Abstract:
A rigid, unitary ladder is provided having one end which attaches to a powerline support tower after the ladder is dropped by helicopter to a lineman previously offloaded onto the tower. The ladder includes a first section which extends outwardly from the tower and a second section which extends angularly downward from the first section to a base which rests on the powerline. The first section is of a length sufficient to space the base a distance from the tower which is beyond the flashover area around the tower.

Description:
BACKGROUND OF THE INVENTION 
     High voltage powerlines carrying voltages in the hundreds of kilowatt range have been constructed to extend from spaced towers high above terrain which is often practically inaccessible to land vehicles. In recent years, a number of methods and devices have been developed to facilitate the repair of remote high voltage powerlines which are accessed by helicopter. 
     Helicopter transported landing platforms and work scaffolds which can be attached to the cross arm of a utility pole have been developed to permit a lineman to disembark from a helicopter and descend vertically along the utility pole to the powerlines supported thereby. Such devices are heavy and bulky, and are often difficult to maneuver. More importantly, however, is the fact that these devices position a lineman in close proximity to a grounded utility pole, so that the powerlines must be deenergized before repair can occur. Otherwise it is dangerous for a lineman to remain in the electrically charged space between energized high voltage conductors and a grounded tower or utility pole. A helicopter-mountable landing platform and work scaffold adapted to be mounted by a utility pole is illustrated in U.S. Pat. No. 3,863,736 to McWilliams. 
     In order to avoid an extensive power outage which might occur when a high voltage powerline is deenergized for repair, it is desirable to provide live maintenance on energized high voltage conductors. To accomplish this, linemen first bond to the energized transmission line to become energized to the same potential as that of the line. This permits the lineman to work safely on the energized line so long as he maintains sufficient spacing from grounded structures, such as support towers, to prevent the risk of flashover. To facilitate live maintenance on energized high voltage powerlines, various lineman support structures have been developed which are suspended from a helicopter over a powerline pair remote from the support towers. These support structures are formed either partially or totally from electrically conductive material and include skids or rollers which engage a pair of parallel lines. Such lineman support structures are shown by U.S. Pat. Nos. 4,478,312 to Kurtgis and U.S. Pat. No. 5,494,132 to Hansen. Not only are these structures bulky and somewhat cumbersome, but it is difficult to position the conductive skids or rollers on the energized powerline pair from a helicopter. Also, these support structures may inadvertently move into dangerous proximity with a grounded tower structure. 
     In an attempt to eliminate bulky, difficult to position, linemen support platforms, systems have been developed for lowering a lineman from a helicopter directly onto an energized powerline. U.S. Pat. No. 4,673,059 to Kurtgis discloses such a system. Not only is there some difficulty in accurately positioning a lineman at the end of an elongate, swinging cable adjacent to an energized powerline for a time sufficient to complete a safe interconnection with the powerline, but once on the powerline, the lineman, through inattention, may move too close to a grounded tower. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a novel and improved helicopter lineman&#39;s ladder and method which permits a lineman to be directly landed from a helicopter onto a stable powerline support tower. 
     Another object of the present invention is to provide a novel and improved lineman&#39;s ladder which is lightweight and which can be lowered from a helicopter and easily positioned between a tower cross arm and energized conductors by a single lineman. 
     Yet another object of the present invention is to provide a novel and improved helicopter lineman&#39;s ladder which will deliver a lineman from a tower to energized powerlines and insure that the lineman is a safe distance from the tower before the lineman bonds to an energized powerline. 
     A still further object of the present invention is to provide a novel and improved helicopter lineman&#39;s ladder which operates to prevent a lineman who has bonded to an energized powerline from inadvertently moving dangerously close to a grounded tower. 
     These and other objects of the present invention are achieved by providing a rigid ladder having one end which hooks to a powerline support bridge after the ladder is dropped by helicopter. The ladder includes a first section which extends out from the support bridge for the flashover distance to the tower or more, and a second section that extends downwardly from the outer end of the first section to a base which rests on the powerlines spaced from the support bridge. A weight on the base holds it against the powerlines, and the complete structure is formed of electrically nonconductive material. Using this structure, when a lineman hooks up to the powerline, he will be more than the flashover distance from the support bridge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of high voltage tower supported powerlines; 
     FIG. 2 is a perspective view of the helicopter lineman&#39;s ladder of the present invention; 
     FIG. 3 is a view in side elevation of the helicopter lineman&#39;s ladder of FIG. 2; and 
     FIG. 4 is a perspective view of a second embodiment of the helicopter lineman&#39;s ladder of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, high voltage AC transmission lines normally include at least three powerline conductors  2 ,  4  and  6 , with each powerline conductor carrying a voltage which is 120° out of phase with the voltages carried by the remaining conductors. These powerline conductors extend over substantial distances and are supported to extend between spaced towers  8  and  10 . Each powerline conductor is suspended beneath a tower cross arm  12  by elongate insulators  14 . 
