T&D World Magazine

Risk Assessment Criteria for Branch Failures

An investigation of branch failures having the potential to cause interruptions to electric service was completed in four phases. These included a review of the literature, interpretation of photos of tree-caused outages, a survey of the industry¡¯s experience, and destructive testing of branches of six species of tree. The small- and medium-sized branches included in this research ranged between two and eight centimeters (¡Ö1-4 in.) in diameter. Qualitative and quantitative observations were analyzed and used in the development of recommendations intended to aid in the assessment and mitigation of the risk of branch failures to an overhead electric distribution system.

This research identified a critical zone of failure within twenty percent of the branch length to the union with the main stem. It also demonstrated the resiliency of small diameter branches in resisting failure under static loads. The role of irregularities in the branch form in concentrating load-generated forces of stress was identified and characterized. The study also found that a relatively small reduction in branch length resulted in a substantial reduction in load-induced stress in branches, and may be an effective means of mitigating the risk posed by branches adjacent to and, particularly, overhanging distribution conductors.

Opportunities to apply various risk assessment criteria were identified. Risk mitigation practices including hazard inspection and branch reduction are included as recommendations....

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