There were fears of an explosion in Belfast after witnesses described a loud bang accompanied by a flash of light in the sky.
The events were later attributed to an incident at an electricity substation in the south west of the city, where a fault interrupted power to thousands of homes.
Fire service crews were reportedly spotted at the site near Lenadoon shortly after the flash was seen on Friday night.
Witnesses described seeing the night sky light up from up to 15 miles away, with the flash accompanied by interference to radio stations.
NIE Networks said their early assessment indicated the fault was not suspicious.
A spokeswoman said: "Due to an equipment fault, several thousands of homes and businesses would have seen a dip in power for a number of seconds on Friday night.
"Network protection systems operated correctly to ensure that power supplies were not lost.
"Engineers from NIE Networks and Soni - the System Operator Northern Ireland - are investigating the cause."
Twitter lit up with speculation over incident.
Michael Doran wrote: "What was that big flash in Belfast? Hoping not a bomb."
Abi Louise tweeted: "The blinds rattled! Flashes have happened in Belfast before - at least this one has an explanation!!!"
Press Association reporter Catherine Wylie described how the sky was illuminated with "dramatic flashes" and her car radio lost signal.
She said: "We were driving home from Belfast International Airport and were around the Nutts Corner area - about 10 miles outside Belfast - when the sky suddenly lit up and there were dramatic flashes at about 11.15pm.
"We didn't think it was lightning as it didn't look like anything we had seen before in the night sky. It was directly in front of us so it was quite a sight.
"It only lasted a few seconds, and in that time the car radio went fuzzy. When the DJ came back on he remarked on how he had lost power or light momentarily, which really intrigued us.
"People contacted the radio show about how they had seen the flashes in the sky or mentioned their lights dimming, and it seemed to be quite widespread."