1. Field of the Invention
The present invention relates to electrical devices. More particularly, the present invention relates to electric cables, heating cables, and the like, having a ground plane layer of conductive polymer and drain conductor for providing ground fault detection.
2. Introduction to the Invention
Heating cables are well known in the art. These electrical devices typically comprise an elongate resistance body of an organic polymer such as a polyethylene or polyvinylidene fluoride having a particulate conductive filler such as carbon black effectively dispersed therein. The body is typically melt-extruded over two or more suitably gauged stranded metal (e.g. nickel or tin-coated copper) wires to produce an inner heater having a generally rectangular, oval or dog-bone cross-section. Many of these types of known electrical devices include a metallic braid which is provided to act as an electrical ground path and also to provide some mechanical reinforcement of the cable device. In many instances the heating cable has a resistance element manifesting a positive temperature coefficient (PTC) which renders the heater self-regulating about a desired temperature generally irrespective of its particular length. Self-regulating heating cables are commonly used as heaters for bodies such as liquid-containing vessels, and structures or substrates such as pipes, within chemical processes or other systems requiring temperature maintenance. Since heating cables may be used in a wide variety of applications and configurations, it is highly desirable that the heating cables manifest a sufficient degree of mechanical flexibility in order to be wrapped around pipes to be heated as well as providing a sufficient degree of toughness, wear resistance, and longevity. Heating cables powered by single phase AC power may extend for up to 1200 feet in length or longer, for example. Three-phase strip heaters may extend much farther, up to 12,000 feet in length or longer, for example.
It is useful and important to monitor the condition of a heating cable that may have been improperly installed in the first instance, or may have sustained physical damage or degradation after installation, such as a cut, puncture, tear, break, abrasion or other failure mode of the outer insulation, or of a ground braid element of the heater, in response to external impact or other externally caused abuse or misuse. By monitoring the heating cable condition one can increase the safety and reduce the possibility that a damaged heating cable will be used or remain in service and protect against hazards to personnel and equipment posed by any continuing use of damaged heating cables such as, for example, an explosion or a fire, particularly within hazardous environments. In order to protect against continued use of damaged heating cables, ground-fault protection devices ("GFPDs") may be employed. GFPDs generally function to sense a current imbalance, trip, and thereupon interrupt a source of electrical power to the strip heater as by opening a circuit breaker or a set of contacts at a power distribution circuit breaker panel. GFPDs may be included within breaker switches. Discrete GFPDs may alternatively be installed at branch circuit breaker panels. GFPD equivalent functions may also be included within temperature/operational control or monitoring apparatus to which a heating cable may be connected. GFPDs for protecting apparatus and equipment are designed to trip at a relatively low fault current detection level, such as 20 mA to 360 mA or higher, and most typically 30 mA. GFPDs typically include, but are not limited to, ground-fault circuit interrupt (GFCI) devices which provide ground fault protection for personnel against shock. GFCI devices are typically set to trip at a 5 mA current level.
One example of a method of monitoring a heating cable for faults is described in U.S. Pat. No. 4,698,583 to co-inventor Chester L. Sandberg, entitled "Method of Monitoring a Heater For Faults", the disclosure thereof being incorporated herein by reference.
With reference to FIGS. 1 and 2 a conventional self-regulating heating cable 10 is shown as including two stranded electrical conductor 12 and 14. In this particular example, the conductor 12 is denominated the phase lead and conductor 14 is denominated the neutral (return) lead. The conductor wires 12 and 14 are effectively and intimately embedded within a heater body 16 most preferably comprising a matrix polymer and conductive particles effectively dispersed therein. The heater body 16 most preferably manifests a positive temperature coefficient (PTC), so that the heating cable 10 is self-regulating about a design temperature following application of operating power, such as about 120 volts (alternating current) for example.
An inner jacket 18 of nonconductive thermoplastic or elastomeric material, such as polyethylene or ethylene-propylene-diene monomer (EPDM), respectively, is extruded over the heater body 16, preferably using a tube-down extrusion technique. The innerjacket 18 and body 16 are then exposed to an electron beam or other ionizing radiation source at a selected energy level and for a controlled time period as to promote polymer crosslinking.
