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