Florida Power & Light issued a preliminary report saying that a field engineer was to blame for Tuesday's failure, which affected more than 584,000 customers, or the equivalent of more than 1 million people.
The engineer was investigating a malfunctioning switch at one of the power company's substations in west Miami when he disabled two levels of protection for the system, officials said.
While he was making measurements of the switch, a circuit shorted, making a loud noise and smoke that was reported as a fire. Normally, the protection system would have contained the consequences of the short circuit, but because both levels of protection had been removed, the problem cascaded to other parts of the system.
In total, 26 of the company's 435 transmission lines and 38 of its 600 substations were affected. Two nuclear reactors and a natural gas unit at Turkey Point south of Miami shut down protectively. Two other FPL plants were also affected.
"We don't know, still, why that particular employee took it upon himself to disable both sets of relays," FPL president Armando Olivera told reporters.
The employee, who had "significant tenure," has been put on paid leave during an investigation, Olivera said. It would have been appropriate for the employee to disable one, but not two, levels of protection while making measurements, he said.
A full investigation could take months, Olivera said.
