(CNN) -- Preliminary results of an investigation show that Tuesday's massive power outage in Florida was caused by human error, Florida Power and Light President Armando Olivera said Friday.
A field engineer was diagnosing a switch that had malfunctioned at FPL's Flagami substation in west Miami.
Without authorization, the engineer disabled two levels of relay protection, Olivera said.
"This was done contrary to FPL's standard procedures and established practices," he said.
Standard procedures do not allow the simultaneous removal of both levels of protection.
"We don't know why the employee took it upon himself to disable both sets of relays," he added.
A fault occurred during the diagnostic process, and because both levels of relay protection had been removed, the fault caused an outage ultimately affecting 26 transmission lines and 38 substations, Olivera said. Find out more about power grids and blackouts »
One of the substations affected serves three of the generation units at Turkey Point -- a natural gas unit and both of the plant's nuclear units.
Both the nuclear units automatically shut down due to an under-voltage condition, he said. Also affected were two other generation plants in FPL's system. The total impact to the system was a loss of 3,400 megawatts of generating capacity.
The error affected 584,000 FPL customers, Olivera said. Another 500,000 non-FPL customers also lost power.
That translates to about 3 million people. See photos of the blackout's impact »
The employee has been suspended with pay as the investigation continues, he added.
"The employee realized something had gone wrong, but I think it's fair to say the employee didn't recognize the extent or magnitude of the problem," Olivera said.
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