The blackouts are “due to the trip on Eskom line in the north that was undergoing planned maintenance earlier today, which affected several of our substations, at least 20 of them with each servicing at least 10 suburbs,” Isaac Mangena, a spokesman for Johannesburg’s City Power, said via text message. “We have a huge outage in most suburbs in Johannesburg, especially the central and northern suburbs. We are currently working closely with Eskom to try and restore.”

Eskom has not implemented rolling blackouts in the city, Andrew Etzinger, Eskom’s acting head of generation, said via a mobile phone text message.

Cash-strapped Eskom, which Goldman Sachs Group Inc. calls the biggest single threat to South Africa’s economy, is surviving on government guarantees after corruption, cost overruns and delays in building new plants left it with ballooning debt. A lack of maintenance on aging plants has led to rolling power cuts that damp economic growth.

“Technicians are on site to determine the cause of the outage,” Khulu Phasiwe, a spokesman for state-owned power utility Eskom Holdings SOC Ltd., said via Twitter. There’s no estimated time for the restoration of electricity at this stage, he said.

Eskom said earlier on Friday that there was a risk of blackouts because the power system “remained tight and vulnerable” and the power could be cut at short notice if there was a shift in plant performance. “This could include a significant loss in generating plant due to unplanned technical breakdowns,” Eskom said in the statement. DM