1.  Field of the Invention
This invention relates to protective channel or conveyance systems for conduits, and more particularly, but not by way of limitation, to corrugated sheathing systems for loosely receiving electrical conductors extended between remotely spaced locations. 
2.  Brief Description of the Prior Art
The National Electrical Code in use in the United States contains certain specific and exacting provisions relative to the types of power cable and electrical conduits which can be used in certain wiring situations and cable conveyance situations, and includes specifications and criteria to be observed in encasing conductors, electrical wires and cables within certain nonmetallic jackets enclosing conduits.  Such specifications apply to both surface extensions of cables or conductors in which the cables or conductors and surrounding nonmetallic jackets are mounted directly on the surface of walls or ceiling, and also aerial extensions in which a supporting cable is used to hang or suspend a cased or jacketed electrical cable or conductor.  The jackets required for the accommodation of such electrical cable and conductors are, of course, in addition to being nonmetallic, flexible so that the necessary turns or bends required to carry the cable over a predetermined course can be utilized.  Good flexibility is thus required of the jacket or conduit which is to contain the electrical cable.  It is further required, of course, that where the nonmetallic jackets or conduits are arranged in end-to-end relation to extend the total length thereof, fittings used for connections shall be such that the electrical conductors are not exposed between the lengths of the conduits or at the fittings.  It is also required that at the terminal point of each conductor, a fitting or box which completely covers the end of the assembly shall be used. 
As a result of certain limitations of structural strength believed to characterize such nonmetallic conduits as those made of polyvinyl chloride and other thermoplastic materials, the specifications of the code require that conduit having a nominal diametric size of 1/2 inch through one inch be supported at intervals which do not exceed 3 feet between points of support.  Also, in order to avoid short circuiting and fire hazards, the brackets used to support spans of the enclosing conduits are required to be attached to woodwork or plaster, and are not permitted to contact any metalwork or other electrically conductive material.  Other requirements exist for aerial cable assemblies supported upon a messenger line or cable. 
In the case of metallic conduits or raceways used to carry electrical conductors, it is required that contact with similar metals (by the metallic raceway) be avoided in order to eliminate the possibility of the occurrence of galvanic action.  Also, certain restrictions are provided upon the manner in which metallic raceways can be placed in contact with, or buried in, the earth.  These restrictions are based upon the possibility of deterioration under conditions of corrosion and rusting. 
Due to the propensity of such metal raceways to crimp or become distorted when they are bent through angles of 90.degree.  or more, the code specifications require that installations which include such bends avoid crimping of the raceway in a manner which will reduce the effective internal diameter thereof.  To this end, limitations are placed on the size of the radius of curves defined by the inner edge of the metallic raceway in certain installations. 
In certain types of installations which use rigid metallic conduit, the material of which the conduit or raceway is constructed, must be flame retardent, resistant to impact or crushing, resistant to distortion from heat under conditions likely to be encountered in service, and resistant to sunlight.  It must also, when used underground, be resistant to moisture and corrosive agents, and sufficiently strong to withstand abuse, such as by impact or crushing loads.  Rigid nonmetallic conduit or raceways are required to be supported along their length at each three foot interval in the case of one-half inch to one inch diameter conduit, and supported at from five up to about eight foot intervals for six inch diameter conduit.  Bends made in such rigid nonmetallic conduit are also required to be formed in such a way that no crimping or reduction in internal diameter of the conduit results. 
In Australian Pat.  No.  155,417, an electrical wiring system is disclosed suitable for installation in building structures, and includes at least one central conduit box and a plurality of tubular conduits extending from the box for receiving insulated electrical conductors.  The tubular conduits in each case include at least one relatively short piece of corrugated flexible tubing, and at least one relatively long piece of rigid tubing connected in a relationship such that there is a piece of the flexible tubing interposed between the conduit box and the adjacent end of a piece of the rigid tubing.  The ends of the rigid tubing are threaded suitably to form a threaded interlock with the corrugations of the flexible sections of tubing. 
U.S. Pat.  No.  3,936,417 to Ronden is directed to a polyvinyl chloride pipe or conduit which is sufficiently flexible to permit it to be coiled up for transport and handling purposes, but has sufficiently high impact and tensile strength to satisfy the most stringent requirements for commercial installation.  The composition prescribed in this patent also has a high distortion temperature. 
In some instances, such as in a flexible cable shield developed by IBM and disclosed in the IBM Technical Disclosure Bulletin, Volume IX, No.  2, July of 1966, flexible corrugated cable shields or conduits have been made in two semicylindrical parts which can be folded into a cylinder about an interconnecting hinge, or coupled together by pairs of mating flanges located at opposite sides of the two semicylindrical parts. 
A modular cellular conduit assembly consisting of a plurality of corrugated conduit or pipe sections adapted for use as an underground installation for carrying telephone lines, electrical lines and similar conductors is described in U.S. Pat.  No.  3,693,664.  The individual corrugated pipe or conduit sections used in the assembly are each flexible to facilitate shipment and installation. 
A desideratum which existed for a number of years, and was allegedly solved in 1955 by the patentees in U.S. Pat.  No.  2,728,356, was the fabrication of a large diameter, thin walled flexible plastic tubing.  A difficulty which existed with tubing of that sort as it had been previously provided was that when subjected to bending, it tended to undergo buckling or crimping, thus undesirably reducing the internal diameter of the tubing.  The patentees proposed to overcome the problem by providing a plurality of contiguous corrugations in the tubing in which the minimum radius curvature of any portion of the ridges and valleys of the corrugations of the tubing was at least three times the wall thickness at that portion of the corrugated tubing.  The patentees provided, moreover, that the crest of the ridges and valleys of the corrugations must constitute long arcs of at least 150.degree. ,  and not more than 270.degree. .  The ridges and valleys of the corrugations were each arcs formed about a radius many times the thickness of the tubing wall.  Further, the patentees contemplated that in the most preferred form, the ridges and valleys of the corrugation have an identical radius.  This allegedly resulted in uniform distribution over a wide area of the stresses arising from bending of the flexible tube, without a concentration of stress in any one narrow zone. 
