Tree failures are a significant cause of electric service interruptions, therefore, utilities dedicate significant resources for vegetation management. A reliable estimate of the percentage and total cost of interruptions caused specifically by trees is not available; however, a 2001 study by the Electric Power Research Institute estimated that power interruptions cost American business US$104 billion to $164 billion annually, and an additional $15 billion to $24 billion for all other power-quality problems.
If even a relatively small fraction of that estimate is attributable to trees, the costs to end users of electricity far exceed the estimated $3 billion in annual utility expenditures on vegetation management. Therefore, even a small improvement in mitigating the risk posed by trees could have significant economic benefits for utilities, the local communities they serve and the nation as a whole.
After lengthy drafting, public review and comment periods, a new American National Standards Institute (ANSI) standard for tree risk assessment was published in late 2011, along with a companion best management practice guide. In addition, recently developed tree risk assessment qualifications for vegetation management personnel are now available, and a new, more comprehensive international program will be rolled out by the International Society of Arboriculture (ISA) later this year.
These standards and associated programs reflect advancements in the science and practice of tree risk assessment. Together, they present an opportunity for utilities to leverage improvements in safety, reliability and fire protection by applying proactive tree risk management and mitigation strategies.
Managing Tree Risk
Utilities have been managing tree risk since the development of the telegraph. Back then, and in the early years of electric service distribution, it was common practice to string wires from existing sturdy trees. The qualifications, if any, of those who inspected and approved the trees for such use are lost to time. Today, utilities recognize the art and science of tree risk assessment as a valued skill.
Utility vegetation management programs have traditionally focused on preventing tree-line contact by obtaining specified clearances. While such programs certainly reduce tree-line contact and prevent some interruptions, many outages are caused by tree and branch failures that originate from outside the specified scope of work. To improve system performance, utilities are increasingly focusing resources on hazard tree abatement, or, more accurately, tree risk management. Utilities can increase the value of their vegetation management investment by systematically concentrating on trees that pose the highest level of risk.
Completely mitigating the risk posed by trees would require utilities to specify pruning or removing any tree with the potential to strike a utility line. Of course, this would be cost prohibitive and raise customer acceptance concerns. More importantly, it would be quite unnecessary, since many trees in close proximity to utility lines pose relatively low risk.
The key to improving the effectiveness of vegetation management efforts is for utilities to determine the relative level of risk posed, allocate resources to benefit the greatest number of customers and establish written specifications that clearly define the scope of work for contracted personnel.
Tree Risk Assessment Principles
All trees pose some risk to their surroundings. Trees that appear perfectly healthy may fail unexpectedly; in fact, every tree will be lying on the ground at some time. The only way to completely eliminate all risk is to remove trees. By systematically assessing the risk, trees or branches more likely to fail can be identified and the risk mitigated before failure causes injury or damage.
Tree risk assessment is performed in varying locations and for many different reasons, but there must be something of significant value — a target — that is put at risk by the tree. A house, playground, street, walkway or other items of value are examples of targets. For utilities, the target is most often a power line.
Mitigating the risk posed can be accomplished either by moving the target out of harm's way, or by eliminating or reducing the risk. In many settings, potential targets can be moved. However, in most utility settings, moving a power line is not a viable option. Utilities must mitigate the risk by specifying the pruning of branches or removal of trees. In many cases, the trees posing the greatest risk may be completely off the right-of-way, outside the normal scope of work.
Targets also have varying importance. On a typical property, a building or a frequently used outdoor seating area normally would be considered a high-priority target. Seldom-used areas with low-value targets are of least concern.
Likewise for utilities, different types of lines, or even segments of lines, have varying priorities as targets. Voltage, number of customers served, types of customers and engineering factors such as fusing and switching all may affect target ranking. For example, a healthy tree that could make contact with a transmission line would be of greater concern than a defective tree that threatens an individual service line, because the consequences of transmission line failure are so much greater than those of a service line.
Also, distribution lines protected by automated switches and fuses may not be as high a priority as unprotected lines coming right out of a substation. Allocating utility resources in a way that reflects both the priorities of the utility and the best interests of the communities served can be a challenging proposition.
