Almost every single thermostat failure is due to a short circuit on the heater that damages the sensitive thermostat triac.
Poor quality bulbs that frequently fail in a particular way, are the main cause of triac failure in Dimming thermostats.
Because of the potential damage that this causes HabiStat are now fitting a cartridge fuse to protect the circuit from such heater failures.
These fuses are specifically designed to protect triacs and only these should be used when replacing the existing triac protection cartridge fuse in the side of the thermostat.
Please note that these are specifically for the replacement of the cartridge fuse and do not replace the normal 3 amp fuse that can still be found in the plug.