Why do power lines sag?
Short answer
Overhead lines are pulled downward by their own weight, so a span between two supports naturally forms a curve. Pulling a line perfectly straight would put very large forces on the wire and its supports, so the designed sag is part of the safety plan.
How it works
The ends of a power line are supported, but the weight of the wire acts along the span between them. The line therefore hangs lower in the middle. The amount of sag depends on the wire’s weight, the distance between supports, and the tension in the wire.
Electric current can heat a conductor. Metal expands when it gets warmer, so hot weather, strong sunlight, or a high electrical load can make a line longer and increase its sag. Wind and ice can also change the load.
Key point
Sag is not necessarily a sign of a fault. Lines are installed with enough clearance for expected temperature and load changes, so that people, buildings, and vegetation remain safely separated from the conductors.