KR20000022269A - Heating cable - Google Patents

Abstract

PURPOSE: A heating cable is used as pipe, tank, the freezing protection of substrate and the maintenance of temperature. The choice of wire for use as an electrode products the heat cable. The heat cable has a consistency and a confidence, though being had a physical and heat stress. CONSTITUTION: The heat cable relates to an automatic controlling electronic heat cable. The heatcable includes the braid which functions as electronic grounding and provides mechanical protection, and the metal grounding layer of tape shape. An electrical heating cable in which at least two elongate conductors are separated by an insulating spacer and are contacted by at least one elongate resistive heating strip. The heating strip. which contacts alternately the first conductor and the second conductor at contact points which are longitudinally spaced apart along the length of the strip and along the length of each of the conductors, is made from a conductive polymer composition which exhibits PTC behavior. The conductors are concentric stranded wires having a lay length L of at most 20 mm or a lay angle beta of at least 11 degree or a lay length L of at most 20mm and a lay angle beta of at least 10 degree.

Description

Heating cable

Long electrical heating cables are well known for use in frost protection and temperature maintenance of pipes, tanks and other substrates. Particularly useful long heating cables consist of (a) a first and a second long electrode and (b) a plurality of resistance heating elements connected in parallel between the electrodes and (c) an insulating sheath surrounding the electrode and the heating element. In addition, the heating cable also often includes a metal ground layer in the form of a braid or tape surrounded by an insulating sheath that serves to electrically ground the heating cable and provides mechanical protection. Because of the parallel structure of the heating elements such heating cables can be cut to a suitable length for use in each application.

For many applications, the resistive heating element preferably comprises a conductive matrix, ie a polymer matrix in which particulate conductive fillers are dispersed. The conductive polymer preferably exhibits active temperature coefficient (PTC) behavior and allows the heating cable to automatically adjust. Two types of long heating cables are common. In the first class, the conductive polymer is in the form of a continuous strip in which the electrode is embedded. Such heaters are described, for example, in U.S. Pat. Batliwalla], 4,344,148 [Kampe], 4,334,351 [Sopory], 4,426,339 [Kamath et al.], 4,574,188 [Midgley et al.], No. 5,111,032 (Batliwalla et al.) And International Patent Publication No. WO91 / 17642 (Raychem Corporation, published November 14, 1991). In the second type of heating cable, the conductive polymer is in the form of a continuous strip wound around the long electrode and alternately contacts the exposed electrode when it extends down the length of the heating cable. In this configuration, the electrodes are separated from each other by a generally insulating spacer. Alternatively, the electrode can be wound around the core comprising the conductive polymer strip. This type of cable is described in US Pat. No. 4,459,473 (Kamath).

Under normal operating conditions, long heating cables are subject to physical stress and deformation when placed in contact with the substrate being heated. For example, when the substrate is a pipe or conduit, the cable is often wound in a spiral fashion around the pipe. Moreover, the cable must be wound around valves, joints and other areas to be heated. Although heating cables are flexible, in some situations, such as small pipe or valve diameters, it is necessary to bend or twist the cables. In addition, under normal use conditions, the heating cable undergoes a thermal cycle from relatively low temperature to relatively high temperature. Both physical stresses due to positioning on the substrate and thermal stresses caused by thermal cycles can cause changes in the heating cable. In particular, when the electrodes are located at opposite edges of the spacers rather than embedded in the conductive polymer strips, the electrodes can move away from the position in the spacers, reducing contact with the conductive polymer strips wound around them.

The present invention relates to heating cables, in particular to self-regulating electrical heating cables.

The invention is illustrated by the following figures.

1 is a plan view of a heating cable of the present invention.

2 is a cross-sectional view taken along the line 2-2 of FIG.

3 is a plan view of another heating cable of the present invention.

4 is a schematic diagram of the conductors used in the heating cable of the present invention.

5 is a cross-sectional view of the conductor used in the heating cable of the present invention.

We now know that the choice of a particular kind of wire for use as an electrode creates a heating cable that suffers less strain. As a result, the heating cable of the present invention has a consistent and reliable performance even under physical and thermal stresses. In a first aspect, the present invention

(1) spaced first and second long conductors, each of which may be connected to a power source, each of the first and second conductors having a concentric stranded wire having a lay angle β of at least 11 °; ),

(2) (i) a long PTC element consisting of a conductive polymer composite extending along the length of the heating strip (ii) exhibiting PTC behavior, (b) along the length of the strip and the length of each conductor An elongated resistance heating strip in alternating electrical contact with the first and second conductors at longitudinally spaced contact points along

(3) When the conductor is connected to a power source, a strip of insulating material placed between the conductors so that all current passing between the conductors passes through the heating strip

It provides an electric heating cable comprising a.

