The present invention is in the area of technology of electrical cables and relates to a cable for data transmission which has a high resistance to fire. As is known, substantial research which has been carried out in the telecommunications sector has led to the development of communication apparatuses capable of transmitting and receiving numerical data at ever increasing transmission rates . For example, current Internet connections easily reach capacities of several Mbps, while computer networks can exchange data at rates of several Gbps .

Performance at these levels is possible as a result of technical improvements in the transmission channels. These channels must have capacities equal to those of the other apparatuses so as not to limit the speed of transmission of the numerical data.

In general, at least one cable segment comprising one or more conductors is present in the transmission channel. For example, in computer networks, a particular type of cables is used, with the cables being called LAN cables or Ethernet cables. These cables have electrical and mechanical characteristics adapted to satisfy the specifications defined by standard protocols for communication between computers. The LAN cables have within them a number of pairs of insulated conductors, commonly simply called "pairs", with the ends of the conductors being typically fixed to the terminals of a connector, e.g. of the type RJ45. In the market it is possible to find different categories of LAN cables which differ in the choice of materials used and in the design of the internal layout.

The categories also define certain important electrical parameters of the conductors, so that an operator, by the mere knowledge of the category, can easily identify the suitability for certain applications and the main characteristics of the given LAN cable.

For example, a cable of category 5e will allow transmission of numerical data up to a nominal frequency of ca. 100 MHz, and will have characteristics of attenuation and interference protection which are less advantageous compared to a cable of category 6, which can transmit data up to a nominal frequency of ca. 250 MHz. A cable of category 7 can transmit up to a nominal frequency of 600 MHz.

As the category number increases, the electrical performance of the cable improves, since the cable has been fabricated with materials having better electrical performance characteristics. The cable interior also has a configuration of its internal elements which is adapted to provide higher electrical performance.

LAN cables can be installed in any environment - home, office, or industrial - and must meet a number of specific safety standards relating to fire behaviour. The choices of materials and cable layout must therefore be made so as to obtain a cable which has the required electrical characteristics and at the same time is able to meet standards for the fire resistance. Typically European Standards EN 50200, EN 50289-4-16, and similar standards are used as references. A further stringent specification imposed by safety standards provides that during the combustion of the materials used in the cable, no release of halogen gases, which are particularly harmful to health, is permissible. As a result of these stringent standards, cables of the type "halogen free" in Category 6 (or higher) currently available on the market only have a fire resistance equal to several minutes .

To increase the duration of fire resistance, cables for data transmission have been developed which employ layers of mica and of glass fibers in their interior.

These two materials are resistant to flames and have electrically insulating properties such that they can be interposed between conductors. The inclusion of these materials modifies the overall electrical characteristics of the cable. To account for these influences, the cable is adapted to the required values by careful design of the dimensions of the insulation.

CN204315291U describes a cable for data transmission between computers, formed of a plurality of insulated conductors around which a layer of fire resistant material is wrapped. An additional layer of insulating material is deposited on the said wrapping material, while the outside of each pair of conductors has superposed on it a thermally insulating layer formed from a polymeric sub-layer onto which glass fibers have been adhesively bonded. A first major drawback of this solution is that such a cable, while significantly improving the overall thermal properties, has a fire resistance comparable to that of currently commercially available data transmission cables. Moreover, the above-described cable is difficult to fabricate because the thermally insulating layer disposed outside of each pair of conductors is formed by joining together its two sub-layers by distributing a thin layer of adhesive. The presence of a layer of adhesive distributed between the polymer sub-layer and the sub-layer with the glass fibers can cause a modification of the performance of the cable. The natural degradation which the adhesive layer suffers over time causes an increase in the stiffness of the cable, and reduces the duration of resistance of the cable to fire. Further, the layer of mica is placed directly above the electrical conductor, and this disposition does not allow optimization of the electrical transmission characteristics of the cable. Therefore, the electrical transmission properties of this cable are limited; e.g. it has high attenuation and low interference reduction, and as a result this conductor cannot be used in LAN connections of higher categories.

GB2448778A discloses a cable having one or more insulated conductive cores surrounded by layers of mica tape with a cross linking zero halogen material.

