GB2514385A

GB2514385A - Heating cable

Info

Publication number: GB2514385A
Application number: GB201309226A
Authority: GB
Inventors: Jason Daniel Harold O'connor
Original assignee: Heat Trace Ltd
Current assignee: Heat Trace Ltd
Priority date: 2013-05-22
Filing date: 2013-05-22
Publication date: 2014-11-26
Also published as: GB201311882D0; GB201309226D0

Abstract

A heating cable comprising a first and second power conductor 21,22 and a failure detector 24 extending along the length of the cable and a heating element body 23. The failure detector comprising a conductor 25 surrounded by an electrically insulating sheath 26 of lower melting point than the element material, the failure detector being located such that it is generally between the first and second conductors, the first and second conductors being in electrical connection with each other via the heating element body. The detector 24 is connected to a control module 29 to interrupt the power supplied from unit 28 in the event of a failure in the cable e.g. a wet fire causing current to flow or a resistance change in the failure detector when localised excessive heat caused by a fault melts the sheath 26. The conductor 25 may be a wire, a foil or an apertured foil and may be alternatively positioned closer to one of the power conductor or close to the protective sheathing 27. The heating element body may be a PTC material, a NTC material or a combination of these. Conductors 21,22 may also comprise foils.

Description

Heating Cable The present invention relates to a heating cable. In particular, the present invention relates to a heating cable having a failure detector for use in preventing fire in a heating cable.

Parallel resistance, self-regulating heating cables are well known. Such cables normally comprise two conductors (known as buswires) extending longitudinally along the cable. Typically, the conductors are embedded within a resistive polymeric heating element, the element being extruded continuously along the length of the conductors.

The cable thus has a parallel resistance form, with power being applied via the two conductors to the heating element connected in parallel across the two conductors.

The heating element usually has a positive temperature coefficient of resistance. Thus as the temperature of the heating element increases, the resistance of the material electrically connected between the conductors increases, thereby reducing power output. Such heating cables, in which the power output varies according to temperature, are said to be self-regulating or self-limiting Figure 1 illustrates a typical parallel resistance, self-regulating heating cable 2. The cable consists of a polymeric matrix 8 extruded around the two parallel conductors 4, 6.

The matrix serves as the heating element. A polymeric insulator jacket lOis extruded over the matrix 8. Typically, a conductive outer braid 12 (e.g. a tinned copper braid) is added for additional mechanical protection and/or use as an earth wire. Such a braid is typically covered by a thermo plastic overjacket 14 for additional mechanical and corrosive protection.

Such parallel resistance self-regulating heating cables possess a number of advantages over non self-regulating heating cables and are thus relatively popular. For instance, self-regulating heating cables do not usually overheat or burn out, due to their positive temperature coefficient (PTC) characteristics. As the temperature at any particular point in the cable increases, the resistance of the heating element at that point increases, reducing the power output at that point, such that the heater is effectively turned down.

However, parallel resistance heaters do possess a number of undesirable characteristics.

A possible failure mode of parallel resistance heaters is one in which water enters the cable and bridges between the two conductors, causing a short circuit. This failure mode may be known as wet-fire.

Heating cables are often deployed in outdoor locations which are exposed to wet conditions. Therefore, the likelihood of a cable becoming wet is high. Water may enter an open end of a heating cable, or a join between adjacent lengths of cable where a protective cover has been dislodged or removed. Once water has entered the cable, a large electrical discharge or short circuit can take place at the location of the water ingress, causing localised heating. This localised heating can cause the polymeric heating material to melt in the region of the electrical discharge, exposing the conductors further away from the point of the initial short circuit. Further water entering the cable, or steam generated from the water causing the initial short circuit, can cause further short circuiting between the newly exposed conductors and further localised heating within the cable body. This in-turn can melt more of the heating matrix, exposing yet more of the conductors, allowing water, and consequently wet-fire to propagate along the heating cable.

While the heating element is exposed to the full effect of the localised heating, the polymeric insulating jacket tends to be protected by the heating element, making it less likely to melt. Additionally, the polymeric insulating jacket may have a higher melting point than the heating element. Therefore, although the heating element is melted by wet-fire, as described above, the polymeric insulating layer may not be melted to the same extent. The conductive outer braid is therefore unlikely to be short circuited to either of the conductors. This prevents detection of the failure by earth current leakage detection. The heating cable can thus fail, but without triggering a safety device based on earth current leakage detection.

