The frost protection thermostats are fitted with liquid-filled probes made of copper that act as temperature-dependant pressure probes. A change in temperature causes a pressure change in the probe, which is transferred to a switch by a bellows system. Probe and bellows system are connected with a copper capillary tube. A compression spring acts as a counteracting force. The switching values are set by changing the initial stress of the compression spring with a setpoint spindle. The probes in the frost protection thermostats have been designed as capillary tubes and are active along their entire length. Switching occurs when the set temperature acts on approximately 30 cm of the probe. The instruments are fail safe, so that they switch off if the probe is damaged or ruptured. A second switch contact may be fitted to the instruments as an option. This allows safety measures to be taken before switching off. The switching difference between both switching levels is 5 K.