In a second aspect, the present invention

(1) spaced first and second long conductors that can be connected to a power source, each of the first and second conductors comprising a concentric strand wire having a twist length L of up to 20 mm (0.8 inch); Wow,

(2) (i) a long PTC element consisting of a conductive polymer composite extending along the length of the heating strip (ii) exhibiting PTC behavior, (b) along the length of the strip and the length of each conductor An elongated resistance heating strip in alternating electrical contact with the first and second conductors at longitudinally spaced contact points along

(3) When the conductor is connected to a power source, a strip of insulating material placed between the conductors so that all current passing between the conductors passes through the heating strip

It provides an electric heating cable comprising a.

In a third aspect, the present invention

(1) spaced first and second long conductors, each of which may be connected to a power source, each of the first and second conductors having a twist length L of up to 20 mm (0.8 inch) and a twist angle of at least 10 ° (β); Comprising a concentric stranded wire having

(2) (i) a long PTC element consisting of a conductive polymer composite extending along the length of the heating strip (ii) exhibiting PTC behavior, (b) along the length of the strip and the length of each conductor An elongated resistance heating strip in alternating electrical contact with the first and second conductors at longitudinally spaced contact points along

(3) When the conductor is connected to a power source, a strip of insulating material placed between the conductors so that all current passing between the conductors passes through the heating strip

It provides an electric heating cable comprising a.

The heating cable of the present invention comprises at least one elongated heating strip composed of elongated PTC elements extending in the longitudinal direction of the heating strip and consisting of a conductive polymer composite exhibiting PTC behavior. As used herein, the term "PTC" has been used to mean a spike in resistivity at temperatures over a relatively small temperature range, ie the composite has an R ₁₄ value of at least 2.5 and / or an R ₁₀₀ value of at least 10. It is preferred that the composite has a R ₃₀ value of at least 6, where R ₁₄ is the ratio of resistivity at the end and the beginning of the 14 ° C. range and R ₁₀₀ is the ratio of resistivity at the end and the beginning of the 100 ° C. range. The composite preferably comprises a polymer component which is a crystalline polymer, ie a polymer having crystallinity before being treated with a composition of at least 20%. Suitable crystalline polymers include polyolefins such as polyethylene or ethylene copolymers; Fluoropolymers such as polyvinylidene fluoride (PVDF), ethylene / tetrafluoroethylene copolymers (ETFE) or tetrafluoroethylene / perfluoroalkoxy copolymers (PFA); Or mixtures of two or more such polymers. Alternatively, the polymer component may comprise an elastomer, ie a thermoplastic elastomer. Particulate conductive fillers such as carbon black, graphite, metals, metal oxides, conductive coated glass or ceramic beads, particulate conductive polymers or composites thereof are dispersed in the polymer component. Additional or other ingredients such as antioxidants, inert fillers, non-conductive fillers, radiation crosslinkers (sometimes called prorads or crosslinking enhancers), stabilizers, dispersants, binders, acidic impurity removers (eg, CaCO ₃ ) Can be provided. Examples of suitable compounds are given in the literature listed above.

The resistivity of the conductive polymer composite at 23 ° C. is generally from 1 Ω · cm to 100,000 Ω · cm, preferably from 100 Ω · cm to 10,000 Ω · cm, in particular from 1,000 Ω · cm to 10,000 Ω · cm, in particular 1,000 Ω · cm to 5,000 Ω · cm. For most applications, the resistivity at 23 ° C. is at least 100 Ω · cm.

The heating strip may be formed by any convenient technique, such as generally good melt extrusion or by passing the substrate through a liquid, such as a liquid of solvent-based composition, and then cooling or going through a solvent removal process. When the strip is produced by melt extrusion, the draw-down ratio has an important effect on the electrical properties of the heating cable. For example, the use of high draw ratios increases the resistance uniformity of the strip, but reduces the expansion of the PTC effect. The optimal draw ratio depends on the particular conductive polymer composite.

The heating strip may have any convenient shape, ie a circular or elliptical cross section or may be in the form of a flat tape. The thickness of the heating strip is generally 0.25 mm to 2.5 mm (0.01 inch to 0.1 inch), preferably 0.38 mm to 2.16 mm (0.015 inch to 0.085 inch), in particular 0.51 mm to 1.91 mm (0.02 inch to 0.075 inch) to be.

