Gardiner Harris and Heather Timmons / New York Times News Service

NEW DELHI — The world’s largest blackout ever crippled roughly half of India for a second consecutive day Tuesday, sending officials scrambling for an explanation.

The power failure spread across 22 of the country’s 28 states, an area whose population is nearly 700 million, almost 10 percent of the world’s population. Hundreds of trains stopped across the region and, in Delhi, the subway system stalled, and massive traffic jams collected as traffic lights stopped functioning.

But despite the scale of the power failure, many Indians responded with shrugs. In the first place, India’s grid is still being developed and does not reach into many homes. An estimated 300 million Indians have no routine access to electricity.

Second, localized failures are routine. Diners do not even pause in conversation when the lights blink out in a restaurant.

Third, so many businesses employ backup generators that, for many, life continued without much of a hiccup.

The root cause of the vast power failure was not immediately clear. India has struggled to generate enough power of its own to fuel businesses and light homes, and the country relies on huge imports of coal and oil to power its own plants.

While top government officials blamed several northern states for pulling more power from the national grid than they had been allotted, those states have been power needy for years.

It is also unlikely that power demands suddenly spiked this week since monsoon rains have lowered temperatures in recent weeks across much of northern India. An investigation has been started, with some government officials pointing to a relay problem near the Taj Mahal as the prime culprit.

The government — which controls much of the nation’s electrical grid and generating capacity — responded to the crisis by announcing that it was promoting the power minister, Sushil Kumar Shinde, to the more important post of home minister. Shinde immediately tried to shift attention to the power-hungry northern states.

“I have asked my officers to penalize those states which are drawing more power than their quota,” Shinde said.

Government officials claimed that power was restored by early Tuesday evening to 90 percent of those who lost power during the day. Still, a senior official at the Uttar Pradesh Power Corp. said much of the state, including rail and water lines, were still without power at 5:45 p.m.