Transmission lines sag. The magnitude of the sag depends on several factors like the distance between the towers or poles, the weight of the conductor and temperature, both the ambient temperature and the heat of the line, which is the function of the load on the line. Sags should not be a problem, since there are defined formulas to calculate the sags as well as rules on required clearance to ground or vegetation. Nevertheless, losing transmission lines due to short circuits caused by tree-to-(power) line contact can be a major reason why blackouts cascade. If the system is already stressed, losing a line results in overloads on remaining lines. The overloaded lines heat up and sag more, maybe beyond emergency ratings or tree trimming has not been kept up, increasing the risk for tree-to-line contact and more lines will be lost. It happens fast, like a cascade, as was the case in the 1996 Western North America blackout, the 2003 Northeast blackout, the 2003 Italy blackout, and many other blackouts.
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