The spinning steel and plastic components of a combine, insulated from the ground by rubber tires and plastic skid shoes on small grain platforms, have been proven to create a static electric charge in machines under certain harvest conditions. Does that electrical charge create a fire risk?
Probably not.
“There is no doubt that static electricity builds up on parts of combines under some conditions, but there is no evidence to support static electricity as a prime cause [of combine fires],” wrote the late Dr. Graeme Quick, a professor at Iowa State University who did extensive research on fires in harvest equipment in his homeland of Australia as well as in the U.S., in “An Investigation into Combine Harvester Fires.”
“The static energy from a [combine] discharge ranges from about 10 millijoules (mJ) to 150 mJ in a single-discharge spark,” said Ben White, a research engineer for the Kondinin Group, an agricultural consortium in Australia. “The static electricity required to ignite a fire [in crop residue] is around 500 mJ in a continuous arc.’
White cited testing done by Chilworth Global in New Jersey, a consulting company that tests, under controlled conditions, the hazardous properties of materials during their processing. Controlled Global found that even an exceptionally large single-discharge static electric spark of 500 mJ didn’t ignite powdered crop residue because it didn’t provide the requisite continuous arc.
White acknowledged buildup of static electricity attracts fine dust on combines, especially under low-humidity conditions. Since surface temperatures of modern diesel engine exhaust systems often exceed 500 degrees, accumulations of dust near exhaust components can grow large enough to ignite without an actual static electric discharge. Dust accumulations become a fuel supply and exhaust heat a source of ignition.
Quick noted in his report a distinct bias for combine fires to be in the left rear quarter of the machines. Exhaust manifolds, turbochargers, mufflers, diesel particulate filters and other exhaust components, sitting downwind from high-volume cooling fans, are prone to collect crop debris. The fact many combine fires are on the left rear sidewall of machines suggests debris lodges near exhaust components, ignites and becomes embers easily dislodged by fan blast, which then fall onto pockets of debris lower on the machine.
Fires in other areas of combines, when investigated, were identified as caused by overheated bearings, sparks from metal components rubbing together or electrical system short circuits.
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