A fire that caused relatively minor damage to a Birnamwood business Sunday night likely was caused by an electrical malfunction. Fire crews responded to a sprinkler alarm at Kersten Lumber at about 9:35 p.m. Sunday, Birnamwood Area Fire Chief Randy Berger said.  When crews arrived, they found heavy smoke coming from the building, he said. One wall had burned, he said, but the damage was not bad. No one was hurt in the incident and the sprinkler system kept the fire contained, he said.