It is not as straight forward as that in determining manual labour. It depends more for your knowledge rather than your physical strength. Lots of occupations involve the use of hands. Surgeons use their hands and butchers also use their hands, but you can't classify surgeons as manual labourers. The argument is actually important here. It is about OT. It is really not sound for your employer to say you are not entitled to overtime because your salary is over 1500. Why should one work overtime when there is no overtime pay?
Sometimes, it is difficult to control overtime work especially in your type of maintenance work. I don't of nature of your work but the following suggestions may help to overcome it:
1. Pay overtime based on the ceiling of 1500 although the actual salary is higher. As such employers may not feel the pinch and you may feel better. This has to be mutually agreed upon.
2. Pay fixed overtime allowance if the overtime work is difficult to forecast or control.
Try to work something out with your employer.
If you read the First Schedule of the Employment Act, those who come under manual labour are mentioned there.