From the desk of the president:
Line-clearance specifications for electric utilities, in many cases, are not much different from those used 30, 40, even 50 years ago. Too often, specifications are simply set vegetation clearances above, below and to the side of the conductors. While this is easy to understand, communicate to the line-clearance worker and audit, it may not be the most effective or economical method available, and is not considered an industry “best management practice.”
The most common variation on the set-distance clearance standard is to have all pruning cuts made according to the guidelines found in ANSI Standard A300. The next level of modification to a set-distance standard is to use many, if not all, of the recommendations found in Best Management Practices: Utility Pruning of Trees. The suggested modifications found in this International Society of Arboriculture (ISA) publication include considering the species of tree, its regrowth potential, and site or location as related to environmental factors.
The UAA and ISA are working to support these best management practices; however, there are two new and important influences every utility must consider going forward. First is John Goodfellow's research into tree-caused interruptions. Through his research, Goodfellow, a vegetation management and reliability researcher, is helping to explain just how tree limbs cause outages when they come in contact with overhead power lines. In “Investigating Tree-Caused Faults” (T&D World's Vegetation Management Supplement, November 2005), Goodfellow states: “All tree contact with energized conductors can result in a fault. The branch provides a pathway for the flow of current. The contact begins as a high-impedance (resistance), very low current event. The vast majority of tree contacts remain this way. Only under the ‘right’ combination of conditions does the fault pathway become more conductive. In these cases, the fault pathway evolves from high to low resistance, resulting in high levels of fault current and ultimately an interruption.”
The right conditions occur when branches or trees fall across phases or lean against conductors, usually as a result of a weather event. Some of these trees and branches originate outside the typically prescribed clearance zone and are not covered by most current utility-contract specifications. Though not all such hazards are detectable, improvements in reliability may be achieved by changing specifications to include the removal of obviously hazardous trees and branches in the vicinity of utility corridors.
The next major influence that must be considered is the minimum cycle mandated by public utility commissions across the country. Illinois and now New Jersey require all power lines be maintained on a four-year vegetation-maintenance cycle. New Jersey goes even further by mandating an annual inspection of all lines.
In “The Economic Impacts of Deferring Electric Utility Tree Maintenance” (UAA Quarterly, Vol. 5, No. 3), ECI's Mark Browning says being on cycle is “where pruning is scheduled just before the trees grow to the point where they have the potential for contacting the conductors. The optimum cycle minimizes the number of trees that have the potential to contact the conductors, which in turn helps to maintain the safety and reliability of the electric system.”
Most utilities operate with a vegetation-maintenance cycle longer than four years and often have as much as 20% to 30% of their distribution system rights of way that have not been trimmed within the self-imposed trimming cycle. Sometimes utilities can be two or more years beyond their stated cycle goal (or out of cycle). Given this, state utility commission minimum-cycle requirements could have a huge impact on future vegetation-maintenance costs.
Utilities need to reach a reasonable balance between cycle and cost. Should every circuit be maintained at the same level of line clearance? Should areas with low population density, few businesses or critical infrastructures be managed at the same level as those with schools, hospitals, firehouses and police stations? Can specifications be modified to achieve longer cycles, often through mechanization or herbicide applications, in rural areas? The UAA is helping to educate regulators at all levels about the interdependence of all these factors and that there is usually no one “quick fix” for improving reliability and safety.
By answering these questions, utilities can incorporate current research to meet internal reliability goals and state-mandated cycle goals. Only by thinking outside the “specification box” can utilities adapt to government mandates and budget challenges as the industry continues to change.
With highest regards,
Utility Arborist Association
Jim Orr is president of the Utility Arborist Association and general manager of technical services for Asplundh Tree Expert Co.