In fault condition, the STB sets the system being monitored to a safe operational state. Surface-mounting thermostats operate on the principle of liquid expansion, with a microswitch serving as the electrical switching device.
Temperature controller TR and temperature monitor TW
When the temperature at the probe exceeds the set limit, the microswitch is operated through the mechanism and the circuit is opened or closed. When the temperature has fallen below the set limit (by the switching differential), the microswitch returns to its initial position.
Lock-out facility on the temperature limiter TB and safety temperature limiter STB
When the temperature at the probe exceeds the set limit, the circuit is opened and the microswitch is locked out mechanically. After the temperature has dropped below the safe temperature limit by about 10 % of span (about 15% for limit setting > +350°C), the microswitch can be reset manually.
Use of the safety temperature monitor STW as safety temperature limiter STB
In this case, the circuit connected to the thermostat must comply with DIN 3440 and with Section 8.7 of DIN/VDE 0116.
Safety temperature limiter STB and safety temperature monitor STW (STB)
In the case of the STB and STW (STB), failure of the measuring system, i.e. a leakage of the expansion fluid, will cause the pressure under the diaphragm to drop, thus permanently opening the circuit. A reset is now impossible.
If the temperature at the probe cools down to below -20°C, the circuit will also be opened. As the temperature rises to above -20°C, the STB has to be reset manually, by pressing the reset button. On the STW (STB), the reset is performed automatically.