US2005/199415A1 discloses a cable exhibiting reduced cross ^¬ talk of any form between transmission media that includes one or more core support-separators having various shaped profiles which define a clearance to maintain spacing between transmission media or transmission media pairs.

WO 2006/031633A1 discloses a cable with at least one pair of insulated conductors and an optional drain wire, which are longitudinally covered in a shielding tape.

The publication "CAT 5e FIRE RESISTANT 120 MINUTES FRNC-LSZH CABLE DESCRIPTION" published by the applicant, discloses cables of category 5 with product features relating to fire protection, with which however higher performance categories, such as category 6 and 7 could not be reached.

The present invention is therefore based on the object of providing an improved cable for data transmission, particularly a cable for local area networks (LAN) and wide area networks (WAN) .

The present invention enables production of a cable for data transmission which is fire resistant and resistant to propagation of flame and fire, at levels considerably improved compared to currently known cables.

A further object of the present invention consists in providing a cable for data transmission which has high flame resistance and has electrical and mechanical characteristics which allow it to comply with the specifications of Category 6 or higher, even when subjected to the action of fire.

A further object of the present invention consists in providing a cable for data transmission which has high fire resistance and which uses materials with chemical and physical characteristics that are stable over time, so as to maintain electrical and mechanical properties of the cable substantially unchanged even after long periods of use and/or after a long period following the date of manufacture.

Another object of the present invention is to provide a cable for data transmission having high fire resistance, with the cable being easy to manufacture and not involving the use of adhesives or other similar materials. The described objects of the invention are achieved by a cable for data transmission having high fire resistance in accordance with claim 1.

In particular, the described cable has at least one pair of wire-like conductors, a first insulation layer superposed on the outer surface of each conductor, and an external insulating sheath which is substantially tubular and which encloses in its interior said at least one pair of wire-like conductors with the corresponding first insulation layers. The cable also comprises a second insulation layer directly superposed on the outer surface of the first insulation layer of each conductor. This second insulation layer is formed with a material selected from the group comprising mica and glass fibers . Other objects of the invention which will be better described herein below are achieved by a cable for data transmission according to the invention which is realized in accordance with the dependent claims.

The advantages and characteristics of the present invention will be evident from the following description of some preferred embodiments of a cable for data transmission, having high fire resistance, which are presented for purposes of example and do not limit the scope of the invention; the description makes reference to the following Figures: Fig. 1 illustrates a lateral view and a plan view, of a segment of a cable for data transmission having high fire resistance according to the invention, provided with a connector of the RJ45 type; Fig. 2 is a circuit diagram of the conductors present in the interior of the cable according to Fig. 1, along with a front view of an RJ45 connector illustrating the distribution of the conductors;

Fig. 3A is a cross section of a first embodiment of a cable for data transmission having high fire resistance according to the invention;

Fig. 3B is an enlarged detail view of a unitary element of the cable from Fig. 3A;

Fig. 4A is a cross section of a second embodiment of a cable for data transmission having high fire resistance according to the invention;

Fig. 4B is an enlarged detail view of a unitary element of the cable from Fig. 4A.

The object of the present invention is a cable for data transmission 1 having high fire resistance. This cable, designated with reference numeral 1 herein below, is intended to be used primarily for connecting electronic apparatuses such as computers, printers, modems, and other similar peripheral apparatuses, such as to form a computer network.

The present cable 1 may be used indoors, to connect apparatuses in the interior of offices, domestic spaces, and/or industrial spaces, and may be used outdoors, to connect two or more apparatuses in a conduit underground or from point to point above ground.

Preferably, the cable 1 for data transmission which is the object of the present invention may be used to realize a LAN or Ethernet cable of Category 6 or higher, adapted to enable transmission of numerical data at a frequency of not less than 250 MHz and a data transfer rate of 1 Gbps or more, over distances of up to 100 meters. As is known for this type of conductor, a connector of type RJ45 (designated with reference numeral 2 in Figs. 1 and 2) may be attached to the end of the cable 1. The use of such connectors has been standardized for all cable connections in telephone networks and data transmission networks. The connector 2 has eight pins 3, some or all of which may be connected to the ends 4, 5 of corresponding electrical conductors 6, 7 present in the cable 1.