An alternative safety device which may be used is a residual current device (ROD). In an ROD the current in two conductors (e.g. live and neutral) is compared. If the current in the two conductors differs by more than a predetermined amount (e.g. 30 mA), then it is determined that the current must be leaking in some way, and that an error has occurred. However, in a wet-fire situation, the short circuit occurs between the conductors, therefore no imbalance will be detected by an F1CD.

A further safety device is a current peak detector, or circuit breaker. A common sign of failure in an electrical system is a sudden increase in current drawn, for example by a short circuit. However, in a wet fire situation, while there is a large localised current, the magnitude of the discharge may be small in comparison to the total current drawn by the heating cable. For example, a 100 m long parallel resistance heating cable may draw 15 A of current. A wet tire failure may involve an additional discharge of current, which may be enough to cause localised damage to the cable, but not enough to trigger a circuit breaker device monitoring the power supplied to the cable by the power supply. The additional discharge may be, for example, 1 DOmA. The circuit breaker device monitoring the power supplied to the cable may, for example, be rated at 16 A. Thus, a current peak detector may fail to detect wet-fire.

It is an object of the present invention to provide a heating cable that obviates or mitigates one or more of the problems of the prior art, whether referred to above or otherwise.

According to a first aspect of the present invention there is provided a heating cable comprising: a first conductor extending along the length of the cable; a second conductor extending along the length of the cable; a failure detector extending along the length of the cable, the failure detector comprising a failure detector conductor surrounded by an electrically insulating material; and a heating element body; the failure detector being located such that it is generally between the first and second conductors; and the first and second conductors being in electrical connection with each other via the heating element body.

The failure detector may be located such that it is closer to each of the first and second conductors than the first conductor is to the second conductor.

The failure detector may be embedded in the heating element body.

The electrically insulating material may have a melting point which is lower than the melting point of the heating element body.

The heating cable may further comprise a protective sheath surrounding the first and second conductors, the failure detector and the heating element body.

The electrically insulating material may have a melting point which is lower than the melting point of the protective sheath.

The electrically insulating material may be high density polyethylene.

The failure detector conductor may comprise a wire.

The failure detector conductor may comprise a metal foil. Metal foil is taken to mean any sheet-like form of metal. However, it will be appreciated that while a foil is usually continuous, it may also be discontinuous. For example, a foil may comprise a sheet of metal containing a plurality of apertures.

The heating element body may comprise a material with a positive temperature coefficient of resistance.

The heating element body may comprise a material with a negative temperature coefficient of resistance.

The heating element body may comprise a material with a negative temperature coefficient of resistance at a first temperature and a positive temperature coefficient of resistance at a second temperature.

According to a second aspect of the invention there is provided a heating cable apparatus comprising a heating cable, the heating cable comprising: a first conductor extending along the length of the cable; a second conductor extending along the length of the cable; a failure detector extending along the length of the cable, the failure detector comprising a failure detector conductor surrounded by a failure detector material; and a heating element body; wherein the failure detector is located such that it is generally between the first and second conductors; and the first and second conductors are in electrical connection with each other via the heating element body; the apparatus further comprising: a power supply; and a failure detector module; the power supply being configured to apply a voltage between the first and second conductors, the failure detector module being connected to the failure detector conductor and being configured to monitor a characteristic of the failure detector, and to trigger an event based on the monitored characteristic of the failure detector.

The monitored characteristic of the failure detector may be the resistance.

The monitored characteristic of the failure detector may be current flowing in the failure detector.

The triggered event may be preventing the power supply from applying the voltage between the first and second conductors.

The failure detector material may comprise an electrically resistive material.

The failure detector material may comprise an electrically insulating material. The failure detector material may be high density polyethylene.

The failure detector may be embedded in the heating element body.

The failure detector material may have a melting point which is lower than the melting point of the heating element body.

The failure detector material may have a melting point which is lower than the melting point of the protective sheath.

The failure detector conductor may comprise a wire.

The failure detector conductor may comprise a metal foil.

