FPL Group's utility Florida Power & Light Company said preliminary results of the investigation into the four-hour blackout on February 26, 2008, indicated human error was the primary cause.

According to the utility, the blackout affected about 584,000 FPL customers in South Florida. A total of about 1 million customers of four utilities across the state lost power as emergency safety systems were activated to prevent long-term damage to power plants and transmission equipment.

FPL said preliminary findings indicated a field engineer diagnosing a problem switch at a west Miami substation disabled two levels of relay protection systems without authorization. Those safety mechanisms are designed to isolate problems to one piece of equipment or one location.

Disabling both systems was contrary to FPL’s standard practice, said Armando Olivera, FPL president. He said a voltage drop at the substation could not be contained. The situation triggered other FPL safety systems, forcing the shutdown of 26 transmission lines, 38 substations and 3,400 megawatts of generation.

The generation loss automatically launched a “load-shedding” program to cut power consumption by 4,000 megawatts across the state. Most service across Florida was restored in less than five hours.

The field engineer was suspended, and FPL has taken steps to review standard procedures with its employees to prevent a recurrence, Olivera said.

“While in this instance, we failed to perform to our expected standards in regard to reliability, safety was never an issue, and the safeguards built into our system worked as intended,” Olivera said in a statement. “To this point, we have no indication that there are any deficiencies with the design of our facilities or with our maintenance procedures.”

FPL’s blackout was the largest to fall under tough, mandatory reliability standards and penalties put in place after the August 2003 Northeast blackout.