Two Hawaiian Electric Co. workers were injured yesterday, one of them critically, in an explosion in an underground electrical vault at Fort Street Mall that left 1,100 Downtown customers without power for most of the day.

HECO spokesman Darren Pai said an investigation is under way to determine the cause of the blast, which occurred at 10:18 a.m. and immediately knocked out power in parts of the Downtown business district.

Lynne Unemori, HECO vice president for corporate relations, said the incident occurred as a crew was "doing scheduled work on the underground electrical system in preparation for replacing a transformer."

"It was like a bomb going off," said Edith Alcalde, who was working across the street at the Aloha Sushi convenience store inside the 1132 Bishop St. high-rise building when the blast occurred.

Alcalde said she looked out the store window about 20 feet from the vault access opening on the Fort Street Mall sidewalk and saw heavy gray smoke coming out.

Honolulu firefighters arrived at 10:20 a.m. and found both injured workers already outside the vault.

"We don't know who pulled them out, but one of them appeared to be more seriously injured than the other," said Honolulu Fire Department spokesman Capt. Terry Seelig.

Bryan Cheplic, spokes- man for the city Department of Emergency Services, said one of the men was initially deemed to be in critical condition but was upgraded to serious condition shortly after his arrival at the hospital.

Yesterday afternoon, HECO said the most seriously injured worker was improving and the other had been treated for minor injuries and released.

Seelig said HECO workers used hand-held extinguishers to snuff out the fire, then pumped carbon dioxide into the vault to displace the oxygen and make sure the blaze was completely extinguished.

The site was cordoned off with yellow "caution" tape and attracted a few dozen curious onlookers, including a number of Hawai'i Pacific University students and faculty members.

BUSINESS DISRUPTED

The explosion disrupted several businesses right before their usual lunch rush. Manager Fely Peria and two of her workers stood in a darkened Subway sandwich shop at 11 a.m., putting things away for the day.

Peria said the shop opens at 7 a.m., with the lunchtime crowd showing up in numbers between 11 and 11:30 a.m.

She said her two workers also described the incident as sounding like a bomb had gone off.

The explosion left several key Downtown intersections on King and Beretania without traffic lights, and police officers were deployed to direct traffic.

Power was restored at 3:15 p.m. Pai said the situation does not pose a danger to public safety.

PROBLEMS IN PAST

Underground electrical cables in the Downtown area have been a source of problems in the past for HECO.

Three covers blew off utility company manholes on Punchbowl Street during a three-week period in May and June 2002. One of the cast-iron covers flew 12 feet into the air, striking the branches of a tree in front of the state Circuit Court building.

In the other two cases, the covers were lifted slightly off the ground.

Two blasts, on Oct. 4, 1996, blew covers off a manhole and utility vault on Richards Street, almost at the doorstep of HECO's corporate headquarters .

There were five more mishaps through January 1997, which led HECO to rewire the Downtown circuit and begin replacing all 2,000 of its solid, 150-pound manhole covers with vented ones.

However, Unemori said that yesterday's explosion did not have anything to do with prior Downtown cable problems.

The state granted the utility company a 3.3 percent rate increase in September 2005. The company said it planned to use the anticipated $41 million in additional revenues to pay for major capital improvements the company had completed during the preceding 10 years.

Among the capital investments listed at that time was the replacement of major cables in the Downtown area.

Unemori said all work was completed and that extensive work had been done to replace almost all of the lower voltage cables in the Downtown area.