Electric utilities sell a potentially deadly product that is transmitted and distributed throughout the communities and urban forests in which we live, work and play. Unfortunately, tragedies occur when members of the public come into unintended direct or indirect contact with energized overhead lines while climbing trees.
Commercial arborists and qualified line-clearance specialists make a living in trees, but their risk is reduced through appropriate work practices, training and specific regulations. The general public, on the other hand, is mostly unaware of any risks or safety standards associated with tree climbing. People climb trees as a form of recreation, to retrieve an object, to harvest fruit or to perform pruning, and if the trees are in close proximity to energized overhead electric conductors, the climber can be at risk of serious injury or even lethal exposure to electrocution.
Vegetation managers recognize the risks posed by “readily climbable trees,” and as a result, they spend more time and care maintaining these trees than the others conflicting with overhead conductors.
In theory, every tree is climbable. Of the trees adjacent to overhead utility lines, only some have structural crowns that are in close proximity to energized conductors. Of these, some may be relatively easy for a child or adult to climb, while others may be difficult enough to climb that a casual climber would require assistance.
The hazard posed by climbable trees can be thought of as a function of the risk they create and the likelihood of an adverse contact: Hazard = Risk (clearance and climbability) × Frequency (exposure).
It is important to recognize that while focusing on public safety is an absolute requirement, it is exceedingly difficult to achieve zero risk of adverse exposure to the tree-climbing public. There is simply too much exposure, considering the thousands of miles of overhead lines interspersed through millions of trees that the average utility strives to maintain. Secondly, there is inherent uncertainty associated with predicting which trees are likely to be climbed.
Vegetation managers commonly use variations of the following questions in assessing risk and identifying readily climbable trees.
Is the structural crown of the tree in close proximity to energized high-voltage conductors, allowing a person to achieve a position of risk?
Is the structural crown of the tree easily accessed from the ground and climbed?
Do individuals who are possibly interested in climbing the tree frequently visit the site that the tree occupies?
Clearance is dependent upon the distance from energized conductors. It also factors in the weight-bearing capacity of branches in the vicinity of a conductor to determine whether it would be possible for a person to occupy a position of risk. On distribution circuits, a readily climbable tree typically would have a stem or major branch capable of supporting the weight of a child or average person within arm's reach of an energized conductor. The key point is whether climbers could achieve close proximity to energized electric conductors, placing themselves at risk of injury by direct or indirect contact.
Line clearances from low-voltage secondary and system-neutral conductors are inherently lower risk. Trees in which the main stem and/or large diameter structural branches have been allowed to develop and remain in close proximity to energized high-voltage primary conductors would be considered higher risk.
Periodic line-clearance maintenance pruning can be effective in reducing the proximity risk. The structural crowns of trees that are repeatedly pruned during routine line-clearance preventive-maintenance work are often held to a height that is at or slightly above the telecommunications under build and well below the system neutral. A climber requires suitable hand and foot positions. It is very unlikely that a person would stand on the upper weight-bearing branches and extend himself or herself vertically with no additional support for stability. When clearance is achieved by good-quality height-reduction utility pruning, in which pruning cuts are made at appropriate branch nodes, the distance between structural elements of the crown and conductors should provide adequate clearance. In cases where previous line clearance was accomplished by “topping,” the resulting profusion of dense, weakly attached regrowth sprouts may impede a casual climber's access to the upper crown.
EASE OF ENTRY AND CLIMBING
Climbability refers to how easy it would be for a casual climber to enter the crown of the tree and climb to a position of risk. The lower limbs of a readily climbable tree provide easy access to the crown so a child or average person could enter the crown without the use of a ladder or other specialized equipment. The average height of adult males in the United States is 5 ft, 10 inches (178 cm), and the average height for adult females is 5 ft, 4 inches (162 cm). Children of course are shorter than adults. A height to the first branch of 6 ft (1.8 m) or more would typically be considered too great for a person of average athletic ability and climbing skill to overcome without some aid.
The height to the first branch may be considered sufficient to preclude easy entry to the crown when there is a clear absence of suitable hand and foot positions. This could occur when the first branch is actually the union of major codominant stems.
Many readily climbable trees are landscape plants that came from a nursery. It's common practice for nurseries to make a heading cut on the main stem of new trees to force development of a more spreading branching structure. Many trees in the urban landscape will demonstrate this form, with a clear stem to a height of 5 ft or 6 ft (1.5 m or 1.8 m).
In addition to ease of first entry, a readily climbable tree would typically have an open growth form with regular scaffolding branches sufficiently close together so the tree can be easily climbed by a child or average person without the use of climbing gear or other forms of assistance. An example would be an open grown conifer such as Eastern White Pine (Pinus strobus) planted as a landscape tree, with regular whorls of branches from ground level to the height of overhead conductors.
Branching habit and structure can also create a significant impediment to climbing within the crown of many trees. Physical impediments, such as species with thorns and the excessively dense regrowth found in some trees following heavy pruning, can be significant enough to eliminate the tree from consideration as a potential readily climbed tree.
SITE RISK CONSIDERATIONS
To determine the likelihood of potential adverse exposure, it's important to focus on sites such as schools or public parks. In these locations, members of the public are likely to be present, and may climb the tree and expose themselves to elevated risk. Utilities should pay particular attention to fruit-bearing trees, as well as to the presence of a tree house or rope swing, which would be a strong indication that the tree represents unacceptable risk.
Achieving and maintaining adequate clearance through preventive-maintenance pruning is one means of mitigating risk. It may be necessary to monitor and maintain these trees on a shorter inspection and maintenance cycle than that used for routine vegetation-management work. It is also possible to further mitigate the risk through means such as additional pruning, including crown lifting through the elimination of lower branches. Tree removal is also an important and effective way to eliminate the risk. However, many climbable trees occur as individual landscape trees or as part of a visual barrier. As such, the property owners involved place value on these trees and may not readily agree to their removal.
Readily climbable trees represent risks to public safety. Utility vegetation management programs should recognize and provide for a means of managing this risk. While all trees in proximity to overhead-energized conductors hypothetically present a degree of risk, criteria can be applied to identify readily climbable trees. Once identified, the risk can be managed by a variety of means.
John Goodfellow is a vegetation management researcher with more than 25 years experience in the electric utility industry. He has been responsible for vegetation management, engineering and field services at three large investor-owned electric and gas utilities. He has also been responsible for managing T&D services for a major contracting organization. He earned his bachelor's degrees in forestry and natural resources management from Syracuse University and the SUNY College of Environmental Science and Forestry. firstname.lastname@example.org