Cape Town - About 3 000 workers at Eskom subsidiary Rotek in Germiston downed tools over performance bonuses on Wednesday.
"Apparently, management is claiming that they didn't reach their target this year," said one protester, speaking on condition of anonymity.
"All the gates are locked, and police have blocked the road," said the National Union of Mineworkers (NUM) member.
He said it was unfair that Eskom employees got 10% bonuses, but they were being offered only 6%.
"Last year we performed, Eskom didn't perform, but they got the same share."
Workers also downed tools on Monday.
Another protester, who belongs to the National Union of Metalworkers of SA, said: "Since we amalgamated with Eskom, we have a wage difference between us."
Eskom spokesperson Khulu Phasiwe confirmed that unionised Rotek Industries employees had downed tools over the bonuses.
He explained that there were certain targets set for everyone in the past financial year, and Eskom had paid bonuses to the bulk of its employees.
But Rotek Industries did not meet its targets, Phasiwe said.
He said the protest action would have no direct impact on power supply because it is not a power station.
It was, however, an important link in power stability, because one of Rotek's functions is to repair transformers.
"As we speak now it is not having any direct impact. We haven't implemented load shedding, and there is no plan to do so."
In the meantime, meetings were planned between management and union representatives to try and resolve the issue.