A manhole cover was blown off as two people walked by on the pavement in Norwich at about 13:30 GMT.
They were treated for shock and asthma attacks by paramedics after the blast which happened in Dereham Road at a link junction between two sub-stations.
The road has now partially reopened at the junction with Old Palace Road and work is under way to restore power.
UK Power Networks said it had to "temporarily" turn off electricity to 65 properties, including Tesco Express.
Police and two fire crews were originally called to reports of a suspected gas leak.
Rob Burling, watch manager at Norfolk Fire and Rescue Service's North Earlham station, said: "Something failed which caused a power outage.
"There was then a power surge, something exploded and blew three covers from a manhole.
"We cordoned off the area and waited for UK Power Networks and they are now digging out the link junction."
Mr Burling said two pedestrians were treated at the scene for panic and asthma attacks after the exploding manhole cover caused them a "shock".
A spokesman for UK Power Networks said it wanted to "reassure customers affected that we will turn their power back on as soon as it is safe to do so".
A main road into a city was sealed off after an electrical fault triggered an underground explosion.