The states affected are Delhi, Punjab, Haryana, UP, Himachal and Rajasthan.
"Yes there are problems with Northern Grid, we are trying to restore it", said SK Soonee, CEO of Power System Operating Co (PSOC) PSOC manages the Northern Power Grid.
Officials sources said while the exact reasons for the grid failure are being ascertained, over drawl of power by states could have led to the problem.
The restoration of the grid may take a few more hours even as the engineers are trying to restore it since early morning. Power supply in some pockets is being restored and the first priority is for public services like hospitals and transport.
"We are hoping to restore the grid in the next one or two hours. We are giving essential loads for services like Railways, Metro and Hospitals," Chairman and Managing Director of Power Grid Corporation AM Nayak said.
Metro services hit
Train services on the 190-km Metro network connecting length and breath of the national capital were affected due to power failure in the Northern Grid, the main power source for Delhi, Uttar Pradesh, Punjab, Haryana, Rajasthan, Himachal Pradesh and Jammu and Kashmir.
The Northern Grid failure also caused power cuts in large parts of Delhi.
"The train services with normal frequency will be resumed only after the supply is restored as it is a major Northern Grid power failure," the official said.
The Delhi Metro normally operates over 2,700 trips a day, covering about 70,000 km and carrying around 1.8 million passengers on week days.
"From 7 a.m., only 25 percent of trains on all lines are being operated. One out of four trains are on time as of now. We hope to operate about 60 percent of the trains by 9 a.m.," a Metro official said.