The TH11 and TH21 consists of two nickel-plated supports, held together with ceramic pins. One support holds the high-performance Klixon® bimetal disc, which, in combination with the sophisticated contact system, guarantees the superior cycling performance. One ceramic pin has a layer of resistive material, functioning as a small heater when a voltage is supplied. A wide temperature range, standard 5K tolerance, different bimetal resistivity and various optional terminal configurations make the TH11 and TH21 suitable for a wide range of applications. Whereas the TH11 operates at 230 Vac.

The TH21 is designed for 120 Vac applications. Because of their identical dimensions, the TH11 and TH21 can be easily exchanged with the auto reset thermal protector TH10. The operating principle of the TH series is both simple and effective. A current flows through the resistive Klixon® bimetal disc. When a fault condition occurs, the increased ambient temperature causes the bimetal disc to snap open the contacts.

The resistive layer spots the voltage over the open contacts and a current flows through the resistor, generating sufficient heat to keep the bimetal warm and the contacts open.
When the power is switched off, the device cools down to a safe temperature and the contacts will close.