Patent Publication Number: US-2002009273-A1

Title: Neutral wire for power distribution systems

Description:
[0001] The present invention relates to neutral wires for use in an above ground power distribution systems.  
       [0002] Electrical power is distributed through conductors carried above ground on poles. Typically, the system will have one or more line conductors carrying AC at different phases and a neutral wire to provide a return path.  
       [0003] The service requirements for a neutral wire require it to be electrically conductive as well as self-supporting. In more recent years use has been made of the neutral wire to also carry a messenger wire so that information can be transmitted along the existing power distribution corridors. The wires have been located within a conductive, usually aluminum, tube which in turn is supported by wire strands that provide the tensile strength for the ground wire. It is also known to incorporate optical fibers with the messenger wire to increase the information carrying capacity.  
       [0004] Because the neutral wire is unsupported between the posts, it is subject to the normal mechanical forces due to its own inherent weight, wind and other atmospheric conditions such as ice build up. These mechanical forces are imposed on the wire strands and the aluminum tube which in turn transmit the forces through to the optic fibers.  
       [0005] To avoid the possibility of damage to any optical fibers used as messenger wires, it is known to arrange the fibers helically on a spacer within the tube. Such an arrangement is relatively expensive due to the need for the additional components and the need to assemble the fiber onto the spacer. As an alternative to a helical spacer it is also known to ensure that an excess length of fiber is provided within the tube so that as the tube is subjected to the elongation due to the mechanical forces, the strain is not passed through to the fiber.  
       [0006] The use of an aluminum tube protects the messenger wire from water and other environmental concerns. However, the aluminum tube is relatively weak in compression and therefore is unable to withstand the radial forces that might be imposed upon it during installation. To provide additional mechanical strength it has been proposed to utilize concentric rings of aluminum wires around the aluminum tube. Typically these wires are steel with an aluminum cladding adjacent the tube and an aluminum alloy as a second band of wires. The increased tensile and radial strengths avoid damage to the tube but at the same time increases the expense of the wire.  
       [0007] It is therefore an object of the present invention to provide a neutral wire in which the above disadvantages are obviated or mitigated.  
       [0008] In accordance with one aspect of the present invention there is provided a neutral wire for use in an above ground power distribution system. The neutral or messenger wire comprises an elongate plastics tube and at least one optical fiber freely moveable within said tube and having an overall length greater than the tube in a free body state. A plurality of metal wire strands are disposed about and extend along the length of said tube.  
       [0009] The provision of a plastic tube to house the optic fibers enables the requisite radial strength to be attained.  
       [0010] Preferably said tube is coated with a protective plastics coating of a dissimilar material. 
     
    
    
     [0011] An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:  
     [0012]FIG. 1 is a schematic illustration of a power distribution system.  
     [0013]FIG. 2 is a perspective view of a portion of the ground wire used in the power distribution system of FIG. 1.  
     [0014]FIG. 3 is an end view of the ground wire shown in FIG. 2.  
     [0015]FIG. 4 is an enlarged view of a portion of an alternative embodiment of distribution system. 
    
    
     [0016] Referring therefore to FIG. 1, an electrical power distribution system generally indicated at  10  includes a pair of posts  12  supporting power conductors  14 . Each of the conductors  14  carries an AC supply and are at different phases. A neutral wire  16 ; also referred to as a messenger wire, extends between the posts  12  below the conductors  14  and is fixed at each of the posts  12  by suitable clamps.  
     [0017] The neutral wire  16  is shown in more detail in FIGS. 2 and 3 and includes a plastics tube  18 . A number of optic fibers  20  are located within the tube  18 . The optic fibers are utilized to carry information and typically the tube will carry a significant number of fibers, sometimes upwards of 200. The fibers  20  are loosely located within the tube  18  and, in a free body state, have a length greater than that of the tube. The excess length of the fibers  20  will be at least 0.05% greater than that of the tube  18  although greater discrepancies may be used. In this way the tube  18  may elongate without inducing a corresponding strain upon the fibers  20 .  
     [0018] An outer film or coating  22  is applied to the tube  18  to provide enhanced protection for the outer surface of the tube.  
     [0019] A plurality of wire strands  24  are helically wound around the outside of the tube  22 . The strands  24  are typically a steel wire  26  having an aluminum coating  28  or are galvanized. Alternatively the strands are made entirely of an aluminum alloy. The strands  24  provide the requisite tensile strength for the tube to span between the posts  12  and provide a conductive path for electrical current.  
     [0020] The tube  18  is formed from a plastics material having a relatively high modulus of elongation to permit extension greater than 1% and typically in the order of 6%. The extension can be approximately 10 times the excess length of the fibers  20 . The tube is formed from an engineering thermoplastic, typically polycarbonate, polybutylene terephtalate (PBT) polyethylene tetraphtalate (PET) or polyethylene naphthalate (PEN). In the preferred embodiment, tube  18  has a diameter of between 3 and 6 millimeters and a wall thickness of between 0.5 and 2.0 millimeter.  
     [0021] In a typical application the tube  18  has a diameter of 5.0 mm and a wall thickness of 0.8 mm.  
     [0022] The outer coating  22  is preferably a plastics material exhibiting good wear characteristics and resistance to environmental factors. Preferably polyethylene is used for the coating and applied at a thickness of 0.1 mm to 1.0 mm to give a diameter of between 3 and 7 mm.  
     [0023] The wire strands  24  are chosen to be of sufficient diameter and number to provide the tensile strength required for the span between the posts  12  and may if preferred be arranged in 2 annular bands.  
     [0024] In use, the plastic tube  18  has sufficient radial strength to withstand the radial loads imposed on the cable due to the clamping at the posts. At the same time the plastic tube provides protection for the fibers from water and ice and the excess length of the fibers in the tubes prevents strain being induced in the fibers when the ground wire is subject to mechanical loading.  
     [0025] It will be appreciated that the dimensions noted above with respect to the preferred embodiment are exemplary only and the individual components may be adjusted to suit the particular mechanical requirements of a given installation.  
     [0026] A further embodiment is shown in FIG. 4 where like reference numerals will be used to denote like components with a suffix ‘a’ added for clarity.  
     [0027] In the embodiment of FIG. 4, the conductors  16   a  are suspended from a neutral or messenger wire  14   a  by a hanger  30 . The hanger  30  has spaced apertures  32  to receive one of the conductors  16   a  and a hook assembly  34 . The hook assembly  34  engages the wire  14   a  to support the conductors  16   a . The structure of the wire  14   a  is identical to that shown in FIGS. 2 and 3 with external wire strands wrapped extending along a plastic tube that holds a bundle of fibers.  
     [0028] The plastic tube  18  resists the radial loads imposed by the hook assembly  34  with the strands providing the requisite tensile strength.