Ice On Power Lines: A Weighty Subject

The added weight of ice on poles can quickly lead to broken power poles and other pole equipment. Ice can also make falling tree branches 30 times heavier and much more likely to break power lines.

For example, on a 300-foot span of 1-inch-thick power lines:
1/2 inch of ice adds 281 pounds of weight
1 inch of ice adds 749 pounds of weight
2 inches of ice adds 2,248 pounds of weight

When ice melts
In addition, melting ice can cause power outages. If ice on the bottom (neutral) line melts before the lines above, it can cause the lines to touch, leading to an outage.

Other icy facts
Damage can begin when ice exceeds 1/4 of an inch.
One-half inch of ice can cause a line to sag up to 12 inches.
Pressure can also be caused by a broken tree limb.
Both ice and melting ice can cause power outages.
Ice can form around power lines in a teardrop shape. This shape acts as a wing, causing the line to gain lift and rise with high winds. The ice can also break off, causing unsafe conditions on the ground.

_{*source: www.safeelectricity.org}