EP3123818B1

EP3123818B1 - Hybrid electrical heating cable

Info

Publication number: EP3123818B1
Application number: EP15707657.1A
Authority: EP
Inventors: Tom Verhaeghe; Filip Lanckmans; Steve Verstraeten
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2014-03-26
Filing date: 2015-03-04
Publication date: 2019-11-27
Anticipated expiration: 2035-03-04
Also published as: CN106105386A; PL3123818T3; RS59829B1; EP3123818A1; WO2015144406A1

Description

Technical Field

The invention relates to the field of electrical heating cables that comprise metal filaments that act as heat generators when electrical current is sent through the cable.

Background Art

Electrical heating cables comprising metal filaments are known. Important parameter for the heating cable is its electrical resistance in ohm/meter length of the cable, as it determines heat generation. Depending on the application, specific values for the electrical resistance in ohm/meter length of the cable are required.
For a number of applications, e.g. for car seat heating applications, the flex fatigue resistance (or flex life) of the heating cable is important. A number of patent documents relate to heating cables with high flex fatigue, e.g. realized by means of appropriate selection of the metal filaments used in the heating cable.
EP1337129 discloses an electrical heating element for heating units of seats and steering wheels. The heating element comprises at least one conductor having at least one core-coated wire. The coating comprises steel and the core comprises copper or a copper alloy, or the coating comprises copper or a copper alloy and the core comprises steel.
Hybrid electrical heating cables comprising a number of metal filaments of different types are known as well.
EP1705957 discloses a heating cable for car seat heating applications that has high flex fatigue properties. The heating cable has outer strands wound around an inner strand that is electrically conductive. The specific conductivity of the inner strand is smaller than the specific conductivity of the outer strands. The inner strand is made of a material having high tensile strength and/or high alternating bending strength than a material from which the outer strands are made. The inner strands can e.g. be made out of stainless steel filaments and the outer strands out of copper filaments.
DE10137976A1 discloses a heating cable for car seat heating. The heating cable comprises a combination of several types of individual wires braided together. The individual types of the individual wires have different electrical properties for setting desired electrical characteristics of the heating cable. At least some of the types of individual wires differ in terms of specific electrical resistance. The combination of copper (or copper alloy) wires and aluminium (or aluminium alloy) wires is specifically favoured.
WO2012/136418A1 (and its corresponding US publication US2014/0008351A1 ) discloses a heating cable comprising between seven and two hundred metallic monofilaments of a first type. The metallic monofilaments of a first type have a diameter ranging from 30 µm to 100 µm. The metallic monofilaments of a first type have a substantially round cross section. The metallic monofilaments of a first type comprise a steel layer with a chromium content of less than 10% by weight. The heating cable has an electrical resistance ranging between 0.1 Ohm/m and 20 Ohm/m when measured at 20°C. A preferred heating cable comprises a metallic monofilament of a second type or one or more bundles of metallic monofilaments of a second type; wherein the second type differs in composition from the first type.
DE19638372 discloses a lead wire for a sensor having a conductor unit including at least stainless steel wires and copper wires. By setting the cross-sectional percentage of the stainless steel wires in the conductor unit within the range of 30 to 70 percent, it is possible to obtain a lead wire having low electrical resistance, and high flexibility, tensile strength and elasticity.
The use of copper wires or copper filaments for heating wires is not always recommended, as copper has a too high conductivity, leading to a high conductivity of the heating cable. Therefore, when heating cables are required of lower conductivity, the use of copper wires or copper filaments is not recommended. Copper wires and copper filaments have low flex fatigue properties.
Electrical heating cables can be connected by means of crimp connectors. Such a crimp connector can be used to connect the heating cable to another cable, e.g. the cable supplying the electrical current. Such a crimped connection of a heating cable and a supply cable is e.g. described in DE202004018709U1 and in US3324441 .
Alternatively, the heating cable can be crimped alone in the crimp connector and the electrical connection is realized e.g. by means of a plug or pin of the crimp connector.
It is a problem that for a range of electrical heating cables that have a high flex life, mechanical crimp connections - especially when the heating cable is crimped alone in the crimp connector - cannot be decently nor reliably made.

