Thermal Fuses (also named thermal cut out or TCO)
Thermal fuses open the circuit if the heater exceeds the selected fuse temperature. They are a one time over temperature protection device that can prevent heater runaway in the event of a control device failure.
As the TCO body is electrically live, they must be covered by a silicone rubber/fiberglass foil or boot vulcanized or bonded to the heater body