Aging underground electrical cables were blamed for both blasts Aug. 15 and Dec. 29, which caused no injuries. City officials had pushed Georgia Power to make short-term safety improvements while it replaces 80 miles of old cable a $51 million upgrade that won't be finished for several years.
Georgia Power expects its initial order of 25 manhole restraints to arrive in March, but they'll take several months to install, company spokeswoman Swann Seiler said.
If another explosion happened, the latches would allow manhole covers to pop up 2 inches to release pressure from expanding gases, while preventing the covers from becoming airborne missiles.
The devices also prevent oxygen from rushing in and fueling secondary explosions, said Jim Justice, vice president of sales and marketing for the manufacturer, Michigan-based Stabiloc. About 1,200 of the company's manhole restraints have been installed by the utility Detroit Edison in southeast Michigan.
Meanwhile, crews in Savannah are working to replace the old cables in the underground network, some of which are 50 to 80 years old.
"We have expanded our work hours, we're working on the weekends," Seiler said. "This project has the full support of the entire Georgia Power organization. It is a priority with everyone."
Both explosions rattled buildings, blacked out power to downtown homes and businesses, and shot columns of smoke and flame from manholes after blasting off their covers. Georgia Power determined the blasts were caused by cable with deteriorating insulation that overheated and caught fire.
Michael Brown, Savannah's city manager, praised the utility's plan for manhole anchors as an important safeguard that can be put in place quickly before the power grid upgrade is finished in 2012.
But Brown said city officials would like to see more short-term measures to reduce the chance of future explosions.
"I'm still worried, in this interim period, to see if we're actively and comprehensively finding all of our potential problem spots," Brown said. "In addition to keeping control of these spots, how can we identify them and somehow detect or predict them?"
Downtown Savannah has about 400 manhole covers, but not all of them will be fitted with safety anchors. Seiler said Georgia Power will likely order more after the first 25 are installed, but she couldn't give an estimate for how many.
She said safety latches will be installed in areas with heavy pedestrian traffic where the old underground cables have yet to be replaced.
Atlanta-based Georgia Power took over the aging, underground network in 2006 when it absorbed Savannah Electric, which provided electricity to the city for more than a century.
The utility began work two years ago to upgrade the underground network and increase its capacity from 4,000 to 13,000 volts. The project was scheduled to take a decade to complete, but the back-to-back explosions prompted Georgia Power to speed up the work and finish the job twice as fast.