The owner of electrical equipment that exploded Friday in Downtown Madison had identified a potential problem last week before the incident that left thousands without power on the hottest day of the year.

In a written statement, American Transmission Co. said it is still investigating the cause of the blast and assessing damage to its equipment at a Madison Gas & Electric substation, where environmental cleanup continued Monday.

During routine inspections of the substation last week, ATC said its workers identified a component “that we felt required additional review,” which was scheduled for this week. ATC said that the unidentified component is part of its investigation, but “we do not want to draw any conclusions until we review all possibilities.”

The company believes the cause was a mechanical failure not related to temperatures, which were in the 80s when the explosion occurred Friday morning.

ATC, which maintains high-voltage lines for moving electricity over long distances, said it expects to complete its investigation by the end of the month and that there is enough capacity in the transmission system to serve the city’s needs while the damaged equipment is repaired.

Just before 7:40 a.m. Friday, Dane County dispatchers began receiving calls of an explosion at MGE’s Blount Street substation at 722 E. Main St. Another fire was reported about 14 minutes later in ATC equipment at a substation about 1.4 miles away near the UW-Madison campus.

ATC said it is likely the first fire triggered the second.

MGE spokesman Steve Schultz said the utility’s distribution system was working at the time of the explosion and fire, but the utility shut down the substation to make it safe for firefighters to battle the blaze.

Cleanup efforts were ongoing Monday as environmental contractors worked to remove hazardous materials from the Blount Street substation and city storm sewers.

The primary concerns are fluorinated compounds known as PFAS contained in some firefighting foam and possible PCBs in the approximately 18,000 gallons of oil contained in the transformer at the Main Street substation, said Christine Haag, director of spill response efforts for the Department of Natural Resources.

ATC spokeswoman Anne Spaltholz said the transformer fluid contained no PCBs.

Haag said it’s not yet known how much foam was used or how much of that oil was released.

The storm sewers near the substation empty into Lake Monona and the Yahara River downstream of the East Washington Avenue bridge, said Hannah Mohelnitzky, public information officer for the city’s Engineering Division.

The city has not said when the storm sewers were closed and whether any water from the site escaped. In its statement ATC said the impact was “largely contained” within the Main Street substation.

“Over the weekend, the remaining insulating fluid from the damaged transformer was removed, and some fluid was recovered from outside the substation,” the statement read. “The second fire at the East Campus Substation resulted in minor damage.”

The fires left about 13,000 homes and businesses without power on the hottest day of the year, with the heat index reaching 108 degrees. Street signals were down and city, county and state government offices closed as a blackout covered most of the Isthmus.

Schultz said most service was restored by 1:25 p.m., but isolated problems persisted until Friday evening.

No injuries were reported as a result of either fire. Both were contained by about 9 a.m., according to the Madison Fire Department.

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