A "major fault" caused a power cut in Derry that, at once stage, affected 10,000 customers.
Traffic lights at Irish street, Dales Corner and at the Waterside end of the Craigavon bridge in the city were all out.
The Foyle Arena, the Waterside Health Centre and some city centre businesses were closed for a short time.
A spokesperson for Northern Ireland Electricity said they were working to restore power.
A team of engineers have managed to restore power to the majority of those affected.