We recently had both of our two incoming high voltage lines out of for a brief period of time (3hours). During this time we were solely running on generator power (3 50 MW generators and 2 75 MW generators). We were running at 61 Hz for 3 hours. The machinery guys were scratching their heads because they could were monitoring vibration levels on their machines because they hadn't gotten vibration alarms on alot of their motor driven gear boxes. They could not explain why all the motors at the plant were running at higher than normal rpm. The higher frequency caused the motors to speed up. I thought it was just worth mentioning to consider the effects of to system of continously running at a frequency of other than 60 hz.