CAST Practice Test 13

Trees and Power Lines

¹Trees are one of the most common causes of power outages. Wind can blow branches into wires, storms can topple whole trees across a line, and limbs that simply keep growing can reach a wire. For this reason, keeping trees clear of power lines is one of the largest and most costly maintenance jobs that many utilities have.

²A tree does not have to touch a high-voltage line to be dangerous. Electricity can arc from a line to a branch that is close to the wire but not touching it, especially when the branch is wet. A person touching the tree or a tool in contact with it can become part of the path to the ground. For this reason, only trained line-clearance tree workers should prune near power lines. Homeowners who see branches close to a line should call the utility rather than cut the branches themselves.

³Line-clearance crews use a method called directional pruning. Instead of shearing a tree into an even shape, they remove the branches that grow toward the wires and cut each one back to a branch or trunk that points away from them. Each cut is made just outside the branch collar, the slightly swollen ring where a branch joins a larger limb, so that the tree can seal the wound. Over time, directional pruning trains the tree to grow away from the line.

⁴Crews avoid topping, which means cutting off the top of a tree straight across. Topping may look like a quick fix, but the tree responds by sending up many fast-growing shoots that are weakly attached and quickly grow back toward the wires. The large cut surfaces also give decay an easy way in.

⁵The best solution begins before a tree is planted. Utilities encourage the idea of "the right tree in the right place": choosing small species for spots beneath power lines and planting tall trees well away from them. Utilities also trim along their lines on a regular cycle, often every few years, so that branches never get the chance to reach the wires.