An accidental, hour-long fire in a 132kV receiving station of CESC on Wednesday morning caused power outage in phases in several pockets of central Calcutta, affecting homes, hospitals, traffic signals and Metro.

The fire, dubbed "small" by the utility, broke out around 9.30am in an abandoned switch room in the Prinsep Street facility off Ganesh Chandra Avenue, near Wellington Square.

It took 11 fire tenders nearly an hour to extinguish the flames. A preliminary probe suggests the fire was caused by a mechanical or human error.

"We enforced a cautionary shutdown between 10.10am and 11.10am to prevent any possible damage to the supply network. The station controls supply to most parts of central Calcutta," a CESC official said.

Calcutta National Medical College and Hospital suffered a power outage between 10.05am and 10.55am. The hospital's medical superintendent and vice-principal, Pitbaran Chakraborty, said an open-heart surgery had to be rescheduled to minimise risk.

The supply to parts of the Calcutta Medical College and Hospital was disrupted for 15-30 minutes.

Many consumers - between Girish Park in the north and Esplanade in the south, BBD Bag in the west and Moulali in the east - went without power in phases for 10-30 minutes.

Traffic lights were on the blink on Central Avenue, between Esplanade and Girish Park, and on Red Road, Queensway, Kidderpore Road and Kingsway.

"Traffic was extremely slow on Central Avenue around 11am. It took me nearly 20 minutes to cross MG Road while heading north," chartered accountant Saurabh Khetan, whose office is in Esplanade, said.

Trams were stranded in the middle of the road at many places. "Many trams came to a halt on Lalbazar Street. We towed them to Rabindra Sarani because Lalbazar Street is a busy thoroughfare," a police officer said.

Metro services were not disrupted though all but the emergency lights went out on the platforms at Chandni Chowk, Central, MG Road and Girish Park stations, causing panic among commuters.

"My cousin from El Paso (Texas) was waiting for a train at Central station. When the lights went out, he thought there was an earthquake and immediately tried to run out of the station. He went back and took the next train after some cops allayed his fears," Dargah Road resident Dipyaman Dutt said.

Officials said that apart from lights, a few radio-frequency identifying gates for entry to the platforms ceased to function for a few minutes.

"The power outage in Metro lasted 12 minutes, from 10.02am to 10.14am," a senior Metro official said.

At Lalbazar, the main control room and some other parts of the Calcutta police headquarters faced a blackout for half an hour. Some police stations in the central division - such as Jorasanko, Amherst Street, Burrabazar and New Market - as well as traffic guards faced power-cut, too.