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A malfunction at a single electrical substation caused a cascading blackout that shut down a nuclear plant Tuesday and briefly cut power to about 3 million people from Daytona Beach through the Florida Keys.

Authorities said there were no safety concerns at the nuclear plant. The outages initially affected about a fifth of Florida's population, but power was quickly restored. Only about 20,000 people lacked electricity during the traffic-snarled evening commute home, but that was mainly due to bad weather.

The equipment malfunction in a substation near Miami disabled two power distribution lines between Miami and Daytona Beach, and in response, Florida Power & Light's Turkey Point nuclear plant south of Miami stopped operating around 1 p.m., Nuclear Regulatory Commission spokesman Kenneth Clark said.

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The utility said it was trying to determine what caused the equipment failure and fire at the substation that led to the outage. The company said such equipment failure should not have caused the widespread blackouts, so teams were investigating.

An FPL spokesman initially said its nuclear plant caused the outages. But the utility's nuclear spokesman, Dick Winn, later said the electric grid problems caused both Turkey Point reactors to shut down.

"All the safety systems worked just like they were supposed to and both of those units are in stable condition right now," he said. Clark agreed that there were no safety concerns.

The outages have no connection to terrorism, Homeland Security Department spokeswoman Laura Keehner said. Miami-Dade County Mayor Carlos Alvarez said the outages were technical, not criminal.

"It's a matter of just a cascading effect," he said.

Florida emergency management officials said the outages cut power to about 3 million people on a day when temperatures soared into the mid-80s.

Bob Wild, a sports marketing consultant who lives in Miami's southern Kendall neighborhood, said he didn't even notice the outage, thanks to his home's generator.

"We're a hurricane family. We've been though Hurricane Andrew and everything before and since," he said. "Our daughter called us from Washington and said she'd seen the blackouts on TV. That's when we found out."

The Federal Energy Regulatory Commission, which has responsibility for electricity grid reliability, said it wants to know whether there were any violations of federal grid reliability rules.

Outages were concentrated in the southeast portion of the state, including Miami, but were also reported in the southwestern and northeastern parts of the state as well as in the Florida Keys.

Jaime Hernandez, a spokesman for Miami-Dade County Department of Emergency Management, said no injuries were reported. Officials said Miami International Airport, the Port of Miami and the area's rail and bus transportation were working normally, although some places briefly relied on generator power.

Several Miami-area hospitals switched to backup generators when the power went out. Students left Miami-Dade schools on time, and school buses were running.

By 2 p.m., most of northern downtown Miami appeared to be back to normal operation, including a campus of Miami Dade College and numerous stores and businesses. In the Florida Keys, spokesman Andy Newman said areas were without power for about 30 minutes.

In Miami's western suburb of Doral, disappointed office workers sat under an awning at a strip mall, unable to buy lunch nor go anywhere else due to a sudden downpour. At Starbucks Coffee Co., employees began handing out sandwiches they feared would go bad.

Nelson Suarez, 35, a manager for Asia sales at World Fuel Services, enjoyed the free lunch.

"I can't work anyway since all the power is out, so at least something good came out of this," he said.

In central Florida, the sheriff's office in the Daytona Beach area confirmed power outages at traffic signals across its jurisdictions. CiCi's Pizza in Port Orange lost power for roughly two hours during their busy lunch rush.

"We told the customers that we were out of power and then they just left because we didn't have any ovens, any pizzas," manager Frances Cruz said. "We lost a lot of money today."

The first of Turkey Point's two nuclear power units started operation in 1972. In March 2006, a tiny hole was found in a coolant pipe at the plant. The FBI determined it was vandalism, not sabotage. An out-of-state contractor hired to do routine maintenance was suspected of drilling the hole, the FBI said at the time. The public's health and safety were not at risk, the Nuclear Regulatory Commission said then.

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