According to a third aspect of the present invention there is provided a method of detecting a failure of a heating cable, the heating cable comprising: a first conductor extending along the length of the cable; a second conductor extending along the length of the cable; a failure detector extending along the length of the cable, the failure detector comprising a failure detector conductor surrounded by a failure detector material; and a heating element body; wherein the failure detector is located such that it is generally between the first and second conductors; and the first and second conductors are in electrical connection with each other via the heating element body; the method comprising: applying a voltage between the first and second conductors; monitoring a characteristic of the failure detector; and triggering an event based on the monitored characteristic of the failure detector.

The failure detector material may comprise an electrically resistive material.

The failure detector material may comprise an electrically insulating material.

It will be appreciated that where features are discussed in the context of one aspect they may be applied to other aspects.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a partially cut away perspective view of a prior art parallel resistance self-regulating heating cable; Figure 2 is a cross-section view of a heating cable in accordance with an embodiment of the present invention; Figure 3 is an illustration of a heating cable system in accordance with an embodiment of the present invention; Figure 4 is a cross-section view of a heating cable in accordance with an alternative embodiment of the present invention; Figure 5 is a cross-section view of a heating cable in accordance with an alternative embodiment of the present invention; and Figure 6 is a cross-section view of a heating cable in accordance with an alternative embodiment of the present invention.

Figure 2 illustrates schematically a heating cable 20 in accordance with an embodiment of the present invention. The heating cable 20 comprises a first conductor 21 which extends along the length of the heating cable and a second conductor 22 which also extends along the length of the heating cable. The first conductor 21 and the second conductor 22 are embedded within a heating element body 23. The first conductor 21 and the second conductor 22 may be wires. The wires may be round wires or stranded metal wires.

The heating cable 20 further comprises a failure detector 24 which extends along the length of the heating cable 20. The failure detector 24 comprises a failure detector conductor 25, which is enclosed within a failure detector sheath 26. The failure detector 24 is itself embedded within the heating element body 23.

The failure detector sheath 26 may be extruded over the failure detector conductor 25.

The heating element body 23 may be extruded over the first and second conductors 21, 22, and the failure detector 24.

The failure detector conductor 25 does not carry any significant current, therefore does not need to be as thick as the first and second conductors 21, 22. The failure detector conductor 25 may be a formed from narrow metal wire. For example, a 7-strand wire having a cross-sectional area of 0.5 mm2 may be suitable for a failure detector conductor 25. Alternatively, a single strand of wire having a cross-sectional area of 0.2 or 0.3 mm2 may be suitable for a failure detector conductor 25.

It may be considered advantageous to use a thin wire for a failure detector conductor 25, so as to minimise the effect of the failure detector conductor 25 on the overall geometry of the heating cable 20. A further advantage of using a thin wire for the failure detector conductor 25 may be to reduce the material cost associated with the metal. However, where the thickness of the first and second conductors is significantly larger than the thickness of the failure detector conductor 25, the cost reduction associated with using a thinner failure detector conductor 25 may be insignificant. The minimum thickness of a suitable failure detector conductor 25 may be controlled by manufacturing processes. For example, a wire may be chosen which is sufficiently strong to withstand an extrusion process without having a high risk of breaking.

The failure detector sheath 26 may be formed from an electrical insulator. The electrical insulator may be formed from high density polyethylene (HDPE). The failure detector sheath 26 may be formed from materials other than electrical insulators. The failure detector sheath 26 may be formed from an electrically resistive material. For example, the failure detector sheath 26 may be formed from HDPE blended with a conductive filler material such as particles of carbon black.

The failure detector sheath 26 may be formed from a material which has a melting point which is lower than or similar to the melting point of the heating element body 23.

The heating element body 23 may be enclosed within a protective sheath 27. The protective sheath 27 may be a polymeric insulator jacket. The protective sheath 27 may be extruded over the heating element body 23.

The protective sheath 27 may be provided with a conductive outer braid (e.g. a tinned copper braid, not shown) which provides additional mechanical protection and may also or alternatively serve as an earth connection. Such a braid may also be covered by a therrno plastic overjacket (also not shown) for additional mechanical and corrosive protection.

The heating element body 23 may be formed of a conductive filler distributed within a matrix of an insulative material. One example of a suitable insulative material is HDPE.

