AD & SD Miniature Thermal Cutoff Devices Bourns AD and SD Miniature Thermal Cutoff (TCO) devices are resettable mini-breaker devices offering advanced overtemperature protection technologies. These TCO devices use a highly-rated corrosion-resistant bimetal mechanism for better endurance in humid environments. The SD and AD TCO devices are either non-automatically resettable (self-hold function) or automatically resettable (non-self-hold function). These TCO devices feature overcurrent protection and control abnormal, excessive current virtually instantaneously, up to rated limits.