The Wikipedia article on thyristors as well as the one on valve halls mentions that thyristor stacks in HVDC applications can be hung from the ceiling or supported by insulators mounted on the floor. What is the reason for hanging them from the ceiling? Is it simply as the article states: to better survive earthquakes? Is there some electrical benefit given the high voltages present in the stacks? Why are mercury arc rectifiers generally mounted on the floor?
I believe it is done for safety reasons. Since high voltage is involved, in the event the stack hits the ground, there would be an arc to ground, causing sparks, heat and possible explosions. By "hanging" them from the roof, the architect is forced to think about the extra weight on the roof and design it to support the extra weight, even in an earthquake situation.