Now, workers must find the spot where the failure occurred and repair it. The failure of an underground power cable is a rare occurrence for KUB, he said.
"We will stay on rotating shifts until we find it and get it replaced," Bolas said.
"These cables are very reliable and can last for 75 to 100 years," Bolas added. "We have very, very few cables like this that have failed, but since they are underground we have to go looking for problems. Tracking it down is the trade-off."
The outage seemed to especially affect power users in the Gay Street and Summit Hill Drive area, including the Crowne Plaza hotel, the Sterchi Building, the Commerce Building and the Emporium.
Ken Knight, general manager of the Crowne Plaza, said the hotel had to give refunds to a number of guests who did not have electricity in their rooms, and some events at the hotel were canceled when power was still not available at 8:30 a.m. April 7.
"We actually had two events that totaled 400 people that were both canceled," Knight said.
The outage primarily affected customers in the 37902 ZIP code area, which includes downtown, said KUB spokesman Jason Meridieth.
Bolas said the city's underground electric power system "is built as a spiderweb of cable, interconnected so that everywhere there is more than one source of power."
The KUB procedure for maintaining the underground electric system includes annual inspections of all accessible parts, such as transformers, manholes and vaults, but Bolas said there is no way to inspect the underground cables themselves. Most of the system was installed in the 1950s through 1970s, but some cable dates back to the 1920s.
KUB will not likely change any of its maintenance procedures because of the outage but will continue an ongoing program of replacing underground cable downtown, Bolus said.
"I can tell you that since 2007 we have spent $4 million replacing cable and we will continue doing that," he said.