SIRIUS 3RN1 Thermistor Motor Protection

Siemens - Thermistor Motor Protection

3RN1 thermistor motor protection relays offer substantial advantages wherever current-dependent protection with circuit-breakers or overload relays is simply not enough: in certain situations overheating of a motor may occur, which won't be detected by the thermal image in the circuit breakers or overload relays.


  • Heavy starting (e.g. centrifuges)

  • Frequency converters

  • Frequent switching

  • Impeded cooling, e.g. due to soiled environment
    (paper and textile industry, cement works)

  • Braking operations

  • "Warning and tripping" function by using two sensor circuits with different response temperatures in order to be able to react before shutdown (e.g. switch in additional cooling, reduce load)

  • Multimotor protection with a single device, e.g. for
    conveyor lines where several motors need to be
    switched off simultaneously

Advantages at a glance

  • One device covers all motor powers

  • Universal use through double terminal labeling for monitoring relays and overload protection equipment 

  • High contact stability due to models with hard-gold-plated contacts - for use under difficult conditions

  • Simple and fast troubleshooting thanks to display of wire breakage and short- circuits in the sensor circuit

  • Wide voltage power packs reduce the number of different versions

  • Various technical approvals: ATEX for dust and gas, UL, CSA, several shipbuilding classifications