By Daniel Kelley

DELANCO, New Jersey (Reuters) - Putrid air hung over a luncheon meats warehouse long after a blaze consumed the building where frustrated firefighters met their enemy: rooftop solar panels.

Loved by the green movement, solar panels pose a growing threat to firefighters, who may suffer electrical shocks from panels that typically cannot be turned off, said John Drengenberg, consumer safety director for Underwriters Laboratories.

Even when systems are equipped with shutoffs, any light can keep panels and their wires energized, Drengenberg said.

Gaining access to roofs gives firefighters advantages such as venting gases, and the panels get in the way, said Ken Willette, who manages the public fire protection division at the National Fire Protection Association.

In Delanco, New Jersey, volunteer fire crews rushed to the burning meat warehouse on Sunday and discovered the roof was covered in solar panels, forcing firefighters to change tactics. It took 29 hours to put out the flames at the Dietz & Watson warehouse, which was left gutted and smoldering in ruins.

"Do I think we'd have had a different outcome if we could get on the roof? Sure," Delanco Deputy Fire Chief Robert Hubler said.

Solar energy has grown rapidly over the past decade, primarily in California, Arizona and New Jersey. Risks to fire responders have prompted building codes and firefighter training, but implementation is spotty and often left to individual jurisdictions.

"We are working very closely with firefighters across the United States on the development of codes and standards. After every incident, we learn from it and improve," said Ken Johnson, a spokesman for the Solar Energy Industries Association trade group. "Firefighters don't have a good idea of how solar works. It's incumbent in us to do a better job in educating them."

Experts warn firefighters might use less aggressive tactics in buildings with solar panels, especially in instances where the fire poses little risk to human life.

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"It's an emerging challenge," Willette said. "We're seeing more of these panels installed in places that we have not seen them before."

Among the risks are shocks from panel wires that might be sliced when firefighters cut into a roof. Those wires also could come into contact with metal roofing material, causing injuries far from the roof cut, according to studies conducted by Underwriters Laboratories.

Those experiments, funded by the Department of Homeland Security, have shown that the light emitted by fire equipment can generate enough electricity in the panels that a firefighter who inadvertently touches an energized wire might not be able to let go, a phenomenon known as "lock on."

Unable to access roofs, firefighters sometimes switch goals - from actively trying to save a building to preventing flames from spreading to neighboring properties - a practice known as defensive firefighting, said Bert Davis, an engineer who performs forensic examinations at fires and studied solar markets at Carnegie Mellon University.

"I've talked to a lot of guys, and they're saying, 'We've never run into it, but if we do, we're going to fight it defensively,'" Davis said.

Emergency responders have begun receiving training to deal with solar panels in the last few years, Drengenberg said.

There is no national standard operating procedure for firefighters dealing with solar systems, but some state and regional agencies do have them, Willette said.

(Editing by Barbara Goldberg and Cynthia Osterman)