Bihar State Electricity Board (BSEB) sources said distribution of electricity was disrupted in the state from 1:02 PM.
All the functional units of NTPC at Kahalgaon stopped generation from 1 PM because of technical problems in the grids, official sources said.
BSEB Chairman P K Rai and other senior officials were conducting meetings to ensure early restoration of electricity. "We are in constant touch with Eastern Region Load Despatch centre," they said.
Patna went totally without power and work was paralysed at the secretariat for several hours due to the grid failure. Officials worked using lamps and emergency set ups.
Several trains, including New Delhi-Patna Magadh Express, Rajgir-New Delhi Shramajeevi Express and passenger trains were stranded at various places.
East Central Railway (ECR) Chief Public Relations Officer Amitabh Prabhakar told PTI that more than 30 trains, including 18 mail and express and over 48 goods trains were stranded at various places since 1 PM.
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"We are making alternative arrangements by mobilising diesel engines so that trains reach places where passengers can buy food items," Prabhakar said.
ECR officials are in constant touch with BSEB for restoration of power, he added.