The HDPE may be blended with a conductive filler. The conductive filler may be conductive particles. The conductive particles may be particles of carbon black. The use of a conductive filler, such as particles of carbon black, may increase the melting point of the blended material over the original insulative material. Therefore, the addition of the conductive filler to the material of the heating element body 23 can result in the heating element body 23 having a higher melting point than the failure detector sheath 26 which may be manufactured from the same insulative material (e.g. HDPE). The use of less of the conductive filler within the insulative material used to form the failure detector sheath 26, when compared to the heating element body 23, can result in the failure detector sheath 26 having a lower melting point than the heating element body 23. The failure detector sheath 26 can therefore be made from similar materials to the heating element body 23, while also having a lower melting point than the heating element body 23, allowing it to be sensitive to wet-fire.

The heating element body 23 may be considered to be electrically resistive. The heating element body has an electrical resistance which is higher than that of first and second conductors 21, 22, but lower than that of an electrical insulator.

Figure 3 shows the heating cable 20 connected to a power supply 28. The first and second conductors 21, 22 are each connected to outputs of the power supply 28. In use, the power supply applies a voltage between the first and second conductors 21, 22. This applied voltage causes a current to flow through the heating element body, which connects the first and second conductors 21, 22. This current causes heat to be generated in the heating element body 23.

The failure detector 24 is used to detect any short circuits between the first and second conductors 21, 22. The failure detector conductor 25, which is embedded within the failure detector sheath 26, is connected to a failure detection module 29.

A cable failure may be caused by water entering the cable housing. For example, an open cable end, a join between cables, or a defect or a damaged part of a protective sheath around a cable may allow water to enter a heating cable. In the event that water enters the heating cable and causes a short circuit between the first and second conductors 21, 22, the heating element body 23 may be melted by the high temperatures generated by the electrical discharge. The failure detector sheath 26, having a melting point which is lower than temperature generated by the electrical discharge, will also be melted. In such a scenario, water in the cable, or steam generated by evaporation of water, will not only cause a short circuit between the first and second conductors 21, 22, but will also cause some current flow between one of the first and second conductors 21, 22 and the failure detector conductor 25. The failure detection module 29, to which the failure detector conductor 25 is connected, is configured to detect any current flowing within the failure detector conductor 25. The current flowing within the failure detector conductor 25 is a characteristic of the failure detector 24 which can be monitored by the failure detector module 29. In this way, the failure detection module 29 can detect any wet-fire failure within a heating cable. On detection of wet-fire failure the failure detector module 29 may take appropriate action such as disconnecting the power supply and/or triggering an alert.

The temperature at which wet-fire detection is triggered will vary depending on the composition of the materials used in the heating cable and the failure detector. A heating element formed from a HDPE based compound material may soften at around 130 C, and may be destroyed at temperatures of around 200 C. In a heating cable which is not provided with a failure detector, a wet fire may occur, resulting in the melting of the heating element body. For example, if the heating cable 2 illustrated in Figure 1 suffered a wet-fire event, the polymeric insulator jacket 10 may prevent the wet-fire from being detected. The insulator jacket 10, which may not melt during the wet-fire, would prevent a short circuit between either of the conductors 4, 6 and the conductive outer braid 12. Therefore, considering again the heating cable 20, to successfully detect a wet-fire event, the failure detector sheath 26 should melt before the protective sheath 27 has melted.

The failure detector sheath 26 described above is described as being formed from a material which has a melting point which is lower than or similar to the melting point of the heating element body 23. However, the failure detector sheath 26 may instead have a melting point which is higher than, even significantly higher than, the melting point of the heating element body 23. The high temperatures generated inside a heating cable 20 by a short circuit may simultaneously melt a heating element 23 with a first melting point, and a failure detector sheath 26 with a second, higher, melting point.

In such an example, some damage may be suffered by the heating element 23, but a wet-fire event may still be detected before the heating cable 20 has been completely destroyed.

As an alternative to the detection of current, the failure detection module 29 may be configured to monitor the resistance between one of the first and second conductors 21, 22 and the failure detector conductor 25. The initial resistance between these conductors should be high, as the failure detector sheath 26 will prevent any significant current flow to the failure detector conductor 25 from one of the first or second conductors 21, 22. However, should water enter the cable and the failure detector sheath 26 become melted, the resistance between the failure detector conductor 25 and the other conductors 21, 22 will be reduced. The resistance between the failure detector conductor 25 and the other conductors 21, 22 is a characteristic of the failure detector 24 which can be monitored by the failure detector module 29.

