Note: Descriptions are shown in the official language in which they were submitted.
3~
7487-266
I.OW VOLTAGE CABLE WITH TWO LAYERS OF POLYVINYL CHLORID~ INSUL~TI~N
The present invention relates to a low voltage cable
which is resistant to heat, and which does not propagate fire
and particularly, to an electric cable which is heat-resistant,
which does not propagate fire and which is included in the cate-
gory of low-voltage cables which are known as "building wires".
The majority of cables which come into said category
have conductor coverings formed from a polyvinvl chloride (P.V.C.)
based compound. One drawback of these compounds, is that they
are thermoplastic and hence, incapable of providing any stabi-
lity of form during the action of heat which might be applied tothe conductor covering.
This is a drawback which could cause serious con-
sequences. For understanding this fact, it is sufficient to con-
sider the risks for the user himself, when, for example, there is
overheating resulting from a bad connection of the cables to an
energy consuming device, a softening of the conductor coverings
results.
Moreover, in the known cables, where the conductor
covering is formed from a thermoplastic compound based on P.V.C.,
it is impossible to have either any significant fire non-propa-
gating properties or any optimization of the mechanical and in-
sulating properties.
In fact, in order to have such fire non-propating
properties, it would be necessary to introduce into the compound
itself, very high quanti-ties of mineral fillers, and this is
unacceptable since it would reduce the mechanical and insulating
properties of the compound and hence, of the conductor covering,
to unacceptabie values.
There have been proposed cables having a conductor
covering made of a mixture again based on P.V.C., but
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cross-linked through radiation. Ifl in this manner, it was
possible to solve the problem of providing stability of form to
the conductor coverings of the cable, under the action of heat,
the other problems, of not propagating fires and of the o~timization
of the mechanical and insulating properties of the cable were not
solved.
In fact, for the cross-linking of the compounds
forming the conductor covering of a cable, it was necessary to
add to the compound an additive selected from divinylbenzene,
polyallyl dimethacrylate esters and polyallyl trimethacrylate
esters, such as, for example, trimethylolpropane-trimethacrylate.
These additives interact with the mineral fillers of
the compound as well as with the P.V.C. during the cross-linking
of this latter, and hence, when the quantity of mineral fillers
present in the compound is increased, the quantities of the ad-
ditives increase.
The cross-linking additives of the p~V.C. have the draw-
back of reducing the insulating properties of the compound if, as
inevitably occurs in practice, they are still present after the
cross-linking of the P.V..~v, and therefore, they reduce the di-
electric properties of the conductor coverings of the cable formed
out of such a compound.
Accordingly, to limit the quantity of the cross-linking
agents present in the compound, the quantity of mineral. fillers
is limited, and hence, cables have the property of not propagating
fire are not obtained.
One object of the present invention is to provide low-
voltage cables in the "building wires" category having a P.V.C.
base covering for the conductor which has an optimum resistance
to heat and to propagation of fire in addition to an optimum of both
the dielectric characteristics and the mechanical characteristics.
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In accordance with the present inventlon, low voltage
cable comprising a conductor with an extruded covering sur-
rounding it has the covering constituted by a primary covering,
which is in contact with the conductor, and by a secondary
covering surroundina the primary covering. Said coverings are
made from a P.V.C. based compound and are characterized by the
fact that the primary covering is of a thermoplastic compound,
based on P.V.C., containing mineral fillers in quantities which
are greater than 60 parts by weight with respect to 100 parts by
weight of P.V.C., and that said secondary covering is a compound
based on P.V.C. cross-linked through radiation.
Other objects and advantages of the present invention
will be apparent from the following detailed description of the
presently preferred embodiments thereof, which description should
be considered in conjunction with the accompanying drawing, the
single flgure of which is a perspective view, with portions
thereof removed,of a length of the cable of the invention.
As can be seen in the drawing, the cable comprises a
conductor 1, constituted either by a single wire or by several
layed-up wires, which is surrounded by a covering comprising an
extruded primary covering 2 which is covered by a secondary
covering 3, said coverings being joined together.
The primary covering 2 is made of a thermoplastic
compound based on P.V.C., and the secondary coverlng 3 is made
of a compound based on P.V.CO cross-linked through radiation and
hence, is made of a compound which, prior to being cross-linked,
contains the necessary additive for cross-linking. The general
characteristics which the P.V.C. based compounds must possess,
for forming the primary covering 2 and the secondary covering 3
of a cable according to the present invention,will be set forth
hereinafter.
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The compounds based on P.V.C., for the formation of the
primary covering 2, are thermoplastics and contain mineral
fillers in such quantities as to endow it with the dielectric
properties and properties of resistance against fire propagation
which are desired~ In particular, the mineral fillers in the
compound in question, such as calcium carbonate, magnesium
carbonate, calcined kaolin and mixtures thereof, are present in
an amount which is greater than 60 parts by weight with respect
to 100 parts by weight of P.V.C. Preferably, the quantity of
mineral fillers present in the compound, is between 60 and 120
parts by weight, with respect to 100 parts by weight of P.V.C. so
as to obtain an optimization of the dielectric characteristics
for the insulation 2 and a resistance against fire propagation.
