Abstract:
A rigid conduit has a hard plastic exterior surface and electrical metallic tubing as the interior surface. The hard plastic exterior forms a smooth male fitting at one end of the conduit and a smooth female fitting at the other end of the conduit. Chemical solvent adhesives join together adjacent conduit sections, the hard plastic male and female fittings fusing together. The interior surface of electrical metallic tubing protects the exterior hard plastic material from damage caused during cabling of the electrical raceways. The exterior hard plastic protects the rigid conduit from corrosive environments, such as underground installations.

Description:
RELATED APPLICATIONS 
   The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/954,997, filed Aug. 9, 2007, which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electrical conduits and raceways and, more particularly, to such conduits and raceways as are resistant to physical damage. More specifically, the present invention relates to electrical conduits and raceways having a metal lining and an outer layer of PVC or like plastics. 
   2. Description of the Related Art 
   Underground installations of electrical raceways, conduit or duct banks for current carrying conductors are designed to be waterproof and corrosion proof. The majority are now fabricated out of PVC (Polyvinylchloride), and used to construct direct burial raceways that can run horizontal or vertical. When a change of course is required, bends are used to go left or right for horizontal runs and bends permit a change in direction of up and down for vertical runs. 
   Electricians are bound by the National Electrical Code to insure the safety and quality of electrical installations, with the Code offered for use in law and for regulatory purposes in the interest of life and property protection. As provided in ARTICLE 300.5 (4) ENCLOSURE OR RACEWAY DAMAGE, where the enclosure or raceway is susceptible to being physically damaged, the conductors shall be installed in one of the following: rigid metal conduit; intermediate conduit; Schedule 80 rigid nonmetallic conduit; or an equivalent. 
   Another set of requirements is present where it is anticipated that pull string will be used to pull rope through the raceway for wire pulls. In such environments, the materials used to fabricate the conduit must prevent the string or rope from cutting or burning through the interior radius of the bends. Openings resulting from such internal radius failure will expose the current conductors to water and corrosive elements outside of the raceway, as well as adversely impact the pulling process. 
   Overhead or exposed raceways are not subjected to the harsh environment of underground installations, permitting the use of Electrical Metallic Tubing (EMT), which has thinner walls than the rigid conduit used to construct all bends used in underground applications. Of course, the EMT walls in above-ground installations also must be sufficiently thick to permit wire pulls without encountering inner radius failure. Structurally suitable for such above ground applications, EMT is more cost effective than its underground counterpart: the thicker-walled, Galvanized Steel Rigid bends. 
   DISCLOSURE OF THE INVENTION 
   In accordance with aspects of embodiments of the present invention, the advantages of this newly developed product with all PVC bends lined with EMT (Electrical Metallic Tubing) would include the following: (1) It will not allow the interior radius of the bends to be cut or burned though when pulling; (2) It will be cheaper priced per unit; (3) It will be less labor intensive to install; (4) It will not require female PVC adapters; (5) It will not require corrosion resistance 10 mil tape; (6) It will be much lighter in weight than its rigid counterpart thus reducing the injury rate of the installers; and (7) It will have a larger glue surface than the female adapter counterparts used with threaded rigid bends, which will make it stronger, more stable, and provide a better seal, securing the conductors within the conduit from attack by outside elements. 
   To the journeyman electrician, these features translate into less labor required during installation, with considerable time saved by gluing the male to female PVC slip coupling on one end and gluing the female to the male slip coupling on the other end of the bend. 
   Another aspect of the present invention provides a rigid conduit adapted for installation of an electrical conductor therethrough, said conduit comprising: a rigid tubing having an interior surface and an exterior surface, said exterior surface comprising a hard plastic and said interior surface comprising an electrical metallic tubing. 
   In a further aspect of the present invention an adapter conduit of configuration suitable for connecting an electrical raceway of rigid metal conduit and a conduit having an exterior surface of a hard plastic, said adapter conduit comprising: a rigid tubing having a first end comprising a solid plastic tube with an exterior surface and an interior surface, said interior surface defining a smooth female fitting, and a second end comprising a rigid tubing having an exterior surface comprising a hard plastic and an interior surface comprising a threaded metal inlay, said threaded metal inlay extending inwardly from said second end to a terminal edge located within said rigid tubing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other aspects of this disclosure are described in detail below in connection with the accompanying drawing figures in which: 
       FIG. 1  is an exploded schematic view, with portions shown in phantom, of a direct burial raceway or conduit construction in accordance with the present invention; 
       FIG. 2A  is a partial side elevation view in cross-section of a conduit member taken along the lines  2 - 2  of  FIG. 1 ; 
       FIG. 2B  is a partial side elevation view, in cross-section, similar to  FIG. 2A , of an alternative conduit member taken along the lines  2 - 2  of  FIG. 1 ; 
       FIG. 3  is a side elevation view, in cross-section, of a curved conduit member of  FIG. 1  taken along the lines  3 - 3 ; 
       FIG. 4  is a side elevation view, in cross-section, of a 90-degree curved conduit member of  FIG. 1  taken along the lines  4 - 4 ; and 
       FIG. 5  is a side elevation view, in cross-section, of an adapter conduit member of  FIG. 1  taken along the lines  5 - 5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference is now made to the drawings wherein like numerals refer to like parts throughout. In  FIG. 1  an underground raceway  10  is schematically depicted, consisting of a plurality of individual raceway conduit members—the number and nature of individual conduit members dependent upon the specifications and requirements called out for a specific project or job. The underground raceway  10  of  FIG. 1  includes a straight conduit  14 , vertically arranged, a ninety-degree (90°) elbow  16 , and a thirty-degree (30°) elbow  18 . It is presently contemplated that these conduits of lesser bend might range from ten degrees (10°) to forty-five degrees (45°)—although degrees outside of this range are also considered within the scope of the present invention. 
