resistance of
` the elementary circuits, by the capaeitance of these circuits, and
in partieular by their eapaeitance dissymmetry. These same
parameters also eontribute to line attenuation, whieh results in a
:::
deerease in the sound level of the transmitted eonversation and
should of eourse be as low as possible.
; The eleetrieal resistance of the circuit is defined by
parameters which can be fairly easily controlled, namely the
resistivity of the metal used for the conductor, the constancy of
this resistivity along the line, and the conductor dimensions.
The capacitance and capacitance dissymmetries, the
influence of which is preponderant in eross-talk questions,
depend on the dieleetrie constant of the insulant utilised, which
. . .
~ , . .. .
- , ...... - ~ : ' , . : : '' ' : . .
, . . , : :
6~3~
is a measurable and reproducible parameter, but they also depend
on other parameters much more difficult to control and which are
related to the geometry of the quad. This geometry results from
the helical assembly of the quad wires, and it is obvious that it
is very difficult to control such geometry with rigorous precision,
and in particular to ensure that no displacement of the wires in
the quad occurs during the further cable manufacturing operations.
For a long time, cables were insulated with spirally
wound paper strip, but this insulating material, which is relatively
fragile, of low productivity and leads to complications in splic
ing, has now been replaced by plastics insulation applied by
extruding machines.
This plastics insulation has the disadvantage of a high-
er dielectric constant, which requires the thickness of the
insulation to be increased to obtain the same line attenuation.
In addition to these d~fects deriving from the nature of
: ;
; the insulation, two other defects may be indicated which derive
indirectly from the use of plastics insulation. First, it is
difficult to ensure accurate centering of the conductor in the
sheath produced by an extrusion machine. Eccentricity of the
.:~ I' .
i conductor has repercussions over the entire length of the wire, and
creates capacitance dissymmetries. Furthermore, even when this
defect is practically non-existant, the stability of the geometry
of quads formed from wires insulated by plastics sheaths is poor
because of the low coefficient of friction between the sheaths, so
that the wires may become displaced and create capacitance dis-
,. . .
symmetries, in particular during the cable manufacturing operations
which follow the manufacture of the quads themselves.
Finally, and in contrast to paper insulation, plastics
insulation offers no protection to the cable against water infil-
tration if the cable envelope becomes defective.
- Among the numerous solutions which have been advocated,
~' '.'
~ -2-
. .. .
.
it has been proposed in sritish patent No 1,40~,068 to fix
cellulose fibres around a plastics sheath to form a hydrophilic
region about the sheath, capable of swelling in the presence o~
water, to become a seal which prevents water progressing along the
cable. According to this method, the purpose of the plastics
- sheath is to insulate the electrical conductor which it envelops,
while the purpose of the cellulose fibres is to prevent water in-
filtrating along the cable by the hydrophilic proper-ty of the
; cellulose.
The object of the present invention is to improve the
capacitance symmetry of telephone circuits, using wires the
insulation sheath of which is surrounded by a plurality of fibres.
~ To this end, the present invention provides a telephone
;~ cable element formed from at least one pair of helically twisted
electrical conductorscomprising, in combination: a respective insulating
sheath enveloping each of said conductors, and a plurality of
fibres embedded in the wall of each of said sheaths so that they
project all round said sheaths, the lengths of these fibres and
their density being chosen so as to maintain between said conductors
`- 20 a determined spacing which is a function of the desired capacitance
between the conductors, the average length of said fibres and the
density at which they are embedded in said sheaths being kept con-
stant so that said spacing remains uniform over the entire length
of the cable element, the projecting portions of said fibres form-
ing members of which the sheath of one conductor is fastened to the
sheath of the other conductor by the mutual interpenetration of the
fibres of the adjacent sheaths, such that the pitch of the helix is
:
kept constant and uniform over the entire length of thecable
element.
~ 30 Telephone cable elements constructed in accordance with
I the invention have shown that, regardless of the nature of the
fibres utilised, the presence of these fibres of given average
,
-3-
~3~
length embedded in the sheath at a constant density enables a - -
spacing to be maintained between the conductors which is a
function of the desired capacitance, and to give said capacitance
a symmetry which is absolutely surprising, and very difficult to
obtain wi-th extruded insulation. Moreover, when bundles and cables
are manufactured using pairs of quads according to the invention,
it is found that these elements have not undergone any deformation,
because of the interpenetration of the fibres in the adjacent
regions of the conductors, and in contrast to that which happens
with all other known forms of insulation, which allow the twisted
conductors to become displaced relative to each other during the
further cable manufacturing operations.
The importance of these advantages and of other
accompanying advantages will be more evident from the description
given hereinafter.
The accompanying drawing represents a diagrammatic
illustration by way of example of cable elements according to the
presentinvent~ion; In the drawing:
Figure 1 is a perspective view of a pair;
Figure 2 is a cross-section through a quad of ~ ~ -
symmetrical pairs; ~
. .
Figure 3 is a cross-section of a star quad;
Figure 4 is a diagram which is useful in defining the
; parameters k which are a measure of capacitance dissymmetries; and
:. : :
Figure 5 is a diagram showing the various factors which
can modify the value of the aforementioned parameters.
