Dear Sami,
I lend support to the presumption of Mr.Saikumar that the T.N.S.E Act,1947 is applicable to your establishment provided that the premises carrying on the activity of designing is not a notified factory under Sec. 85 of the Factories Act,1948 by the Govt. Having said that, I would like to elaborate the views of Saikumar little bit more academically.Basically, a contract of employment, I think, is a species of the genus of 'the contract' under the Law of Contract but with the exception for the implicit possibility of creating rights and obligations beyond the terms of the contract.In order to cicumvent this, always novel methods such as contract labour, fixed term employment etc., are employed by certain employers world-wide.That is the reason for the non-emergence of a Comprehensive Labour Code applicable to all systems and ventures of employment.Coming to your specific query, yours is an 'establishment' as defined u/s 2(6) of the TNSE Act,1947. Section 2(12) of the Act defines a "Person employed" to mean as a person wholly or principally employed therein in connection with the business of the establishment.When mach. eng. designing is the business of your estt, the people engaged to do the job in the same place will be the' persons employed' irrespective of their mode of employment.However, their eligibilty for the benefits you've mentioned is dependant on the tenure of their contract with your establishment.