In normal operation, the failure detector conductor 25 will not carry any significant current. A moderate voltage may be applied to the failure detector conductor 25, however, a large current (such as that which is supplied to the first and second conductors 21, 22) will not be caused to flow in the failure detector conductor 25. The purpose of any flow in the failure detector conductor 25 is for monitoling, rather than the delivery of energy in the form of heat. The failure detector conductor 25 is not capable of allowing a large flow of current without being damaged. The failure detector conductor 25 is not configured to be a power conductor. Where the failure detector sheath 26 is formed from an insulating material, the insulating nature of the material will prevent any significant current flow from occurring. When a wet-fire event occurs, the insulating properties of the failure detector sheath 26 will be compromised, and a current will flow in the failure detector conductor 25.

In addition to detecting a cable failure, monitoring the resistance between one of the first or second conductors 21, 22 and the failure detector conductor 25 may allow the location of a failure to be identified. If a short circuit occurs between one of the first or second conductors 21, 22 (power conductors) and the failure detector conductor 25, the resistance between them will be a function of the distance of the short circuit from the ends of the cable. Each conductor will have a characteristic resistance per unit length, which will be known, meaning that the detected combined resistance should allow an estimate of the failure location to be calculated.

Although the heating element body 23 is described above as being formed from a blend of HDPE and carbon black particles, it may also be formed from a number of other suitable materials. Table 1 summarizes a typical range and variation of materials which may be suitable for use to form the heating element body 23. Any one or more of the listed materials could be utilised, from any one or more of the listed types.

__________ Heating Element Body: Range of Formulations Type Compounds could include but not be limited to Addition __________ _______________________________________________ Range Conductive Carbon Black 2% -80% Graphite Carbon fibre Nanotubes Metal Powders Metal strand Metal coated fibre Insulative HDPE: High Density Polyethylene 20% -98%.

MDPE: Medium Density Polyethylene LLDPE: Linear Low Density Polyethylene Fluoropolymers -PEA: Copolymer of Tetrafluoroethylene and Perfluoropropyl vinyl ether -MFA: Copolymer of Tetrafluoroethylene and Perfluoromethylvinylether -FEP: Copolymer of Tetrafluoroethylene and Hexafluoropropylene -ETEE: Copolymer of Ethylene and Tetrafluoroethylene -PVDF: Polyvinylidene fluoride Ethylene Acetate/Acrylate Copolymers -EMA: Ethylene methyl acrylate -EEA: Ethylene ethyl acrylate -EBA: Ethylene butyl acrylate -EVA: Ethylene vinyl acetate Other Polymers -PP: Polypropylene -PA: Folyamide (nylon) -Polyester ___________

TABLE 1

The heating element body 23 may have a positive temperature coefficient of resistance, such that resistance of the heating element body 23 increases with temperature. The use of a PTC material provides the advantage of allowing a heating cable to be self-regulating, effectively turning down at a target operating temperature, and preventing a cable from over-heating by reducing heating as a target temperature is approached. A further advantage of self-regulation is that the use of energy to continue heating above a target temperature is reduced, reducing unnecessary energy use, and hence reducing heating costs.

Alternatively the heating element body 23 may have a negative temperature coefficient of resistance, such that resistance of the heating element body 23 decreases with temperature. The use of a negative temperature coefficient (NTC) material provides an advantage when a heating cable is initially connected to a power supply when cold.

When a heating cable is connected to a power supply, a large surge of current may be drawn which may be damaging to the supply. The use of an NTC material increases the resistance of the heating element at low temperatures (such as those experienced by an unpowered heating cable), preventing a significant surge of current at turn-on.

As the heating cable begins to generate heat, increasing the temperature of the heating element, the resistance will then reduce, allowing more current to flow, and more heat to be produced, without producing the same surge which would have been produced by a heating element without a NTC characteristic.