The heating cable of the present invention comprises first and second elongated conductors connected to a power source, such as a power supply or a cord hole in a wall, using a suitable plug or electrical element as necessary. Conductors (also referred to herein as electrodes or wires) are preferably composed of metals such as nickel, copper, tin, aluminum, nickel clad copper, tin clad copper, metal alloys or other suitable materials. The conductor has a concentric twisted structure, ie the central (core) wire is surrounded by one or more layers of helical twisted wire (strand) twisted in the reverse direction so that the continuous layer is arranged with opposite twisting directions. Each continuous layer of wire strand in the concentric structure has a greater twist length than the bottom layer. The twist length L is the axial distance required for the individual wire strand to be wound 360 ° around the center (core) wire. In this application, the specific twist length is the length of the outermost layer of the spiral twisted wire. This structure comprises (1) a single twisted structure in which the center wire is surrounded by one or more spiral twisted wires and each layer of the spiral twisted wire has the same twisting direction and the same twisting length and (2) the same twisting direction but each layer It is different from a unidirectional structure in which the central wire is surrounded by one or more layers of helical twisted wire with increasing twist length of. Although the size of the conductor used in the heating cable of the present invention depends on the applied voltage, the required current carrying capability and the required cable length, most cables of the present invention have a diameter of up to 14 AWG, or 1.93 mm (0.076 inch). On the other hand, small diameter wires such as 16 AWG (0.06 inch), 18 AWG [1.27 mm (0.05 inch)] or 20 AWG [1.02 mm (0.04 inch)] or larger Diameter wires, such as wires having a diameter of 12 AWG (2.16 mm (0.085 inch)) or 10 AWG (3.05 mm (0.12 inch)), may be suitable for some applications. The conductors used in the heating cable of the invention have a twist length L of at most 20 mm (0.8 inch), preferably at most 18 mm (0.7 inch), in particular at most 15 mm (0.6 inch). Generally the twist length L is at least 14 mm (0.55 inch). The conductors used in the heating cables of the invention have a twist angle β of at least 10 °, preferably at least 11 °, in particular at least 12 °, in particular at least 13 °. The twist angle β is a function of the helix angle α as shown in Figure 4 below (ie the winding angle of the wire strand around the center wire). β is [(π / 2) -α], ie [(3.14 / 2) -α], and β is affected by the diameter of the center wire and the diameter of the strand wrapped around the center wire. β is usually up to 16 °. In a preferred embodiment, the conductors used in the heating cable of the present invention have a twist length L of up to 20 mm (0.8 inch), preferably up to 18 mm (0.7 inch), and at least 10 °, preferably at least 11 It has a twist angle β of. Preferred wires for use as conductors in the heating cable of the present invention have a twist length L which can be defined by the diameter d _c of the core wire. Thus, for conductors with seven strands, L is preferably less than 36d _c , in particular less than 31d _c , in particular less than 28d _c ; For conductors with 19 strands, L is preferably less than 72d _c , especially less than 65d _c , especially less than 55d _c ; For conductors with 37 strands, L is less than 110d _c , in particular less than 97d _c , in particular less than 80d _c .

The heating cable of the present invention generally includes two long conductors, but in some applications there may be three or more conductors in continuous contact by a heating strip provided such that the conductors are properly connected to one or more suitable power sources. When three or more conductors are provided, the conductors can be arranged such that different power outputs can be obtained by connecting different pairs of conductors to a single phase or two phase power source. When three conductors are provided, the conductors can be arranged such that the heating cable is suitable for connecting to a three phase power source.

In some applications, the conductor may be coated with a layer of conductive material such as low resistivity ZTC (zero temperature coefficient of resistivity) conductive polymer composite, graphite filled emulsion, silver filled emulsion or carbon black filled emulsion or carbon black powder. The conductor may be coated before and after contact with the heating strip.

