Section 9(4)(a) in The Essential Services Maintenance Act, 1981
(a)no employer in relation to an establishment to which the Order applies shall lay-off or continue the lay-off of any workman (other than a badli workman or a casual workman) whose name is borne on the muster-rolls of such establishment unless such lay-off is due to shortage of power or to natural calamity and any laying-off or continuation of laying-off shall, unless such laying-off or continuation of laying-off is due to shortage of power or to natural calamity, be illegal;