In the embodiment illustrated, the cable 1 for data transmission according to the invention comprises at least one pair of wire-like conductors 6, 7, the ends 4, 5 of which are adapted to be electrically connected with the connector 2 of type RJ45.

In the configuration of the cable 1 illustrated in the figures, there are four pairs of wire-like conductors 6, 7 which have their ends 4, 5 connected to all the pins 3 of the respective RJ 45 connectors 2.

Further, the connections of the conductors 6, 7 to the connectors 2 may be according to the "EIA/TIA-568A pin to pin" standard, as illustrated schematically in Figs. 1 and 2, or the "EIA/TIA-568B" standard.

Advantageously, the conductors 6, 7 may comprise a single copper conductor 8, 9 with a substantially circular cross- section between 0.1 and 0.5 square millimeters. Alternatively the conductors 6, 7 may comprise numerous elongated wire elements .

In the event that the described cable 1 is in Category 6, each conductor 6, 7 preferably has a diameter D of ca. 0.65 mm, corresponding to a copper cross section of ca. 0.4 mm ² .

Further, a first insulation layer 10 is provided on the outer surface 11, 12 of each wire-like conductor 6, 7 of a respective pair, as best seen in Figs. 3A and 3B.

The first insulation layer 10 may have a thickness si between 0.1 and 0.5 mm, and may be fabricated from a material chosen from the group comprising polyolefin and/or polypropylene and/or polyethylene, and/or similar materials.

In addition, the polymer material with which the first insulation layer 10 is fabricated must be mechanically and electrically compatible with the polymer material with which the connector 2 is fabricated.

The described cable 1 further comprises an outer sheath 13 which is substantially tubular and is adapted to accommodate the pairs of wire-like conductors 6, 7 and the corresponding first layer of insulation 10 inside it.

The sheath 13 may be fabricated from a polymeric material selected from the group of electrical insulators with excellent resistance to propagation of fire.

Preferably, the outer sheath 13 is fabricated from a polymeric compound of the type FRNC-LSZH (Fire Retardant, Noncorrosive, Low Smoke, Zero Halogen) which has a high retarding capability for propagation of combustion and does not generate halogen gases which are particularly harmful to human health and corrosive to other mechanical and/or electronic devices.

E.g., the outer sheath may be fabricated from an FRNC-LSZH mixture based on EVA (ethylene vinyl acetate) . As best seen in the cross sectional views in Figs. 3A and 4A, the cable 1 which is the object of the present invention is characterized by the fact that it has a second insulation layer 14 which is disposed directly over the outer surface 15 of the first insulation layer 10. The second insulation layer 14 will cover completely the first insulation layer 10 and may be fabricated from a mixture of insulating materials selected from the group comprising mica and comprising glass fibers.

Advantageously, the second insulation layer 14 may be comprised of a tape 16 of the glass-mica type having a thickness s2 between 0.05 and 0.20 mm.

Such a tape 16 may be wrapped around or placed around the first insulation layer 10 so as to cover its entire outer surface 15 without leaving exposed areas. In particular, the tape 16 may be wrapped around or placed around the outer surface 15 so as to achieve an abundant overlap between different windings or passages.

Advantageously, the second insulation layer 14 may be formed by the permanent combination of two sub-layers (not shown in the Figures) . In a particularly preferred embodiment, the tape 16 is a synthetic double-layered polyester (PET) /mica tape or a mica tape combined with a PET-tape. Preferably, the tape width is about 3-10 mm. The PET tape protects the mica tape, which improves the fire resistance. The PET tape also improves - li the uniformity of the outer surface of the second insulation layer 19 which improves the overall cable properties.

The first sub-layer may comprise natural mica or a synthetic compound with similar chemical and physical properties, whereas the second sub-layer may comprise a glass fiber tape.

The second insulation layer 14 may comprise, in addition to the abovementioned materials, an additional chemical compound (e.g. muscovite and phlogopite) comprised of potassium (K) , magnesium (Mg) , aluminum (Al) , and silicon (Si) . The introduction of these materials makes it possible to considerably increase the resistance to temperature and to propagation of fire, of the tape 16.

