State regulators on Friday stepped up their inquiry into the safety of Peco Energy Co.'s smart-meter installation program, which the utility suspended last month after some devices caught fire.

The Pennsylvania Public Utility Commission scheduled an informational meeting for Thursday at its Harrisburg offices and has summoned representatives from Peco and its three meter vendors.

Peco, in a formal written response Friday to questions from the PUC, updated the number of meters that have overheated from 15 to 26. Some resulted in fires that damaged the area where the meter is mounted to the wall, and three caused fires that spread beyond the meter, said Cathy Engel Menendez, Peco's spokeswoman.

In the most recent incident, a meter flashed and caused minor damage to the exterior of a home on Sycamore Ridge Drive in Levittown, said James McGuire, the Middletown Township fire marshal.

Peco's suspension of its smart-meter installation program has complicated the utility's $650 million plan to upgrade the region's electrical-distribution system. The utility received a $200 million federal grant to install 600,000 meters by the end of 2013.

Pennsylvania requires all large electric utilities to install the meters, which take frequent readings and allow suppliers to offer hourly pricing. The meters also improve utilities' ability to manage outages.

In its filing Friday, Peco blamed seven of the 26 overheating incidents on "pre-existing" conditions with customer equipment and seven on "localized" conditions, including water infiltration. Twelve incidents are still under investigation.

Some of the incidents with water infiltration were attributed to the way the installers had secured a metal ring around the meter, Engel Menendez said. One involved a meter situated beneath a leaky gutter. Another occurred after a homeowner had power-washed the wall around the meter, she said.

All 26 incidents involved meters manufactured by Sensus Metering Systems Inc., but the utility is not blaming the devices.

"We're not seeing the meter as necessarily being the catalyst," Engel Menendez said. She said the utility has hired two outside forensic experts to evaluate the meters.

"We want to determine what's going on so we can move ahead," she said.

As of Wednesday, Peco had installed 201,642 meters, including 155,326 manufactured by Sensus, which is based in Raleigh, N.C., and 46,000 manufactured by Landis & Gyr AG, a Swiss company. About 30,000 of those were swapped for Sensus meters.

Peco is also buying meters from Elster Group S.E., a German company for industrial and commercial customers.

Peco is installing new software so the meters shut down automatically if they overheat.

Customers with concerns about their smart meters can call Peco at 855-741-9011.