To ensure that the heating cable maintains its shape, the first and second conductors are formed by a strip of insulating material placed between the conductors so that all current passing between the conductors passes through the heating strip when the conductors are connected to a power source. Separated from each other. Insulating strips (or spacers) are generally composed of electrically insulating materials, such as polymers, ceramics or glass that remain substantially shaped during the preparation and use of heating coils despite thermal expansion and heat shrinkage during normal use. In some applications, the insulating strip may comprise elements that enhance the thermal conductivity of the spacer, such as a metal strip or thermally conductive particulate filler surrounded by the insulating strip. The insulating strip generally has the same general structure as the conductor, i.e. if the conductor is straight, the insulation strip is straight, and if the conductor is wound the insulation strip is also wound together. In order to improve the physical stability of the heating cable, it is preferred that the insulation strip is configured such that the conductor can be kept in close proximity to the spacer, for example by a concave or grooved face or edge sized to receive the conductor.

At least one heating strip is provided in the heating cable of the present invention and in contact with the conductor, but two or more heating strips may be provided. If multiple heating strips are provided, the multiple heating strips do not necessarily have to be always parallel to each other along the length of the heating cable. The multiple heating strips are preferably of the same material, but may differ in material and / or dimensions. For a particular heating strip, heating cables of the same power output can be obtained by a single strip wrapped at a relatively low pitch (high circuit count per unit length) or by a plurality of parallel heating strips wrapped at a relatively high pitch. The use of a plurality of strips lowers the voltage stress on the heating strips.

The conductor and the heating strip can be positioned in various configurations to create the electrical contact required at spaced separation points. In general, the conductors are straight and the heating strips follow a regular sinusoidal path or vice versa. The path may, for example, be generally spiral, sinusoidal or Z-shaped (including generally circular and flat circular spirals). However, it is also possible for both the conductor and the heating strip to follow a regular sinusoidal path with different or opposite shapes or pitches, or only one or both of the conductor and the heating strip follow an irregular sinusoidal path. In one preferred embodiment, the heating strip is wound around a pair of straight parallel conductors and the conductors can be kept spaced apart by the distance required by the separator strip. In another configuration, the heating strip is wound around the separator strip and the wound strip is later contacted with a straight conductor. In another preferred embodiment, the conductor is wound around at least one straight heating strip and at least one straight insulating core. In another preferred embodiment, the conductor is wound around the insulating core, which is later in contact with the straight heating strip. It is convenient to have a generally helical structure such that the wound element can be obtained using a conventional wire winding device. For the best heat transfer to the substrate, it is often good for the heater to have a generally rectangular shape with rounded corners.

Once the heating strips and conductors are wound, the ZTC conductive polymer composite can be placed on the junction between the conductors and the heating strips to enhance electrical contact between the conductors and the strips and form fillets.

The heating strip can be crosslinked, for example by irradiation, before and after the heating strip is assembled into the heating cable.

Additional heating cable construction and fabrication techniques are described in US Pat. No. 4,459,473 (Kamath).

The invention is illustrated by the figures, in which FIG. 1 is a plan view of the heating cable of the invention, and FIG. 2 is a sectional view along line 2-2 of FIG. The single heating strip 2 is wound spirally around the first conductor 3 and the second conductor 4 separated by an insulating strip (spacer 5). Electrical contact between the heating strip 2 and the conductors 3, 4 is strengthened by a low resistivity material 8 which forms a fillet between the strip and the conductor at the point of contact (joining). The polymer insulation sheath 7 is wound around the heating strip 2, the conductors 3, 4 and the insulation strip 5.

3 is a plan view of another heating cable of the present invention, in which two heating strips 2, 9 are wound around the conductors 3, 4 and the insulating strip 5;

4 is a schematic diagram of a conductor 10 that is a concentric strand wire. The helix angle α and the twist angle β are shown.

5 is a cross-sectional view of seven strand conductors 10 having a core wire 11 and a spiral wire 12. The core diameter d _c is also shown.

The present invention is illustrated by the following examples, Example 2 is an example of the present invention and Examples 1 and 3 to 6 are comparative examples. Examples of each heating cable are prepared according to a three step process which generally follows the process described in example 2 of US Pat. No. 4,459,473, the disclosure of which is incorporated herein by reference.

Preparation of the heating strip

The dry mixture of tetrafluoroethylene / perfluoroalkoxy copolymer (PFA) and carbon black is mixed in a twin screw extruder. The mixture was made into small spheres, extruded through a 1.52 mm (0.060 inch) round die fitted in an extruder and dried. Extrusion is performed to provide a heating strip having a diameter of 0.94 mm (0.037 inch).

Preparation of spacer

The glass fiber filled PFA was extruded through a die having a concave side and a flat top and dried to provide an insulating spacer having a concave side with dimensions of 1.9 × 3.0 mm (0.075 × 0.120 inch).

