About 15,000 workers in power stations across the country were said to be participating in the strike.
"The employer obtained an interdict against the strike late last night, but our members decided to go ahead with the stoppage," said Livhuwani Mammburu, the spokesman for the National Union of Mineworkers (NUM).
"They believe their demands are valid," he said.
The union is demanding a 10 per cent increase for the lowest paid workers, 8.5 per cent for the highest earners and a housing allowance worth 3,000 rand (USD 226).
The union accuses the utility of structuring wages according to race, with white employees said to be paid more than their black colleagues.
"The NUM members are very disturbed and angry with the attitude of Eskom in refusing to close the apartheid wage gap," said a statement.
"White employees are still being paid more than black employees."
Eskom said talks were underway to try end the strike, adding that the country's electricity supply had not been disrupted.
The firm said in a statement power station workers were prohibited by law from striking as they provided a critical service.
The union was adamant that its members would continue the illegal strike until their demands were met.
Eskom generates most of its electricity from coal and exports some to neighbouring countries.
The utility was last year hit by shortages that caused nationwide blackouts, as its ageing facilities struggled to meet a rising demand.
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