"Based on the preliminary investigation, it is expected that Unit 5 will remain out of service for a minimum of three months," said Eskom Group Chief Executive Phakamani Hadebe on Thursday.
This follows Wednesday's incident at the Free State-based power plant's Unit 5.
"The loss of Unit 5 will have a negative effect on the power system given that Lethabo power station is one of the most cost-effective generators with adequate access to a steady coal supply. All efforts will be focused on the safe and swift return of this unit to the national grid," said Hadebe.
The incident resulted in the death of an employee while another sustained serious injuries.
In Wednesday's incident which occurred at around 3:23 pm the live steam pressure from the boiler of Unit 5 ruptured, sending a plume of dust into the air.
Eskom management has since visited the site and a thorough investigation has been initiated as per regulatory requirements.
In addition, a work stoppage has been initiated in which the power station manager, CEO of Eskom Rotek Industries and the Group CEO addressed staff.
Meanwhile, an employee assistance programme has been arranged to assist all employees with counselling.
Counselling has also been arranged for family members of the two directly affected employees.
Hadebe again conveyed the company's condolences to the families of the two employees.
"On behalf of the Eskom Board, executive management and all the Eskom employees, we would like to send our condolences to the family and friends of the deceased, and also wish the colleague who is in the hospital a speedy recovery. Our thoughts and prayers are with the families, friends and immediate colleagues of the two affected employees during this difficult period."
(With inputs from South African Government press release)