If your gross salary is equivalent or less than 21000 rupees you are bound to be covered under the ESI scheme.
'Overtime allowances will be considered as wage for the purpose of charging the contribution only and will not be considered for the purpose of the coverage of the employee under the Scheme.
The same view was held by the Supreme Court in its judgment delivered on 6.11.96 in the case of Indian Drugs & Pharmaceuticals Ltd. Vs. ESIC, in Civil Appeal No.2777 of 1980.'
So even if you are paid the salary/wages more than 21k, you are to be covered and there is no exception in this case. Your contribution will be on the wages plus the overtime allowances.
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