U.S. Pat.  No.  3,060,069 discloses a plastic corrugated tubing useful for enclosing electrical conductors, and particularly of value in providing heat insulation for high resistance electrical conductors. 
A corrugated flexible conduit which can be transversely flexed or bent without danger of collapsing or crimping, but which is incapable of undergoing axial stretch, is disclosed in U.S. Pat.  No.  2,891,581.  This tubing, however, is made of a rubber composition. 
An objectionable characteristic of many types of corrugated tubing proposed as raceways or sheaths for electrical conductors in the prior art is that after bending through a relatively short radius, a tendency exists for the tubing to spring back or to undergo a reduction in radius of curvature at the bend location due to the elastic properties of the material, and the nature of the corrugations formed therein.  This difficulty is allegedly overcome by the particular material of construction used in a corrugated tubing having unconventional corrugations therein, as shown and described in U.S. Pat.  No.  3,908,704.  In short, the side walls of each corrugation are of unequal length, or stated differently, the principal axis of each corrugation extends at an acute angle to the longitudinal axis of the tubing or conduit in which the corrugations are formed.  This enables each corrugation to fold in under an adjoining corrugation without kinking or reduction in the effective internal diameter of the tubing when bends are made on a very small radius.  A suitable plastic of which this tubing may be made is flexible polyvinyl chloride.  It is pointed out in the patent that where the corrugated tubing is used as a cable or harnessed dielectric for conductors, a substantial advantage is obtained as a result of the very good dielectric properties of the plastic of which the tubing is made. 
U.S. Pat.  No.  3,132,415 indicates it as a criteria of corrugated electrical sheathing or conduits that such conduits be manually coilable in bends of various radii which can be as short as five times the outside diameter of the conduit if necessary.  It is also desirable that the conduit be sufficiently resilient that it can be restored to its original straight form, without residual bend deformations as it is installed.  The conduit has high impact resistance (of the order of 0.9 foot-pounds per inch according to ASTM D-256).  The corrugated conduit or conductor made in accordance with this patent is said to be capable of readily elongating and contracting with temperature changes, without bowing out between points of support, or buckling at supports.  Moreover, the conduit can elongate or shorten as bridge members move as a result of expansion or contraction, without any harm being done to the assembly. 
A corrugated electrical conduit for containing electrical conductors is disclosed in U.S. Pat.  No.  3,892,912.  In this conduit, the corrugations have flat troughs and crests which extend parallel to the axis of the conduit, and the side walls of each of the corrugations are of equal length.  The crests, however, are of greater length, as measured longitudinally along the conduit, than are the troughs. 
The standards and specifications established by Underwriters Laboratories for rigid nonmetallic conduits constructed of polyvinyl chloride, polyethylene and similar materials require that the conduit be used at a temperature not higher than 50.degree.  C. with wiring rated, in any case, not higher than 90.degree.  C. The specifications further provide that such conduit or fittings shall provide a smooth internal raceway for the pulling in of wires and cables, and that such conduits shall not be threaded.  For a so-called heavy wall PVC conduit, specifications of Underwriters Laboratory require that a wall thickness ranging from 0.109 to 0.129 inch be characteristic of conduit having a nominal 1/2 inch diameter, and that a wall thickness ranging from 0.28 to 0.314 be characteristic of heavy wall PVC conduit having a nominal diameter of 6 inches.  For a so-called thin-walled PVC conduit, the 1/2 inch conduit must have a wall thickness ranging from 0.060 to 0.080 inch.  Four inch conduit of the thin-walled type must have a wall thickness of from 0.15 to 0.17 inch. 
The Underwriters Laboratory specifications further require that elbow bends in a conduit effected through connection of elbows to a straight section of the conduit be such that the bend is free of kinks and creases.  It is further required that for this straight walled, noncorrugated conduit, no bends shall be sharper than 90.degree. .  For a 1/2 inch conduit, the radius of a bend in the conduit can be no smaller than four inches.  For a six inch conduit, the radius needs to be no smaller than thirty inches.  For all types of PVC conduit, the tensile strength must be not less than 5,500 pounds per square inch.  The specifications and standards of Underwriters Laboratory further provide certain crushing specifications.  For example, a 1/2 inch straight wall conduit must not undergo a reduction of more than 30% in its internal diameter when a six inch specimen of the conduit is subjected to a 1000 pound loading imposed upon the central portion conduit.  For thin walled PVC conduit, the same reduction in internal diameter must not be exceeded when a load of 300 lbs.  is centrally imposed upon a six inch specimen.  For a six inch diameter conduit, the specified reduction in internal diameter must not be exceeded under a load of 850 lbs.  in the case of the heavywalled conduit.  Certain impact strength requirements are also imposed upon both the heavy-walled and thin-walled conduit. 
Corrugated conduit suitable for underground burial and for the containment and conveyance of telephone cables and the like is manufactured and marketed under the trademark "CORFLO" by Haskon, Inc. of Middletown, Del. The CORFLO underground corrugated duct is claimed to have high crush resistance, and high flexibility which allows elbows and bends to be easily formed in the duct.  The CORFLO corrugated duct is made of high density polyethylene, and is sold in sizes of from 11/2 inches up to 4 inches.  The impact resistance of the CORFLO polyethylene corrugated duct ranges from about 40 foot-pounds for 11/2 inch duct up to about 150 foot-pounds for 4 inch duct as determined by ASTM D-2444-70 TUP B.