Reliability-Centered Maintenance

Traditional Vegetation Management Programs

It has long been recognized that trees pose a significant threat to the reliable operation of overhead electric distribution lines. It is estimated that the industry spends in excess of 4 billion dollars annually maintaining vegetation growing in close association with conductors. Contemporary vegetation management or “tree trimming” programs emphasize the completion of preventive maintenance on a scheduled cycle in an effort to mitigate this threat.

The focus of preventive maintenance work is to create and maintain clearance between conductors and trees. This is accomplished by establishing and applying uniform clearance specifications. Vegetation maintenance is typically conducted as a discrete program, with an emphasis on achieving efficiency in completing line clearance work. 

Application of RCM to Distribution System Maintenance and Vegetation Management

Recent work in applying Reliability Centered Maintenance (RCM) to a traditional distribution vegetation management program has confirmed that there is a significant opportunity for improvement in reliability and cost efficiency. Development of a RCM-based approach to overhead distribution maintenance has led to the realization that while it has been useful to manage traditional preventive maintenance efforts as discrete programs for the efficiency’s sake, they need to be coordinated so that their composite effect is to optimize the performance of the system.  

RCM focuses the allocation of available maintenance resources on the preservation of system function based on risk. The analysis process starts by identifying the important systems and the function to be preserved, which is reliable electric service. The process then moves to the identification of the important modes and causes of failure. With a clear understanding of the way interruptions occur, RCM uses a logical decision hierarchy to select preventive maintenance tasks that will be most effective in mitigating the identified risks to system function.

Moving from a traditional “corridor clearing” approach to VM to a reliability-centered approach requires a significant paradigm shift for not only UVM managers, but also engineers and other internal and external stakeholders (tree contractors). A willingness to make this shift can result in significant savings as well as electric service reliability improvements. 

Key components of this paradigm shift include:

• Considering changes to conductor orientation and alignment 
• In some high risk conditions, considering conversion from overhead to underground 
• Embracing a new maintenance paradigm - prescriptive reliability 
• Selecting trees to be worked based on risk, not trimming more trees 
• Implementing out-of-the box vegetation maintenance alternatives 
• Considering the changes in overcurrent protection 
• Identifying and assessing opportunities for changes to infrastructure 
• Understanding the mode & cause of tree-related system failures

Taking a risk management approach to distribution vegetation management can yield tremendous dividends in terms of improved electric service reliability and reduced cost. It must also be balanced with worker and public safety. Too often, traditional vegetation maintenance focuses on just achieving clearance, and not on the ultimate goal, which should be reliability.

Prescriptive reliability represents an opportunity to refocus maintenance resources on what counts; improved reliability. This philosophy relies on condition assessment and the development of a specific maintenance prescription. A much wider range of maintenance alternatives are available than are typically found in the traditional program. The resulting integrated maintenance solution provides for a more effective allocation of resources and improvement in reliability.