With more than 500,000 Connecticut residents still without power from Tropical Storm Isaias, fire departments are issuing warnings about the potential dangers of running a generator indoors — or inadvertently setting one on fire.

Generators should be shut off and allowed to cool before homeowners try to refuel them, according to the National Fire Prevention Association.

“Never refuel a generator while it is hot,” the association said in an infographic shared by the Danbury Fire Department on Facebook.

Generators should be run outside, several feet away from a home — at least 5 feet from any doors, windows, or other openings potentially dangerous exhaust fumes could make their way into living quarters.

That means never running a generator in an attached garage, even if the sliding door is left open, officials said.

“We’re seeing a huge uptick in carbon monoxide-related incidents related to generators,” Brookfield Fire Chief Andrew Ellis said Thursday.

He said crews have responded to at least a dozen incidents of carbon monoxide alarms being triggered by generator fumes. One was set up so the exhaust was drawn into a central air conditioning intake, according to Ellis.

A small amount of carbon monoxide — an exhaust gas produced by an internal-combustion engine — can poison a person over a long period.

“Carbon monoxide is a poisonous gas, and will kill you. It cannot be smelled. it can only be detected with special equipment. You need working smoke and carbon monoxide detectors that are less than 10 years old,” Redding Fire and EMS Co. 1 said in a Facebook post Wednesday.

Generators used to power household appliances should be hooked up to the home’s electrical system by an electrician — otherwise residents risk “backfeeding” electricity into the grid, which can pose problems for anyone handling electrical wires.

It’s another reason to avoid touching downed electrical wires.

“The only people that can do this are power line crews, and they're on their way. Even if there is a 0.0000000001 percent chance of a line being live, there is a 100 percent chance of it killing you if it's live,” the Redding fire post said.