Bird droppings may have short-circuited a unit at the largest nuclear power complex in the United States, causing the plant to shut down last month, investigators said.
"There were eyewitnesses," said Kwin Peterson, a spokesman for the Western Electricity Coordinating Council, which is looking into the incident at the Palo Verde Nuclear Generating Station that caused outages all the way to Canada. "There was a bird on a 230 kilovolt power line west of Phoenix, and as the bird took off, it let loose as birds often do."
Investigators said they think excrement contaminated an insulator, and electricity flashed to the tower, creating a short.
At the time, utility officials said all three units at Palo Verde, plus the natural gas-fired Red Hawk power plant, turned themselves off because of a disturbance in the transmission system. But they did not know the cause at the time.
There was no radioactivity leakage or danger to plant workers or nearby residents, plant officials said. However, the loss of the 5,000 megawatts caused short power outages across Arizona to New Mexico, central California and as far as the Canadian province of Alberta.
A final report has not been completed on the cause, but evidence is strong that a bird initiated the incident, Peterson said. Normally, such an incidents would cause a circuit breaker to trip and contain the problem, but a backup relay also failed.
Peterson said the electricity council doesn't consider it to be a major incident because most of the 50,000 customers who lost power had it back on within two hours.
A final report is due in about two months.