Grass fires burned about two acres each at two neighboring ranches in Nicasio late Monday night, sparing humans and animals from injury. Firefighters were dispatched at 10:57 p.m. and contained both fires before midnight. The flames started when fallen tree limbs hit power lines down-range of the ranches, according to Mark Brown, Marin County Fire Department deputy chief. The resulting electrical surge caused fuses to trip and sparks to fly, though Mr. Brown notes that such sparks are not common. Mr. Brown said that Monday was not a windy night. “Sometimes, it’s just time for a limb to fall.”