At 3:05 p.m. on August 14, 2003, a 345-kilovolt transmission line in northern Ohio shut down after its conductors came into contact with a tree. Tree Contact Removed Key Transmission LinesElectrical transmission lines hang between towers rather than remaining completely straight. High electricity use, hot weather, and limited wind can all raise conductor temperatures and reduce the distance between a power line and nearby vegetation. Credit: Tim Harrison/MSUWhen a high-voltage conductor approaches a tree, electricity can travel through the air or vegetation toward the ground. New Reliability Rules Followed the BlackoutBefore 2003, many North American electricity reliability rules were voluntary.