In connection with the use of electrical heating cables for various heating purposes a heating cable, in use, will comprise the actual part of the cable to be used for the specific heating purpose and a connection cable which connects the heating cable to a power supply.

Electrical heating by means of the above-mentioned heat- ing cables naturally has to satisfy rather great safety requirements, it being borne in mind that the heating cables are frequently used in e.g. moist environments in which even small fractures or openings in the insulation around the electrically conducting threads may have seri- ous consequences.

This is particularly pronounced in the junction between the actual heat conductor cable and the connection cable, as this junction must frequently be made as a discrete connection between the heat conductors and a low-ohmic connection cable, said connection comprising inter alia a housing provided around the established connection.

CH Patent Specification No. 607 863 discloses an electri- cal heating cable in which a heating thread is wound around two end-to-end connected core elements, which con- sist of a conducting core part and a non-conducting core part, respectively. The heating cable, however, is weak in the junction between these two core elements and is moreover difficult to manufacture in continuous lengths.

US patent no. 4,531,049 discloses a heating wire in which the above-mentioned drawbacks have been dealt with as the ends of the cold terminal and the tension wires are specifically mechanically coupled, thus obtaining a strengthened mechanical connection. A serious drawback of the disclosed heating wire is, however, that the wire is very expensive as the necessary coupling between the wires results in a complex manufacturing process.

It is the object of the invention to obtain a heating wire having a strong coupling between the cold lead section and the heating wire and, moreover, to obtain a cheap process of making such wire.

Summary of the invention When, as stated in claim 1, the core element comprises at least two stranded electrical heat resistance conductors, and each cold lead section of the heat cable comprises at least one low-ohmic section which is arranged continuously relatively fixed to the core and which is in galvanic connection with at least one of the stranded heat resistance conductors, a heating cable which has an integrated cold terminal, is simple to manufacture and has a great ultimate strength, is obtained. Thus, the invention provides a heating cable whose junction between cold terminal length and heat conductor length may be integrated directly within a common layer of insulation.

It should also be noted that the heating cable may be manufactured by an automated manufacturing process without thermal or mechanical loading of the said junction between the cold terminal length and the heat conductor length.

The basic properties of the invention thus ensures that the stranded heat conductors themselves are able to fix the cold lead sections in the cable itself and during the manufacture of the cable as well.

According to the invention continuously relatively fixed means that each mutually spaced cold lead section is connected continuously to the core.

Thus, according to the invention discrete couplings between the ends of the cold lead sections and the heating sections or tension wires can be avoided.

The cold cable may e.g. be arranged inside the core, be wound around the core, be positioned along the core or at other suitable positions, as long as galvanic connection is maintained with at least one of the stranded heat re- sistance conductors.

Further, a reduction in material and a reduction in costs when manufacturing the cable are obtained over prior art heating cables.

A further object of the invention is to obtain an environmentally friendly heating concept, both in use and in manufacture.

It should be noted that the cold cable may be made of any low-ohmic conductor suitable for the purpose.

The cold cable may also be made of a solid or a tubular length or lengths of wire suitable for the purpose, and the form and the cross section may be suitably adapted to the purpose.

Moreover, it should be noted that the cable has no separate mechanical couplings between terminated ends of longitudinally arranged wires.

When, as stated in claim 2, the heat conductors are stranded in the longitudinal direction of the heating cable around the cold lead sections, a particularly ad- vantageous embodiment of the invention is obtained, as the cold conductor may be fed continuously into the heat- ing cable when this is stranded. Also, optimum retention and thereby galvanic contact between the cold terminal length and the heat conductor length are ensured, as the stranding of the heat conductor threads is performed di- rectly on the cold cable. The heating cable may thus be manufactured under economically favourable conditions.

When, as stated in claim 3, each low-ohmic section is formed by a stranded length of wire, a practical and economical embodiment of the invention is obtained, a stranded cable being preferred in many connections as a connection cable to the mains supply, as the windings of the cold cable may constitute longitudinal depressions and guides for the heating threads wound around the cold cable. This provides an improved mutual fixing as well as a galvanic connection between the heat conductors and the individual conductors in the cold cable.

The stranded cold cable will preferably be made of copper or surface-treated copper.

When, as stated in claim 4, the heating cable comprises three to fifteen stranded electrical heat resistance conductors, an advantageous embodiment of the invention is obtained.

When, as stated in claim 5, the electrical heat resis- tance conductors are stranded with at least two revolutions per metre of heating cable, an additional embodiment of the invention is obtained.

When, as stated in claim 6, each core is enclosed by a flexible layer of insulation, a heating cable satisfying common standards for insulation requirements between heating cable and surroundings is obtained.

The layer of insulation may be applied to the heating cable in a known manner by extrusion of known insulation materials suitable for the purpose.

It is noted that the insulation of the heating cable also encloses the critical junction between the cold terminal length and the heat conductor length so that the junction is integrated in the heating cable.

When, as stated in claim 7, the heating cable comprises at least two cores, an additional advantageous embodiment of the invention is obtained, if a two- or multi-conduc- tor cable is desired.

When, as stated in claim 8, the heating cable comprises one or more visual markings on the insulation layer of the heating cable to indicate the junctions of the heating cable between cold lead sections and heating sections, a advantageous embodiment of the invention is obtained, in the event that it should be desired for various reasons that cutting to shorten a continuous heating cable according to the invention should be per- formed manually, e.g. at the mounting site.

The visual markings may according to the invention comprise of colour markings, mechanical markings or the like.