A heating element body 23 having an NTC characteristic may be manufactured by from a matrix of an insulative material (e.g. HDPE) blended with an NTC material (e.g. a mixture of manganese oxide (Mn203) and nickel oxide (NiO)). The use of a filler, such as a mixture of Mn203 and NiO may increase the melting point of the blended material over the original insulative material. The addition of NTC filler to the material of the heating element body 23 can result in the heating element body 23 having a higher melting point than the failure detector sheath 26 which may be manufactured from the same insulative material (e.g. HDPE). Any suitable material with an NTC characteristic may be used as an NTC filler material. Ceramic materials such as, for example, oxides of iron, cobalt or manganese may be used as an NTC filler material. Ceramic materials having a spinel structure may be particularly suitable as NIC fillers. Semi-metallic elements, such as silicon, germanium and gallium also may be suitable for use as NTC fillers.

An NTC material may be used in combination with a FTC material. A FTC material will have a small resistance at a low temperature. Therefore, when a voltage is applied to a cold heating cable with a FTC heating element, a large current surge may be observed. The addition of an NIC material increases the resistance of the heating element at low temperatures (such as those experienced by an unpowered heating cable), preventing a significant surge of current at turn-on. As the heating cable begins to generate heat, increasing the temperature of the heating element, the resistance will then reduce, allowing more current to flow, and more heat to be produced. As the temperature is further increased, the FTC characteristic of the heating element will begin to dominate over the NTC characteristic, and the resistance will again begin to rise. This self-regulating behaviour effectively turns down the heating cable at a target operating temperature, and prevents the cable from over-heating. A further advantage of self-regulation is that the use of energy to continue heating above a target temperature is reduced, reducing unnecessary energy use, and hence reducing heating costs.

Any combination of PIG and NTC materials may be used to achieve a desired combination of cold-start surge limiting and self-regulating characteristics, as required by a particular application. For example, a heating element may comprise a heating element body having a FTC characteristic with the first and/or second conductors being provided with an NTC coating. In an alternative arrangement, the heating element body 23 may be composed of a mixture of PTC and NTC materials.

Although the failure detector sheath 26 is described above as being formed from HDFE or a blend of HDFE and carbon black particles, it may also be formed from a number of other suitable materials, such as, for example polymers or polymer composites.

Examples of other suitable polymers may be: PE (polyethylene), PP (polypropylene), PA (polyamide), EMA (ethylene-methyl acetate), EVA (ethylene-vinyl acetate), EBA (ethylene-butyl acetate), TPE (tetraphenylethylene), MFA (copolymer of tetrafluoroethylene and perfluoromethylvinylether), PFA (copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether), PVDF (polyvinylidene fluoride), ETFE (copolymer of ethylene and tetrafluoroethylene), ECIFE (ethylene chlorotrifluoroethylene), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), PUR (polyurethane), and polyesters.

The failure temperature of a failure detector sheath 26 formed from any of the listed materials may be the melting temperatures of those particular materials, this being the temperature at which the failure detector sheath 26 would become molten, and detached from the failure detector conductor 25. Such detachment of the failure detector sheath 26 would allow the failure detector conductor 25 to contact the heating element body 23 and a current to flow in the failure detector conductor 25.

The failure detector sheath 26 is shown in Figure 2 as being distinct component from the heating element body 23, with an abrupt transition between the failure detector sheath 26 and the heating element body 23. However, in an alternative embodiment a gradual transition between the failure detector sheath 26 and the heating element body 23 may be used. A gradual change in loading of the filler materials in an insulative matrix can be used to bring about a change in properties of the material between the heating element body 23 and the failure detector conductor 25. For example, a matrix of insulative material (e.g. HDPE) may be blended with a PTC filler in a first region, to form a heating element body 23, and used without additives in a second region, to form an insulating failure detector sheath 26. The transition from the first region to the second region may be gradual. In this example, there is no abrupt boundary between the heating element body 23 and the failure detector sheath 26 as the transition between materials and the materials' resulting electrical properties may be gradual.

It will be appreciated that while wet fire is a particular failure mode associated with known heating cables, the failure detector discussed herein may be applicable to other failure modes. For example, should a mechanical failure be caused in a heating cable, the failure detector 24 may become severed or short circuited to another one of the conductors 21, 22. The failure detection module 29 may be operative to detect such an event and cause the system to enter a safe mode in which power is not supplied to the heating cable 20.

