Ventilation Fan Fires: Overheated Windings Lead to Failed Thermal Limit Switch

Over the last several years, the forensic community has experienced an increase in the number of fires which have been attributed to ventilation fans. These failures have been anecdotally observed in Jakel motors, specifically model numbers J239-050-5138 and J238-050-5494, manufactured between 2000 and 2003. While there have been a number of variations to motors within exhaust fans over the preceding decades, including different winding size, winding metal, and thermal cut-off (TCO) manufacturers, two design implementations have led to the higher incidents of failure. The first design change is the implementation of 27 American Wire Gauge aluminum winding (magnet) wire. The second, and most important, is the implementation of the Tamura Thermal Device Corporation (TAM) TCO to replace the Elmwood Sensor/Honeywell (Elsen) TCO. Through investigations and testing, the Tamura TCO has been found to be unreliable at opening the circuit within the motor when exposed to temperatures above its specified cut-off temperature (136°C). Thermal testing of a Tamura TCO showed that even when exposed to temperatures as high as 260°C, the flux of the fusible thermal link is incapable of breaking down the oxides created by the metal alloy. The hypothesized failure mechanism for the Tamura TCO include increased exposure to high temperatures during fan operation and/or air infiltration from cracks in the epoxy sealant due to improper installation, along with the corrosive effects of the chlorinated flux. This work presents case studies, experimentation, and analysis of the failure mechanisms related to fires associated with the aforementioned motors.