UPDATE 1-US FERC opens probe into Florida power blackout

(Adds details, background)

WASHINGTON, March 19 (Reuters) - The Federal Energy Regulatory Commission (FERC) on Wednesday said it has launched a formal probe into the power blackout that hit Florida on Feb. 26.

FERC said it was initiating a nonpublic, formal investigation into whether mandatory federal reliability standards were violated in the event, which affected about 1 million customers of four utilities across the state.

FPL Group's (FPL.N) Florida utility Florida Power & Light said it would cooperate with the FERC probe, and its own preliminary investigation found that human error was a "primary factor" in the event.

The blackout affected about 584,000 FPL customers in South Florida.

Florida's grid operator and the the North American Electric Reliability Corp (NERC) -- the U.S. power grid watchdog group -- are also investigating the event.

FPL's blackout was the largest to fall under tough, mandatory reliability standards and penalties put in place after the August 2003 Northeast blackout, which left 50 million people without power.

Under the new rules, FERC can levy civil penalties up to $1 million a day for violations.

"The Florida blackout was a significant event that affected millions of Florida residents," FERC Chairman Joseph Kelliher said. "We must now determine whether the Florida blackout was related to violations of the standards and to apply any lessons we learn to improve the system."

FPL said a field engineer who was working on a power substation in Miami disabled key relay protection systems without authorization.

Disabling the systems triggered a severe drop in system voltage that caused 3,400 megawatts of electric generation capacity, including the Turkey Point nuclear plant, to "trip" offline as a protective measure.

Most service was restored in less than five hours. (Additional reporting by Eileen O'Grady in Houston)