Here at Western Area Power Administration (WAPA), our line crews are responsible for the operation and maintenance of approximately 17,000 miles of power lines within a 15-state region of the central and western U.S. Within that region are geographic areas where vegetation hazards can pose a threat to the reliability of some of our power lines. To identify these hazards, WAPA utilizes both routine aerial and ground patrols to collect and monitor vegetation data. The criteria we use to establish vegetation minimum clearance distances is based on the OSHA 29 CFR 1910.333 minimum approach distance for nonelectrical workers, rounded up to the nearest foot, plus 5 feet to account for conductor and tree movement due to wind and ice loading, or increased conductor sag as a result of thermal loading. In addition, another 5 feet is added to allow for an average tree growth of 12 inches per year and a retreatment interval of no fewer than five years.

After attending a Monday morning safety meeting, a lineman is assigned the task of driving to a remote county road to measure the conductor height of an energized 115-kV transmission line. A rural farmhouse in the vicinity is scheduled to be moved and subsequently would pass directly underneath the transmission conductors. The lineman’s foreman wants to know if the top of the house will encroach on the minimum approach distance to live parts as it passes underneath the conductors.