The Siemens 3RB1253-0FM30 is a Size S6 overload relay for full motor protection, rated 50-205 A with adjustable Class 5-30 trip curve. It covers the higher end of the SIRIUS 3RB1 family — this is the relay you spec for a 90-110 kW motor at 400 V, not a small pump. Main circuit connection is via busbar (no screw terminals on the power side), while the auxiliary circuit uses screw terminals. The control supply is AC 220-240 V, 50/60 Hz. It supports manual, automatic, and remote reset modes. Integrated ground fault detection is built in — this is a differentiator versus basic thermal overloads. The relay provides monostable outputs for overload warning, meaning the signal drops on trip and stays dropped until manually reset or power-cycled, depending on the reset mode selected.
If you need this exact Size S6, 50-205 A relay with ground fault detection, the sourcing posture is: we source it from our multi-region network against an RFQ. Genuine is genuine — same part, different passport — but the warranty path runs through us, not through Siemens direct.
The 50-205 A range and Class 5-30 adjustment means this relay covers a wide motor FLA band — you can set it for a 55 A motor on a small conveyor or a 200 A motor on a large compressor, and dial the trip class to match the start-up time. Class 5 for short acceleration (centrifugal pumps), Class 10 for normal start (fans), Class 20 for high-inertia loads (flywheels), Class 30 for extreme starts (crushers). Size S6 is a physical frame — it mounts on a busbar system or individually. In a panel, that means you need the S6 busbar adapter or direct busbar drilling pattern; it does not snap onto a contactor like smaller S0/S2 sizes. Plan the panel layout accordingly — it is a substantial block, not a DIN-rail clip-on. Ground fault detection is integral, not an add-on module. That saves a panel component and wiring, but verify the ground-fault trip level is compatible with your system's leakage — it is designed for solidly-grounded systems, not IT or corner-grounded delta configurations without checking the application note.