Firefighters were called to a small electrical fire at a library in Aldenmanbury, City of London.
A high voltage transformer in an electrical substation had overheated and burnt out. Twenty people left the building before the Brigade arrived.
The Brigade was called at 0637 and the incident was over for firefighters by 0804.
Crews from Dowgate, Whitechapel, Dockhead and Islington Fire Stations were at the scene.