Assembly of the heating cable

Two wires are located within the concave side of the spacer, and four separate heating strips are wound around the conductor and the spacer strip in a spiral fashion using a winding device. The spacing between each adjacent heating strip is 4.3 mm (0.170 inch). The conductors as well as the heating strips in the areas where the conductors are in contact with the heating strips are covered with graphite emulsion. As a result, the heating cable is first jacketed with a PFA layer, followed by a tinned copper braid and finally with a second PFA layer. The jacketed heating cable is heat treated and cooled.

Examples 1-7

The 14 AWG wire shown in Table 1 is used as a conductor in a heater manufactured according to the above process. Each wire is nickel coated copper (ie electroplated (NPETP) with 2% nickel plating) available from Hudson International Conductors.

Yes Wire type Strand number Strand Gauge (AWG) Wire diameter (mm / inch) Strand Diameter (mm / inch) Twist Length L (mm / inch) Twist angle β (。) One Concentric 7 22 1.93 / 0.076 0.64 / 0.025 22.2 / 0.875 10.3 *2 Concentric 7 22 1.93 / 0.076 0.64 / 0.025 14.5 / 0.571 15.5 3 Concentric 19 27 1.80 / 0.071 0.36 / 0.014 25.4 / 1.000 10.1 4 Single kink 19 27 1.80 / 0.071 0.36 / 0.014 20.3 / 0.800 12.6 5 Single kink 19 27 1.80 / 0.071 0.36 / 0.014 15.2 / 0.600 16.6 6 Single kink 37 30 1.78 / 0.070 0.25 / 0.010 19.1 / 0.750 14.1

* Example of the present invention

The heating cables of Examples 1 to 6 were tested according to the following Tests A to E. For each test, the results for each heating cable were graded from highest performance to lowest performance. These results are shown in Table 2. The heating cable of Example 4 had poorer performance than the heating cable of Example 5 for each test. The heating cable of Example 2, which is an example of the present invention, has the best overall performance.

Performance Exam A Exam B Exam C Test D Test E Best 2 2 2,1 (tie) 2 6 6 6 One 2 3 3 5 6 3 5 5 3 3 5 lowest One One 6 5 One

Test A: Bend

This experiment measured the ability of a heating cable to withstand bending conditions where one wire was placed in tension and the other wire was in compression. This kind of deformation can occur during installation where the heating cable is positioned flat relative to the substrate but is pressed around an object such as a flange or bolt.

The heating cable with a length of 0.91 m (3 ft) is placed flat on the substrate (bottom table of the press), and two mandrels with a diameter of 6.4 mm (0.25 inch) each have a cable at the cable edge. It is located halfway down the cable from one side on both sides of the side. The distance between the two conductors was measured. The top table of the press is later positioned on the top of the cable and mandrel to hold the mandrel tight, but frees the cable so that it slides forward and backward but does not rotate. Both ends of the cable bend to a first position around one mandrel (ie, both ends bent at 90 ° from the starting position) and start again around a second mandrel (ie, both ends bent at 180 ° from the first position). Bent into position. In the first position, one of the conductors is in the tensioned state and one is in the compressed state, and in the second position, the conductor that was originally in the tensioned state is compressed and the conductor that was originally in the compressed state is tensioned. Any deformation of the conductor was also recorded and the distance between the conductors measured at the nearest point was recorded. The test was repeated using a mandrel having a diameter of 25 mm (1 inch) and 51 mm (2 inch). The best performance was for the cable with the distance between the conductors closest to the starting point.

Test B: Compression

This test measured the ability of a heating cable to withstand axial compression. Such compression may occur when the heating cable is attached to a rigid substrate, such as a bundle of tubes that is bent such that the heating cable is on its inner diameter.

One end of the sample of a heating cable having a length of 133 mm (5.25 inch) is located on the first (top) side of the test fixture, the other end is located on the second (bottom) side of the fixture, and the cable [5.8 mm ( 0.23 inch)] center area is without a fixture. The distance between the conductors was measured. The fixture was inserted into an Instron® apparatus and a pressing force was applied to the test fixture to compress the 4.6 mm (0.18 inch) cable at a speed of 2.5 mm / min (0.1 inch / min). The pressing force was released, the cable was removed from the test fixture, and the shortest distance between the conductors was measured. The best performance was for the cable with the distance between the conductors closest to the starting distance.