Disclosure of Invention

The primary object of the invention is to provide a heating cable that has excellent flex fatigue properties and that can be easily and reliably connected by means of a mechanical crimp connection.
It is a specific objective to provide such a heating cable in the resistance range of 0.1 - 5 Ohm/meter.
According to a first aspect of the invention an electrical heating cable is provided. The electrical heating cable comprises two or more strands of metal filaments twisted or cabled together. At least one, preferably at least two, and more preferably each of the strands, comprises a first set of metal filaments and a second set of metal filaments, both sets of metal filaments for conducting electrical current and generating heat when the heating cable is in use. The metal filaments of the first set of metal filaments comprise a steel layer. The metal filaments of the second set comprise a steel layer. The metal filaments of the second set have an annealed microstructure, involving that the steel layer of the metal filaments of the second set has an annealed microstructure. The metal filaments of the first set have an end-drawn microstructure. A strand is defined as a combination of filaments that are parallel to each and/or are twisted together, whereby untwisting of the strand results in parallel filaments.
The first set of metal filaments and the second set of metal filaments will conduct electricity and contribute to the heat generation when the heating cable is in use.
The heating cable of the invention has excellent flex fatigue resistance. It can be crimped easily and reliably, with all metal filaments being properly crimped. This is a consequence of the presence of the metal filaments of the second set that have an annealed microstructure. The metal filaments of the second set are compressed during crimping, ensuring good electrical contact between the metal filaments of the crimped connection. Quality control of the crimp connections shows that a proper crimp connection is made. An additional benefit is that the contact resistance of the crimped connection is lower, certainly compared to similar crimp connections of other prior art heating cables with high flex fatigue. A low contact resistance is beneficial as it avoids overheating of the crimp connection due to heat generation at the crimp connector when the electrical heating cable is in use.
The combination of metal filaments of two different types in one strand puts an additional burden to the production of the strands. When combining the strand (e.g. when feeding the different filaments together when twisting the filaments or before twisting a strand) metal filaments of two different types have to be provided, e.g. unwound from their respective spools, kept under appropriate tension and processed. Such processing is more difficult - due to their differing mechanical properties - compared to the processing of only one type of metal filaments.
Preferably, the specific electrical resistance of the metal filaments of the second set is higher than the specific electrical resistance of the metal filaments of the first set.
In a preferred embodiment, the equivalent diameter of the metal filaments of the first set differs at maximum 20%, preferably at maximum 10%, and more preferably at maximum 5%, from the equivalent diameter of the metal filaments of the second set. The equivalent diameter is the diameter of a circle with the same surface area as the cross section of the metal filament which is not necessarily round.
Preferably, the filaments of the first set of metal filaments and the filaments of the second set of metal filaments are combined in the strand via twisting, wrapping, or cabling.
In a preferred embodiment, the first set of filaments comprises between 5 and 150 filaments. More preferably more than 5, e.g. more than 10, e.g. more than 20, e.g. more than 30, e.g. more than 50, e.g. more than 70, e.g. more than 90, e.g. more than 100. More preferably less than 150, e.g. less than 125, e.g. less than 100, e.g. less than 80, e.g. less than 65, e.g. less than 50.
A preferred hybrid electrical heating cable has an electrical resistance in the range of 0.1 - 5 Ohm/m, more preferably in the range of 0.1 - 3 Ohm/m.
Preferred for the filaments of the first set of metal filaments and/or for the filaments of the second set of metal filaments are filament diameters between 35 µm and 100 µm, more preferably between 50 µm and 80 µm.
The heating cable can have a corrosion resistant sheath. The corrosion resistant sheath can comprise a polymer layer. Preferred polymer layers comprise fluorine in the polymer, resulting in superior corrosion resistance and high temperature resistance. Further preferred, the corrosion resistant sheath of the heating cable comprises perfluoroalcoxy (PFA), FEP (fluorinated ethylene propylene), TPE-C, PPS (polyphenylene sulfide) or MFA (perfluoromethylalcoxy). The heating cable of such embodiments does - contrary to heating cables made from hard drawn filaments only and having such a corrosion resistant sheath - not show flaring after stripping of insulation coatings, facilitating crimping of the heating cable.
In a preferred embodiment, the metal filaments of the first set of metal filaments and the metal filaments of the second set of metal filaments are intermingled. With the metal filaments of the first set and the metal filaments of the second set are intermingled is meant that the metal filaments of the first set of metal filaments are distributed in the strand in between metal filaments of the second set of metal filaments, and the metal filaments of the second set of metal filaments are distributed in the strand in between metal filaments of the first set of metal filaments.
The metal filaments of the first set of metal filaments have an end-drawn microstructure. This has shown to provide excellent crimping in a crimp connector.
Preferably, in the cross section of the heating cable, the cumulated cross sections of metal filaments of the second set is at least 25%, more preferably at least 30%, and even more preferably at least 40 % - and preferably less than 70% - of the combined cumulated cross sections of metal filaments of the first and second set.
Preferably, the specific electrical resistance, expressed in ohm*mm²/m, of the metal filaments of the second set is at least 2 times, and more preferably at least 3 times, more preferably at least 3 times, and more preferably at least 10 times higher than the specific electrical resistance of the metal filaments of the first set.
An exemplary metal filament out of high carbon steel with a coating layer out of zinc has a specific electrical resistance of 0.202 ohm*mm²/m.
An exemplary metal filament out of low carbon steel with a coating layer out of zinc has a specific electrical resistance of 0.110 ohm*mm²/m.
