Electric utilities must maintain adequate clearance of overhead circuits to prevent interruptions to their customers’ service. Uncontrolled tree growth commonly causes service interruption by contacting energized conductors and creating momentary faults, complete outages, or mechanical breakage of utility facilities.
The overall objective of the EPRI Project “. . .is to design and produce prototype equipment capable of lowering costs and improving efficiency, safety, and flexibility of line clearance treetrimming operations.” The scope of the project is limited to positioning and cutting equipment and does not include removal of debris.
Tree/wire Relationships
Ideally, trees are trimmed back from electric conductors to allow them to grow 2 to 4 years before they again become hazardous to the line. Trees are either top, side, overhang (under), or through trimmed according to their relationship to overhead conductors and their potential for interference (Figures 1 and 2). Basic trimming methods include: pollarding, shearing or rounding over, and natural trimming (Figures 2 and 3). Because of long-term line clearance limitations, pollarding and rounding over are undesirable methods. Natural trimming is the method recommended by most professionals.
Tree/wire relationships
Natural trimming methods
Undesirable trimming methods
Natural Trimming
Natural trimming consists of cutting potentially interfering branches flush to a suitable parent limb toward the center of the tree crown. This method is sometimes called “drop crotch” or “lateral” trimming. Generally, cuts are made with a saw and minimal pole pruner work is required. The result is a natural looking tree. Natural trimming is also directional trimming, since it guides growth of the tree away from the wires.
Crew Types
Utility lines located along streets and highways are maintained most economically by treetrimming crews working from vehicle-mounted aerial devices. Utility lines traversing areas inaccessible to vehicles are normally maintained by manual crews using climbing ropes and belts. Selection of the type of line clearance crew to be used in a given area depends on a combination of factors. The number of men and type of equipment assigned to a crew is determined after evaluating such variables as:
Location of utility lines — roadside, rearlot, or cross country
Tree type, growth rate, wood strength, branching habit
Tree conditions
Terrain
Climate
Labor poor — availability, cost, experience
Urban or rural utility system
Customer expectation
Budget considerations
A national survey shows that 56 percent of all tree-trimming crews are lift crews; 44 percent are manual crews. The most common crew types are 2- or 3-man lift crews (44 percent) and 3- or 4-man manual crews (27 percent). It is important to note that, while nationally 56 percent of the tree-trimming crews have been mechanized with lift trucks, 44 percent of the crews are operating essentially as they did 40 years ago. Improvements and innovations to line clearance tools for manual crews have been minimal since the 1940’s.
Trimming Tools
A survey of tree trimming contractors and utilities was made to determine how tools and equipment used by line clearance crews may be improved. The most frequent requests were for:
Lighter tools
More reliable and maintenance-free tools
Less expensive tools
These suggestions present conflicting requirements. This establishes the need for careful value engineering analysis of the long-term economics of changing tools that have been used, refined, and improved over many years.
New Concept Analysis
The Scientific Method, which provides guidance to define the problem, gather relevant data, formulate concepts from the data, and empirically test the concepts, was used to identify new concepts for tree trimming tools and equipment. The problem, tree trimming for line clearance, was defined by using a work measurement system that identified the day’s total activity and the operations required. The lack of repetition in operations makes defining the task of tree trimming more difficult than defining assembly line work. The cutting and positioning requirements of tree trimming contain natural elements that continually change. Data were collected by a literature review, a patent search, and a study of work situations. New concepts were developed from creativity workshops and review of data gathered. These concepts were subjected to an initial screening process to separate those with potential from those with little or no potential for success. Concepts were then subjected to a decision analysis which tested each concept for its practical contribution to program objectives. Only nine, of more than 40 ideas for new tools and equipment that merited screening, were worthy of complete analysis.
Conclusions and Recommendations
After the decision analysis and study of adverse consequences, five concepts emerged (Figure 4) as potential improvements on line clearance equipment:
Pantograph-Type Linkage — support for powered pole saws and pruners.
Mobile Platform Lift — similar to those used in the orchard industry.
Boom-Mounted Tool — replacement of the bucket on existing lift trucks.
Servo-Assisted Pole Saw — for trimming from the ground in backlot locations.
Cut/Bundle/Chip Vehicle — mechanized vehicle for continuous operations of tree trimming.
New tree-trimming concepts
The estimated development cost, benefit/cost ratio, and savings to an average size utility for these concepts are shown below.
The pantograph concept, in conjunction with higher horsepower pole tools, is recommended for short-term development. The adaptation of this concept to existing lift trucks insures a high probability of success.
An engineering study should precede development of other recommended concepts to substantiate their benefits to utilities throughout the United States.
There are unknowns in all development programs such as market demand, operating costs, and actual productivity of the concept after development. Regardless of the unknowns, the economic analysis supports the belief that the development of equipment for tree trimming has the potential for high return on investment.
Footnotes
↵1 Presented at the annual conference of the International Society of Arboriculture in Louisville, Kentucky in August 1982.
- © 1983, International Society of Arboriculture. All rights reserved.
