About 25,000 to 45,000 homes on O'ahu lost electric power this week as gusting winds of up to 69 mph knocked branches into power lines, causing them to short out.

Power was cut for several hours in some neighborhoods, forcing at least one school to close and leaving scores of traffic lights dark.

To prevent similar outages, the power company would have to reinforce overhead lines or place lines underground at a cost of billions of dollars, utility experts said.

"The cost of the outage is lower than the cost of the upgrade," said Gerald Heydt, professor of electrical engineering at Arizona State University and a leading expert on electric power. "It's probably impossible to build a system that would absolutely protect against power outages caused by the wind."

It would cost anywhere between $8.2 billion and $11.3 billion to place power lines, as well as cable television and telephone lines, under ground, according to a 2003 report by the Honolulu chapter of the American Institute of Architects.

Even when spread out over 20 to 30 years, the costs would add as much as $162 to the average consumer's monthly electric bill.

NOT AS BAD AS OCT. 15

Unlike the Oct. 15 earthquakes that caused an islandwide blackout for most of the day, Hawaiian Electric Co. said this week's outages were relatively brief, in some cases lasting as little as 15 minutes.

Hawaiian Electric Co. spokesman Peter Rosegg said most of the service disruptions occurred when winds sent tree branches and other debris into power lines, causing short circuits.

On several occasions, the high winds caused overhead power lines to knock into each other, causing what's known as a "swing short."

Hundreds of HECO customers in the Waimanalo area lost power Wednesday when an ironwood tree knocked down a utility pole near the Waimanalo Polo Fields.

University of Hawai'i meteorologist Tom Schroeder said Windward area residents experienced the strongest winds due to their southeasterly direction.

The so-called kona winds, which flow in the opposite direction to the usual northeast trade winds, pick up speed and sometimes double their pace as they cross over the Ko'olau Range and down the mountains' steep windward side, Schroeder said.

"The shape of the mountain helps amplify the winds as it passes over the mountain," said Schroeder, director of the UH Joint Institute for Marine and Atmospheric Research.

"It can result in the doubling of the winds."

Rosegg said HECO typically trims trees next to power lines to prevent trade winds from causing contact with branches and other vegetation.

PHONE COMPANY DID OK

With kona winds, trees and palms that normally would be blown in the opposite direction from the poles and lines are pushed toward them, causing contact, he said.

Unlike HECO, Hawaiian Telcom said it suffered virtually no service disruptions this week. The local telephone company jointly shares 55,000 overhead power lines on O'ahu with Hawaiian Electric.

But Hawaiian Telcom's lines do not short out since they don't carry electricity. And the phone lines are reinforced with steel wires, the company said.

"Of course we don't want to tempt Mother Nature, but Hawaiian Telcom has been taking steps in recent years to prepare for events like the recent winds," said company spokesman Dan Smith.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com.