About 1,000 homes and businesses were left without electricity in Londonderry on Wednesday evening after a fault with an underground cable.
Rush hour motorists had to navigate treacherous roads in the dark without traffic lights.
NIE said power was restored to 800 homes in the Laurence Hill/Northland Road area within 20 minutes.
Sinn Fein councillor Maeve McLaughlin said it was frightening for older residents, but praised NIE's response.
"They did respond fairly quickly to the initial 800 homes, and I know that where I referred them to addresses, they rang the householder personally to reassure them."
The mayor of Derry, Colum Eastwood, said questions had to be asked.
"Firstly, could this have been avoided?
"Secondly, is there a connection between this incident and today's planned essential works, which is obviously a concern.
"Thirdly, can we be assured that this will not happen again during these planned essential works today?
"I do take the point that NIE did respond quite quickly, but it is very difficult for people to accept this is happening at all."