An exemplary metal filament out of low carbon steel with a coating layer out of tin has a specific electrical resistance of 0.124 ohm*mm²/m.
A stainless steel filament has a specific electrical resistance of 0.682 ohm*mm²/m.
An exemplary metal filament comprising a low carbon steel core and a copper cladding has a specific electrical resistance of 0.043 ohm*mm²/m.
An exemplary metal filament comprising a copper core and a stainless steel cladding has a specific electrical resistance of 0.046 ohm*mm²/m.
Preferably, the electrical conductivity of the heating cable is for more than 75%, preferably for more than 85%, determined by the electrical conductivity of the metal filaments of the first set.
In exemplary embodiments, the steel layer of the metal filaments of the first set of metal filaments is a low carbon or a high carbon steel layer, preferably comprising a corrosion resistant metal layer (e.g. zinc or nickel) around the low carbon or high carbon steel layer.
In exemplary embodiments, the metal filaments of the first set of metal filaments comprise a layer of a metal or metal alloy of specific electrical conductivity higher than the specific electrical conductivity of the steel layer of the metal filaments of the first set of metal filaments. In such embodiments, preferably the steel layer of the filaments of the first set of metal filaments is a stainless steel layer or a low carbon steel layer.
Preferably, the layer of a metal or metal alloy of specific electrical conductivity higher than the specific electrical conductivity of the steel layer is concentric with the steel layer.
The layer of a metal or metal alloy of specific electrical conductivity higher than the specific electrical conductivity of the steel layer can be a copper layer or a copper alloy layer.
For example, the metal filaments of the first set of metal filaments can comprise a copper or copper alloy core and a - preferably circumferential - stainless steel layer around it. Preferably, such metal filaments have an end-drawn microstructure.
Another example of the metal filaments of the first set is metal filaments comprising a steel core (e.g. a low carbon steel core) and a - preferably circumferential - layer of copper or of a copper alloy around it. Such filaments are known as copper cladded steel filaments. Preferably, these metal filaments have an end-drawn microstructure.
In a preferred embodiment, the metal filaments of the second set of metal filaments are stainless steel filaments, e.g. with polygonal cross section shape, e.g. bundle drawn, or e.g. with substantially circular cross section, e.g. single end drawn.
In exemplary embodiments, the steel layer of the metal filaments of the second set of metal filaments is a low carbon or a high carbon steel layer, preferably comprising a corrosion resistant metal layer around the low carbon or high carbon steel layer.
For the invention, with high carbon steel is meant a steel grade with carbon content between 0.30 and 1.70% by weight and a chromium content of less than 10% by weight. For the invention, preferably high carbon steel grades with carbon content between 0.40 and 0.95% by weight are used, even more preferably high carbon steel grades with carbon content between 0.55% and 0.85% by weight. The high carbon steel grades can contain alloy elements; but for the invention, the high carbon steel grades preferably have a chromium content of less than 2.5% by weight and a nickel content of less than 1% by weight, preferably a nickel content of less than 0.1 % by weight, even more preferably a nickel content of less than 0.05% by weight. And preferably a chromium content of less than 1% by weight.
The use of a high carbon steel grade has a number of additional benefits. The strength of metal filaments comprising a high carbon steel layer is high, especially if the metal filament has an end-drawn microstructure. The result is that heating cables comprising such filaments have a high strength and a high flex life.
For the invention, with low carbon is meant a steel alloy where - possibly with exception for silicon and manganese - all the elements have a content of less than 0.50 % by weight, e.g. less than 0.20 % by weight, e.g. less than 0.10 % by weight. E.g. silicon is present in amounts of maximum 1.0 % by weight, e.g. maximum 0.50 % by weight, e.g. 0.30 % by weight or 0.15 % by weight. E.g. manganese is present in amounts of maximum 2.0 % by weight, e.g. maximum 1.0 % by weight, e.g. 0.50 % weight or 0.30 % by weight. Preferably for the invention, the carbon content ranges up to 0.20 % by weight, e.g. ranging up to 0.06 % by weight. The minimum carbon content can be about 0.02 % by weight. In a more preferred embodiment, the minimum carbon content can be about 0.01 % by weight. The low carbon steel composition has mainly a ferrite or pearlite matrix and is mainly single phase. There are no martensitic phases, bainite phases or cementite phases in the ferrite or pearlite matrix.
For use in the first set or in the second set of metal filaments, low carbon or high carbon filaments are preferably provided with a corrosion resistant coating layer. The corrosion resistant coating layer can e.g. be selected from the group consisting of zinc, tin, silver, nickel, aluminum, or an alloy thereof. Preferably, the corrosion resistant metal coating is between 1 and 10 % by weight of the metal filament. More preferably, between 2 and 6 % by weight. Even more preferably between 3 and 5 % by weight. As the metal coating layer is low in weight percentage of the metal filament, it is not affecting the electrical resistance of the filament to a significant extent. The benefit of the metal corrosion resistant coating on the steel filament is that the steel filaments are better resisting staining and corrosion. This is of interest for the production process of the heating cable of the invention and for storage of half-products during the production process, but also during installation and use of the heating cable.
Specific examples are the use of a nickel coating on steel filaments; the coating layer being between 2 and 6% by weight of the steel filament. More preferably the nickel coating is between 3 and 5% by weight of the steel filament. Specific examples for a nickel coating layer are on a steel filament comprising low carbon steel or comprising high carbon steel.
Another specific example is use of a zinc coating on a steel filament; the coating layer being between 0.5 and 5% by weight of the steel filament. More preferably the zinc coating is between 1.5 and 2.5% by weight of the steel filament. Specific examples for a zinc coating layer are on a steel filament comprising low carbon steel or comprising high carbon steel.
Preferably the metal filaments of the second set of metal filaments have been produced by means of single end drawing.
Examples of preferred combinations of first and second set of metal filaments are