For the formation of the secondary covering 3, the
compounds based on P.V.C. contain a cross-linking agent selected
from among the divinylbenzene, polyallyl-dimethacrylate esters
and polyallyl-trimethacrylate esters, such as, for example, the
trimethylolpropane-trimethacrylate, in an amount not exceeding
16 parts by weight, with respect to 100 parts by weight o~ P.V.C.
and may contain mineral fillers in quantities not exceeding 10
parts by weight with respect to 100 parts by weight of P.V.C.
Moreover, the mineral fillers, for the compound
forming the secondary covering 3, are constituted by calcium
carbonate and by magnesium carbonate, and preferably, are of the
type obtained through precipitation, i.e. having a high specific
surface area. Even mixtures of calcium carbonate and magnesium
carbonate, of the types obtained through precipitation, can be
utilized as fillers in the compound for the formation of the
secondary covering 3.
Set forth hereinafter, solely by way of example, are
preferred compositions of a compound for the covering 2 and of a
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compound for the covering 3 of a cable according to the in-
vention.
A preferred compound for the formation of the primary
covering 2 of a low voltage cable, according to the invention,
may have the following composition, the components being ex-
pressed in parts by weight
POLYVINYL CHLORIDE (P.V.C.)........... l00 p. bv w.
PHTALATE PLASTICIZER (example:
dioctylphithalate or disodecyl-
phthalate~............................ .50 p. by w.
MINE~AL FILLERS (comprises by a
mixture of equal parts of calcium
carbonate and magnesium carbonate).... .80 p. by w.
TRIBASIC LEAD SULPHATE................ ..5 p. by w.
LUBRICANT (for example: stearic acid
or lead-basic stearate)............... .Ø5 p. by w.
ANTI-OXIDANTS (for example: triphenol-A)0.2 p. by w.
A compound for forming the secondary coverin~ 3, of a low voltage
cable according to the invention, may have the following
composition, the components being expressed in parts by weight.
POLYVINYL CHLORIDE(P.V.C.)............ .l00 p. by w.
PHTHALATE PLASTICIZER (example:
dioctylphthalate or disodecylphthalate 35 p. by w.
MINERAL FILLERS....................... ...5 p. by w.
TRIBASIC LEAD SULPHATE........... ~.................. ...5 p. by w.
LEAD BASIC STEARATE..................... ............ ..Ø5 p. by w.
BISPHENOL-A (anti-oxidant).............. ............ ..Ø2 p. by w.
TRIMETHYLO1PROPANE-TRIMETHACRYLATE
(cross-linking a~ent)................... ............ ...8 p. by w.
With the particular compounds given above, a low voltage cable,
according to the present invention, has been manu~actured, by
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extruding over the conductor 1, simultaneously or with suc-
cessive operations, the primary covering 2 and *he compound for
forming, after the cross-linking through radiation, the secondary
covering 3.
In particular, the conductor 1 was formed by thirty
wires, each one having a diameter of 0.Z5 mm, layed-up together
and having a cross~sectional area of 1.5 mm2.
The primary covering 2 had a thickness of 0.7 mm, and
the superimposed layer, which, after the cross-linking through
radiation, will constitute the secondary covering 3, had a
thickness of 0.1 mm and, in any case, the thickness of the
covering 3 should not exceed 15~ of the thickness of the primary
covering 2.
The conductor l,covered by the extruded layers de-
scribed above, underwent radiation treatment by causing it to
pass through an apparatus, of a known type, which furnished it
with a dose of radiation equal to 10 M rad, so as to cross-link
the secondary covering 3 of the cable 1. The speed of pro-
duction of the cable was of the same order of magnitude as that
of the known cables having a conductor covering constituted
entirely of a P.V.C. based thermoplastic compound.
With a cable according to the present invention, ex-
perimental tests were carried out, according to the provisions of
CEI 20-11, for the purpose of determining, in the conductor
covering composed of the assembly of the primary and the
secondary coverings:
a) the values of the insulation constant Ki at 20C
expressed in M Qper Km;
b) the resistance to thermo-compression, defined by
the crushing force to be exerted upon a flat sample of
conductor covering, heated to a temperature of 120C,
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for reducing the thickness to a value equal to 60%
of the initial thickness; and
c) the mechanical characteristics of the rupture load
and the elongation at rupture.
Moreover, with a cable according to the invention,
experimental tests were carried out for determining the capacity
of maintaining a sta~ility of its ~orm, at high temperatures,
by suspending a cable-length inside a chamber which is heated
-to 200C.
