ESI Deduction - do we deduct ESI on his Gross Salary or on Gross + OT Salary?

Hi, Can u plz let me know, if an employee is working in a factory and his Gross salary is 15,000/- but he is also doing OT (Over Time) and with OT he is getting 30,000/-. So do we deduct ESI on his Gross Salary or on Gross + OT Salary.

OT is optional, and though ESI deduction is payable on the OT also, the employee is eligible to ESI deduction,
when his gross salary without OT or retention allowance exceeds 21000, he will opt out

Hi,
Thanks for the answer. I have some more questions and want the clarification. Plz help me.
You say if the basic salary or with OT salary cross 21,000/- employee needs to be taken out of the ESI.
If we take out the employees who exceeds 21ooo/- payout in any concern from the ESI benefit and some mis happening happens in factory and the employees who are out of the ESI benefit, who would compensate or pay the medical expenses or the partial/total loss to the employee or their family members?

Hi,
Plz help me in this.
If we take out the employees who exceeds 21ooo/- payout in any concern from the ESI benefit and some mis happening happens in factory and the employees who are out of the ESI benefit, who would compensate or pay the medical expenses or the partial/total loss to the employee or their family members?

Any one plz help me in this regard, its really very urgent.