The explosion took place in Chancel Street - close to the Edward Edwards House sheltered housing complex and Transport for London's taxi and private hire test centre - at about 6pm on Tuesday.
Engineers from UK Power Networks were already working in the adjacent electrical substation investigating a fault that had been reported earlier that afternoon
No-one was injured in the explosion but one passer-by was hit by flying debris.
"A fault occurred on an underground electricity cable affecting the local area," said UK Power Networks.
Businesses affected by the power cut included The White Hart pub in Great Suffolk Street.