Mechanical thermostat for controlling filter fans and heaters The contact system of the regulator is subjected to environmental influences and contact restistance may change which can lead to a voltage drop and/or self-heating of contacts If the Normally Closed contact is used, the switch temperature difference could be reduced by connecting terminal "N" (RF heating resistor). It causes thermal feedback which is subject to surrounding conditions and thus has to be determined for each individual application. Voltage only needs to be specified if the optional use of the RF resistor is desired