Johannesburg - The National Union of Mineworkers (Num) said on Thursday that it had given power utility Eskom the option between signing a minimum service level agreement or face industrial action.
It said the minimum service level agreement "would allow a certain number of Eskom workers to go on strike should there be any labour disagreement."
"It would be the first point of war between the Num and Eskom, wage negotiations ... the second," it said.
Officials at Eskom were not immediately available for comment. Job Matsepe, Num's national energy sector coordinator, told Reuters the union had 16 000 members with Eskom.
An Eskom spokesperson said the utility employed around 40 000 people.
Eskom, grappling with tight supplies for its customers, is embarking on steep rate hikes that could increase inflationary pressures in Africa's largest economy.