first set of metal filaments out of high carbon steel or out of low carbon steel; preferably with an end-drawn microstructure; and preferably with a corrosion resistant coating layer, e.g. out of zinc or nickel; in combination with a second set of metal filaments out of stainless steel, with an annealed microstructure. Preferably the stainless steel filaments of the second set of metal filaments are single end drawn.
first set of metal filaments comprising a steel core (preferably a low carbon steel core) and a copper or copper alloy cladding, the filaments preferably have an end-drawn microstructure; in combination with a second set of metal filaments out of stainless steel, with an annealed microstructure. Preferably the stainless steel filaments of the second set of metal filaments are single end drawn.
first set of metal filaments comprising a copper or copper alloy core and a stainless steel cladding or layer, the filaments preferably have an end-drawn microstructure; in combination with a second set of metal filaments out of stainless steel, with an annealed microstructure. Preferably the stainless steel filaments of the second set of metal filaments are single end drawn.
first set of metal filaments comprising a steel core (preferably a low carbon steel core) and a copper or copper alloy cladding or layer, the filaments preferably have an end-drawn microstructure; in combination with a second set of metal filaments out of high carbon or low carbon steel, with an annealed microstructure; and preferably with a corrosion resistant coating layer, e.g. out of zinc or nickel.
first set of metal filaments comprising a copper or copper alloy core and a stainless steel cladding or layer, the filaments preferably have an end-drawn microstructure; in combination with a second set of metal filaments out of high carbon or low carbon steel; with an annealed microstructure; and preferably with a corrosion resistant coating layer, e.g. out of zinc or nickel.
first set of metal filaments out of low carbon steel with an end-drawn microstructure; and preferably with a corrosion resistant coating layer, e.g. out of zinc or nickel; in combination with a second set of metal filaments out of out of low carbon steel with an annealed microstructure; and preferably with a corrosion resistant coating layer, e.g. out of zinc or nickel.
first set of stainless steel filaments with an end-drawn microstructure; in combination with a second set stainless steel filaments with an annealed microstructure. The stainless steel filaments of the first set can e.g. be single end drawn; the stainless steel filaments of the second set can e.g. be single end drawn

