Electrical power cable

Technical Field

This invention relates to the production of flexible electrical cables for conveying low voltage electricity and relates to an electrical power cable of the type comprising at least one metal conductor; at least a first layer of insulating material covering the metal conductor and at least a second layer of material covering the first layer of insulating material.

Background Art

It is known that electrical cables of the type indicated above always comprise a conductor, which may be made of copper, aluminium or other conducting materials, which is covered with a first material that has good dielectric properties (insulation) and which is often additionally covered with another layer of material (sheath) which is resistant to the use for which the cable is intended, that is to say, to the environment in which the cable must be installed.

For example, a cable which must be used in an industrial environment will have a sheath that is resistant to oil, whilst a cable for a mine will have a sheath that is resistant to abrasion and tearing; a cable used for welding booths must be particularly resistant to ozone. Cables which do not have the sheath are generally called "insulated wires".

To obtain a multicore cable several non-sheathed insulated cables (from two to several dozen) must be united in a single cable.

This multicore cable has an irregular cross-section which is not perfectly circular and which is more irregular the fewer the number of non-sheathed insulated cables used.

The area between the cross-section of the multicore cable and the circle tangent to the outside of the individual non-sheathed insulated cables is called the "stellar area", whilst the tangent points of the non-sheathed insulated cables are called "crests".

A cable circular cross-section is a good quality of the manufactured article, since, irrespective of appearance, it makes the cable easier to handle, to install and to join in tidy bundles in raceways.

Moreover, a smooth outer surface increases the resistance of the manufactured article, both against any mechanical action and against any chemical attack.

To obtain a multicore cable with a circular cross-section it is therefore necessary that the sheath also fills the stellar area of the cable, which involves the use - for cable construction - of large quantities of high quality material, with an evident waste of material and economic disadvantage.

To overcome this problem, filling of the stellar area of the cable is often obtained using extremely inexpensive materials.

Various types of materials are used for this purpose. For example: waste PVC, strands of synthetic fibres or specially formulated PVC-based materials, as well as polyolefins or rubber. These materials are generally called "fillers". On the outside of the fillers the sheath is then extruded in the minimum specification thicknesses established by the various regulations.

For that purpose, all industrialised nations have special Technical Specifications which define the properties and construction methods for electrical cables. Said Specifications are governed by the various National Bodies: in Italy the CEI (Italian Electrotechnical Commission, in England the BS (British Standard) and so on. In Europe, said Specifications were subsequently harmonised under the HAR mark.

The materials used for the production of these cables may be thermoplastic or cross-linked. In the category of thermoplastics, the most widely used material is certainly PVC.

The reasons for the success of PVC are the low cost of the starting material, the simplicity of transformation and easy recycling, both of PVC, and of the cable in the event of production defects.

The weak points or disadvantages of PVC are mostly environmental and relate to the safety of the cable laid which, when subjected to strong heating, for example during a fire, develops highly toxic substances which are very often deadly for persons present.

PVC develops hydrochloric acid in the form of black fumes which: on one hand poison persons affected by a fire, and on the other hand make locating escape routes difficult. Statistics drawn up on the causes of deaths during fires indicate, beyond reasonable doubt, that most deaths were caused not by burns, but as a result of poisoning by hydrochloric acid and its derivatives.

Many studies have been carried out in the past to find a solution to this problem, proposing that PVC be abandoned in favour of materials which, if subject to fire, produce small quantities of non-toxic, clear fumes.

More than a decade ago products with such properties were identified, but the cost of alternative materials to PVC is much higher than the cost of PVC. That, together with greater process difficulties (cable extrusion) until now has not allowed the long-hoped-for replacement of PVC. Disclosure of the Invention

The aim of this invention is therefore to overcome these disadvantages by producing a cable, insulated with a material which is extremely economical and easy to extrude, covered with a thin layer of a polyolefin which fixes its dimensions and gives the assembly the mechanical properties required for use.

Brief Description of the Drawings

The technical features of the invention, with reference to the above aims, are clearly described in the claims below, and its advantages are more apparent from the detailed description which follows, with reference to the accompanying drawings which illustrate a preferred embodiment of the invention provided merely by way of example without restricting the scope of the inventive concept, and in which:

Figure 1 is a perspective assembly view of a first embodiment of a cable in accordance with the invention, having a single-core structure;

Figure 2 is a perspective assembly view of a second embodiment of a cable in accordance with the invention, having a multicore structure.

