About 40,000 customers were without power for 45 minutes, although some customers reported outages lasting several hours.

HECO had de-energized one of two transmission lines that bring power from the Ko'olau substation in Kane'ohe to the Pukele substation in Palolo because of scheduled maintenance. To do that, workers opened circuit breakers at each end just after 6 a.m.

A digital microwave communication system is used to ensure that the line remains de-energized. But at 7:42 a.m. the system malfunctioned, prompting the circuit breakers on the other transmission line to open, HECO spokesman Chuck Freedman said. During its investigation, HECO found four other parts of the communication system that could fail under similar circumstances and modified them as well, Freedman said.

"This is the first time on record that a protective signal was misrouted and inadvertently de-energized a transmission line on the HECO system," Freedman said.

HECO has replaced the defective equipment and is in the process of modifying the signal used by the microwave communications system to ensure that a misrouted signal does not accidentally open a circuit breaker, Freedman said.

"If we hadn’t taken the actions we have taken, those places on the line would have been vulnerable to an outage," Freedman said. "We fixed it."

Customers in Waikiki, Manoa, Palolo, St. Louis Heights, McCully, Mo'ili'ili, Kaimuki, Diamond Head and Kapahulu lost power. HECO estimates that represented about 14 percent of the electricity demand that morning.

HECO did its own investigation but also brought in Power Engineers Inc., a Mainland consulting company, to review its findings.

"This was a very serious failure and a reminder that unforeseeable events can happen on utility systems which can lead to large-scale outages," Freedman said.

The line that failed — a 138-kilovolt transmission line from Kane'ohe to Palolo Valley — is one of two that cross the Ko'olau Mountains. It belongs to a network of power that HECO sought to bolster with a third line until public pressure forced the company in 2002 to seek an alternative route underground.