The temperature detector P operates independent from any current supply. Temperature detection is effected by means of a bimetal disk which was first dimensioned in accordance with the required cut-off temperature TA. When this fixed cut-off temperature TA is reached, this bimetal disk  will snaps over, breaking a contact system and thereby interrupting the electric circuit of the device to be protected. After cooling down and reaching the cut-in temperature TE, the bimetal disk will automatically return into its original position and thus re-make the contact. The electric circuit is closed again.