Working in cooperation with the U.S. Consumer Product Safety Commission (CPSC), Whirlpool recalled the units because they had received seven complaints of fire and did not want to risk customer injury or property damage. The CPSC helps protect the general public from unreasonable risks of serious injury or death from consumer products under the agency’s jurisdiction. More often than not, manufacturers of such products work with the CPSC to voluntarily recall products from the market that may pose a risk to individuals.
Seven cases were reported to the CPSC where a fire resulted in the microwave unit. In those cases, the microwave unit was in operation at the time it caught fire. Exponent performed a fault tree analysis led to the identification of a “hotspot” (location of high intensity microwave energy) situated at the top of the unit above a mica plate separating the cooking chamber from the distributor fan and wave guide duct. Because the mica cover at the top of the unit was porous, it was susceptible to the seepage of splashed food oils through its surface. Once across the porous cover, oils condensed at the base of the wave guide where the hotspot caused ignition. The upper steel surface of the wave guide duct was then heated to temperatures high enough to ignite adjacent plastic ducting that served the vent hood. It was ignition of the hood’s plastic ducting that caused the observed cases of fire.
To correct the design, Whirlpool replaced the mica plate with a nonporous material, sealed the air duct from food vapors, and added a high limit thermostat at the top of the unit to monitor temperature and ensure safety.