While a failure detection module 29 is shown in figure 3 as being separate from the power supply 28, the failure detection module 29 may instead be integrated with the power supply 28 (it may, for example, be in the same housing).

Figure 2 shows the heating cable 20 in accordance with an embodiment of this invention which has a particular configuration. However, the heating cable may have other configurations. For example, the failure detector 24 may be arranged in different locations, or take another form than that which is shown in Figure 2.

Figure 4 shows an alternative embodiment in which a heating cable 40 comprises a first conductor 41, a second conductor 42, a heating element body 43 and a failure detector 44. The failure detector 44 is formed from a failure detector conductor 45 and a failure detector sheath 46. The failure detector sheath 46 may be an insulating layer.

The failure detector 44 is not centrally disposed between the first conductor 41 and the second conductor 42, but instead is located nearer to the first conductor 41 than the second conductor 42. If a significant portion of the heating element body 43 is melted by a short circuit, then the electrically insulating coating (failure detector sheath 46) of the failure detector 44 will also be melted. This will result in a detectable change in the resistance (or other electrical characteristic) monitored by a failure detection module connected to the failure detector 44.

Providing the failure detector 44 at a location which is not central between the conductors may provide a more rapid detection if wet-fire is more likely to be initiated at a particular one of the conductors. For example, where a DC voltage is applied to a first conductor 41, and a the second conductor 42 is connected to earth, the region of the heating element body 43 immediately adjacent to the first conductor 41 is likely to melt momentarily before the region of the heating element body 43 immediately adjacent to the second conductor 42. Therefore positioning the failure detector 44 closer to the first conductor 41 than the second conductor 42 may provide an earlier indication of wet-fire than a centrally positioned failure detector.

Figure 5 shows a further alternative embodiment in which a heating cable 50 comprises a first conductor 51, a second conductor 52, a heating element body 53 and a failure detector 54. The failure detector 54 comprises a failure detector conductor 55 which is formed from metal foil. The failure detector conductor 55 is embedded within a failure detector sheath 56. The failure detector sheath 56 may be an electrically insulating layer. The failure detector 54 is disposed between the first conductor 51 and the second conductor 52.

Metal foil is taken to mean any sheet-like form of metal. However, it will be appreciated that while a foil is usually continuous, it may also be discontinuous. For example, a foil may comprise a sheet of metal containing a plurality of apertures.

Metal foil may be used as the conductor within the failure detector 54 instead of a round wire or stranded metal wire, as shown in other embodiments. The use of metal foil may be beneficial in some instances as it will present a smaller cross-sectional area to current flowing between the first conductor 51 and the second conductor 52. A smaller cross-sectional area will result in reduced disruption to current flow between the conductors 51, 52, resulting in a similar performance between similar heating cables with and without failure detectors.

In some embodiments, the failure detector may be located at the edge of the heating element body. Figure 6 shows one such embodiment in which a heating cable 60 comprises a first conductor 61, a second conductor 62, a heating element body 63 and a failure detector 64. The failure detector 64 comprises a metal foil 65 embedded within an electrically insulating layer 66 which is disposed at the edge of the heating element body 63. Arranging the failure detector 64 at the edge of the heating element body 63 advantageously allows minimal disruption of the current flow between the first and second conductors 61, 62 by the failure detector. Although not directly between the first and second conductors 61, 62, the failure detector 64 is generally between the first and second conductors 61, 62. The failure detector 64 could be a round wire or stranded metal wire, as shown in Figures 2 and 4.

The failure detector can be in any location, provided it can detect a wet-fire event. For detection of a wet-fire event to be possible, the failure detector should be in contact with the heater element body, and in particular a region of the heater element body which would be melted should a wet-fire event occur. Therefore, while being described as being located between the first and second conductors, there is no requirement for the failure detector to be directly between the first and second conductors. The failure detector location may be described as being located generally between the first and second conductors. Alternatively, the failure detector location may be described such that it is closer to each of the first and second conductors than the first conductor is to the second conductor, while also being in contact with the heater element body.

In some embodiments, the first and second conductors may not be embedded within the heating element body, but may instead be located at an edge of the heating element body. In all embodiments, the first and second conductors are in electrical contact with the heating element body. The first and second conductors may take the form of wires, as shown in Figures 2 to 6. However, the first and second conductors may alternatively take the form of a foil, or any form appropriate for a particular application.

