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Residents of Coulee Dam reported hearing, and feeling, a loud explosion coming from Grand Coulee Dam Wednesday afternoon. It was basically man-made thunder.

Workers getting ready to test a generator under repair in the left powerhouse opened a gate to the penstock that flows water from behind the dam to the turbine that powers G-8, the generator under repair, explained Public Affairs Officer Lynne Brougher.

Excess water in that process overflows through a bypass valve and out onto the face of the dam through a large tube that opens onto the concrete face of the dam behind and above the powerhouse.

Coulee Dam residents reported hearing a loud explosion from as far away as the post office, where windows rattled.

That shock wave came as a 230 kilovolt line went to ground through water spilling from the too-close overflow.

No one was hurt in the incident, although a YouTube video taken by a contractor working on the transformer deck below recorded the arcs and workers’ audible reactions. The video has since been taken down.

A statement from the Bureau of Reclamation’s Boise office explains the incident:

“As the penstock filled, the air released through the vent valve on the face of the dam also forced water out of the vent. The water stream was close enough to an energized power line to cause a path for electricity to flow from the line to the source of the water at the vent outlet. Some minor surface damage occurred to the concrete on the face of the dam at the fault location.”

There was no damage to power lines, the generator or transformers.

Reclamation is performing technical analysis on the cause of the fault to prevent similar events in the future.