Moreover, the invention provides an improved degree of security as the mounting of the cable in e.g. heating systems will be supported to ensure that the heating sections are excluded from the mains installation. This is a particularly important feature when utilizing relatively long cold lead sections, as no heating should be enclosed in e.g. mains tubes.

When, as stated in claim 9, the heating cable is continu- ously stranded of two or more heating threads and the cold lead sections are produced by continuously feeding mutually spaced lengths of low-ohmic sections into the heat conductors when stranding the heat conductors around the low-ohmic cable, an effective manufacture of heating cables with "incorporated" cold cable is obtained, as the manufacture may be performed entirely automated with a reduced consumption of resources.

Furthermore, it should be noted that the process requires no separate mechanical coupling between the ends of longitudinally arranged wires.

Drawing The invention will be described below with reference to the drawing, in which fig. 1 shows a junction of a cable according to the invention, fig. 2 shows a cross-section of the cable of fig. 1 along the line II-II,

fig. 3 shows a cross-section of the cable of fig. 1 along the line III-III, fig. 4 shows a cross-section of an additional cable according to the invention in the heat conduc- tor length of the cable, fig. 5 shows a cross-section of the same cable ac- cording to the invention in the cold conductor length of the cable, and fig. 6 shows a general view of a heating cable ac- cording to the invention.

Example Fig. 1 shows a junction on a cable according to the in- vention.

The heating cable constitutes a plurality of two parts, viz. a cold terminal length 6 and a heat conductor length 5.

The heat conductor length 5 comprises four heat conductor threads 1, 2, 3, 4 which are stranded in the longitudinal direction of the heating cable.

The cold conductor length 6 additionally comprises a cold cable 10 around which the four heat conductor threads 1, 2, 3 and 4 are stranded.

According to the embodiment shown, the relative movement of the cable in the longitudinal direction is increased by the insertion of the cold conductor length 6. Accord- ing to other embodiments of the invention it is preferred

that the revolution rate is maintained with respect to the movement in the longitudinal direction.

The cold cable 10 may be formed by a low-ohmic structure of e.g. stranded copper wires selected for the purpose, or solid conductor lengths. In the embodiment shown, it is preferred, inter alia owing to the flexibility of the cable, to construct the cold cable 10 as a stranded copper wire. In other applications, it might e.g. be preferred to use tubular conductor lengths or the like.

It is a prerequisite, however, that an electrical contact can be obtained between the cold conductor length and the heat conductor threads, which can be achieved, of course, according to the present example.

It is noted that the embodiment shown provides a continu- ous electrical coupling between the cold conductor length and the heat conductor length, which is maintained during a subsequent extrusion of an insulation layer.

It should also be noted that, optionally, the extrusion rate may advantageously be regulated in certain applica- tions, if it should be desired to have additional control of the extrusion and consequent control of the thickness of the insulation layer in the cold conductor length 6 as well as the heat conductor length 5.

Moreover, it should be noted that the cold conduction length according to the present embodiment is continuously mechanically fixed within the heat conductor threads.

Fig. 2 shows a cross-section of the heat conductor length, in the section II-II in fig. 1, from which it ap-

pears that the cable comprises four heating threads 1, 2, 3 and 4.

Fig. 3 shows a cross-section of the cold terminal length 6 in the section III-III in fig. 1, illustrating that the cable comprises the four heating threads 1, 2, 3 and 4 which are stranded around a cold cable 10.

Fig. 4 shows a cross-section of an additional cable ac- cording to the invention in the heat conductor length of the cable.

The heating cable consists of six heating threads 21, 22, 23, 24, 25 and 26 of an alloy of stainless steel which <BR> <BR> <BR> together have a cross-section of about 0.25 mum2. . The length of the heat conductor length is e.g. 25 metres ac- cording to an embodiment of the invention.

Fig. 5 shows a cross-section of the same cable according to the invention in the cold terminal conductor length of the cable.

In addition to the six heating threads 21, 22, 23, 24, 25 and 26, the cold terminal conductor length consists of an embedded cold cable which is stranded of seven copper threads 31-37 and has a total cross-section of about 1 mm2. The total cross-section is thus 1.25 mm2.

The length of the cold conductor terminal length is e.g.

5 metres according to an embodiment of the invention.

According to the example, the cold cable is continuously inserted into the heat conductor threads in discrete lengths of 5 metres during the stranding, without inter- ruption of the stranding process, at intervals of 25 metres.

The heating cable is covered by a layer 20 of insulation in fig. 4 as well as in fig. 5.

As an individual conductor cable, the heating cable will be cut into lengths of 25 m which additionally comprise a connection cable of 2.5 m at each end.

Fig. 6 shows a greater section of a heating cable accord- ing to the invention, said heating cable comprising the heat conductor length 5 of the length L1 and the cold conductor length 6 of the length L2. The heating cable additionally comprises a cold cable 10 around which heat- ing threads (not shown) are stranded. A cold cable 10 is embedded in the cold conductor length 6, and a flexible PEX insulation layer 8 is applied to the entire cable by extrusion.

The cable may e.g. be made with a different thickness of the insulation layer 8 around the cold conductor length 6 and the heat conductor length 5, if this should be de- sired.

According to the invention it is decisive that the core is stranded of more than two heat conductors, the stranded core being "self-supporting".

According to the invention, a colour marking or the like may be applied to the outer surface of the insulation layer applied to the heating cable to indicate the junc- tions between the heat conductor lengths and the cold terminal lengths.

It is also understood according to the invention that multi-conductor heating cables are manufactured, it being possible to manufacture the individual heat conductors as single conductors, which may subsequently be embedded in the same cable by additional extrusion.