Heating cables according to the embodiments shown in figures 2 to 6 are generally shown having first and second conductors which extend along the length of the heating cable in a substantially planar arrangement. However, in some embodiments a heating cable may extend along the length of the heating cable in a non-planar arrangement.

For example, the first and second conductors may each follow helical paths along the length of the heating cable. The failure detector may be located generally between the first and second conductors. For example, the failure detector may be at the central axis of the helices.

Claims

CLAIMS: 1. A heating cable comprising: a first conductor extending along the length of the cable; a second conductor extending along the length of the cable; a failure detector extending along the length of the cable, the failure detector comprising a failure detector conductor surrounded by an electrically insulating material; and a heating element body; the failure detector being located such that it is generally between the first and second conductors; and the first and second conductors being in electrical connection with each other via the heating element body.
2. A heating cable according to claim 1 wherein the failure detector is located such that it is closer to each of the first and second conductors than the first conductor is to the second conductor.
3. A heating cable according to claim 1 or claim 2 wherein the failure detector is embedded in the heating element body.
4. A heating cable according to any preceding claim wherein the electrically insulating material has a melting point which is lower than the melting point of the heating element body.
5. A heating cable according to any preceding claim further comprising a protective sheath surrounding the first and second conductors, the failure detector and the heating element body.
6. A heating cable according to claim S wherein the electrically insulating material has a melting point which is lower than the melting point of the protective sheath.
7. A heating cable according to any preceding claim wherein the electrically insulating material is high density polyethylene.
8. A heating cable according to any preceding claim wherein the failure detector conductor comprises a wire.
9. A heating cable according to any one of claims 1 to 7 wherein the failure detector conductor comprises a metal foil.
10. A heating cable according to any preceding claim wherein the heating element body comprises a material with a positive temperature coefficient of resistance.
11. A heating cable according to any one of claims 1 to 9 wherein the heating element body comprises a material with a negative temperature coefficient of resistance.
12. A heating cable according to any one of claims 1 to 9 wherein the heating element body comprises a material with a negative temperature coefficient of resistance at a first temperature and a positive temperature coefficient of resistance at a second temperature.
13. A heating cable apparatus comprising a heating cable, the heating cable comprising: a first conductor extending along the length of the cable; a second conductor extending along the length of the cable; a failure detector extending along the length of the cable, the failure detector comprising a failure detector conductor surrounded by a failure detector material; and a heating element body; wherein the failure detector is located such that it is generally between the first and second conductors; and the first and second conductors are in electrical connection with each other via the heating element body; the apparatus further comprising: a power supply; and a failure detector module; the power supply being configured to apply a voltage between the first and second conductors, the failure detector module being connected to the failure detector conductor and being configured to monitor a characteristic of the failure detector, and to trigger an event based on the monitored characteristic of the failure detector.
14. A heating cable apparatus according to claim 13 wherein the monitored characteristic of the failure detector is the resistance.
15. A heating cable apparatus according to claim 13 wherein the monitored characteristic of the failure detector is current flowing in the failure detector.
16. A heating cable apparatus according to any one of claims 13 to 15 wherein the triggered event is preventing the power supply from applying the voltage between the first and second conductors.
17. A heating cable apparatus according to any one of claims 13 to 16 wherein the failure detector material comprises an electrically resistive material.
18. A heating cable apparatus according to any one of claims 13 to 16 wherein the failure detector material comprises an electrically insulating material.
19. A method of detecting a failure of a heating cable, the heating cable comprising: a first conductor extending along the length of the cable; a second conductor extending along the length of the cable; a failure detector extending along the length of the cable, the failure detector comprising a failure detector conductor surrounded by a failure detector material; and a heating element body; wherein the failure detector is located such that it is generally between the first and second conductors; and the first and second conductors are in electrical connection with each other via the heating element body; the method comprising: applying a voltage between the first and second conductors; monitoring a characteristic of the failure detector; and triggering an event based on the monitored characteristic of the failure detector.
20. A method of detecting the failure of a heating cable heating according to claim 19 wherein the failure detector material comprises an electrically resistive material.
21. A method of detecting the failure of a heating cable heating according to claim 19 wherein the failure detector material comprises an electrically insulating material.