WALNUT CREEK -- Metallic Mylar balloons caused an hourlong power outage Tuesday, disrupting power for an estimated 13,000 customers, trapping people in elevators and knocking out traffic signals.

The outage was triggered by Mylar balloons that touched down on a power transmission line located next to PG&E's substation in Walnut Creek, said agency spokeswoman Tamar Sarkissian.

"That 115-kilowatt line enters the substation, so when the balloons made contact, they actually damaged equipment inside the substation, as well, triggering a sizable outage," Sarkissian said.

The power outage threw out traffic signals throughout Walnut Creek and caused at least two elevators to shut off with people still inside, according to the Contra Costa County Fire Protection District.

Power was restored about one hour later and no accidents or injuries were reported.

Metallic balloons triggering power outages is a frequent occurrence, with more than 57 such instances reported last year throughout the East Bay, according to PG&E.

"A lot of people will release balloons after a party, but they don't realize that the metal in the balloon conducts electricity and can bring down a power line," Sarkissian said.

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