A second aspect of the invention is an assembly of a hybrid electrical heating cable as in the first aspect of the invention and a crimp connector, wherein the hybrid electrical heating cable is crimped in the crimp connector.
In a preferred assembly, the crimp connector is connectable to an electrical power supply without overlap of an end portion of the heating cable with an end portion of an electrical power supply cable, preferably via an alternative connection than a crimp connection, e.g. via a pin or plug connector.
A preferred assembly comprises an electrical power supply wherein the crimp connector is connected to the electrical power supply without overlap of an end portion of the heating cable with an end portion of an electrical power supply cable, preferably via an alternative connection than a crimp connection, e.g. via a pin or plug connector.
A preferred assembly comprises an electrical power supply wherein the crimp connection is connected to the electrical power supply with an overlap of an end portion of the heating cable with an end portion of an electrical power supply cable in the crimp connector. Such an electrical power supply cable can advantageously comprise copper filaments.
Crimping can be performed in a way that two crimps take place simultaneously, the cable crimp and (if an insulation is present) the insulation crimp. The wire crimp forms the mechanical-electrical connection between the heating cable and the terminal (crimp connector). For a lot of applications, the connection must be gastight, meaning that there should be no voids between the filaments of the heating cable and the terminal (crimp connector) as corrosion could occur through such voids.
It is possible to strip an insulating coating layer fully or partly before crimping.
Several types of crimps exist and can be used for the invention. Examples are B-crimp and O-crimp, wherein the name of the crimp is derived from the shape of the crimp.
The hybrid heating cable of the invention can e.g. be used in car seat heating systems and in industrial heating systems

Brief Description of Figures in the Drawings

Figure 1 shows an example of a heating cable according to the invention. Figures 2 and 3 show examples of crimp connections comprising a heating cable of the invention.

