White Westinghouse refrigerator door was accidentally left open over night when it was humid, and moisture condensed on the interior. In the morning the arc fault circuit breaker was tripped. Tried letting the unit dry out, but it would still trip the breaker as an arc fault. Started unplugging various components, compressor, evaporator fan, finally the defrost heater before it could be plugged in without tripping the breaker. Although the defrost heater's resistance measured normally, when it was isolated from any metal and plugged in, 110 vac could be measured between the outside of the heater and chassis ground. Apparently, the moisture had broken down the insulation inside the defrost heater, although it could not be measured with an ohmmeter. A new defrost heater was ordered, it came promptly, and the refrigerator is back to normal.