In particular, an insulating tape 16 thus realized can maintain its chemical and physical characteristics substantially unchanged up to temperatures close to 950 °C, while also maintaining electrical insulation of the pair of conductors 6, 7.

The described cable 1 may further comprise a third insulation layer 17, best seen in Figs. 3A and 3B, and 4A and 4B, which is superposed directly over the second insulation layer 14.

In particular, the third insulation layer 17 may comprise a further glass fiber tape 18 having a thickness s3 between 0.08 and 0.2 mm.

Further, this glass fiber tape 18 may be partially wrapped around or placed around the outer surface 19 of the second insulation layer 14 of a given pair of wire-like conductors 6, 7. In other words, the third insulation layer 17 may be wrapped around or placed around the outer surface 19 of the second insulation layer 14 so as to obtain a unitary element 20 formed by corresponding conductors 6, 7 and the three layers of insulation 10, 14, 17.

Accordingly, each cable 1 will have the same number of unitary elements 20 as it has pairs of conductors 6, 7.

In the embodiment of the invention illustrated in Figs. 3A and 4A, therefore, four pairs of conductors 6, 7 and four unitary elements 20 are present.

Also in this case, the glass fiber tape 18 which forms the third insulation layer 17 may be spirally wound around the outer surfaces 19 of the second insulation layer 14 so as to completely cover the latter without leaving exposed areas. In this case, the application of the tape 18 on the outer surface 19 of the individual second layers of insulation 14 may give rise to abundant overlaps.

Further, the tape 18 may be realized by means of a yarn obtained by twisting glass fibers which have been previously impregnated with silicone resins.

Also, the tape 18 has a dielectric strength greater than 15 kV/mm, a high dielectric constant greater than 2.5, along with a high resistance to temperature of approximately 500 °C.

In a preferred embodiment of the present invention, an additional shielding layer (symbolically shown in Fig. 3b as layer 180) is placed directly on the tape 18. This additional shielding layer 180 is preferably made of aluminum foil. Preferably, the layer 180 covers the underlying tape 18 completely . In the embodiment of the cable 1 illustrated in Fig. 3A, a cable comprising two or more pairs of conductors 6, 7 has a spacer element 21 disposed inside the outer sheath 13.

The spacer element 21 is fabricated from an insulating polymeric material, and has corresponding accommodating spaces 22 for accommodating corresponding unitary elements 20 so as to keep them spaced apart and to prevent them from contacting one another.

In a case in which the cable 1 has four pairs of conductors 6, 7, the spacer element 21 may have a transverse cross section substantially in the shape of a cross, so as to define four elongated accommodating spaces 22 for the corresponding unitary elements 20.

The shape of the spacer element 21 may be adapted without impeding or impairing the overall properties of the cable 1. In particular, the spacer element 21 can also be designed as a filler cord.

Preferably, as best illustrated in Fig. 3A, the cable 1 may also comprise a shielding layer 23 disposed in the interior of the sheath and adapted to enclose the pair of conductors 6, 7, the three insulation layers 10, 14, 17, and the spacer element 21.

This shielding layer 23 may be fabricated from a thin sheet of aluminum or, alternatively, from a metallized polyester sheet. For data transmission cables 1 which require a high degree of shielding from interference, a second shielding layer 24 is provided which is disposed directly above the other shielding layer 23 so as to be contained within the interior of the outer sheath 13. This second shielding layer 24 may be fabricated from a braid of copper wires (not shown in the Figures) , the structure of which will be suitably chosen so as to cover at least 65% of the outer surface 25 of the other shielding layer 23. Additionally, in order to further reduce electromagnetic interference inside the cable 1, the wire-like conductors 6, 7 of each pair, along with the respective first and second insulation layers 10 and 14, are mutually intertwined with progression along the length of the cable 1. The number of twists per meter may vary depending on the type of cable 1 which one seeks to realize.

In the embodiment of the invention shown in Figs. 4A and 4B, the cross section of a cable 1 of Category 7 is illustrated. The cable 1 is adapted to transmit data with a working frequency of up to 600 MHz. In this particular cable 1, for each unitary element 20, an additional shielding layer 26 is provided which is superposed on the outer surface 27 of the third insulation layer 17.