Mode(s) for Carrying Out the Invention

Figure 1 shows an example of a heating cable 105 according to the invention. A strand 110 of 7 metal filaments has been twisted in a first twisting operation. In a second twisting operation, 10 of such twisted strands 110 have been twisted together into a cable with an outer diameter of 1.60 mm. The cable is extrusion coated with a PFA coating layer 120, e.g. with a wall thickness of 0.22 mm. The strand 110 of 7 metal filaments comprises a first set of metal filaments, wherein the set consists out of two metal filaments 130 that have a copper core (36 % by weight of the metal filament) and a stainless steel layer (64 % by weight of the metal layer), e.g. AlSl 316L. Preferably the stainless steel layer has a substantially constant thickness over the full circumference of the metal filament.
The strand 110 of 7 metal filaments comprises a second set of metal filaments, wherein the set consists out of five metal filaments 140 which are single end drawn stainless steel filaments (e.g. AlSl 316L).
The metal filaments of the first set 130 and the metal filaments of the second set 140 each have a diameter of 75 µm. The metal filaments of the first set 130 and of the second set 140 had been randomly distributed in the strand 110, resulting in an intimate blend in the strand 110 (and in the heating cable 105) of the two sets of metal filaments and a homogeneous distribution over the heating cable of the two sets of metal filaments. The stainless steel filaments (the filaments of the second set) have an annealed microstructure; and the metal filaments of the first set can have an end-drawn microstructure.
The electrical resistance of the heating cable is 0.46 Ohm/meter.
After stripping the insulating coating, the heating cable was crimped in a B-crimp connector and via a plug connected to a power supply. The heating cable could be reliably crimped in the crimp connector.
The heating cable showed excellent flex fatigue resistance.
A comparison of the contact resistance of the crimp connection of such a hybrid electrical heating cable according to the invention compared to a similar heating cable but without the second set of metal filaments (thus without the single end drawn stainless steel filaments, e.g. AlSl 316L) showed that the contact resistance of the crimp connection of the inventive heating cable was much lower than the contact resistance of the crimp connection with the prior art cable: 3 mOhm with the inventive heating cable compared to 30 mOhm with the prior art heating cable.
Another example of a hybrid electrical heating cable according to the invention comprises 9 strands twisted together, each of the 9 strands consists out of 7 metal filaments twisted together. Three filaments of each strand are high carbon steel filaments with a corrosion protecting coating of Zn and a diameter of 60 µm. The metal filaments of the first set of filaments have an end-drawn microstructure. Four filaments (the filaments of the second set of filaments) of each strand are single end drawn stainless steel filaments of 60 µm diameter. The single end drawn stainless steel filaments have an annealed microstructure. The position of the seven metal filaments in the strand is random. The hybrid heating cable has a resistance of 2 Ohm/meter. The heating cable was crimped in in a B-crimp connector and via a plug connected to a power supply. The heating cable could be reliably crimped in the crimp connector.
Another example of a hybrid electrical heating cable according to the invention comprises 11 strands twisted together, each of the 11 strands consists out of seven metal filaments twisted together. Five filaments of each strand are high carbon steel filaments (the first set of metal filaments) with a corrosion protecting coating of Zn and a diameter of 60 µm. The metal filaments of the first set of filaments have an end-drawn microstructure. Two filaments (the filaments of the second set of filaments) of each strand are single end drawn stainless steel filaments of 60 µm diameter. The metal filaments of the second set of metal filaments have an annealed microstructure. The position of the seven metal filaments in the strand is random. The hybrid heating cable has a resistance of 1.2 Ohm/meter. The heating cable was crimped in in a B-crimp connector and via a plug connected to a power supply. The heating cable could be reliably crimped in the crimp connector.
Another example of a hybrid electrical heating cable according to the invention comprises 6 strands twisted together, each of the 6 strands consists out of seven metal filaments twisted together. Two filaments of each strand are low carbon steel filaments with a corrosion protecting coating of Zn and a diameter of 60 µm; these filaments are forming the first set of metal filaments. The metal filaments of this first set of filaments have an end-drawn microstructure. Five filaments (the filaments of the second set of filaments) of each strand are single end drawn stainless steel filaments of 60 µm diameter. The metal filaments of the second set of filaments have an annealed microstructure. The position of the seven metal filaments in the strand is random. The hybrid heating cable has a resistance of 2.5 Ohm/meter. The heating cable was crimped in in a B-crimp connector and the crimp connector has been connected to a power supply via the plug of the crimp connector. The heating cable could be reliably crimped in the crimp connector.
Another example of a hybrid electrical heating cable according to the invention comprises twelve strands twisted together, each of the twelve strands consists out of seven metal filaments twisted together. Two filaments of each strand are high carbon steel filaments with a diameter of 60 µm (the first set of metal filaments). The metal filaments of the first set of filaments have an end-drawn microstructure. Five filaments (the filaments of the second set of metal filaments) of each strand are single end drawn stainless steel filaments of 60 µm diameter. The metal filaments of the second set of metal filaments have an annealed microstructure. The position of the seven metal filaments in the strand is random. The hybrid heating cable has a resistance of 1.7 Ohm/meter. This is cable A for the comparison. The heating cable was crimped in a B-crimp connector and via a plug connected to a power supply. The heating cable could be reliably crimped in the crimp connector.
This hybrid electrical heating cable was compared with another inventive hybrid electrical heating cable (cable B). The hybrid heating cable comprises 8 strands twisted together, each of the 8 strands consists out of seven metal filaments twisted together. Five filaments of each strand are high carbon steel filaments with a diameter of 60 µm (the metal filaments of the first set of metal filaments). The metal filaments of the first set of filaments have an end-drawn microstructure. Two filaments (the filaments of the second set of filaments) of each strand are single end drawn stainless steel filaments of 60 µm diameter. The metal filaments of the second set of metal filaments have an annealed microstructure. The position of the seven metal filaments in the strand is random. The hybrid heating cable has a resistance of 1.6 Ohm/meter.
The performance of cable A and cable B has been compared with the performance of a prior art cable (cable C) comprising 6 strands twisted together, each of the 6 strands consists out of 7 metal filaments twisted together; metal the filaments are high carbon steel filaments with a diameter of 60 µm. The metal filaments have an end-drawn microstructure. Cable C has a resistance of 1.6 Ohm/meter.
Comparative experiments with cables A and B (inventive cables) and cable C have shown that

