Zambia has started rationing electricity supply to mining firms following reduced power generation after a big drop in water levels in lake Kariba, the chairman of state-owned power utility Zesco said on Tuesday. Water levels in the lake were down at 1.66% of usable storage on Monday for the Kariba North Bank Power Station in Zambia and the Kariba South Bank Power Station on the Zimbabwean side of the lake, said the Zambezi River Authority, which manages the dam.