Load shedding will move to Stage 4 from 2pm this afternoon after Koeberg Unit 1 tripped, Eskom announced on Tuesday.
"The unit has been disconnected from the grid due to a fault on the turbine side, however the nuclear reactor remains safe," Eskom said in a statement.
"The teams are investigating the root causes of the fault, and will advise of the remedy as soon it is established."
Eskom spokesperson Sikonathi Mantshantsha told Fin24 on Tuesday that there was no safety risk "whatsoever" related to the fault.
"This has got absolutely nothing to do with the reactor," he said. "It is the sea water pump that cools the reactors. We use sea water, not fresh water. [The fault] is not in the reactor, so there is absolutely no risk or potential risk of any nuclear safety concerns."
It was not immediately clear how long it would take to return the unit to capacity or whether there was a further risk of escalated load shedding.
Mantshantsha said an update would be provided as soon as the information was available.
"We will let you know as soon as we know," he said.
Koeberg supplies around 1 860 MW. Around 930 MW of power were lost with the disconnection of Unit 1.
On the risk of escalated load shedding, Mantshantsha said: "In every statement we have said things can change at short notice for the best or the worst. That has not changed."
Koeberg's Unit 1 was for scheduled maintenance recently, with Eskom spokesperson Dikatso Mothae confirming in January that it had come back online after an extended period.
However, it is not clear whether the part in question was undergoing maintenance at the time.
* This is a developing story