Several hundred residents in the east end of Lansdale lost power for about 90 minutes Wednesday night when a tree branch fell on a cable at the 4th Street Substation and shorted it out.   Some residents that live nearby reported seeing a flash just before the lights went dark.   It's been about a year since the last outage this significant.  The fire department was on the scene while utility workers cut power at the substation.  No one was injured getting the situation under control.