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Determining the interval between scheduled line clearance vegetation activities is one of the fundamental challenges utility vegetation managers face. In theory, the optimal cycle period is the point at which the cost of prevention equals the cost of deferral. In other words, it is the point of balance between proactive preventive maintenance and reactive corrective maintenance. This relationship between the cost of preventive and corrective maintenance often is depicted as a classic bathtub curve, where the optimal interval is the point of lowest combined cost. If only it was that simple.
The problem with this approach is determining costs. The cost of preventive maintenance would be relatively straightforward if not for the uncertainty as to what is being maintained. One simple approach is to define it as clearance between trees and energized conductors. The assumption is trees grow, so clearance loss is a function of time. That may be so, but regrowth rates vary over time post-pruning. Most interruptions are a result of tree and branch failures rather than incidental tree-conductor contacts, which further complicates the analyses. And, what assumption should be made as to the efficacy of the preventive maintenance action? Interruptions still occur on circuits recently maintained.
Determining the cost consequences of tree-caused incidents is even more problematic. The cost of unscheduled vegetation maintenance work, or hot spotting, can be estimated; the demand for this work is assumed to increase over time, but it, too, varies over time. What about other costs such as the repair of tree-related damages, loss of energy sales and the cost of outages to customers?
These are some of the questions being addressed in a project funded by the newly established Utility Arborist Research Fund. The Utility Arborist Association (UAA) has identified the need to develop a means of assessing preventive maintenance cycle periods as a top research priority.
Maintenance and Risk Management
Utility vegetation management (UVM) programs exist and maintenance practices are performed in an effort to reduce risk to a system. As risk can never be removed completely, it is important to understand the relationship between risk reduction, maintenance efficacy and economic outlay. An important factor of the maintenance optimization study, as defined by UAA, is the relationship between the relative costs of preventive versus corrective...