In particular, this additional shielding layer 26 may be comprised of an inner sub-layer 26' made of aluminum, placed directly in contact with the outer surface 27 of the third insulation layer 17, and an outer insulating sub-layer 26'', typically fabricated from a polyester material. The outer sub ^¬ layer 26'' faces towards the base shielding layer 23. Also, this particular embodiment of the cable 1 lacks the insulating spacer 21 present in the Category 6 cable the cross section of which (latter) is illustrated, by way of example, in Fig. 3A. A spacer element 21 could however be added to the cable 1 from Fig. 4A without impeding the overall properties. The above-described cables 1 are characterized by excellent electrical properties and fire resistance. Below, some of the electrical parameters and transmission parameters are presented which were measured experimentally on a LAN cable of Category 6 of the type described above, with measurements at a reference temperature of 20 °C.

In Table I, some general electrical parameters of the cable 1 measured at a temperature of 20 °C and with fixed reference frequencies are presented:

In Table II, additional parameters of the cable 1 at various frequencies of transmission of the signal and various thermal conditions of resistance to fire, in accordance with the European Standards EN50200 and EN50289-4-16, are reported: Table II

Freq. Attenuation PS Next PS EL-NEXT Return Loss (Mhz) (dB/100m) (dB) (dB) (dB)

1 st Test 2nd Test 1 ^st Test 2nd Test 1 st Test 2nd Test 1 st Test 2nd Test EN50200 EN50200 EN50200 EN50200 EN50200 EN50200 EN50200 EN50200

1 1 .2-2.5 1 .7-2.8 70-90 70-80 65-85 70-80 15-30 20-30

16 6.2-7.6 6.8-7.6 58-70 60-70 55-68 58-68 25-40 25-32

62 13.6-15.4 14.8-15.4 55-70 50-60 42-53 42-50 25-36 22-28

125 18.6-22.4 22-22.4 45-53.2 48-52.4 38.6-45.2 39.5-40.6 24.8-27.5 20.5-21.6

200 26.8-28.2 28.4-29 39.7-45.2 38.8-41.5 30-45 30.4-32.7 18-28 19-21 .6

250 30.1 -31.4 32.3-33.1 38-45 38.6-39.3 28-33 27.4-28.6 21 -25 18.2-19.3

The column "PS Next" indicates the value of the electromagnetic interference induced in the pairs of conductors 6, 7 which are inactive, detected at the ends 4, 5 which are closer to the source.

The column "PS EL-Next" indicates the value of the electromagnetic interference induced in the pairs of conductors 6, 7 which are inactive, detected at the ends 4, 5 which are farther from the source.

The column "Return Loss" indicates the power loss associated with the impedance mismatch. The values entered in the columns "2nd Test EN50200" were detected experimentally after placing the cable 1 at a distance of 10 m from a flame for ca. 120 minutes, with measurements according to the method described in European Standards EN50200 and EN50289-4-1.

The present invention is achievable in numerous other variants, all falling within the scope of the inventive features described and claimed. The technical characteristics may be replaced by other technically equivalent elements, and other suitable materials may be used. Further, the shapes and dimensions of components of the invention may be varied as desired, to other values which may be compatible with the intended application. Additional layers may also be inserted between any two layers described above for certain applications.

The reference numerals and designations inserted in the claims and in the specification have the sole purpose of increasing the clarity of the text, and should not be considered as elements which limit the technical interpretation of the objects or processes identified therein.

- IS

Reference numerals

1 cable

2 RJ 45 connector

3 pin of connector

4, 5 end of electrical conductor

6, 7 electrical conductor

8, 9 copper conductor

10 first insulation layer

11, 12 outer surface of conductor

13 outer sheath

14 second insulation layer

15 outer surface of first insulation layer

16 tape

17 third insulation layer

18 glass fiber tape

180 additional shielding layer

19 outer surface of second insulation layer

20 unitary element

21 spacer element

22 accommodating space

23 shielding layer

24 second shielding layer

25 outer surface of shielding layer

26 additional shielding layer

26' inner sub-layer

26' r outer sub-layer

27 outer surface of third insulation layer si Thickness of first insulation layer s2 Thickness of tape

s3 Thickness of glass fiber tape