Several pockets of the city and Howrah had to go without power for about an hour on Thursday evening because of a snag.
Parts of Tollygunge, Jadavpur, Gariahat and Ballygunge in the south, Bagbazar, Maniktala and Belgachhia in the north and pockets of Howrah were affected by the outage that lasted from 5.55pm to 7.05pm.
At 5.55pm, a busbar a high-voltage conductor in a switch gear at the West Bengal Power Development Corporation Limiteds (WBPDCL) Kolaghat generating station tripped, snapping supply to the Howrah substation of the West Bengal State Electricity Distribution Company Limited (WBSEDCL).
CESCs southern receiving station, connected to the WBSEDCL through the Howrah substation, isolated itself to save CESC-served areas in Calcutta and Howrah from a cascading impact that could have caused a blackout for several hours, said sources.
The attempt to isolate also led to the shutdown of the two 67.5MW generating units that CESC has at the southern station.
As a result, at 5.55pm when the demand was around 1,500MW CESC was able to import 175MW less than what it needed from the state-run utility and was without its own generation of 135MW from the southern plant.
The isolation saved CESC-served areas of several hours of power outage. Had all the CESC generating units tripped in a cascading impact, there would have been a deficit of 1,500MW and the system could have taken all night to be restored, said a state power department official.
For around 20 minutes, a shortfall of around 300MW caused an outage in many pockets of north, central and south Calcutta, besides parts of Howrah.
By 6.30pm, the deficit was cut down to 160MW and supply was normalised in half an hour.
There was a power outage in our neighbourhood, with even the streetlights going out, for around 15 minutes from 6pm, said Anurag Basak, a 28-year-old homemaker from New Alipore. But it didnt really hurt because the weather was very pleasant.
We worked on a war footing to restore essential services in a few minutes. The situation was fully normal by 7.05pm, said a CESC spokesperson.
The state-run utilities first fixed the problem at Kolaghat, then reconnected with Howrah and finally re-established synchronisation with CESC through the southern station.