Resources for Tree Risk Assessment
The new ANSI A300 (Part 9) Tree Risk Assessment standard was published in December 2011. This publication describes the steps for assessing risk posed by trees and is a good framework for developing a risk management specification. The publication describes three levels of risk assessment:
Level 1, or limited visual, is from a specified perspective such as foot, vehicle or aerial patrol. The assessment could be of an individual tree or a population of trees near specified targets such as a utility line. A level 1 assessment specified by an electric utility might include instructions to look for dead or declining trees, obvious structural defects, dead or weak branches, signs of disease, storm damage or safety concerns such as readily climbable trees or tree houses.
Level 2, or basic, is a detailed visual inspection of the entire tree and its surrounding site. This level requires the inspector to walk completely around the tree and may include the use of simple tools, such as a mallet, probe or trowel, to identify hidden defects.
Level 3, or advanced, is a thorough examination that includes a basic assessment but also uses specialized tools and techniques to find hidden defects that cannot be determined by a level 2 assessment. Sonic tomography, root excavations and resistance drilling are examples. An advanced level may be used in certain circumstances after a basic inspection reveals concerns that require more attention. Due to the time and expense involved, the use of advanced techniques is usually reserved for high-value trees.
The scale of utility operations, and the vast number of trees involved, makes a level 1 (limited visual) assessment the only practical method for utilities to commonly specify, except in certain circumstances.
Copies of ANSI A300 standards are available on the Tree Care Industry Association or the ISA websites.
Also in 2011, the ISA published Tree Risk Assessment Best Management Practices. Written by an internationally recognized team of experts, this guide spells out, in detail, what is outlined in the ANSI A300 standard, including specific descriptions of what assessors should look for when assessing trees for risk. Additionally, the guide offers methods for systematically prioritizing risk and allocating resources for mitigation. It is available on the ISA website.
Best Practices for High-Fire-Risk Areas
The risk of fire started by trees contacting power lines also is a concern, especially in areas prone to wildfires but increasingly in areas stricken by drought. Drought conditions are compounded by associated tree mortality and failure, which, when combined, can pose unique challenges for utilities as they allocate and prioritize resources for tree pruning and removal. Those resources should be incorporated into a utility-directed program that effectively reduces the risk of wildfire.
In 2009, the Utility Arborist Association (UAA) published Tree Risk Assessment and Abatement for Fire-Prone States and Provinces in the Western Region of North America. This guide was written to help Western utilities manage the extraordinary risk posed by trees falling on utility lines in high-fire-risk areas. It is available on the UAA website.
The guide recommends that utilities conduct periodic line patrols from the ground (foot or vehicular) or air (essentially a level 1 assessment) to identify areas or individual trees that require either mitigation or further investigation. Further investigation may reveal the need for additional mitigation or for an abatement plan with a time frame and assignment of responsibility.
The point is, by specifying systematic monitoring and abatement, utilities can reduce the probability of catastrophic tree failure and wildfire. However, the guide also points out it is not reasonable to expect every tree to be closely monitored or every defect detected and abated.
Clearly, with many thousands of trees that could affect a typical utility system, it is not practical to expect utilities to thoroughly assess every tree and mitigate all risk. However, by applying the growing body of scientific knowledge of how trees fail and the practice of tree risk assessment, resources can be better focused on improving the safety and reliability of utility systems.
To leverage tree risk assessment programs to the greatest effect, a utility should first prioritize lines and line segments based on their importance. Risk assessment should then be performed to the utility specification by qualified personnel, and trees or areas requiring additional assessment or mitigation should be identified. Finally, the utility should allocate resources so the trees posing the greatest level of risk to the highest number of customers receive the highest priority.
The specified interval between assessments also may depend on the priority of the line; for example, critical transmission lines might be patrolled annually or more frequently if conditions warrant. Distribution lines may be assessed at appropriate intervals depending on the relative priority of the line.
Though this may sound fairly straightforward, in reality, deploying a tree risk management strategy across any utility system is a complex endeavor. To maximize effectiveness, utilities must gather information, including a comprehensive understanding of tree failure patterns by size, species and type of failure, along with regional and site-specific features, and have a thorough understanding of priorities, system characteristics, the application of federal and local regulations, and reporting requirements.
The quality of the utility's risk assessment program is only as good as the personnel doing the job. When allocating resources to manage tree risk, utilities should ensure qualified personnel are available to perform risk assessment as well as any necessary mitigation measures, and set objectives accordingly. Tree risk assessment personnel also must possess good communications skills and be prepared to answer questions from the public about how and why the work must be performed.
