Bathroom Fan Fires

Bathroom fans usually fail because of wear to the motor bearing. Almost all bathroom fans use a shaded pole motor which has the lowest starting torque of any induction motor. Hence, if there is any mechanical resistance in the barring, there is a good chance that the motor will not start. This is a locked rotor condition. Experimental results show that with the rotor locked, the temperature of a bathroom fan motor can reach in excess of 340 °F. Bathroom fan motors are typically NEMA Class B, which corresponds to a maximum insulation temperature of 266 °F. Thus, the varnish insulation on the wire will start to fail, and depending on how the motor winding fails, this can cause a fire. This problem is compounded by lint build up and plastic propellers, which can ignite and/or add fuel to the fire. To prevent this, new bathroom fans have a thermal fuse, which opens at approximately 260 °F. The accompanying photographs depict actual cases and experimental results.

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