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Principal stand liable for all workers, either direct or through contractor,
for payment towards casual ancillary jobs, viz electrician, plumber etc departments usually ignores the payments made, as it is very low in comparison to regular employees,
but as you mentioned in reply, "Modification of materials" by TP, it amounts to manufacture under all respective laws, and considered principal activity to business, if (PE) principal denies it, then GST will catch the ball and fix liability of GST applicable on manufactured goods to the TP. which may cause more complex issues
Get in touch with local legal assistant/ CA, they can analyze the situation and can guide better