A metal wire braid 20 is woven or otherwise placed over the inner jacket 18. A standards-specified ground plane braid, such as wire braid 20, has a woven strand mesh density such that a 1 mm diameter probe passing through an outer jacket 22 at any arbitrary location will necessarily come into electrical contact with one or more strands of the braid. The braid 20 forms a ground plane for the heating cable 10.
Using a tube-down extrusion technique, an outer jacket 22 of nonconductive material, which may be of the same type as the inner jacket 18, is extruded over the wire braid 20. Accordingly as shown in FIGS. 1 and 2, the conventional self-regulating heater cable 10 includes (progressively from its periphery to its center) the outer insulative jacket 22, the wire braid 20, the inner insulative jacket 18, and the conductive polymer matrix heater body 16 which envelopes and electrically connects to the phase and neutral conductor wires 12 and 14.
An alternative conventional heating cable construction 25 is shown in the FIG. 1A view. In this example, the phase and neutral stranded copper bus wire electrodes 12 and 14 are spaced apart by a nonconductive polymeric spacer 15. A plurality of self-regulating conductive polymeric-fiber heating elements 17 are wrapped around, and connected to, the phase and neutral electrodes 12 and 14. The construction 25 includes a conventional tinned-copper wire braid jacket 20, and a nonconductive outer jacket 22 of e.g. fluoropolymer. Heating cables in accordance with the FIG. 1 cable construction 25 are described in greater detail in U.S. Pat. No. 4,459,473 to Kamath, entitled "Self-Regulating Heaters", the disclosure of which is incorporated herein by reference.
As shown in FIG. 3, electrical power is supplied to the cable 10 from a breaker panel 24 including a circuit breaker 26 for selectively connecting the phase conductor 12 to a phase bus 28. The neutral conductor 14 is typically returned to a neutral bus 30 at the breaker panel 24. A GFPD 32 typically located at the breaker panel 24 is connected to the conductors 12 and 14, and to the neutral bus 30. Braid 20 is then connected to ground. Any imbalance in current between the phase conductor 12 and the neutral conductor 14 is detected by the GFPD 32, and if the imbalance is above a predetermined trip threshold, such as 30 mA, the GFPD 32 trips breaker 26 which thereupon disconnects the phase conductor 12 from the phase bus 28. One reason for a current imbalance is an unwanted ground fault between the wire braid 20 and the phase conductor 12, such as a current-leakage path 34 at some location along the cable 10. The current-leakage path 34 may be the result of abuse such as cutting, tearing or abrasion of the cable 10, or may be caused by excessive blows or compression applied to the cable 10 at the site of the current-leakage path 34.
Whatever the reason for the fault, the GFPD 32 functions to detect the ground fault and trip breaker 26. Of course, if the current-leakage path 34 constitutes a very low-resistance direct short which passes significantly more current than the rating of the breaker 26, the breaker 26 will ordinarily trip normally without GFPD intervention, and disconnect the phase conductor 12 in conventional fashion.
Preferred methods for making a self-regulating strip heater such as cable 10 are taught in U.S. Pat. No. 4,426,339 to Kamath et al., entitled "Method of Making Electrical Devices Comprising Conductive Polymer Compositions"; and U.S. Pat. No. 5,300,760 to Batliwalla et al., entitled "Method of Making an Electrical Device Comprising a Conductive Polymer", the disclosures thereof being incorporated herein by reference.
There are several recognized drawbacks arising from the use of a braided ground plane layer, such as wire braid 20. For one thing, a wire braid requires using a relatively slow wire braiding machine for braiding multiple strands of wire and applying the braided strands to the heater body and inner jacket composite in the manufacturing process. Also, broken wire strands or bunching up of the wire braid can result in defects in the outer insulative jacket and can reduce yields in downstream manufacturing operations. Another drawback stems from the fact that if moisture contacts the wire braid, as when a cut or tear or other defect through the outer jacket 22 permits moisture to enter, corrosion of the wire braid layer 20 can develop and progressively extend along a considerable length of the cable. One further drawback stemming from the wire braid 20 is the difficulty in preparing a heating cable end for electrical connection. In this regard, the outer jacket 22 must be stripped off, and the wire braid 20 then parted into a separate conductor for connection to ground. Thus, it would be very desirable to provide a "braidless" elongate electrical cable, such as a heating cable, with effective ground-fault detection wherein the cable does not require or include a woven wire strand ground plane braid component or layer within the cable construction.