Hawaiian Electric Co. "could not have reasonably prevented" an islandwide blackout on Oahu that was triggered by a lightning strike on Dec. 26, 2008, the Public Utilities Commission has ruled.

Conclusions the PUC reached in its investigation of the incident were similar to those in earlier reports by private consultants for HECO and the state Office of the Consumer Advocate.

The 73-page PUC report released Tuesday said the "triggering event" for the 2008 outage was a lightning strike or strikes on all three wires that make up HECO’s main transmission line between the Kahe and Waiau power plants. The lightning touched off cascading events that left most of Oahu in the dark for more than 12 hours.

The outage drew international attention because President-elect Barack Obama and his family were vacationing in Kailua at the time and were left without power.

"Due to the relative infrequency of lightning-induced three-phase faults on Oahu, as well as the large magnitude of the particular strike events which triggered the power outage, the commission finds that HECO could not have reasonably prevented the damaging effects of the lightning strikes to prevent the power outage," the PUC commissioners wrote in the decision and order.

The PUC said HECO responded to the outage in a "prudent and efficient" manner that minimized delays in bringing the grid back up.

HECO said it has made several improvements to its systems as a result of what it learned from the blackout.

The 2009 addition of a 110-megawatt combustion turbine generator with "black start" capability at Campbell Industrial Park means that HECO will be able to more quickly energize its electrical grid in the event of a large-scale power outage, said spokesman Darren Pai. HECO also made changes to the way a 208-megawatt plant, operated by an independent power producer, is connected to the grid so that it can continue to operate when the rest of the grid is down. In addition, HECO expanded training programs for its workers and is required to submit to the PUC annual progress reports through 2014.

The PUC’s decision also means that HECO can begin to issue final determinations on the 700 claims filed by customers for things like spoiled food and damage to computers and other electrical appliances, Pai said. The claims will be decided on a case-by-case basis, he said.