   The underground raceway  10  is shown terminating in a concrete slab  22 , such as a housekeeping slab for the placement of electrical equipment (not shown in the drawings). An adapter conduit  26  is provided to connect the underground raceway  10  to an exposed raceway  28  located above ground. The exposed raceway  28  terminates in a threaded metal male fitting  32 . The adapter conduit  26  is provided a threaded metal female fitting  34  of appropriate dimension to permit the formation of a threaded metal connection between the adapter conduit  26  and the exposed raceway  28 . 
   As is shown in  FIG. 1 , each of the raceway conduit members is provided a smooth male fitting  36  and a smooth female fitting  38 . In  FIG. 1  the smooth male fitting  36  of the straight conduit  14  is received by the smooth female fitting  38  of the adapter conduit  26  to complete the connection between the underground raceway  10  and the exposed raceway  28 . 
   It is presently contemplated to fabricate an outer surface of the raceway conduit members from PVC (polyvinyl chloride) or a similar hard plastic material. In addition to providing structural rigidity to the raceway conduit members, the use of PVC as the outer surface permits formation of mechanically secure and air/water-tight connections between adjacent conduits. In a known manner, the application (during field assembly) of chemical cements to both mating surfaces causes a fusing together of those surfaces, forming permanent connections between adjacent conduits that are virtually impervious to leakage. 
   As noted previously, some raceway assemblies require wire pulls for placement of the electrical wiring within the raceways. Curved sections of conduit risk damage from inner radius frictional wear during these wire pulls, requiring a material resistant to “burn-throughs”—-typically a metal. Additionally, when installation is underground, corrosion concerns frequently result in the specification of Galvanized rigid conduit (GRC), which is heavy, expensive, and requires the on-site formation of threaded connections to adjacent conduit, a tedious process. 
   In  FIG. 2A  a multi-layer straight conduit  44  utilizes a hard plastic outer layer  46 , such as PVC, and an inner metal tubing  48 , such as electrical metallic tubing (EMT). The metal used to form the tubing can be ferrous or non-ferrous; the latter, for example, aluminum, is often specified for reasons of weight or for spark/ignition sensitive areas. The hard plastic outer layer  46  provides corrosion resistance, is lighter in weight than the metal it replaces, and permits the installer to form secure and leak-proof connections in the same manner as with PVC conduits. 
   Such connections are obtained using the conduit of  FIG. 2A , where the multi-layer straight conduit  44  terminates at one end in a smooth male fitting  36  and at the other in a smooth female fitting  38 . Use of the PVC-style connections can, in some circumstances, enable construction of the underground raceway  10  (see  FIG. 1 ) in whole or in large part outside of the trench. Complete or substantially complete segments can then be lowered into place, greatly simplifying raceway construction. 
   “Burn-through” is not considered an issue for straight conduits, and in  FIG. 2B  a plastic conduit  54  is provided with substantially identical outer dimensions to the multi-layer straight conduit  44  of  FIG. 2A , lacking only the inner metal tubing  48 , utilizing instead a solid plastic tube  56 . The plastic conduit  54  terminates in the smooth male fitting  36  and the smooth female fitting  38  that are formed in the solid plastic tube  56  in a similar manner to that of the multi-layer straight conduit  44  discussed above. 
   Internally, a taper  62  is preferably formed, slightly narrowing of the inner diameter of the solid plastic tube immediately adjacent the female fitting  38 . The taper minimizes the opportunity for wire snags at the connection between two conduits when wire is being fed through the connected tube raceway. 
   Of course not all raceway conduit can be straight, and in  FIG. 3  a curved conduit  64  is provided, also having the hard plastic outer layer  46  and the inner metal tubing  48 . As previously mentioned, the inner metal tubing  48  is particularly advantageous in protecting the hard plastic outer layer  46  from burn-through during wire pulls. The degree of bend can vary, with pre-fabricated bends of 45°, 30°, 22½°, and 15° presently contemplated. Also contemplated for both straight and curved conduits, diameters ranging from ½-inch to 6 inches—although larger and smaller diameters are considered to fall within the scope of the present invention. 
   The 90° elbow  16  of  FIG. 4  provides a significant change in raceway direction, enhancing the risk of burn-through. Note that in both the curved conduit  64  and the 90° elbow  16 , the inner metal tubing  48  minimizes the need for the taper  62  found beneficial when using the solid plastic tube  56  of  FIG. 2B . 
   It is presently contemplated that the raceways described herein may be utilized in both underground and above ground installations, with the connections formed as discussed above. However, in the instances and at those locations where connections are required between the raceways of the present invention and rigid conduit, the adapter conduit  26  (best shown in  FIG. 5 ) is provided. 
   Somewhat resembling a spliced-together creation, the adapter conduit  26  provides a portion having the solid plastic tube  56  with a smooth female fitting  38  formed therein. A second portion has the hard plastic outer layer  46  receiving a threaded metal inlay  68 , which provides a threaded metal female fitting  34  suitable for connection to a rigid conduit such as Galvanized Rigid Conduit. Additionally, as was the case with the plastic conduit  54  of  FIG. 2   b , the solid plastic tube  56  of the adapter conduit  26  is provided with the taper  62  to minimize the opportunities for wire snags. 
   Our invention has been disclosed in terms of a preferred embodiment thereof, which provides an electrical conduit with internal lining that is of great novelty and utility. Various changes, modifications, and alterations in the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention encompass such changes and modifications.