A buildup of dust and dirt mixed with light rain on insulators atop power poles can cause electricity to ‘track’ over and lead to a fire.

Since early March, about 100 of CPS Energy’s wooden electricity poles have caught fire across San Antonio. On Tuesday, blazes caused at least two power outages in parts of the utility’s service area.

CPS officials blame the fires on drought conditions and high pollen levels, causing dust and dirt to collect on insulators. Those are the devices, made of porcelain or polymer, that hold up power lines at the top of the pole and keep the power lines from touching.

When a light rain fell Monday, it didn’t dump enough water to clean off the dirty insulators sitting atop CPS’s 400,000 utility poles.

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Instead, in some instances, the sprinkle of water mixed with the dirt and pollen to create a “slush” on the poles, which can conduct electricity. When electricity “tracks” off the insulator and through the slush, it can heat up the wooden pole and cause a small fire, said Richard Medina, the vice president in charge of the city-owned utility’s massive system of power lines.

“For a lot of utilities, this is really a common thing that happens in the springtime,” Medina said. “Whenever there’s drought conditions and then there’s a forecast of rain, all our system operators are hoping for a heavy rain,” which washes away the dust and dirt accumulation.

CPS distributes power through a network of about 700 circuits across its service area.

A buildup of dust and dirt mixed with light rain on insulators atop power poles can cause electricity to ‘track’ over and lead to a fire.

When a pole goes ablaze, the utility cuts power to the circuit the pole is located within so operators can put the fire out and make repairs. Since several thousand households and businesses can be served by any one circuit, stopping the flow of power to a circuit can cause sizable outages.

CPS blacked out 900 customers when it turned off one circuit on Tuesday morning, and another 3,000 ratepayers lost power when the utility was putting out a different pole fire later the same day, Medina said.

“They de-energize the circuit so they can make repairs very quickly,” he said.

By late afternoon Tuesday, however, just seven customers remained without electricity, according to CPS.

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The blazes don’t usually cause major damage to power lines. To repair a pole that caught fire, CPS workers might replace the wooden cross arm that holds up the wires, or cut off the top of pole if the tip caught fire, Medina said.

“It’s unusual that we’ll have to replace a whole pole,” he said.

CPS Energy has about 6,000 miles of power lines underground, where vegetation and Mother Nature are less likely to halt the flow of power.

The utility has allocated $10 million to bury more wires in areas “where we have a lot of vegetation and things growing in lines and we can’t get to them,” Medina said.

But CPS still has over 8,000 miles of overhead wires, so burying the whole system would likely be prohibitively expensive.

“It is a lot of infrastructure,” Medina said.

CPS Energy urges residents who see a pole on fire to stay far away. A fire could break part of a pole and cause the power line to fall.

Ratepayers can call 210-353-4357 to report an energy emergency.