Note: Descriptions are shown in the official language in which they were submitted.
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LOW VOLTAGE CABLE PROVIDED WITH IMPROVED INSULATING hAYER
The present invention relates to an improved low voltage
cable, comprising a single conductor and insulated with~`a poly-
vinylchloride (PVC) compound, said cables being well known as
wiring cables for consumer appliances.
In the state of the art, these cables are generally made
compatible with the electrical, mechanical and aging charac-
teristics determined by standards in force while keeping in
mlnd the need to keep costs to a minimum. It follows that the
insulating layer of the wiring cables for consumer appliances ~
is always realized with compounds having a very high mineral ;
iller content, such fillPr content being up to 30 parts by
weight per 100 parts by weight of resin.
The insulating layer of low voltage cables used up to now, ~-
has always a quite rough surface, owing t`o the high content of
mineral fillers. The high friction coefficient due to the
roughness, produces difficulties during drawing of the cable
nto place and abrasions because of the contact with the inner
surface of the conduits. Moreover, in spite o~ the presence of
the~mineral filler~the content of polyvinylchloride is always ;~
~20~ high~with re9pect to said mineral filler. The polyvinylchloride
is~a~thermoplastic material and in case of overheating it melts,
strlpping~the in~sulation from the conductor, with serious risks
of~a~short~c~ircuit~
The~ present~invention has, as one object, the improving
f~tha~properties~of~the ~iring cables used for consumer appli-
anaés~up~to now while~eliminating the dra~backs, and, at the
sàme~time~, malntalning~a~very low cost for the product and equal
electrical characteristics.
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The cable of the invention has an insulating layer which,
even if it contains a high mineral filler, much higher than the
present concentrations used in the art, has such a structure as
to offer acceptable mechanical characteristics, a very low co-
efficient of friction, and a high resistance to flames. Further-
more, said insulating layer can be easily stripp~d, and there-
fore, easily applied, and permits a high reliability in the case
of overheating.
More precisel~, the object of the invention is a low
voltage cable, having a conductor constituted by one or more
wires laid up or stranded together and covered with an in-
sulating layer formed by a compound based on polyvinylchloride,
said compound containing also at least a mineral filler and a
plasticizer, characterized by the fact that said mineral filler
is contained in the compound in proportions from 40 to 500 parts
by weight per 100 parts by weight of polyvinylchloride, said
plasticizer being contained in the compound in proportions from`
60 to 120 parts by weight per 100 parts by weight of polyvinyl-
chloride and by the fact that there is a skin covering the
surface of said insulating layer, said skin having a thickness
from 5 to 30~ o~ the thickness oE said insulating layer and being
constituted by a compound based on polyvinylchloride and con-
taining at least a plasticizer in the quantity of 10 to 40 parts
by we'ght per lOO parts by weight of polyvinylchloride.
The accompanying sheet of drawing illustrates, by way of
example, a practical embodiment of the invention and in the
drawing: ~
Fig. 1 is a schematic, perspective view of a cable length
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~ incorporating the invention; and
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Fig. 2 illustrates diagramatically a test path for testing
cables of the invention.
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The low voltage cable 10 shown in Fig. 1 comprises a
conductor 11 constituted by a single wire, but it is to be
understood that said conductor could also comprise a plurality
of wires laid up or stranded together. The conductor 11 is
covered with an insulating layer 12 extruded thereon. The
insulating layer 12 is constituted by a compound based on poly-
vinylchloride and containing at least a plasticizing mineral
filler.
The mineral filler is inserted in the compound in the
proportions from 40 to 500 parts by weight per 100 parts by
weight of polyvinylchloride.
Among the various substances which could be added to the
compound as a mineral filler the following are preferred:
- calcium carbonate in preferred proportions of
400 parts by weight per 100 parts by weight of PVC;
- magnesium carbonate in preferred proportions of
400 parts by weight per 100 parts by weight of PVC;
- calcined kaolin in preferred proportions of 300
parts by weight per 100 parts by weight of PVC.
The compound of the insulating layer 12 further contains
a plasticizer in proportion from 60 to 120 parts by weight per
100 parts by weight of polyvinylchloride. In a preferred embodi-
ment, the plasticizer is contained in a quantity of 100 parts
by weight per 100 parts by weight of PVC.
The insulating layer 12 is covered on its surface with a
skin or small layer 13, constituted by a compound base~ on poly-
vinylchloride and containing at least a plasticizer. Preferably,
the plasticizer is~Di 2 -~ethyl - hexyl - phthalate (D.O.P.).
The quantity of plasticizer contained in the s]cin is from 10 to
~- 30 4q parts by weight per 100 parts by weight of P~C. Its preferred
content is of 30 parts by weight per 100 parts by weight of PVC.
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The thickness of this skin 13 represents only a small
part of the insulating layer 12 thickness. The skin thickness
may be between 5 and 30% of the insulating layer 12 thickness
and preferably, is about 15% of the insulating layer thickness.
