An explanation of Tampa’s recent “boil water” advisory, which was the result of cascading failure (squirrel on power lines, arcing sagging power lines, and a failed switch preventing backup diesel pumps from activating). Remarkably detailed for a newspaper account.
http://www2.tbo.com/news/business/2013/feb/25/several-events-knocked-out-tampa-water-service-ar-643509/
They don’t spell it out in the article, but I think you’re right that this is a cascading failure. In particular, sagging is due to thermal expansion which is proportional to load. The first line going down caused the sagging second to sag further and short, destroying the switch. Brad Baird said “You had three unlikely events occurring in a row”; if he believes three independent events happened in the same day, he’s wrong. The second line had been essentially useless for months; the switch was never designed for the input line being shorted; the only new thing was the squirrel.