In addition, tests were carried out, according to the
provisions of CEI 20-22, for checking the resistance against fire
propagation. Tests were also carried out for determining the
characteristics of the cable according to our invention with
respect to the resistance to cable slipping inside a duct. For
this latter test, a cable length of 50 m, made according to the
invention, was introduced into a rigid P.V.C. hose, shaped in
the manner indicated below and having an inner diameter of 30 mm
and a length of 10 m.
The shaped hose has a configuration constituted by a
broken-line of equal segments and having curved corners, formed
by the assembly of hose sections of an S-shape and of an L-shape.
On the extremity of the cable, facing one extremity of the hose,
there was applied the force necessary for extracting the cable
from the hose, by causing it to slide within it, and the minimum
force was determined.
Moreover, for showing the improved properties of a
cable according to the invention, due to the simultaneous
presence of a primary and a secondary cover having the above-
stated characteristics, for comparison purposes in the above-
described experimental tests, a conductor having a covering
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formed with only the P.V.C. based thermoplastic compound, used
for the primary covering 2 of a cable according to our invention,
was made.
Identical experimental tests were carried out for a
known cable having a conductor covering entirely formed of a
P.V.C. based thermoplastic compound and for another known cable
having a conductor covering entirely formed out of a P.V.C.
based compound cross-linked through radiation.
The cable for comparison purposes, described previously,
and both of the known cables that underwent the e~perimental
tests, had conductors identical to the conductor of the cable ac-
cording to our invention, and a thickness for the conductor
covering that was equal to the sum total of the thicknesses of
the primary and the secondary coverings of the cable according
to our invention. The results of these experimental tests are set
forth in the following table:
CABLE OF CABLE FOR KNOWN CABLE KNOr1N RADIATION
THE C~ARISON WITH THE~- CROSS-LD~
D~NTIO~l PUR OSES PIASTIC SHEATH SH~ C~E
Ki at 20C 2000 M Q 2000 M Q 1000 M Q750 M Q
per Km. per Km. per Km. per Km.
RESISTANCE
TO THERMO-
COMERESSION 9N 5N 4.5N 4.5N
RUP~mE LOAD
UNDER TR~.CTION 18 N/mm2 8 N/mm2 15 N/mm2 15N/mm2
ELONGATION AT
RU~ 150% 100% 130% 130
STABILITY OF
XNSULATION-FORM
XNSIDE FURNAOE does not does not does not
AT 200C. melt down melt down melts down melt down
FIRE-PROPAGATION
BY STAI~RD does not does not spreadsspreads
OEI 20/22 TEST spread fire spread fire fire fire
DUCT SLIPPING
RESISTANOE 17N 20N 30N 17N
where Ki is the insulation dielectric constant and N is the force in Newtons.
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After examining the results of the tests carried out,
the following conclusions can be reached.
First and foremost, it can be seen that the presence
of a secondary P.V.C. based covering cross-linked through
radiation, did not bring about any reduction in the insulation
dielectric constant Ki of a cable, according to the present in-
vention, which could have been expected on observin~ the values
of the insulation dielectric cons-tant of a known cable having a
conductor covering entirely formed out of a P.V.C. based com-
pound, cross-linked through radiation.
It is true that, in the cable according to the present
invention, the thickness of the secondary covering is extremely
reduced, but it would be expected that even such a reduced thick-
ness would have had an influence on the value of the insulation
dielectric constant of the conductor covering of the cable.
Th~ fact that the dielectric constant has not been
reduced, signifies that the secondary-covering 3 of a cable ac-
cording to this invention, has, surprisingly, optimum insulation
properties in spite of its being formed out of a P.V.C. based
compound, cross-linked through radiation.
Moreover, from examining the results of the experi-
mental tests that are given in the table, it is found that, with
a cable according to the invention, all the previously stated
proposed aims are achieved.
Finally, with a cable according to the invention, there
can be obtained a good production speed in spite of the need for
cross-linking of the secondary covering 3 through radiation.
In fact, the manufacturing speed of a cable according
to our invention, expressed in meter/minute, is in the same
order of magnitude as that of the "known" cables having a con-
ductor covering made of a P.V.C. based thermoplastic compound
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whereas the manufacturing speed of -the known cables, havin~ a
conductor covering of a P.V.C. based compound cross-linked
through radiation, is generally slower by 30%.
The reason for this is not due solely to the fact that
the thickness of the secondary covering 3, of a cable according
to our invention, is small. It is also due to the fact that
mineral fillers are absent or are present in minute quantities
which allows for keeping to the minimum the quantity of cross-
linking agents for the P.V.C. and hence, also permits keeping
the dose of radiation energy which has to be furnished, to the
minimum.
Although preferred embodiments of the present invention
have been described and illustrated, it will be apparent to those
skilled in the art that various modifications may be made without
departing from the principles of the invention.
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