Overload relays protect a motor from sustained overcurrent without tripping on normal starting inrush. Siemens splits the range in two: 3RU thermal relays, the low-cost workhorse for standard motors, and 3RB solid-state electronic relays, which hold a wider setting range and add phase-failure detection. Both mount directly onto the matching SIRIUS 3RT contactor or stand alone on a support.
The setting range must bracket the motor's full-load current — a relay set at its extreme end will nuisance-trip.
Match the trip class to the load: Class 10 for normal starting, Class 20 for high-inertia loads such as fans and centrifuges.
Check the frame size — a relay for an S00 contactor will not mount on an S2.
Where phase loss is a real risk (long cable runs, poor supply), the 3RB electronic range is worth the cost premium.
Frequently asked questions
Should I choose a 3RU thermal or a 3RB electronic relay?
3RU is fine for standard direct-on-line motor starting and is cheaper. Choose 3RB when you need a wide setting range on one part number, phase-failure protection, or repeatable tripping in a hot panel — its electronic sensing is less affected by ambient temperature.