TREE-CAUSED ELECTRIC OUTAGES

In 1995, Eastern Utilities was among 14 utilities in the United States and Canada participating in a study to collect and evaluate data about the phenomena of how trees cause outages. Results indicate that failure of trees accounted for 40% of the preventable tree-caused outages in the Brockton territory of Eastern Utilities. Even though line clearance tree trimming continues to be the primary strategy, programmatic changes were made in Eastern Utilities' Tree Management Program. A Danger Tree Project was initiated to identify and mitigate trees with structural weaknesses along the 3-phase portion of the electrical distribution circuits. Approximately 4% of the trees in the portion of the urban forest managed by Eastern Utilities will either be removed or stormproof pruned. Reliability has improved by 20% to 30% along the circuits where hazardous trees have been removed or storm-proof pruning has been done. Proactive communication within the community and participation in communitybased tree planting has accompanied this project. Electric utilities trim trees to minimize treecaused electrical outages. (Outages are the interruption of electrical service to customers.) Typically, the expense to trim trees is a significant percentage of the maintenance budget of an electric utility. In spite of this expenditure, trees continue to be the greatest single cause of outages for many electric utilities. To minimize tree-caused outages, maintaining clearance between tree growth and energized conductors is the strategy for most utility tree management programs. Yet the phenomenon of tree-caused outages has not been empirically researched. In 1995, 14 utilities in the United States and Canada subscribed to the Tree-Caused Interruption Research Project conducted by Environmental Consultants, Inc. (ECI). The outage study consisted of a survey to collect and analyze data about individual tree-related outage events. The goal of the study was to develop regional and overall databases to identify and analyze the factors involved with tree-related outages. Eastern Utilities was a participant in the study. Results of the study led to programmatic adjustments in Eastern Utilities' approach to tree management, consisting of a modification of practices for line clearance tree trimming and initiating a Danger Tree Mitigation Project. These changes were based on the study's cumulative analysis for all participants, a study analysis specific to Eastern Utilities, a survey of the tree population adjacent to primary electrical circuits in one of Eastern Utilities' service territories, and a review of the outage history from trees at Eastern Utilities for the years 1989 to 1994. Methods The study was designed to collect data on 22 factors that describe the conditions of a treerelated electrical outage. A standardized data collection form was designed by ECI with input from the participants of the study. Data compilation and analysis was done by the ECI staff. Eastern Utilities also conducted a survey of the urban forest adjacent to the primary electrical circuits of the distribution network in its Brockton service territory and reviewed the outage history from 1989 to 1994. The primary electrical circuits consist of pole spans with single and multiple electrical conductors, called phases. The survey was designed to locate trees on the 3-phase portions of these circuits with the characteristics that are most likely to cause an outage. Also, an outage history of the primary circuits was reviewed to identify areas that have been historically prone to treecaused outages. Data collected for outage study. The study was organized to collect data on tree-caused outages by the following 6 categories: 1. cause of the outage 2. characteristics of the tree 3. surrounding environment 4. orientation of tree to electrical conductor 118 Simpson and Van Bossuyt: Tree-Caused Outages 5. weather conditions at the time that the outage occurred 6. components and design of the electrical system at the site of the outage. In addition, the arborist assessed whether the outage was preventable or nonpreventable. This assessment was based on whether the tree would still have caused the outage after the normal cycle of line clearance trimming was done. For instance, if a tree with adequate clearance and no apparent indicators of structural weakness fails during a sunny winter day with 25 mile per hour winds, it was considered nonpreventable. The categories describing the cause of the outage were broken down into growth or failure. The 3 growth-related indicators are top growth, side growth, or bending limb. Tree failures were matched with a choice of 6 descriptors. The type of contact made between the energized conductor and tree, such as tree to conductor or tree broke conductor, was recorded. Characteristics of the tree classify typical physical features of the tree such as size and species. The condition of the tree that failed has 14 subcategories that describe the apparent structural defect (or lack of) that probably resulted in an outage. Also, the last time the tree was trimmed, if ever, and the technique used was logged. Information about the surrounding environment, orientation of the tree to the electrical conductor, and weather conditions at the time the outage occurred was collected. These factors describe the conditions that likely triggered a treecaused outage event. The data about wind conditions were categorized using the Beaufort Scale and orientation of the tree to the electrical conductor. The weather was classified by 10 general descriptions. The type of construction and type of conductor at the site of an outage were recorded. Six data were collected that depict the electrical engineering portion of the outage event. The factors are 1. protective device, such as fuses 2. construction type, such as armless, vertical, spacer cable, and crossarm 3. conductor type 4. conductor size 5. line voltage 6. number of phases involved. Tree population survey. A survey of the tree population was conducted in the Brockton territory of the Eastern Utilities system. The territory consists of 17 communities and covers approximately 300 square miles. The objective of the survey was to locate trees with the characteristics that present the greatest risk from failure to the 3phase portion of the distribution circuits. The survey was a simple random sample of 40% of the number of pole spans for the 3-phase portion of the primary electrical circuits. Random pole spans were selected on a map of primary circuits and the characteristics of the trees within those pole spans were categorized. The trees were categorized along the following 6 indicators of their potential to cause an outage from failure: 1. no predictor of potential failure 2. visible structural defect 3. overhanging limbs 4. weak species 5. white pines with co-dominant stems 6. no potential low growing species.