Detailed Description of the Preferred Embodiments of the Invention

With reference to Figure 1 of the accompanying drawings, the numeral 1 denotes in its entirety a low voltage, flexible electrical power cable, basically comprising a metal conductor 2, made of copper, aluminium or other equally conducting materials, which is covered with a first layer 3 of insulating material, positioned so that it directly covers the conductor 2, and a second layer 4 of flexible material positioned so that it covers the first layer 3 of insulating material.

The first layer 3 of insulating material basically comprises a mixture of hydroxides of metals from the third and/or fourth periods of the periodic table of elements, in particular Al, Mg, Ca, Na, K, of ethylene - propylene copolymers and of butadiene copolymers.

The metal hydroxides are contained pure or mixed with each other in weight ratios of between 50 and 90% of the total weight of the mixture.

The ethylene - propylene copolymers are selected in families of copolymers containing ethylene in a percentage of between 10 and 90%. The ethylene - propylene copolymers are contained in weight ratios of between 5 and 15% of the total weight of the mixture.

The butadiene copolymers are selected in families of copolymers in which the butadiene is contained between 10 and 80%. Said butadiene copolymers are contained in the composition of said first layer 3 of insulating material in weight ratios of between 5 and 15% of the total weight of the mixture.

The composition of the material of which the first, insulating layer 3 consists preferably also comprises phthalates, adipates, sebacates contained pure or mixed with each other and present in the composition of the material of which the first layer 3 consists in weight ratios of between 0.5 and 2% of the weight of the mixture.

Soaps selected in the families of metals comprising calcium, zinc, lead, sodium, aluminium, potassium are preferably also present in weight ratios of between 0.5 and 2% of the weight of the mixture.

The first layer 3 of insulating material is sized in terms of thickness and quantity of materials in such a way that it constitutes between 10 and 90% of the cross-section of the cable.

In contrast, the second layer 4 of material, which covers the first layer 3, comprises polypropylene homopolymer or copolymer with the addition of coloured pigments if necessary.

By suitably associating different thicknesses appropriately selected for the first layer 3 and the second layer 4 it is possible to modulate both the fire resistance and the mechanical properties of the cable 1 relative to the requirements of the specific use for which the cable is intended.

Figure 2 shows a cable 10 with a multicore structure which may be obtained by associating a plurality of basic cables 1 , that is to say, insulated wires which have the single-core structure of Figure 1, on which a first layer 30 and a second layer 40 are deposited, respectively forming the shared insulation and sheath, encompassing all of the insulated wires in an overall multicore structure.

Relative to the process for the production of cables 1, 10 made in this way, it is possible, with minimum plant engineering implementations, to adapt a conventional PVC cable production line so that it can produce the type of cables 1 or 10 according to this invention, as already indicated, having two layers 3 and 4, 30 and 40 of separate materials covering one or more central conductors 2 and basically designed: the first to contain the costs, the second to give the cable 1 or 10 the necessary mechanical properties.

The mixture of raw materials of which the first layer 3, 30 is composed can be prepared using conventional mixers (for example, Banbury or Continuous mixer or in continuous lines, such as two- or one-screw extruders or extruders of the Ko-kneader (Buss) type).

Whatever the plant engineering solution adopted, the material for production of the first layer 3 or 30 is preferably packaged in pellets and is intended to then be extruded over the conductor 2 of the cable 1 or 10 so as to give the cable 1 , 10 fire resistance and non-toxic properties. A special feature of the composition of the material of which the first, insulating layer 3,30 is made is that of only developing water during combustion, consequently emitting a small quantity of clear fumes.

The material of which the second layer 4 or 40 is made, that is to say, the material of the outer sheath of the cable 1 or 10, which can also be extruded on the insulating layer 3, 30, can easily be purchased on the market, since it is produced on a large scale by the world's leading petrochemical companies.

Obviously, extrusion of the first layer 3 or 30 and the second layer 4 or 40 on the power cable 1 or 10 may be performed simultaneously according to a co- extrusion process, or one after another with a "tandem" process.

In short, the invention fulfils the preset aims and is particularly suitable for producing cables 1, 10 with mechanical strength and flexibility comparable to those of PVC, having excellent resistance to flames and very low flame propagation in the event of a fire. Moreover, the fumes produced by combustion of the cable are clear, non-toxic and are emitted in a very small quantity.

The cost of the materials is comparable to that of PVC. The cost of extrusion is also comparable to that of PVC. Finally, the production plants for said cables may advantageously be the same that are currently used for the PVC production technology.

The invention described above is susceptible of industrial application and may be modified and adapted in several ways without thereby departing from the scope of the inventive concept. Moreover, all details of the invention may be substituted by technically equivalent elements.