Dear Parag,
Understanding Casual Employment
One of the meanings of the term "CASUAL" is occasional or irregular. When it is associated with the terms "work" and "worker" as an adjective, it denotes that the work is basically "on and off" or intermittent and thus not regular in nature. This also has a specific connotation for the person engaged in such work as a purely temporary hand whose services are co-terminous with the occasional work. At times, a worker can be engaged in regular or permanent work or perennial work for a shorter duration, and there also he is called a temporary worker. Therefore, the casualness of such employment is determined by both the nature of the work and the duration of the employment of the worker in such work. So the mode of payment of wages and the statutory deductions, if any, made thereof cannot alter the basic character of such employment.
Legal Framework in Tamil Nadu
In the State of Tamil Nadu, there is a special enactment called "The Tamil Nadu Industrial Establishments (Conferment of Permanent Status to Workmen) Act, 1981," which mandates that every temporary workman completing 480 days of continuous service in an industrial establishment covered by the Act within a period of 24 calendar months should be made permanent.
Implications of the Industrial Disputes Act, 1947
Whether there is such a law in your state or not, I would like to point out further that the Industrial Disputes Act, 1947, declares the practice of keeping workmen on a temporary roll for years together to deprive them of the status and privileges of permanent workmen as an unfair labor practice on the part of the employers [item no. 10 - Part I - Schedule V].
Therefore, it is better to make such casual workmen permanent considering the length of their service and the perennial nature of the work in which they are engaged, irrespective of its type being core or incidental.