[G6ONEDave]

The device works by passing current through the wire windings which act like a heating element. I would guess that the wire is nichrome or similar. The heat from the wire is transferred through the insulator into the bi-metal strip, which in turn if hot enough will bend and therefore break the circuit. Some of the domestic appliances using this method would auto reset, such as thermostats, whilst others had a small reset button to remake the circuit. It would need to be exceedingley hot to make it trip by ambient air temperature. Given that it's tripping after a period of time suggests a slow increase of temperature, possibly caused by a slight over current. Perhaps due to a leaky motor run capacitor or other component that is likewise becoming leaky. Things to check are the supply voltage ratings, is it designed for 250, 240, 230 or 220v mains supply etc. If it is designed for 220 operation, then 230 might be enough to upset it over time. Also what frequency was it designed to work on 50 or 60HZ. It's possible that the equipment is not actually faulty, just being over run supply wise. This is just my thoughts about it.
Dave