New Delhi, Jan 5 :     The Central Electricity Regulatory Commission (CERC) today announced a scheme of availability-based tariff (ABT) for central power generating units in a bid to foster grid discipline and prevent frequent power fluctuations. This marks a shift from power tariffs being determined on the basis of plant load factor to plant availability. ABT, which will be applicable for users of the National Thermal Power Corporation (NTPC), the National Hydel Power Corporation (NHPC) and Neyveli Lignite Corporation (NLC), has two parts comprising a fixed charge and a separate energy fare. For Nuclear Power Corporation, the CERC, after consulting with experts, will first assess whether that unit comes under its jurisdiction and will suitably announce ABT for that purpose, commission chairman S.L. Rao told reporters here. The CERC's order will come into effect in the south on April 1, while it will be applicable for the east from June 1. The northern and western regions will come under its purview from August 1 and October 1 respectively. Under the order, annual fixed charge includes interest on loan, depreciation, operational and maintenance expenses, return on equity and interest on working capital. For NTPC stations, the basis for recovery will be between zero and 80 per cent availability in the first year and zero and 85 per cent availability in the next year.