A consultant hired by Hawaiian Electric Co. to investigate December's lightning storm power outage on Oahu concluded the utility could not have reasonable been able to prevent the initial outage or stop it from spreading island wide.

The overall investigation was conducted by Power Engineers of Hailey, Idaho. HECO additionally hired Electric Power Research of Palo Alto, Calif., to perform a lightning performance study on the main part of HECO's system that showed signs of being struck by lightning during the outage on Dec. 26.

Among other things, the investigations concluded the outage was triggered by lightning strikes on or near HECO's 138 kilovolt transmission system. Of five short circuits attributed to the lightning strikes, one was a "rare short circuit of all three phases of the Kahe-Waiau transmission line that set into motion a series of automatic protective relays and actions to protect the electrical system generation and transmission facilities," HECO said in a news release.

"These events ultimately resulted in the island-wide shutdown of the Oahu electrical system."

The two consultants made several recommendations, largely technical in nature.