FPL: Florida power outage caused by human error

(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.