The radial clearances shown below are recommended minimum clearances that should be established, at time of trimming, between the vegetation and the energized conductors and associated live parts where practicable. Reasonable vegetation management practices may make it advantageous for the purposes of public safety or service reliability to obtain greater clearances than those listed below to ensure compliance until the next scheduled maintenance. Each utility may determine and apply additional appropriate clearances beyond clearances listed below, which take into consideration various factors, including: line operating voltage, length of span, line sag, planned maintenance cycles, location of vegetation within the span, species type, experience with particular species, vegetation growth rate and characteristics, vegetation management standards and best practices, local climate, elevation, fire risk, and vegetation trimming requirements that are applicable to State Responsibility Area lands pursuant to Public Resource Code Sections 4102 and 4293.
|10-foot tree clearances, from DEIR|
This 10-foot clearance drives the number of trees impacted by the project: 2,200 are due for removal and another 3,600 for pruning according to the DEIR.
The standard configuration of the overhead contact system places poles on the outside of the tracks, near vegetation. The clearances look like this:
To keep high-voltage components away from vegetation, it is possible to locate the poles between the tracks in the middle of the right-of-way. This requires slightly more space between the tracks; the regulatory minimum is approximately 18 feet (2x 8'3" minimum from track center line to pole face, plus the width of the pole itself, plus some error margin). The diagram below shows 19-foot spacing, with taller poles to carry both feeders with adequate clearance from each other:
Portal gantries are basically the same as side poles, with a cross-bar across the top. Like center poles, the portal arrangement allows the 50 kV feeders to be located in the middle away from vegetation. Unlike center poles, portals do not require the tracks to be further apart than 15 feet:
Most of the 2,200 trees threatened by electrification probably aren't worth saving. They are typically not "heritage" trees, and consist mostly of unremarkable species that have grown haphazardly into the right-of-way. But there are surely certain trees worth saving, and for those, the support arrangement of the overhead contact system can be engineered to keep high-voltage feeders away from vegetation, over the tracks.