You can make the workers work for 8 hours and 30 minutes with 30 minutes for lunch. Beyond that normal working hours, you have to give overtime and if the total hours is 12 hours including one break of 30 minutes, then the overtime will be 3.5 hours. But if that overtime hours exceeds 4 hours after the normal working hours then there should be one more interval of 30 minutes and that interval should be paid at double the rate. In a 24 hours run factory, the working hours of 8 hours will be inclusive of intervals and in such situation each hour extended beyond 8 hours will be overtime only.
Allowing or compelling an employee to do 15 hours work is illegal even if you pay overtime wages at double the rate. Similarly, there is a total overtime hours fixed under the Act.