Investigators said that a transformer explosion and fire in an electric substation that serves downtown Madison, Wisconsin, originated in the equipment's voltage regulating component and was due to a mechanical failure.

The July 19 accident involved an American Transmission Co. transformer that exploded and caught fire at Madison Gas & Electric’s Blount Street Substation. A short time later, a second fire occurred at the East Campus Substation. The substations are electrically connected by an underground transmission line.

Both fires were extinguished about 90 minutes after they started. Roughly 13,000 customers were without power after the accident.

The explosion and fire knocked out power to around 13,000 customers. Source: KHOW TVIn reporting on the accident cause in early October, ATC said that an expert from the Electric Power Research Institute joined in inspecting the damaged 138 kilovolt transformer. Investigators disassembled and inspected the components to understand the accident's cause.

Prior to the accident, ATC said it had been monitoring a component on the Blount transformer because of what it called "observed anomalies." That company said that it had spoken with the component manufacturer, conducted additional inspections in mid-July and made plans to take the transformer out of service for a detailed internal inspection on July 22.

The transformer suffered considerable damage in the fire, but the inspection and teardown showed that the failure originated in the voltage regulating component known as a "load tap changer" due to mechanical failure. The accident sequence generated combustible gases and resulted in an explosion and fire.

ATC said it is conducting internal reviews and consulting industry peers to identify if any changes in work practices are necessary.