Surprisingly, it has been found that, even if the com-
pound of the insulating layer 12 is loaded with more filler than
is normally used in the prior art, the structure of the cable
10 inclusive of the skin 13, has a resistance to tensile stress
sufficient for the handling of the cable since the mechanical
stresses occuring, for example, during the winding o the cable
on a drum are taken up predominantly by the skin 13. Also, the
electrical characteristics of said cable are good. `
The outer surface of the cable 10 is, moreover, very
smooth and very bright, owing to the presence of polyvinyl-
chloride in the skin 13, so that the cable has a very low
friction coefficient wlth respect to that of the cables in use
up to now. Therefore, said smoothness and brightness permit a
drawing of the cable inslde conduits with a gentle and con-
tinuous sliding.
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Another advantage ofered by the cable 10, is the ease of
strippability of the conductor 11, i.e. the obtaining of a
oomplete detachment o the insulating layer 12 from the conduator
during the joining and sealing operations. In fact, it is
sufficient to cut circularly around the skin 13 and the in-
sulatin~ layer 12 up to the conductor 11 and to remove a hollow
pin which~comprises the skin 13 and the underlying insulating
layer 12;to which it adheres perfectly because it is constituted
by a compound hav1ng the same polyvinylchloride base. The hollow
pln cavit~ corresponds to the space previously occupied by the
~ conductor 11.
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In the cable according to the invention, in case of over-
heating, the melting of polyvinylchloride should never strip the
conductor 11, due to the presence of a high mineral filler ~.
content which should remain compact, preventing the melted PVC
from draining away, and instead, should keep the latter in
place.
The fact that the cable according to the invention, to-
gether with the high content of mineral fillers, has always a
sufficiently high plasticizer-resin ratio contributes to giving
the cable itself a good cold 1exibility.
A further property of cable lO is a good behaviour with
respect to thermocompression because of the essentially "mineral"
structure of the compound. Said structure, being a compound
with high mineral filler content, improves also the thermal con-
ductivity of the cable 10, with respect to the conventional
formulations. It also follows that there is a more rapid
cooling of the cable.
Moreover, in contrast to conventional wiring cables, the
compound according to the invention does not include any
coloring pigments, especially in the insulating layer 12, with
a minimum risk, therefore, of decay of the insulating character-
istics.
oreover, the high mineral filler content makes the
cable I0 particularly resistant to flames since the high
quantity of fillers, besides being incombustible, improves the
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~; ~ compatibility of PVC resin with chloroparaffins.
Cable 10 has a urther advantage deriving from the good
impermeability of skin 13 as formed, which lowers greatly the
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absorption of water by the assembly.
The adv~ntages of the invention will be more evident from
the results obtained with a cable having a conductor 1.41 mm in
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diameter, coated with a covering having a thickness S of 7 mm,
where S = Si + Sp = 0~6 mm ~ 0.1 mm, where Si is insulating
layer 12 thickness and Sp is the skin 13 thickness.
The compounds used for providing the skin 13 and the
insulating layer 12, respectively, were as follows:
Skin:
Polyvinylchloride 100 parts by weight
Di 2 - ethyl - hexyl - phthalate36 " " "
Tribasic lead sulphate 7 " " "
Calcined kaolin 7 " " ~'
Bibasic lead stearate 0.5 " " "
Insulating layer:
Polyvinylchloride 100 parts by weight
Chlorinated paraffinic plasticizer 100 " " "
Magnesium carbonate 400 " " "
Calcium stearate 10 " " "
Tests made on said cable have given the following results:
electrical characteristics: insulation resistance of 0.2
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M Ohms/km, measured under water at 70C;
flame resistance: "oxygen index" of 28. The test has been
made with ASTM D 2863-70 method, on the cable after having
removed the conductor;
cold flexib~ility: the tests have been passed according to
~; standards CEI 20-lI Quality ~ issue 1968 - this latter test
has been chosen since it is one o~ the most severe among
standardlzed tests t
absorption: the cable made by way of example has been kept
under water at 100 during 24 hours. At the end of this
period, it has been found an ahsorption of 10 mg/cm2, a
value to be consid~red extremely satisfactory for a cable
having such a high content of mineral fillers.
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The cable lO was also found to have a very low co-
efficient of friction which was measured by determining the
force F necessary to draw a bundle of three cables through a
conventional corrugated PVC tube, for a path (see Fig. 2)
+ 12 + 13 + 14 six meters long and having angles ~
A, (where ~= ~ = 90 and ~- 135). ~ is the angle between the
lengths ll and 12, ~, the angle between the lengths 12 and 13
and ~, the angle between the lengths 13 and 14. The bends at
the angles ~ and ~ are arcs of a circle having a radius r = lO
cm. Each of the three cables has the following dimensional
characteristics:
conductor diameter = 1.41 mm
insulating layer 12 plus skin thickness 13 = 0.7 mm
(insulating layer thickness = 0.6 mm skin thickness =
0.1 mm).
The force (F) necessary for the drawing of the three
cables was 2.5 kg. -
The advantage is considerable if it is recognized that
~ to draw a bundle of three cables, with dimensions equal to those
; 20 of the bundle on which the experimental tests were performed,
but using two cables of conventional type, it is necessary to
~ apply a force of 9 kg.
;~ Although preferred embodiments of the present invention
have been described and illustrated, it will be apparent to
those skilled ln the art that various modifications may be
made without departing from the ~rinciples of the invention.
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