According to a Reuters report, parent company Southern Company shut down all four units at the 3166-megawatt facility near Atlanta after an explosion.

The blast at unit 2 rocked the site and caused serious damage to the unit 2 generator as well as the unit 1 and 2 control room and the plant’s switchyard.

Bowen units 3 and 4 returned to service on Wednesday, though the company said 1 and 2 remained offline.

The company told the news service that damage sustained by unit 1 was unknown, though internal investigators said its probe indicated the blast stemmed from a combustible mixture of hydrogen and air inside the generator and not from equipment failure.

“The explosion occurred as personnel were taking steps to prepare unit 2 for work during a planned maintenance outage, which included a multi-step process to purge hydrogen from the generator,” Georgia Power told Reuters.

The generator said the incident would not impact the 2.4 million customers in its service region.

Just after the explosion, it tweeted that there were four minor injuries sustained.

The Atlanta Journal-Constitution local newspaper reported there was a minor leg injury that was treated at a local hospital and minor injuries to three other people who were treated at the scene.

Plant Bowen is Georgia Power’s second-largest coal-fueled plant in the state and employs more than 400 people.