Vast stretches of the city plunged into darkness for several minutes on Wednesday evening following a snag, the cause of which is yet to be established.
Power went out in many areas of north and central Calcutta, and some pockets of the south, at 6.01pm. The streetlights, too, did not function in most of the affected areas.
Though key establishments and facilities, such as hospitals, Metro Railway, Raj Bhavan and Writers' Buildings were spared, many offices in and around the central business district functioned with backup power.
The situation started improving at 6.31pm, when CESC started restoring supply in phases. All affected areas got back power by 6.50pm.
The areas that suffered the power cut include Ultadanga, Patipukur, Lake Town, Shyambazar, Bagbazar, Hatibagan, the central business district, Alipore, New Alipore, Jadavpur, and Dhakuria.
"I was under the impression that Calcutta doesn't face power cuts in winter. It seems I was wrong," said retired banker Manju Mukherjee, a US resident vacationing in the city.
CESC, the private utility that supplies power to Calcutta and Howrah, attributed the outage to the simultaneous tripping of three swtichgears at a Kasba sub-station, from where 250MW of power was being imported for the two cities.
The sub-station is run by the West Bengal State Electricity Distribution Company Limited.
"The three 132kV switchgears tripped together and our network was isolated. Because the import went haywire, the city suffered a deficit of around 250MW. The demand at that time was 1,300MW," said a CESC spokesperson.
The cause of the glitch is yet to be ascertained, though the private utility has launched a probe to look into the failure.
"We'll have to see if it was a problem at our end or theirs (the state-run utility's)," the spokesperson added.
Shortfall is rare during winter and, what in Calcutta can loosely be called, spring as the demand during the period is much lower compared with the summer need.
A source at the state-run utility said the switchgears trripped "probably" because CESC was "overdrawing" power. "There was no problem at our end. These things can happen when there is overdrawing," said the source.
Power department officials, however, said CESC was unlikely to have overdrawn power as there was a provision for importing up to 500MW.
"Whatever be the reason, a snag during a low-demand month is unacceptable. We have been asking CESC to undertake extensive upgrade and maintenance of its system," he added.