The PRM10 resistive relay is used to protect electrical devices (e.g. motor) against a dangerous increase in temperature. Temperature measurement is carried out by external PTC temperature sensors that can be connected in series. An increase in the temperature on the sensor beyond the safe value causes the output relay to operate. The PTC sensor is galvanically isolated from the mains by an internal isolation transformer. The set includes a PTC sensor that causes the system to trip at a temperature of about 100 °C.