,
Figure 1 shows a pair 25 formed from two wires 24 twisted
into a helix of constant pitch. Each of the wires comprises a
i conductor 2 enveloped by a plastics sheath 19 in which a plurality
{ 30 of fibres 21 are anchored. The pair 25 is designed to constitute
a telephone cable element forming a telephone circuit or line. As
can be seen in Figure 1, the fibres 21 which cover the respective
... ..
. ~ .
insulating sheaths 19 mutually interpenetrate in the adjacent
parts of the twisted wires. Consequently, these fibres act as
distant pieces between the insulating sheaths formed about the
conductors 2, the diameter of which in this example is 0.6 mm,
the sheaths being preferably of expanded polyethylene about 0.2
mm thick, the avarage length of the fibres then being about 1 mm
and their diameter 25 denier. The polyethylene layer ensures good
mechanical strength and sufficient electrical strength. The
fibres, which are preferably cellulose fibres, have a good
insulation resistance when dry, and a low dielectric constant which
is less than that of the plastics material alone.
If the surrounding cable envelope (not shown in Figure
1) is ruptured and water penetrates into its interior, the
cellulose fibres have a duel function of impeding, by their swell-
ing the progression of the water along the cable and, through a
lowering of their electric resistance, sharply increasing the
leakage current and thus signalling the existance of a defect.
.'~
~ The blocking of the flow by the swollen fibres is highly effective
.
and limits the damage to a short length of-cable. The location of
the leak can be readily pinpointed by measurements of the current
flow and the voltage drop along the line.
Even if the effecti~e length of the projecting fibres
varies somewhat about its mean value, the spacing of the conductor
cores remains substantially unchanged during handling so that no
`~ significant variations in the shunt capacitance between the -
conductors occur. Stabilization of this shunt capacitance at a
predeterminedmagnitude is essential for the suppression of cross-
talk between circuits of a ~uad including the conductor pair of
Figure 1, as more fully discussed hereinafter with reference to
Figure 4.
In a specific instance, fibres of the dimensions given
above have a density of about 400 g/km.
.',
--5--
.
.
.~ . j , '' .
;, . .
The fact that the sheath is not extruded but is produced
by the melting of polyethylene powder, as described in the
abovementioned British patent, also contributes to the mainten-
ance of the desired symmetry inasmuch as any irregularities in
the coating process tend to be distributed at random around the
conductor axis; on the other hand, any irregularity of an extrusion
nozzle leads to a distinct eccentricity of the sheath. The inter-
leaved fibres also resist any relative axial shifting of the
conductors which in the absence of the fibres could occur during -
handling, thereby unbalancing the circuit.
; Figure 2 shows two balanced pairs 25 of the type
illustrated in Figure 1 within a common, flexi.ble envelope
` illustrated diagrammatically at 26.
Figure 3 shows a star quad 25' disposed within an
envelope 26; the array 25' consists of four insulated conductors
- 24 twisted about a common axis, the conductor axes lying at the
: corners of a square, in any transverse plane. In this instance, ~ .
- the fibres 21 interpenetrate not only in the spaces between adjoin- .
.i ing conductors but also in a central channel 27 which would be
.; . . ~., . :
. 20 completely empty in a quad composed of conventionally sheathed .
conductors including those with paper wrappings. While such .~
, wrappings could form a barrier between adjoining conductors, they .
would.not:swell sufficiently to block the flow of water in the
vicinity of the cable axis. Envelope 26 may, of course, embrace a .
multiplicity of arravs 25 and/or 25'. ..
We shall now refer to Figure ~ for a discussion of the ~
part played by the various shunt capacitances in a quad forming
i three signalling circuits as noted above. In Figure 4 the four ~.. -
conductors are designated a,b,c and d; the interconductor
e Cac, Cbc, Cad and Cbd; and the shunt capacitances
with reference to ground are Cao, Cbo, CCO do
circuit consists of wires a (outgoing) and b (incoming), the
.' ":
., ~ ,.
.. . . . ; . : .. ;. : .
6~
second circuit consists of wires c (outgoing) and d (incoming),
and the third or phantom circuit consists of wires a, _ (outgoing)
and c, d (incoming). We can then define the cross-talk among
those circuits in terms of three parameters, namely a factor k
relating to the first and second circuits, a factor k2 relating -to
the first and third circuits and a factor k3 relating to the second
and third circuits. These parameters are glven by the following
equations;
klCaC + Cbd Cbc Cad ':
10 k2bc Cbd Cac Cad 2 ao
k = C ~ C - C - C + C C
3 ad bd ac bc do - co
In the ideal case, kl = k2 = k3 = 0.
In practice, deviations from this ideal case are
determined by the following parameters indicated in Figure 5:
dielectric constants ar~ b~ c~ d
wire radii ra', rb', rc', rd'
; outer radii of insulation ra", rb", rc", rd"
~ eccentricities Ra, Rb, Rc, Rd of wire axes
;, .
angular spacing ~a, ~b, ~c, ~d of axial planes.
The conductor insulation according to.the present
invention insures the essential constancy of the foregoing para-
: . .
meters over the entire length of the cable.
A cable according to the invention can be manufactured
in a relatively simple manner and is lighter as well as more
flexible than those of the paper-insulated type while being also
considerably easier to splice with the aid of automatic equipment.
The risk of unravelling, as can happen with paper wrappings, is
eliminated.
',
- . . . :" . : .
,: ,, .. , ... . ,, . . : , , : .: . ,.
