Three people were injured Tuesday evening at Callaway Energy Center by an electrical arc that produced an intense burst of heat. A fourth was injured while trying to help the other three.The incident, which occurred while a piece of equipment in the switch yard was being worked on, happened at 5:06 p.m. The switch yard, which distributes power from the facility to customers, is outside the protected area of the nuclear plant, which is about 25 miles northwest of Jefferson City.A Callaway County emergency management official said a 911 caller reported several burn victims, including two with burns to the face.Company communications director Rita Holmes-Bobo said that three people were taken to hospitals, but she would not identify them or describe their condition. Callaway County emergency management director Michelle Kidwell said a medical helicopter crew was among the emergency responders dispatched to the scene.The cause of the electrical arc, which did not affect power generation, is under investigation by the Occupational Safety and Health Administration.Bill McDonald, the St. Louis area OSHA director, said his office got a call about Tuesday evening's accident on Wednesday. OSHA's official investigation started Thursday morning, and McDonald said it will likely take 30 to 60 days to complete.Ameren Missouri is also investigating the incident.Ameren Missouri said in a statement, "Our first concern is for the well-being of our co-workers. Out of respect for the privacy of the employees and their families, we are not providing details of their status at this time."Beginning the week of April 7, the facility will begin its 19th planned refueling and maintenance outage. These happen every 18 months and last for several weeks. During the outage, energy to about 20 percent of Ameren Missouri's 1.2 million customers will instead come from other Ameren Missouri energy centers.