The Model TTS is a water tank temperature supervisory switch preset to give a low temperature signal at 40°F/4,5°C and a high temperature signal at 140°F/60°C (±5°F/3°C). It is UL Listed and FM Approved. A bi-metal thermostat is used for low temperature sensing and a bi-metal thermostat for high temperature sensing. A diode is used in this unit as a testing aid. When the polarity of DC current is reversed at the feed wires to a TTS, the diode in the circuit prevents the flow of current, proving the absence of a short circuit fault in the wiring or the device itself.