UPDATE 1-FPL cites human error as cause of Florida blackout

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HOUSTON, Feb 29 (Reuters) - FPL Group's (FPL.N: Quote, Profile, Research, Stock Buzz) Florida utility Florida Power & Light said preliminary results of the investigation into Tuesday's four-hour blackout indicated that human error was the primary cause.

The blackout affected about 584,000 FPL customers in South Florida, the Juno Beach-based company said in a release on Friday.

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 that 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 FPL President Armando Olivera.

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, a more extensive outage than described by the North American Electric Reliability Corp (NERC) earlier in the week.

The generation loss automatically launched a "load-shedding" program to cut power consumption by 4,000 megawatts across the state. Most service 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.