Power utility Eskom has confirmed that an explosion at unit 4 of Medupi power station on Sunday night caused "extensive damage" to the generator.
"The incident occurred during the activity to displace hydrogen with carbon dioxide and air respectively, for the purposes of finding an external leak," said Eskom spokesperson Sikonathi Mantshantsha. "Following the power station preliminary investigation, it appears that while performing this activity air was introduced into the generator at a point where hydrogen was still present in the generator at sufficient quantities to create an explosive mixture, which ignited and resulted in the explosion."
"It also appears that there was a deviation from the procedure for carrying out this activity."
Eskom will place those employees who were responsible to manage and execute this work under precautionary suspension pending the conclusion of an investigation into the explosion.
While no injuries have been reported, emergency services attended to seven employees for shock. All employees and contractors have been accounted for, said Eskom.
Eskom said the explosion took place at about 22:50 on Sunday.
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"Investigations are underway into the cause of the incident and Eskom will update the public on developments, as well as to what extent this unfortunate incident will impact the national electricity grid."
The incident is thought to have caused unit 5 to trip. The coal-fired power station's other four generating units are in operation.
Eskom said that unit 4, where the blast occurred, had been on a "short-term outage" since 6 August when the blast occurred.
The utility said it was in the process of returning unit 5 to service.
A week ago, Eskom announced that Medupi, the fourth-largest coal-fired power station in the world, had achieved commercial operation.
It took 14 years to complete the power station, and cost R122 billion.
Eskom estimates it could reach R135 billion on completion of the balance of the plant over the next 24 months. The plant still requires large changes to be made because of initial design defects which are linked to the contract with Japanese construction company Hitachi. Eskom will address its boiler design defects for the next two years. Thereafter it is expected to deliver power to the national grid at full capacity.
Medupi was initially meant to cost R80 billion.