It was more than just a squirrel.
Yes, one of the critters did gnaw Friday into a power line feeding Tampa's David L. Tippin Water Treatment Facility.
But city and Tampa Electric Co. officials say that was the first in a chain of improbable events that caused the water works to lose power, leading to an unprecedented boil-water alert in effect for 37 hours for the entire city.
"We really want to figure out what caused those additional dominoes to fall," TECO Energy spokeswoman Cherie Jacobs said. "The squirrel was the first domino, but the squirrel by himself couldn't have done this. It was a combination of factors."
On Monday, Tampa Water Department director Brad Baird outlined this sequence of events.
First, the squirrel.
Before dawn Friday, it crawled into a vertical stand pipe that shields an electrical supply line that drops down the length of a power pole to go underground to the water treatment plant.
The squirrel apparently became stuck in the pipe, and, as it tried to turn around and get out, chomped into the power supply line. About 5:30 a.m., the line shorted out, killing the squirrel and catching fire.
But a water treatment plant the size of Tampa's is required to have two power service lines from two different Tampa Electric substations. So instead of getting half its power from each, the plant began drawing all its power from the second line, while the utility dispatched seven people to fix the first line.
Trouble was, the second line was sagging about 100 yards away from the plant. (City officials said they had told the utility about the sag before Friday.) About 1:30 p.m., it sagged close to the line directly below it. Electricity arced between the two. Blue flames rose. In an instant, instead of 13,200 volts going into the plant, there were 26,400.
That surge shorted out the second line and caused a third problem. A switch that controls the treatment plant's backup power exploded and caught fire.
Typically, that switch turns on the plant's diesel-powered generators when, for example, lightning knocks out power from one or both electrical substations — something that happens two or three times a year.
While the water treatment plant has advanced surge protection, this switch was not designed to withstand a surge of twice the power it usually gets, Baird said.
Once the switch was knocked out, it took 31 minutes for water plant technicians to get the generators going on a second internal power circuit that serves the plant. During that time, water pressure in the 2,200 miles of city water mains dropped. In some spots, it dropped from 70 pounds per square inch to zero.
The city restored water pressure by 2:30 p.m. Friday, but the chance that untreated water or contaminants had leaked into the system meant that, under health department rules, the city had to advise residents to boil their water until testing showed that it was safe. After tests at 25 spots found no contamination, the city lifted the alert early Sunday.
The city's response was appropriate and what environmental regulations require, said Paul A. Chadik, an associate professor in the Engineering School of Sustainable Infrastructure and Environment at the University of Florida.
True, Chadik said, a drop in water pressure doesn't necessarily mean the water supply is compromised, but it's too important a service not to play it safe.
"If people were drinking that water, would they be harmed?" he asked. "Maybe not, but you don't want to take that chance."
City officials said it would be several days before they add up the costs of the outage, which include overtime and the expense of replacing the switch that blew and some fuses.
Tampa Electric is investigating the condition and maintenance of the lines and poles involved in Friday's problems, Jacobs said.
It is common practice for the utility to close the ends of its stand pipes with animal-excluding rubber-and-neoprene caps that act like a cork in a bottle.
In all likelihood, a cap was in place, Jacobs said, although squirrels sometimes eat away at the caps and get around them. The company cannot say for sure a cap was in place, though, largely because it could have been blasted away or burned when the squirrel touched the live wire and set the pole on fire.
Power lines are designed to sag, Jacobs said. Current moving through a line creates heat and causes it to expand and sag, though the utility had not determined what caused the second service line to sag.
Tampa Electric also is investigating the city's statements that it had reported that the second service line was sagging, but as of Monday afternoon had not found much on the matter.
Engineer and consultant Paul Amato of St. Petersburg said Friday's problems underscore the importance of inspecting, testing and maintaining all parts of critical systems.
Infrastructure built by previous generations often is not getting the testing or maintenance it now needs, said Amato, the former chief engineer for the U.S. Navy base on Guam.
"I think as Americans we're going to see a lot more of these things," he said. "It's little things and it adds up. It's got nothing to do with rodents. It's basic testing, inspection and maintenance."
Tampa Electric places a high priority on system maintenance, Jacobs said, adding that the utility "invests more than $100 million every year to inspect, improve and maintain our equipment to ensure our customers receive safe and reliable electricity every day."