The Middleton Street-Little Russel Street crossing, near one of the entrances to The 42. Sanat Kr Sinha

Sparks from a welding job on the 53rd floor of The 42 triggered a fire that spread downwards at the Chowringhee highrise under construction on Saturday evening.

Once complete, the 64-storey building — 265m tall — will be the tallest residential tower in the city.

A preliminary investigation by the fire department suggests that a nylon safety net installed outside the air conditioner ducts caught fire. The fire did not enter the building.

A service lift outside the building was used to reach the site of the fire and pour buckets of water on the flames.

The fire was reported at 4.55pm and put out around 5.30pm, officers at Lalbazar said. Three fire tenders were pressed into action.

Because of the height, the fire fighters, too, had to take recourse to the service lift. The highest ladder the state fire and emergency services department has — 68m-high Bronto Skylift — can only reach up to 21 storeys.

“We used the service lift to reach the spot and spray water on the flames. Once the fire was controlled, the safety net was pulled off to prevent it from spreading further,” said Samir Chowdhury, deputy director, fire services department.

Mayor and fire minister Sovan Chatterjee visited the spot. “Air-conditioner ducts were being installed. During the welding, the nylon safety-net, which ideally should have been fire-proof, caught fire. The fire... did not spread inside. They used fire extinguishers and water to douse the flames. I am not an expert to comment on the net. DG (director-general) fire will submit a report,” Chatterjee said.

The area — dotted with consulates, offices and upscale residences — witnessed a traffic congestion as onlookers gathered in large numbers at the gates of The 42. The flames revived the memories of the devastating fires at Stephen Court on Park Street, and Nandram market and Bagree Market in Burrabazar.

A team of forensic department officials will visit the spot to collect samples.

A statement issued by the The 42 hours after the fire was spotted said: “Owing to a small accidental occurrence while welding work was in progress at our site a while ago, the safety net that is cladding the upper floors had caught fire and the same brought under control and extinguished by our efficient team at the site within 9 minutes from its detection....”