A brief explosion created by an arc flash from a 600V electrical panel that injured three power plant workers has been ruled accidental. The Massachusetts state fire marshal's office conducted the investigation and made the conclusion.

The accident happened at Ware River Power in Ware, Mass., on April 17 of this year. The three employees were alone in the plant when the panel arced. Two workers were seriously injured, and a third worker, although hospitalized, returned to the scene to assist investigators.

The Republican, a paper in Massachusetts, obtained the investigation report. It shows that the Federal Occupational Safety and Health Administration, which initially was expected to conduct an investigation, did not do so because of the low minimum number of employees.

The report says investigators "determined that the incident was not suspicious and was likely the result of some type of human or mechanical accident."

The report also shows that an investigation by the insurance company handling the worker's compensation claims has not been completed. The report says the insurance company "believed that there may be a mechanical issue with the fuse disconnect switch" involving the electrical panel.

The report says the worker who was the most seriously burned from the heat of the arc flash pulled a lever on the 600V box to disconnect power while the other two injured workers were behind him.