     In accordance with the method of the present invention, a helicopter  16  offloads one or more linemen  18  directly onto a tower  8 . This can be accomplished by having linemen sit on a helicopter skid  20  or upon a platform placed across the helicopter skids which extends laterally of the helicopter. By having the helicopter directly deposit linemen onto a stable tower structure which is insulated from the powerline conductors, unstable tethers which can swing a lineman into an energized powerline are eliminated. 
     Once the lineman is on the tower, the problem to be solved is how the lineman can safely descend to the energized powerline conductors  2 ,  4  and  6  and bond onto a conductor to raise his body voltage to the powerline conductor voltage. This cannot be done from the grounded tower structure or even from within an area close to the tower structure where flashover can occur. A flashover area, indicated at  22 , must be avoided if live maintenance on energized powerline conductors is to be safely accomplished. OSHA safety regulations require that a flashover area of at least eleven feet from the tower be maintained for energized high voltage transmission line repair and that this area must be avoided by linemen bonded to a powerline conductor. 
     In accordance with the present invention, a one piece, unitary lineman&#39;s ladder indicated generally at  24  is provided to ensure that a lineman can safely descend from the tower, bond onto an energized conductor, accomplish conductor repair, and ascend back to the tower without danger of ever entering the flashover area  22  while energized. The lineman&#39;s ladder  24  includes a first elongate section  26  having a free end which terminates in a pair of spaced connectors  28  which are adapted to engage and be connected to the tower cross arm  12  of a tower  8 . These connectors are shown as hooks in FIG. 1, but may take a number of forms to permit a lineman to positively lock the free end of the elongate section  26  to the tower cross arm. These connectors can be, for example, hooks with pivoted locking arms, hooks in combination with belts to secure the ladder to the cross arm, belts alone, or other known locking connectors to secure the ladder to the cross arm. 
     A second elongate section  30  of the lineman&#39;s ladder extends from the end of the section  26  at an angle no less than ninety degrees and which is preferably greater than ninety degrees. The section  30  terminates in a curved base bar  32  which extends outwardly on either side of the section  30  and terminates in downwardly turned ends  34  and  36 . Extending below the base bar on a flexible tether  38  is a weight  40 . 
     The elongate sections  26  and  30  of the lineman&#39;s ladder  24  are formed as ladder sections with spaced side bars  42  joined by cross rungs  44 . All components of the lineman&#39;s ladder, including the sections  24  and  30 , the connectors  28 , the base bar  32 , the tether  38  and the weight  40  are preferably formed of electrically nonconductive material. The ladder is very light weight and maneuverable and can easily be carried and lowered from a helicopter and secured in place by a lineman who was previously offloaded onto the tower  8 . 
     An important feature of the lineman&#39;s ladder  24  is the length of the elongate section  26 . The length of this section must be sufficient to insure that a lineman descending down the section  30  to an energized line is outside the flashover area  22 . Ideally, the length of the section  26  is eleven feet or more to position the section  30  outside the flashover zone. The section  30  will normally be shorter than the section  24  as its length must merely be sufficient to enable a lineman to reach the conductors supported below the tower cross arm  12 . 
     In accordance with the present invention, once the lineman is offloaded from the helicopter onto the tower, the lineman&#39;s ladder  24  is lowered to the lineman from the helicopter. Rings  46  may be secured to the sidebars  42  between the sections  26  and  30  to facilitate attachment to a helicopter cable. The lineman positions the ladder over a conductor or conductors and attaches the spaced connectors  28  to the tower cross arm  12 . Then the base bar  32  is lowered into engagement with a conductor or conductors and is held in place by the tethered weight  40 . The lineman now descends to the base bar, and while standing on the insulated base bar, bonds to the energized conductor. Now, the lineman can step onto the conductor to perform repairs. With the lineman&#39;s ladder in place, access to the flashover zone is blocked to prevent the lineman from inadvertently straying into the flashover zone while his attention is focused on repairing an energized conductor. Once the repair has been completed, the lineman steps back on the base bar  32  and terminates the bond with the energized conductor. Now the lineman can ascend back onto the tower  8 , and assist in removal of the lineman&#39;s ladder by the helicopter. 
     FIG. 4 illustrates a second embodiment of the unitary lineman&#39;s ladder which is indicated generally at  48 . Here structural elements which are the same as those previously described with reference to FIGS. 2 and 3 will be designated with like reference numerals. 
     In the lineman&#39;s ladder  48 , the ladder rungs  44  in elongate section  26  are eliminated and a solid ramp  50  extends between the sidebars  42 . Also handrails  52  are attached to the sidebars  42  of the elongate section  26  on either side of the ramp  50 . These handrails can be attached to a helicopter lowering cable. 
     In the lineman&#39;s ladder  48 , the base bar  32 , tether  38  and weight  40  of the lineman&#39;s ladder  24  are replaced by a platform  54  secured to the bottom of the section  30 . This platform supports weights  56  shown by broken lines in FIG. 4 which hold the platform in engagement with an energized conductor. Downward spaced projections  58  formed on either side of the bottom of the platform  54  extend on either side of a conductor which the platform engages. Preferably, all components of the lineman&#39;s ladder  48  are formed of electrically nonconductive material.