The National Union of Mineworkers warned on Tuesday, signalling workers there may embark on a wildcat, or unofficial, stoppage.
Industrial action at Eskom, which provides 95 percent of the power to Africa's biggest economy, may force vital industries such smelters, mines and factories to shut down.
Africa's most advanced economy is struggling after a series of work stoppages. The latest has been a week-long strike by more than 200,000 members of the National Union of Metalworkers of South Africa (NUMSA), which follows a crippling walkout by platinum miners that lasted five months and ended two weeks ago.
The NUM, which represents around 16,000 workers at Eskom, has already rejected a 5.6 percent wage offer and is demanding a 12 percent increase.
"If doctors can strike in South Africa, why can't Eskom workers strike?" NUM General Secretary Frans Baleni told a news conference. "Our members are angry. Eskom is sitting on a time bomb."
Baleni said the workers, who are not allowed to strike as they are deemed to provide an essential service, were angry, adding the union would not stop a wildcat strike.
"Anything is possible at Eskom. If workers decide to react in various forms including an unprotected strike, we cannot be blamed," he said.
Separately, the metalworkers union was "very close" to sealing a wage deal with employers, a labour ministry spokesman said on Tuesday.
The workers walked out last week and are asking for increases of between 12 to 15 percent just two weeks after miners in the platinum industry ended a five-month walkout.
That strike ended when the country's top three platinum producers struck a deal with the militant Association of Mineworkers and Construction Union (AMCU) to increase wages by about 20 percent.
"In terms of the wage adjustments, they are very close," ministry spokesman Mokgadi Pela told Reuters. "The differences are in terms of fractions." He declined to give further details, saying negotiations were at a delicate stage.
NUMSA officials were not immediately available for comment.
Pretoria has intervened to try to break a deadlock that is costing South Africa's metals industry an estimated 300 million rand ($28 million) a day.
South African companies have always had a fractious relationship with workers but tensions ratcheted up with the police shooting of 34 miners in 2012.
The unrest has subdued business confidence, which edged up slightly to 89.7 in June from 88.9 in May, hurting an already fragile economy.
Last month's settlement in the platinum belt ignited a separate wildcat strike by 2,000 NUM-affiliated workers at Impala Platinum's Marula mine on Friday.
But those miners are now back to work and negotiations have started on their demands that Implats match the deal obtained by AMCU in other platinum mines.
"They are all back to work. We are now engaging with them about their concerns," Implats spokeswoman Alice Lourens said.