Scotland: The devastating fire erupted at the £30,000 substation near the Scottish parliament and the plastic box it was encased in caused thick, acrid smoke.
Thousands of people were evacuated from offices and flats in the area.
A Scottish Power spokesman said: "We are looking into what happened with urgency to find out the cause of the mechanical failure. We are treating this very seriously."
More than 400 homes and businesses lost power during the major incident. The area was cordoned off for about four hours. There were no reports of any injuries.
People reported hearing about 12 loud explosions and others said it sounded like a building was crashing to the ground.
A new substation was installed at the site on Friday.
Police Scotland said: "A joint investigation is under way between police and the Scottish Fire and Rescue Service to establish the cause and full circumstances surrounding the fire."