cables A and B could be connected very well by means of a crimp connection; whereas cable C did not show a good crimp connection; and
a flex fatigue test showed that cable A had a lifetime of 11791 cycles to failure, cable B had a lifetime of 7319 cycles to failure; whereas cable C only had a lifetime of 5411 cycles to failure.

Figure 2 shows an example of a crimp terminal (or crimp connector) 200. A heating cable 205 according to the invention is crimped in the crimp terminal 200. The crimp terminal 200 has a pin 215 that is used to make contact with a power supply. Alternatively, the crimp terminal can have a plug instead of a pin.
Figure 3 shows a cross section 303 of a heating cable 310 crimped in a B-type crimp terminal 317. As an alternative to B-type crimp terminals, O-type crimp terminals can be used as well.

Claims

Electrical heating cable (105) comprising two or more strands (110) of metal filaments twisted or cabled together,
- wherein at least one of the strands (110) comprises a first set of metal filaments (130) and a second set of metal filaments (140), both for conducting electrical current and generating heat when the heating cable (105) is in use;

- wherein the metal filaments of the first set of metal filaments (130) comprise a steel layer;

- wherein the metal filaments of the second set of metal filaments (140) comprise a steel layer;
wherein the metal filaments (140) of the second set have an annealed microstructure; characterised in that the metal filaments (130) of the first set have an end-drawn microstructure.
Electrical heating cable (105) as in the preceding claim, wherein the specific electrical resistance of the metal filaments (140) of the second set is higher than the specific electrical resistance of the metal filaments (130) of the first set.
Electrical heating cable (105) as in any of the preceding claims, wherein in the strands (110) comprising metal filaments (130) of the first set and metal filaments (140) of the second set, the metal filaments of the first set and the metal filaments of the second set are intermingled.
Electrical heating cable (105) as in any of the preceding claims, wherein in the cross section of the heating cable (105), the cumulated cross sections of metal filaments of the second set is at least 25% of the combined cumulated cross sections of metal filaments of the first and second set.
Electrical heating cable (105) as in any of the preceding claims, wherein the specific electrical resistance, expressed in (ohm*mm²/m), of the metal filaments of the second set is at least 2 times higher than the specific electrical resistance of the metal filaments of the first set.
Electrical heating cable (105) as in any of the preceding claims, wherein the electrical conductivity of the heating cable (105) is for more than 75% determined by the electrical conductivity of the metal filaments of the first set.
Electrical heating cable (105) as in claims 1 - 6, wherein the steel layer of the metal filaments of the first set of filaments is a low carbon or a high carbon steel layer.
Electrical heating cable (105) as in claims 1 - 7, wherein the metal filaments of the first set of filaments comprise a layer of a metal or metal alloy of specific electrical conductivity higher than the specific electrical conductivity of the steel layer.
Electrical heating cable (105) as in claim 8, wherein the layer of a metal or metal alloy of specific electrical conductivity higher than the specific electrical conductivity of the steel layer is a copper layer or a copper alloy layer.
Electrical heating cable (105) as in claims 1 - 9, wherein the metal filaments of the second set of metal filaments are stainless steel filaments.
Electrical heating cable (105) as in claims 1 - 9, wherein the steel layer of the metal filaments of the second set of metal filaments is a low carbon or a high carbon steel layer, preferably comprising a corrosion resistant metal layer around the low carbon or high carbon steel layer.
Assembly of a hybrid electrical heating cable (205) as in any of the preceding claims and a crimp connector (200), wherein the hybrid electrical heating cable (205) is crimped in the crimp connector (200).
Assembly as in claim 12, comprising an electrical power supply and wherein the crimp connector (200) is connected to the electrical power supply without overlap of an end portion of the heating cable (205) with an end portion of an electrical power supply cable.