Most utilities already perform regular patrols on lines, whether expressly for risk assessment, or when notifying or planning vegetation management work. If available personnel are competent in risk assessment, specifying a level 1 risk patrol to these functions may be accomplished with relatively little added expense. This step could be the first toward creating a comprehensive tree risk management program, with potentially significant improvements in system reliability.
Inspection Certifications and Qualifications
ISA certified arborists have demonstrated experience and have passed a comprehensive exam covering a broad range of subjects, including a fundamental knowledge of tree risk assessment principles. Utility specialists are ISA certified arborists who have passed an additional exam specific to utility concerns. Utilities are increasingly requiring these credentials for utility and contractor staff. Information on ISA certification can be obtained on the ISA website.
Currently, the only widely available qualification for tree risk assessors in North America is the Tree Risk Assessment Course and Exam (TRACE) offered by the Pacific Northwest chapter (PNW) of ISA. Details about the program, including costs, schedules and experience requirements, can be found on the PNW website. By late 2012, PNW's TRACE will be absorbed into a new ISA risk assessment qualification that will be available internationally. To obtain the qualification, a candidate will need to have demonstrated the requisite background and experience, and then must attend a course and pass a comprehensive exam. Credential holders will need to be retested every five years to maintain their qualification.
Safer, More Reliable Service
In a modern economy, consumers expect a safe and reliable energy supply delivered sustainably by their utilities. There are obviously locations on a utility system, such as under a critical transmission line, where the presence of any tree poses an unacceptable level of risk. However, in the many instances where the presence of trees is unavoidable, utilities must allocate resources for pruning and removal. In these cases, the public is better served by ensuring these resources are focused on reducing the greatest amount of risk.
In fact, many of the benefits provided by large trees, such as air-quality improvement, carbon sequestration, and peak load reduction through heat island reduction and shading of structures, are in line with utility interests. Therefore, in many settings, the preservation of large, healthy trees and tree canopy serves both the interests of utilities and the communities they serve — but only if the utility takes an active role in ensuring the trees pose minimal risk to utility service.
By developing and specifying systematic tree risk assessment with qualified personnel, utilities can ensure their investment in vegetation management provides the greatest possible return in the form of safer and more reliable electric systems.
Geoff Kempter is manager of technical services for Asplundh. He has served on the ANSI A300 Tree Care Standards Committee since 1996 and is the past chair of the ISA Certification Board, which is creating the new tree risk assessment credential. He is the author of the ISA's Best Management Practices Guide for Utility Pruning of Trees.
Why “Tree Risk” and Not “Hazard Tree” or “Danger Tree”?
The terms “hazard” and “danger” commonly have been used to characterize trees situated near power lines. These terms generally connote imminent harm or catastrophe. To more accurately describe potential for harm, the trend is to express these concepts in terms of relative risk posed.
ANSI A300 (Part 7) Integrated Vegetation Management defined a danger tree as “a tree on or off the right-of-way that could contact electric supply lines” and a hazard tree as “a structurally unsound tree that could strike a target when it falls” (in this case, the target is the utility line).
The concept of a danger tree is most relevant in settings with an extremely low tolerance for risk, such as a critical transmission line or a substation. In distribution settings, where a higher level of tree risk is unavoidable, nearly all trees near power lines could be characterized as danger trees, so the concept is less meaningful.
According to the standard, a hazard tree differs from a danger tree only in that a hazard tree must be structurally unsound. In the case of an engineered structure such as a bridge, structurally unsound can be quantified in terms of poor design or by a measurable amount of structural damage. However, since trees are not designed to any particular standard, it is quantitatively impossible to determine the point at which the combined defects present in any tree cross the threshold of structurally unsound.
Furthermore, in utility settings, by definition, all hazard trees are danger trees, but not all danger trees are hazard trees. Due to the inherent confusion, the definitions are being moved from the body of the 2012 revision of ANSI A300 Part 7 into an informational annex. The 2012 revision of Part 7 recommends monitoring trees for risk in accordance with ANSI A300 (Part 9) Tree Risk Assessment.
A risk management approach acknowledges all trees pose some risk, and not all risk can be identified, but systematic analysis by trained personnel may allow more effective and efficient allocation of available mitigation resources.