While a faulty pump at Koeberg – which reportedly tripped after "marine life" clogged an inlet – has now been repaired, stage four load shedding is set to continue until Friday.

On Wednesday evening, Eskom said the pump was running again, but that it still had to get "regulatory approval" to synchronise the generating unit.

"In the meantime, we expect that load shedding, at various stages, may continue into the weekend," Eskom said.

In a statement issued on Thursday, Eskom said that Koeberg's Unit 1 had been tripped manually by operators "in line with operating procedure". This was a safety precaution to keep temperatures in check, the power utility said.

"The reason for the manual trip was as a result of the increasing temperature on the secondary side of the plant due to degraded heat removal (or cooling) capability because the pump that remained in service was supplying a heat exchanger that was degraded and not able to sufficiently remove heat.

"The circulating cooling water system pump that tripped was due to low level in the suction pit as a result of the drum filter that was clogged by an acute ingress of marine life (jellyfish and fish)."

Usually, Koeberg units can survive a trip of one circulating cooling water system pump. Operators are required to reduce power to below 60% and ensure that the temperatures of various components on the secondary side stabilise, Eskom said. However, in this instance, the temperature did not stabilise as the heat exchanger was still in service and had reduced heat transfer efficiency. It was planned for maintenance this week, the power utility added.

"The excess marine life and debris has been cleaned off the drum filter and it is back in service," the statement said. The pump was not damaged as initially feared, according to Eskom.

Energy expert Chris Yelland of EE Publishers had told Fin24 that there was a trip in the so-called tertiary loop section of Koeberg, after the marine life had clogged an inlet, causing water levels to fall.

His interpretation of events was explained in a series of tweets on Thursday morning:

Koeberg supplies around 1 860 MW to the national grid. Around 930 MW of power was lost with the disconnection of Unit 1. Koeberg Unit 1 was being switched back on early in January after it was out of commission for planned maintenance.

Eskom said on Thursday that the necessary technical assessments and regulatory approvals had been obtained to start up and safely return Unit 1 to the grid, with the expected synchronisation date being 14 March.