Test C: Temperature Cycle

This test measured the cable's ability to withstand repeated heating and cooling to create thermal relaxation stresses in the cable. Such thermal cycles can occur during normal use of the heating cable, and in this test the cables were exposed to more stringent temperatures than would normally be encountered.

Conductors in a heating cable with a length of 0.76 m (2.5 ft) are intentionally distorted by bending the cable 90 ° in each direction as in test A. It was sufficiently bent until the distance between the conductors was between 0.51 mm (0.02 inch) and 1.0 mm (0.04 inch). After recording this initial distance, the cable was placed in a heat chamber and received 10 cycles between -71 ° C (-95 ° F) and 204 ° C (400 ° F). Then, the distance between the conductors was measured. The best performance was for the cable with the smallest difference between the initial and final distances.

Test D: Install

This test measured the ability of a heating cable to withstand externally applied stresses and forces causing deformation during installation.

A 10 m long heating cable was installed on the 76 mm (3 inch) valve by winding the heating cable around the valve. The cable is later removed and re-installed on the valve so that bending occurs in a direction opposite to the bending formed during the initial installation. The cable was removed from the valve and inspected to determine the total number of modified wires and the shortest distance between the conductors. The best performance was for cables with few strained wires and the longest distance between conductors.

Test E: Cyclic Bending

This test measured the ability of a heating cable to withstand repeated small bendings where one wire is in tension and the other wire is in compression. Such bending can occur due to repeated bending during maintenance of cables and substrates, such as pipes.

A 0.3 m (1 ft) heating cable is located between two mandrels, each mandrel having a diameter of 12.7 mm (0.50 inch) and halfway down the cable on both sides of the cable (positions similar to those described in Test A) Was located). The end of the cable is fastened into the test fixture, and an ohmmeter is attached to the conductor to measure the resistance of the cable. The sample is pressed around the first mandrel (front bend) to put one wire in tension and the second wire in compression, and the first through a full angle of 120 ° at a rate of 11 seconds each per complete cycle. The wire was pressed around the second mandrel (reverse bending) to put the wire in compression and the second wire in tension. The test was continued until the indication (based on resistance) that the conductors were pressed to come into contact with each other or until the initial 40 cycles. The best performance was for the cable with the most cycles.

Claims (10)

In the electric heating cable (1), (1) spaced first and second long conductors 3, 4, which may be connected to a power source, each of the first and second conductors having a maximum of 20 mm (0.8 inch), preferably a maximum of 18 mm (0.7 inch) Comprising a concentric strand wire having a twist length L of (2) (i) a long PTC element consisting of a conductive polymer composite extending along the length of the heating strip (ii) exhibiting PTC behavior, (b) along the length of the strip and the length of each conductor An elongated resistive heating strip 2 in alternating electrical contact with the first conductor 3 and the second conductor 4 at longitudinally spaced contact points along (3) When the conductor is connected to a power source, a strip of insulating material placed between the conductors so that all current passing between them passes through the heating strip (5). Electrical heating cable comprising a. The electrical heating cable of claim 1 wherein L is at least 14 mm (0.55 inch). The electric heating cable according to claim 1, wherein the wire has a twist angle β of at least 10 degrees, preferably at least 11 degrees. In the electric heating cable (1), (1) spaced first and second long conductors 3, 4, which may be connected to a power source, each of which first and second conductors have a twist angle β of at least 11 °, preferably at least 12 °. Including concentric strand wires having, (2) (i) a long PTC element consisting of a conductive polymer composite extending along the length of the heating strip (ii) exhibiting PTC behavior, (b) along the length of the strip and the length of each conductor An elongated resistive heating strip 2 in alternating electrical contact with the first conductor 3 and the second conductor 4 at longitudinally spaced contact points along (3) When the conductor is connected to a power source, a strip of insulating material placed between the conductors so that all current passing between them passes through the heating strip (5). Electrical heating cable comprising a. 5. The electrical heating cable of claim 4 wherein β is at most 16 °. The electric heating cable of claim 1, wherein the heating strip is comprised of a melt-extruded conductive polymer composite. The electrical heating cable according to claim 1, wherein the conductive polymer composite has a resistivity at 23 ° C. of at least 100 Ω · cm. The electrical heating cable of claim 1, comprising a plurality of heating strips wound around the conductor. The electrical heating cable of claim 1 comprising a sheath of the ZTC conductive polymer composite on the junction between the conductor and the heating strip. The electrical heating cable of claim 1, wherein the first and second conductors are comprised of 14 AWG wire having a diameter of 1.93 mm (0.076 inch).